General Meteorology and Climatology

Atmosphere

The atmosphere is the gaseous envelope of air that surrounds the Earth and is held to it by gravity. It is a mixture of gases — mainly nitrogen (about 78%) and oxygen (about 21%), with small amounts of argon, carbon dioxide, water vapour, ozone and other gases — together with suspended dust and water droplets. It extends from the Earth's surface up to about 10,000 km, though most of its mass lies within the lowest 30 km. It is essential for life, protects us from harmful solar radiation and meteorites, and drives weather and climate.

Vertical structure of the atmosphere

The atmosphere is divided into distinct layers on the basis of temperature variation with altitude. The boundaries between successive layers are called pauses.

1. Troposphere (0–8 km at poles, up to 18 km at equator)

• The lowest and densest layer, containing about 75% of the atmospheric mass and almost all water vapour and dust.
• Temperature decreases with height at an average lapse rate of about 6.5 °C per km.
• All weather phenomena — clouds, rain, storms, fog — occur here. It is called the weather sphere.
• Its upper boundary is the tropopause, where temperature stops falling.

2. Stratosphere (up to about 50 km)

• Temperature is nearly constant in the lower part and then increases with height.
• Contains the ozone layer (15–35 km), which absorbs ultraviolet radiation and protects life on Earth.
• Very dry, cloudless and calm; jet aircraft fly in its lower part.
• Its upper boundary is the stratopause.

3. Mesosphere (50–85 km)

• Temperature decreases again with height, reaching about −90 °C at its top — the coldest part of the atmosphere.
• Meteors burn up in this layer.
• Its upper boundary is the mesopause.

4. Thermosphere (85–600 km)

• Temperature rises sharply with height, reaching over 1000 °C, due to absorption of solar radiation.
• Contains the ionosphere, which reflects radio waves and produces auroras.
• Its upper boundary is the thermopause.

5. Exosphere (above about 600 km)

• The outermost, extremely thin layer where gas molecules are very far apart.
• Atoms and molecules escape into space; it gradually merges with interplanetary space.

Conclusion

Thus, the atmosphere is a layered gaseous envelope whose temperature varies non-uniformly with altitude, giving rise to the troposphere, stratosphere, mesosphere, thermosphere and exosphere, each with its own characteristic features.

Meteorological stations are the basic units of any weather observing network. They provide the actual, measured data on which all weather forecasting and climate study is built.

Importance of meteorological stations in weather forecasting

1. Source of real data. Instruments at these stations measure temperature, pressure, humidity, wind speed and direction, rainfall, cloud cover and sunshine. Forecasts cannot be made without such observed values.
2. Input to numerical weather prediction (NWP) models. The data are fed into supercomputer models that solve the equations of the atmosphere. More and better station data mean more accurate model output.
3. Initial and boundary conditions. Forecasting is an initial-value problem; the present state of the atmosphere must be known at as many points as possible. Stations supply these initial conditions.
4. Weather maps and synoptic analysis. Observations plotted on synoptic charts reveal highs, lows, fronts and pressure systems, which the forecaster interprets to make predictions.
5. Nowcasting and warnings. Dense station networks detect local storms, heavy rainfall, hail and wind changes quickly, allowing timely warnings for aviation, agriculture and disaster management.
6. Verification and model improvement. Comparing forecasts with later station observations shows forecast errors and helps improve models.
7. Climatology and long-range outlook. Long records from stations give normals, trends and the climatological background needed for seasonal and long-range forecasts.
8. Special services. Stations serve aviation (take-off and landing safety), agriculture (crop planning, irrigation), hydrology (flood forecasting) and industry.

Criteria for selection of a site for a surface station

The site must give observations that are truly representative of the surrounding area, so the following criteria are applied:

1. Representativeness. The site should represent the general terrain, land use and climate of the region, not an unusual spot such as a deep valley or a hill top.
2. Open and level ground. A fairly flat, open area is preferred. Sloping ground distorts wind and radiation measurements.
3. Freedom from obstructions. No tall trees, buildings or walls near the instruments. As a rule, the distance of an obstruction from the instrument should be at least ten times its height above the instrument level.
4. Adequate fetch. For wind measurement, an open fetch of about 100 times the height of the surrounding obstacles is desirable so that the wind is undisturbed.
5. Avoidance of local influences. The site should be away from lakes, rivers, irrigated fields, rocky surfaces, steep slopes and heat sources that create local microclimates.
6. Distance from buildings and roads. At least 30 m from buildings and paved roads, and away from chimneys, exhausts and dust sources.
7. Drainage. Well-drained ground is needed so that water does not collect around the instruments after rain.
8. Soil and ground cover. The ground should be covered with short, natural grass, kept trimmed, and representative of the area.
9. Accessibility. The site must be easily reachable for daily readings, inspection and maintenance, yet secure from public interference.
10. Standard exposure. Instruments should be installed at standard heights — e.g. thermometers about 1.2–1.5 m above ground in a Stevenson screen, rain gauge 30 cm above ground, wind instruments 10 m above ground — and the site should be marked and its coordinates and elevation recorded.
11. Long-term stability. The site should be free from likely future construction or land-use change so that a long, homogeneous record can be obtained.
12. Communication facility. Availability of power and communication links helps in transmitting data quickly to forecasting centres.

Thunderstorm

A thunderstorm is a local storm produced by a cumulonimbus cloud, always accompanied by lightning and thunder, and usually by heavy rain, strong gusty winds and sometimes hail. It develops where the atmosphere is conditionally unstable, that is, where warm moist air near the surface is forced to rise and continues to rise on its own because the rising air remains warmer (and hence lighter) than its surroundings. The rising air cools, water vapour condenses, latent heat is released and this keeps the updraught going. The storm is thus a heat engine driven by the release of latent heat, and its lifetime is short, typically less than an hour for a single cell.

Phases of the life cycle

A single thunderstorm cell passes through three stages.

1. Cumulus (developing) stage

• A parcel of warm, moist air is lifted by some trigger — surface heating, a mountain slope, a front or convergence of winds.
• The parcel rises, cools dry-adiabatically until it reaches the condensation level, and a cumulus cloud forms.
• Latent heat released on condensation makes the parcel warmer than its environment, so the updraught strengthens and the cloud grows upward into a towering cumulus.
• The updraught keeps the cloud droplets suspended, so no precipitation reaches the ground yet. The cloud is composed mainly of water droplets.
• Duration: about 10–15 minutes. The storm is still in its youth.

2. Mature stage

• The cloud reaches the cumulonimbus stage and precipitation begins to fall.
• Falling raindrops drag air down with them (frictional drag) and evaporation of rain cools this air further. The cooled, dense air sinks rapidly as a downdraught.
• The downdraught spreads out at the ground as a cold gusty wind called the gust front or squall line, which undercuts the warm moist inflow.
• The storm now has both a strong updraught and a strong downdraught side by side — the most intense phase.
• Heavy rain, hail, lightning, thunder and strong gusty winds all occur. The top of the cloud flattens into the characteristic anvil shape when it hits the tropopause.
• Duration: about 15–30 minutes. This is the most violent and hazardous stage.

3. Dissipating stage

• The downdraught now dominates the whole cell and cuts off the supply of warm moist air from below.
• The updraught weakens and finally dies, so the release of latent heat stops.
• The cloud begins to evaporate from the edges, the anvil drifts away and only light rain or drizzle falls.
• The storm dies out, leaving a cold pool of air at the surface. Duration: about 30 minutes.

Summary

The whole life cycle of a single cell lasts about one hour. In nature, however, thunderstorms often occur as multicell or supercell storms, in which new cells keep forming on the inflow side so that the storm as a whole may last for several hours.

Bergeron–Findeisen theory of precipitation (also called the ice-crystal or Wegener–Bergeron–Findeisen process) explains how precipitation forms in cold and mixed-phase clouds, where temperatures are below $0^\circ$C.

1. Coexistence of ice and supercooled water
   In a mixed cloud between about $0^\circ$C and $-40^\circ$C, ice crystals and supercooled water droplets exist together. The air may be saturated with respect to water but is then supersaturated with respect to ice.

2. Vapour pressure difference
   At the same temperature, the saturation vapour pressure over water is greater than that over ice: $e_{sw} > e_{si}$ For example, at $-10^\circ$C, $e_{sw} \approx 2.86$ hPa while $e_{si} \approx 2.60$ hPa. This difference creates a vapour pressure gradient between water droplets and ice crystals.

3. Growth of ice crystals
   Water vapour diffuses from the supercooled droplets towards the ice crystals. The droplets evaporate to supply vapour, while the ice crystals grow by deposition of water vapour. Thus ice crystals grow at the expense of water droplets.

4. Formation of precipitation
   When ice crystals become large and heavy enough, they fall under gravity. During fall, they may collide and aggregate with other crystals, or collect supercooled droplets and form graupel or hail. If they pass through a layer warmer than $0^\circ$C, they melt and reach the ground as rain; otherwise they fall as snow.

5. Limitations
   The theory works well for temperate and cold clouds, but it cannot explain precipitation from warm clouds in tropical regions, where ice may be absent. Such rainfall is explained by the coalescence theory. Also, the process requires ice nuclei and sub-zero temperatures.

Thus, the Bergeron–Findeisen theory is a major mechanism of precipitation in cold and mixed clouds, often acting together with coalescence and accretion processes.

Solar radiation is the short-wave electromagnetic radiation emitted by the Sun (mainly in the wavelength range 0.15–4 μm, with maximum intensity near 0.5 μm in the visible region). It is the primary source of energy for the Earth–atmosphere system and is often called insolation when it reaches the top of the atmosphere.

Terrestrial radiation is the long-wave radiation emitted by the Earth's surface and atmosphere (mainly in the range 4–100 μm, with maximum intensity near 10 μm in the infrared region). It is also called long-wave or infrared radiation and is responsible for cooling the Earth.

Atmospheric window

The atmosphere is not equally transparent to all wavelengths. Certain gases absorb radiation selectively:

• Solar (short-wave) radiation: Ozone (O₃) absorbs ultraviolet radiation; water vapour and CO₂ absorb some infrared. However, a large part of the visible and near-infrared region passes through almost unabsorbed.
• Terrestrial (long-wave) radiation: Water vapour, CO₂, ozone, methane and clouds strongly absorb most infrared radiation.

However, in the wavelength band roughly 8–12 μm, the atmosphere is almost transparent to outgoing terrestrial radiation. Through this band, a large fraction of the Earth's long-wave radiation escapes directly to space without being absorbed. This relatively transparent spectral region is called the atmospheric window.

Significance:

1. It allows the Earth to lose a substantial part of its heat to space, thus maintaining the global heat balance.
2. If this window were closed (e.g. by increased greenhouse gases), the Earth would become much hotter — the basis of the greenhouse effect.
3. It is used in remote sensing and satellite meteorology, since sensors operating in the 8–12 μm window can measure surface temperature directly.

Precipitation is the process by which water vapour in the atmosphere condenses into water droplets or ice crystals, which then grow large enough to fall to the Earth's surface under gravity. It is the primary mechanism by which atmospheric moisture returns to the land and oceans, and it forms the main input to the hydrological cycle. Precipitation occurs when moist air is lifted, cools adiabatically, and reaches saturation, after which condensation occurs on cloud condensation nuclei.

Forms of Precipitation

1. Rain — Water droplets larger than 0.5 mm in diameter that fall to the ground as liquid. It occurs when cloud droplets coalesce or ice crystals melt before reaching the surface. Rainfall is the most common form in tropical and temperate regions.

2. Snow — Precipitation in the form of ice crystals or aggregates of crystals (snowflakes) that fall when the temperature throughout the falling path remains below freezing (0 °C). Snowflakes have a characteristic hexagonal crystalline structure.

3. Drizzle — Very fine water droplets less than 0.5 mm in diameter that fall slowly and appear to float. It usually occurs from low stratus clouds and reduces visibility.

4. Sleet — Frozen raindrops or ice pellets formed when rain falls through a layer of sub-freezing air near the ground and freezes before reaching the surface. It is a mixture of rain and snow.

5. Hail — Balls or lumps of ice, often 5–50 mm or more in diameter, formed in towering cumulonimbus clouds where strong updrafts carry raindrops repeatedly through freezing layers, adding concentric ice shells. Hail causes severe damage to crops and property.

6. Dew and Frost — Dew forms when water vapour condenses directly onto cool surfaces (grass, leaves) at night when the dew point is above freezing; frost forms when it deposits directly as ice crystals when the surface temperature is below 0 °C.

7. Glaze (Freezing Rain) — Supercooled rain that freezes instantly on contact with cold surfaces, forming a smooth ice coating. It is common in cold mountainous regions.

Note: Fog and mist are suspended moisture near the ground and are not true precipitation, since they do not fall to the surface.

Thus precipitation occurs in liquid (rain, drizzle, glaze), solid (snow, hail) and mixed (sleet) forms, while dew and frost are deposits formed on surfaces, depending mainly on the temperature profile of the atmosphere through which it falls.

Trade Winds

Trade winds are the steady, persistent planetary winds that blow from the subtropical high-pressure belts (about 30°N and 30°S) towards the equatorial low-pressure belt (the Inter-Tropical Convergence Zone, ITCZ). Because of the Coriolis effect, they are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Hence:

• North-East Trade Winds blow from the north-east towards the south-west in the Northern Hemisphere.
• South-East Trade Winds blow from the south-east towards the north-west in the Southern Hemisphere.

They are called "trade" winds because in the age of sailing ships they were used for regular commercial voyages (the word trade originally meant "course" or "track"). They are most regular over the oceans, where they blow throughout the year, and are characterised by high constancy of direction, moderate and steady speed, warm temperatures and high humidity. Where they converge near the equator, the ascending moist air gives heavy convectional rainfall and the calm, windless condition known as the Doldrums.

Economic Importance

1. Navigation and shipping: Their steady direction and speed made them the main highways of sailing ships for centuries. Even today they assist sailing and reduce fuel costs on some ocean routes.
2. Rainfall and agriculture: Moist trade winds rising at the ITCZ cause heavy rainfall, supporting dense tropical rainforests and plantation crops such as rice, sugarcane, cocoa, rubber, coffee and spices in equatorial and tropical regions.
3. Trade and commerce: The regularity of these winds historically promoted international trade between Europe, Africa, Asia and the Americas, helping the growth of port cities and commercial centres.
4. Wind energy: Constant trade winds are used to generate electricity through wind farms, especially in coastal and island areas.
5. Fisheries and marine resources: Wind-driven ocean currents and upwelling associated with trade winds bring nutrient-rich water to the surface, supporting rich fishing grounds.
6. Tourism and recreation: Steady breezes favour sailing, surfing, windsurfing and beach tourism in tropical coastal areas.
7. Climate regulation: By carrying heat and moisture from the tropics towards higher latitudes, trade winds help moderate global temperature and maintain the general circulation of the atmosphere.

Thus, trade winds are not only a major element of the global wind system but also an important natural resource influencing navigation, agriculture, trade, energy and tourism.

Circulation refers to the large-scale movement of air (wind) in the atmosphere, driven by differences in pressure, temperature and the rotation of the Earth. It is the process by which heat energy is transferred from surplus regions (equator) to deficit regions (poles), thereby maintaining the global heat balance.

Variations in the circulation of the northern hemisphere:

1. Latitudinal (zonal) variation — the three-cell model: In the northern hemisphere, circulation is divided into three cells:

   • Hadley cell (0°–30°N): Air rises at the equator (ITCZ), moves poleward aloft, sinks at about 30°N, and returns equatorward as the north-east trade winds.
   • Ferrel cell (30°–60°N): Air moves poleward at the surface as the south-westerlies (westerlies) and equatorward aloft, giving rise to the sub-polar low at about 60°N.
   • Polar cell (60°–90°N): Cold dense air sinks at the pole and moves equatorward as the polar easterlies, meeting the westerlies at the polar front.

2. Seasonal variation: The whole system shifts with the apparent movement of the Sun.

   • In summer (June–July), the pressure belts and wind belts shift northward; the ITCZ lies over the Tropic of Cancer, and the sub-tropical high is displaced poleward.
   • In winter (December–January), the belts shift southward; the ITCZ moves toward the equator and the sub-tropical high comes closer to 30°N.

3. Variation due to land–sea distribution: Because of unequal heating of land and sea, the zonal belts are broken into pressure cells:

   • Winter: Siberian (Asiatic) high over cold Asia and Aleutian low over the warm North Pacific.
   • Summer: Asiatic low over hot Asia and North Pacific (Hawaiian) high over the cooler ocean.

4. Monsoon variation: The seasonal reversal of winds — the south-west monsoon in summer and the north-east monsoon in winter over South and South-East Asia — is a major regional variation of northern hemispheric circulation.

5. Upper-air (jet stream) variation: The sub-tropical westerly jet and the polar front jet shift with the seasons and control the tracks of temperate cyclones and monsoon onset.

Thus, the circulation of the northern hemisphere is not uniform but varies with latitude, season, land–sea contrast and upper-air flow.

Rainfall vs Drizzle

Key point: The basic difference lies in drop size — rainfall has drops larger than 0.5 mm in diameter, while drizzle has drops smaller than 0.5 mm.

Mountain and Valley Winds

Mountain and valley winds are local, diurnal (daily reversing) winds caused by the differential heating and cooling of mountain slopes and the adjacent valley floor.

Valley wind (daytime): During the day, the mountain slopes are heated more strongly than the air at the same level over the valley floor. The air in contact with the slopes becomes warm and light, rises up along the slopes, and is replaced by cooler air moving up the valley toward the mountains. This upslope, up-valley flow is called the valley wind (anabatic wind). It is strongest in the early afternoon.

Mountain wind (nighttime): At night, the slopes cool rapidly by long-wave radiation. The air in contact with them becomes cold and dense, sinks down the slopes under gravity, and collects in the valley, producing a downslope, down-valley flow called the mountain wind (katabatic wind). It is strongest just before sunrise.

Key features:

• They are thermal (diurnal) local winds, reversing direction between day and night.
• Valley wind blows upslope/up-valley by day; mountain wind blows downslope/down-valley by night.
• They arise from the unequal heating and cooling of slopes and valley air at the same altitude.
• They are shallow, weak winds confined to the lower few hundred metres above the terrain.
• They influence local weather — daytime upslope flow favours cumulus cloud and afternoon showers over ridges, while nighttime downslope flow brings clear, calm, and cooler conditions in the valley (often causing temperature inversion and frost pockets).

Atmospheric stability vs instability

In short, stability resists vertical air movement while instability favours it; the deciding factor is the comparison between the environmental lapse rate and the adiabatic lapse rate.

Cloud Condensation Nuclei (CCN)

Cloud condensation nuclei are tiny solid or liquid particles suspended in the atmosphere on which water vapour condenses to form cloud droplets. They are of two types:

• Hygroscopic nuclei — particles having strong affinity for water (e.g. common salt, sodium chloride, ammonium sulphate). Condensation begins on them at relative humidity slightly less than 100%.
• Hydrophobic nuclei — particles having little or no affinity for water (e.g. dust, smoke, clay particles). Condensation on them requires relative humidity greater than 100% (supersaturation).

Their typical size ranges from about 0.001 µm to 10 µm. In the absence of CCN, condensation in the atmosphere would require very high supersaturation (several hundred per cent), which practically never occurs. Hence CCN are essential for cloud formation.

Cloud Condensation Level (CCL)

The cloud condensation level is the height (or the level) in the atmosphere at which the air becomes saturated and water vapour begins to condense into visible cloud droplets. It is the level at which the mixing ratio of the rising air parcel equals its saturation mixing ratio, i.e. where relative humidity becomes 100%.

It is determined by lifting a parcel of air from the surface: as the parcel rises it cools dry-adiabatically until it reaches saturation, and the height at which this occurs is the CCL. In practice, the CCL corresponds to the base of cumulus clouds and is given approximately by

$h = 125\,(T - T_d)\ \text{m}$

where $T$ is the surface air temperature and $T_d$ the surface dew point (in °C). Thus the greater the difference between temperature and dew point, the higher the cloud base.

Trade Winds vs Westerlies

In short: Trade winds are steady, warm, equatorward winds of the tropics, while westerlies are variable, cool, poleward winds of the mid-latitudes.

Definition of Climate

Climate is the aggregate or sum total of the long-term weather conditions of a place or region, expressed through the average and extreme values of temperature, precipitation, humidity, wind, cloudiness and sunshine over a sufficiently long period — conventionally 30 years or more (the standard WMO period). It is the statistical character of the atmosphere, whereas weather is its instantaneous state. Climate is often summarised as "average weather plus its variability."

The climate of any region is not accidental; it is the product of several climatic controls — permanent or semi-permanent factors that modify the amount and distribution of solar radiation and determine the pattern of temperature, pressure, wind and rainfall. Four important controls are explained below.

1. Latitude (Distance from the Equator)

Latitude is the most fundamental control because it governs the angle of incidence of solar rays and hence the amount of insolation received.

• Near the equator the sun's rays fall almost vertically throughout the year, so insolation is concentrated over a small area and the region is hot (tropical climate).
• Towards the poles the rays become increasingly oblique, spreading the same energy over a larger area and passing through a greater thickness of atmosphere, so less heat is received (polar climate).
• Latitude also determines day length and seasonal contrast. At the equator day and night are nearly equal (12 hours each) all year, giving uniform temperature; at higher latitudes the difference between summer and winter day length increases, producing strong seasonality.

Thus temperature generally decreases from the equator to the poles, giving the broad belts of torrid, temperate and frigid climate.

2. Altitude (Elevation above Sea Level)

In the troposphere the normal lapse rate is about 6.5 °C per 1,000 m of ascent. Hence temperature falls with increasing height, and high mountains remain cool even in low latitudes.

• For example, the Terai plains of Nepal (about 100–300 m) are hot with temperatures above 30 °C in summer, while the Himalayan region at 4,000–5,000 m remains below freezing for much of the year, although both lie at nearly the same latitude.
• Altitude also affects pressure, air density and precipitation. Pressure decreases with height, and mountains force moist air to rise, cool and condense, producing heavy orographic rainfall on windward slopes and a rain-shadow on leeward slopes.
• High altitude gives greater diurnal temperature range and intense solar radiation because the atmosphere is thin and less able to absorb and scatter heat.

3. Distance from the Sea (Continentality)

Water has a much higher specific heat than land. Therefore the sea heats and cools slowly, while land heats and cools rapidly. This produces two contrasting types of climate:

• Maritime (oceanic) climate: places near the coast have small annual and diurnal temperature ranges, high humidity and frequent rainfall, because the moderating influence of the sea is felt throughout the year. Example: Mumbai, London.
• Continental climate: places far inland have very hot summers and very cold winters, i.e. a large annual temperature range, with low humidity and scanty rainfall. Example: Jaisalmer, Ulaanbaatar, Siberia.

The sea also supplies moisture to prevailing onshore winds, so coastal areas generally receive more rainfall than interiors. This is why the interior of Asia is a desert while its coasts are humid.

4. Ocean Currents

Ocean currents redistribute heat between low and high latitudes and thus modify the climate of coastal regions.

• Warm currents raise the temperature of adjoining coasts and make them humid. The warm Gulf Stream and North Atlantic Drift keep the ports of Western Europe ice-free and mild in winter, far warmer than places at the same latitude in Canada.
• Cold currents lower coastal temperatures and, by cooling the air above them, reduce its moisture-holding capacity and cause fog but little rain. The Peru (Humboldt) Current off Chile–Peru and the Benguela Current off southwest Africa are associated with coastal deserts (Atacama, Namib).
• Where warm and cold currents meet, as off Newfoundland, dense fog and rich fishing grounds occur.

Other Controls (brief mention)

Besides these, climate is also controlled by prevailing winds and pressure belts (onshore winds bring rain, offshore winds bring dryness), relief and mountain barriers (Himalayas block the cold Central Asian winds from entering India and cause heavy monsoon rain), vegetation and soil cover, and increasingly by human activities such as deforestation, urbanization and emission of greenhouse gases.

Conclusion

Latitude fixes the basic thermal belt, altitude and relief redistribute it locally, distance from the sea and ocean currents moderate or exaggerate it, and winds and pressure belts complete the picture by controlling moisture. The interaction of these controls explains why two places at the same latitude — say Kathmandu and Kolkata — can have markedly different climates.

Monsoon is a seasonal reversal of wind direction, caused mainly by the differential heating of land and sea. In South Asia, the word refers to the seasonal wind that blows from the southwest (sea to land) in summer, bringing heavy rain, and from the northeast (land to sea) in winter, which is dry.

Summer monsoon system in Nepal

1. Onset and duration. The summer (southwest) monsoon normally enters Nepal from the southeast around the first week of June (about 10 June) and withdraws from the west by the end of September. It lasts roughly four months, June to September.

2. Mechanism. In summer the Asian landmass, including the Tibetan Plateau, heats strongly and forms a low-pressure area (the monsoon trough). The Indian Ocean and the Bay of Bengal remain relatively cool and form a high-pressure area. Air therefore flows from the high-pressure ocean towards the low-pressure land as the southwest monsoon. The Himalaya acts as a barrier: moist air is forced to rise, cools, and condenses, giving heavy orographic rainfall on the southern slopes. The Bay of Bengal branch and the Arabian Sea branch both feed moisture into Nepal. The Tibetan Plateau also acts as a heat source and helps maintain the low pressure.

3. Rainfall pattern. About 80% of Nepal's annual rainfall falls in this season. Rainfall decreases from east to west and from south to north. The eastern Tarai and the Chure/Siwalik and Middle Hills receive the heaviest rain (over 2500 mm in places such as Ilam, Dhankuta), while the trans-Himalayan region (Mustang, Manang, Dolpa) lies in a rain shadow and gets very little rain. The monsoon arrives earlier and stronger in the east and weakens towards the west.

4. Associated features. The monsoon is not continuous; it comes in active and break spells. It brings cloud cover, high humidity, and lower temperatures in summer. It also causes floods, landslides, and soil erosion in the hills and Tarai, and sometimes cloudbursts. Weak monsoon years cause drought; strong years cause flooding. Local effects such as the Koshi, Gandaki, and Karnali river systems swell during this period.

Economic importance

1. Agriculture. Nepal's economy is agriculture-based (about 24% of GDP and around 60% of employment). Monsoon rain is the main source of water for paddy, maize, millet, and other crops. A good monsoon means a good harvest; a weak monsoon means crop failure, food shortage, and higher prices. Paddy transplantation depends directly on monsoon onset.

2. Water resources and hydropower. Monsoon rain recharges rivers, glaciers, lakes, and groundwater. It supports hydropower generation (run-of-river projects depend on monsoon flow), drinking water supply, and irrigation canals.

3. Forests and biodiversity. Monsoon moisture maintains forests, grasslands, and biodiversity, which support timber, fuelwood, fodder, and non-timber forest products.

4. Employment and income. Monsoon agriculture provides farm employment to a large rural population. It also supports agro-based industries (sugar, tea, jute, rice mills) and trade.

5. Negative economic effects. Floods and landslides damage roads, bridges, hydropower plants, houses, and crops; they cause loss of life and property, displace people, and require heavy relief and reconstruction spending. This reduces GDP growth and increases poverty. In some years, monsoon disasters push up inflation and slow development.

Conclusion. The summer monsoon is the lifeline of Nepal's agriculture and water economy, but its irregularity and excess cause floods and landslides. Therefore, proper forecasting, embankment construction, afforestation, and planned water management are needed to get maximum benefit from the monsoon.

Climatic Classification

Climatic classification is the systematic grouping of the world's climates into distinct types or zones on the basis of selected climatic elements (mainly temperature, precipitation, and their seasonal distribution) and, in some schemes, vegetation. Because climate varies continuously over the earth's surface, classification imposes boundaries on a continuum so that regions of similar climatic character can be identified, mapped, and compared. Its main purposes are:

• To describe and delimit climatic regions in a simple, orderly way.
• To explain the distribution of natural vegetation, soils, and landforms that depend on climate.
• To provide a basis for agricultural planning, water-resource management, and comparison of climates between places.

Climatic classifications are broadly of two types: genetic (based on the causes/controls of climate, such as air masses, pressure belts, and fronts) and empirical (based on observed climatic elements such as temperature and rainfall). Köppen's scheme is empirical; Thornthwaite's is partly genetic and based on moisture and thermal efficiency.

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Köppen's Classification of Climate

Wladimir Köppen (1846–1940), a German climatologist, proposed his classification in 1900 and revised it several times. It is the most widely used empirical classification because it uses easily available data — monthly mean temperature and monthly precipitation — and relates climate boundaries to the limits of major vegetation types. He selected certain critical values of temperature and rainfall that coincide with the boundaries of vegetation zones.

Basis (criteria used)

1. Temperature: mean temperature of the warmest month, mean temperature of the coldest month, and annual mean temperature.
2. Precipitation: annual total, seasonal distribution (summer, winter, or all-year rainfall), and the relation of rainfall to temperature (aridity thresholds).
3. Vegetation: climate types are named after the dominant natural vegetation, since vegetation is the best visible expression of climate.

The five main climatic groups (A, B, C, D, E)

Sub-groups and sub-types

Group A — Tropical humid

• Af — Tropical rainforest (rain every month, no dry month).
• Aw — Tropical savanna (distinct dry winter season).
• Am — Tropical monsoon (short dry season, very heavy rainfall).

Group B — Dry

• BWh / BSh — Hot desert / hot steppe.
• BWk / BSk — Cold desert / cold steppe.
• The h (hot) and k (cold) letters distinguish hot from cold dry climates; W = desert, S = steppe.

Group C — Warm temperate

• Cfa / Cwa / Csa — Humid subtropical (no dry season / dry winter / dry summer, i.e. Mediterranean).
• Csb — Mediterranean with warm summer.
• Cfb / Cwb / Cfc — Marine west-coast and temperate highland types.

Group D — Cold snow-forest

• Dfa / Dfb / Dfc / Dfd — Humid continental, with increasing severity of winter.
• Dwa / Dwb / Dwc / Dwd — Continental with dry winter (e.g. Manchurian type).

Group E — Polar

• ET — Tundra (warmest month 0–10 °C).
• EF — Ice cap / perpetual frost (warmest month below 0 °C).
• H — Highland (added later for high mountains, where climate changes with altitude).

The letter code system

Köppen used a simple letter notation:

• First letter — main climate group (A, B, C, D, E).
• Second letter — precipitation pattern: f = no dry season, m = monsoon, w = dry winter, s = dry summer, W = desert, S = steppe.
• Third letter — temperature character: h = hot, k = cold, a = hot summer, b = warm summer, c = cool summer, d = very cold winter, F = frost, T = tundra.

Merits

• Simple, quantitative, and easy to apply anywhere in the world.
• Uses data that are widely available.
• Boundaries agree well with vegetation limits, so it is useful in biogeography and agriculture.
• The notation is compact and internationally understood.

Demerits

• Purely empirical — it does not explain why a climate exists.
• Boundaries are arbitrary and sharp, whereas climate changes gradually.
• It gives too much weight to temperature and rainfall and ignores evaporation, wind, humidity, and continentality.
• Some climatic types are very broad and internally varied.

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Thornthwaite's Classification of Climate

C. W. Thornthwaite (1889–1963), an American climatologist, proposed a classification (1931, revised 1948) based on precipitation effectiveness (P/E index) and thermal efficiency (T/E index) — that is, on the moisture and heat available to plants rather than on rainfall alone. He introduced the concept of potential evapotranspiration (PE), the water a vegetated surface can lose to the atmosphere if water supply is unlimited. Climatic types are defined by:

• Moisture index (Im) — derived from precipitation, potential evapotranspiration, and water surplus/deficit, giving humidity provinces from A (perhumid) through B, C, D, E to F (super-arid).
• Thermal efficiency — giving temperature provinces from A′ (tropical) through B′, C′, D′ to E′ (frost).
• Seasonal variation of moisture — letters r (little or no deficiency), s (summer deficiency), w (winter deficiency), d (year-round deficiency).

Thus a place may be described, for example, as "BB′ r" — a humid, mesothermal climate with no marked dry season.

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Differences between Köppen's and Thornthwaite's Classification

Conclusion: Köppen's classification is a simple, empirical, vegetation-based scheme that is easy to apply and therefore most widely used, while Thornthwaite's classification is a more refined, moisture-and-heat-efficiency-based scheme that is better suited to agricultural and hydrological studies. The two differ mainly in criteria, treatment of evaporation, and the number and naming of climatic types.

Greenhouse gases and global warming — justification

Greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapour and chlorofluorocarbons (CFCs) are atmospheric gases that absorb infrared (long-wave) radiation emitted by the Earth's surface and re-radiate part of it back towards the Earth. This natural process, called the greenhouse effect, keeps the average surface temperature of the Earth at about 15 °C instead of about −18 °C, and is therefore essential for life. The problem arises when their concentration is excessive, because the outgoing heat is then trapped in larger amounts and the global temperature rises — this is global warming.

Mechanism of the increase

1. Absorption of infrared radiation: Solar radiation reaches the Earth mostly as short-wave radiation, which passes through the atmosphere and heats the surface. The warmed surface emits long-wave infrared radiation back to space. GHG molecules have vibrational modes that absorb this infrared radiation and re-emit it in all directions, including back to the Earth's surface.
2. Radiative forcing: When the concentration of GHGs increases, the amount of infrared radiation escaping to space decreases. The Earth must then warm up until the outgoing energy again balances the incoming solar energy. This imbalance is called positive radiative forcing (about 2.7 W/m² for the present level of GHGs).
3. Enhanced greenhouse effect: The extra warming caused by human-added GHGs over and above the natural greenhouse effect is called the enhanced (anthropogenic) greenhouse effect, and it is the direct cause of the observed rise in global mean temperature.

Evidence of the increase

• Atmospheric CO₂ has risen from about 280 ppm in pre-industrial times to over 420 ppm today, mainly due to burning of fossil fuels, deforestation and cement production.
• Methane concentration has more than doubled due to paddy fields, livestock, landfills and natural gas leakage; CFCs and N₂O have also increased from refrigeration, fertilizers and industrial activity.
• The global average surface temperature has increased by about 1.1 °C since the late 19th century, with most of the warming occurring after 1950 — the period of maximum GHG emission.
• Direct correlation is seen in ice-core data: periods of high CO₂ in trapped air bubbles coincide with high global temperature, and the present rise is far steeper than any natural variation.

Consequences that confirm the warming

• Melting of glaciers and polar ice caps, retreat of Himalayan glaciers (including in Nepal), and thinning of Arctic sea ice.
• Rise in sea level (about 20 cm in the last century) threatening coastal areas and islands.
• More frequent heat waves, droughts, floods, cyclones and erratic monsoon rainfall.
• Shift in climatic zones and habitats, loss of biodiversity, coral bleaching, and adverse effects on agriculture and human health.

Conclusion

Since GHGs absorb and re-emit infrared radiation, their excessive emission reduces the heat escaping to space and disturbs the Earth's radiation balance. The measured rise in CO₂ and other GHGs closely matches the observed rise in global temperature, and the resulting glacier melt, sea-level rise and extreme weather all confirm it. Hence, excessive emission of greenhouse gases is directly responsible for increasing global temperature.

The energy budget of the atmosphere describes how the incoming solar radiation (insolation) is balanced by the outgoing terrestrial radiation, so that the Earth–atmosphere system remains in a state of thermal equilibrium. The atmosphere is largely transparent to short-wave solar radiation but absorbs long-wave radiation from the Earth, so its budget is maintained mainly through indirect processes.

1. Incoming solar radiation (100 units) Of the total solar radiation reaching the top of the atmosphere:

• About 30 units are reflected back to space by clouds, aerosols, and the Earth's surface (albedo).
• About 19 units are absorbed directly by the atmosphere (ozone, water vapour, dust).
• About 51 units reach and are absorbed by the Earth's surface.

2. Outgoing terrestrial radiation from the surface The Earth's surface, heated by absorbed solar radiation, emits long-wave (infrared) radiation:

• About 6 units escape directly to space through the atmospheric window.
• About 111 units are absorbed by the atmosphere (mainly by water vapour, CO₂, and clouds).
• About 23 units are lost from the surface through latent heat (evaporation).
• About 7 units are lost through sensible heat (conduction and convection).

3. Atmospheric energy balance The atmosphere gains:

• 19 units from direct absorption of solar radiation,
• 111 units from absorption of terrestrial radiation,
• 23 units latent heat + 7 units sensible heat = 30 units from the surface.

Total gain = 19 + 111 + 30 = 160 units.

The atmosphere loses this energy by:

• Emitting 64 units of long-wave radiation to space,
• Emitting 96 units of counter-radiation back toward the Earth's surface.

Total loss = 64 + 96 = 160 units.

Thus the atmosphere's energy budget is balanced.

4. Overall Earth–atmosphere balance

• Incoming solar radiation = 100 units
• Reflected to space = 30 units
• Net radiation received = 70 units
• Outgoing long-wave radiation to space = 70 units (6 from surface + 64 from atmosphere)

Hence, incoming = outgoing, and the mean temperature of the Earth remains nearly constant. Any imbalance (e.g., due to increased greenhouse gases) disturbs this budget and leads to global warming or cooling.

Impacts of Climate Change in Nepal

1. Melting of Himalayan glaciers and reduced snow cover Nepal's Himalayas are warming faster than the global average. Glaciers are retreating and snow cover is shrinking. This threatens the dry-season water supply of rivers such as the Koshi, Gandaki and Karnali, which feed irrigation, hydropower and drinking water systems. It also increases the risk of glacial lake outburst floods (GLOFs) — for example, the 2021 Melamchi flood and the Imja Lake threat.

2. Increased frequency of floods and landslides Erratic and intense monsoon rainfall, together with melting ice, has made floods, landslides and debris flows more frequent and severe. The 2017 Terai floods, the 2021 Melamchi disaster and the 2024 Kathmandu floods caused heavy loss of life, displacement and damage to roads, bridges and settlements. Steep terrain and weak geology make Nepal especially vulnerable.

3. Shifts in agriculture and food security Rising temperatures, changing rainfall patterns, droughts and new pests are reducing crop yields, especially of rice, maize and wheat. Apple cultivation is shifting to higher altitudes, while lower hill areas face water stress. This threatens the food security and livelihoods of a largely agrarian population.

4. Loss of biodiversity and ecosystem degradation Warmer conditions push plant and animal species to higher elevations, disrupting forests and alpine meadows. Species with narrow ranges, such as the snow leopard and Himalayan medicinal herbs, face habitat loss. Invasive species and forest fires are also increasing, weakening ecosystem services.

5. Effects on human health and water resources Climate change expands the range of vector-borne diseases such as malaria, dengue and kala-azar into new hill districts. Water sources are drying up, causing drinking water scarcity in hill villages. Heat waves, air pollution and disaster-related injuries add to public health burdens.

6. Socio-economic impacts and migration Repeated disasters destroy homes, farmland and infrastructure, forcing people — especially from vulnerable hill and mountain communities — to migrate to cities or abroad. This increases urban pressure, loss of productive land and climate-induced displacement.

(Any four of the above, each explained in two to three sentences, would earn full marks.)

Air mass is a large body of air, extending over hundreds to thousands of square kilometres and several kilometres in height, whose physical properties — chiefly temperature and moisture content — are nearly uniform horizontally. It acquires these properties by remaining for a long time over a large, homogeneous surface called its source region (e.g. a snow-covered continent, a warm ocean, a desert). When such air moves away from its source, it is called an air mass and it modifies the weather and climate of the regions it invades.

Air masses are classified mainly by (i) the nature of the source region — continental (c) over land, dry; maritime (m) over sea, moist; and (ii) the latitude of the source region — Arctic/Antarctic (A), Polar (P), Tropical (T), Equatorial (E). Combining the two gives the standard types: cA, cP, cT, mP, mT, mE.

1. Continental air mass (c)

• Source: large land masses — e.g. Siberia, Central Asia, Sahara, interior of North America and Australia.
• Properties: dry (low moisture), and either very cold (cP, cA) in winter or very hot (cT) in summer; large annual and diurnal temperature range; stable or conditionally unstable.
• Impact on regional climate:
  • Brings dry weather with clear skies, because there is little water vapour to condense.
  • In winter, cP air from Siberia/central Asia gives cold, dry spells and frost to northern India, Nepal, Bangladesh and Southeast Asia; the winter monsoon of South Asia is essentially a continental air mass.
  • In summer, cT air over the Sahara and Thar desert produces hot, dry heat-wave conditions (e.g. loo in north India, hot winds in the Tarai).
  • Causes drought when such air persists, and sharp temperature contrasts when it meets maritime air (fronts).

2. Polar air mass (P)

• Source: high-latitude regions — Arctic Ocean, Antarctica, northern Canada, Siberia (cP), and the sub-polar oceans (mP).
• Properties: very cold, dense and dry when continental (cP); cold and moist when maritime (mP); stable near the surface with a low-level inversion.
• Impact on regional climate:
  • Outbreaks of cP air bring severe cold waves, snowfall and frost to mid-latitudes (Europe, North America, East Asia); in South Asia they cause cold waves in the northern plains and Himalayan region.
  • mP air, on passing over warm oceans, becomes unstable and gives heavy rain/snow on the western sides of continents (e.g. the Pacific Northwest) and, after modification, feeds the Western Disturbances that bring winter rain/snow to northwest India, Pakistan and western Nepal.
  • Polar fronts, where polar and tropical air masses meet, generate cyclones, storms and frontal rainfall in temperate latitudes.
  • In the long term, cold polar air masses help maintain glaciers, permafrost and tundra climate, and their southward push is linked with cold waves and crop damage.

Conclusion: Air masses are the basic units of weather systems. Continental air masses mainly control dryness, continentality and seasonal extremes, while polar air masses control cold waves, frontal activity and precipitation in mid- and high latitudes. Their interaction (polar front theory) explains most temperate cyclones and, indirectly, the monsoon and winter weather of South Asia including Nepal.

Ice Crystal (Bergeron–Findeisen) Theory of Precipitation

Basic idea: This theory, proposed by Tor Bergeron (1933) and later developed by Walter Findeisen, explains precipitation formation in cold (mixed) clouds where ice crystals and supercooled water droplets exist together. It states that precipitation forms when ice crystals grow at the expense of supercooled water droplets, because at the same temperature the saturation vapour pressure over ice is less than that over supercooled water.

Steps in the process:

1. Coexistence of ice and water: In a mixed cloud at temperatures between $0\,^\circ\text{C}$ and $-40\,^\circ\text{C}$, both ice crystals (formed by freezing of some droplets or by deposition around ice nuclei) and supercooled water droplets exist together.

2. Difference in saturation vapour pressure: At a given sub-zero temperature, the saturation vapour pressure over ice ($e_i$) is lower than that over supercooled water ($e_w$). For example, at $-10\,^\circ\text{C}$, $e_w \approx 2.86\ \text{hPa}$ while $e_i \approx 2.60\ \text{hPa}$.

3. Vapour pressure gradient: Since the air in the cloud is generally saturated with respect to water, it is supersaturated with respect to ice. A vapour pressure gradient is thus set up from the water droplets (higher $e_w$) towards the ice crystals (lower $e_i$).

4. Growth of ice crystals: Water vapour diffuses from the droplets onto the ice crystals and deposits as ice. The ice crystals therefore grow rapidly by sublimation/deposition, while the supercooled droplets evaporate and shrink.

5. Fall and further growth: As the ice crystals become heavy, they begin to fall. During fall they collide and coalesce with supercooled droplets (riming), growing into snowflakes, graupel or hail.

6. Melting and reaching ground: When these ice particles fall through the layer above $0\,^\circ\text{C}$, they melt and reach the ground as rain; if the air below is cold enough, they reach as snow, sleet or hail.

Significance and limitation: The theory successfully explains precipitation from mixed clouds in temperate and cold regions, where the ice phase is essential. However, it cannot explain rainfall from warm clouds (entirely above $0\,^\circ\text{C}$), for which the coalescence theory is used. In practice, both processes operate together in most rain-bearing clouds.

Collision vs. Coalescence

In brief: Collision is merely the meeting of droplets, whereas coalescence is their actual merging into one bigger droplet. In cloud physics, collision must occur first, and only then can coalescence take place for raindrop formation.

Low-latitude steppe climate vs. middle-latitude steppe climate

Key point: The essential difference is thermal, not pluvial — both are semi-arid (steppe) climates with similar low rainfall, but the low-latitude type is hot all year, while the middle-latitude type has a strong annual temperature range with cold winters.

Solar radiation vs. terrestrial radiation

Key point: By Wien's law, λ_max ∝ 1/T, so the hot Sun radiates short waves while the cooler Earth radiates long waves — this difference is why the atmosphere behaves differently toward the two.

Weather vs. Climate

In short, weather is the instantaneous state of the atmosphere, while climate is the statistical summary of weather over a long period.

Highland climate vs Low-latitude steppe climate

Key point: Highland climate is altitude-controlled and found on high relief at any latitude, whereas low-latitude steppe climate is latitude- and continentality-controlled, hot and semi-arid, found in tropical/subtropical interiors.