Precipitation — evaporation; condensation — dew, frost, fog, mist and cloud; rainfall — types and world distributon
Precipitation, Evaporation & Condensation
- Water Vapour in the Atmosphere
- Water vapour in the air varies from zero to four percent by volume and plays an important role in weather phenomena.
- Water exists in the atmosphere in three forms: gaseous, liquid, and solid.
- The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration.
- There is a continuous exchange of water between the atmosphere, oceans, and continents through evaporation, transpiration, condensation, and precipitation.
Water in the Atmosphere: An overview of water's role in atmospheric processes such as cloud formation, humidity, and precipitation. - Humidity
- Absolute Humidity: The actual amount of water vapour present in the atmosphere, expressed in terms of grams per cubic meter (g/m³).
- The ability of the air to hold water vapour depends on its temperature.
- Relative Humidity: The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature. It is greater over oceans and least over continents.
- The air containing moisture to its full capacity at a given temperature is said to be saturated, and the temperature at which this occurs is known as the dew point.
Absolute Humidity: Conceptual diagram showing the amount of water vapor in the air and its measurement as absolute humidity. 
Relative Humidity: Explanation of relative humidity, its relationship with air temperature, and its role in weather conditions. - Evaporation and Condensation
- Evaporation: The process by which water transforms from a liquid to a gaseous state, driven by heat. The temperature at which water starts evaporating is known as the latent heat of vapourization.
- Increased temperature increases the air's ability to absorb and retain water vapour. Movement of air also enhances evaporation by replacing saturated air with unsaturated air.
- Condensation: The transformation of water vapour back into liquid, caused by the loss of heat. If water vapour condenses directly into a solid, it is known as sublimation.
- Condensation occurs around small particles called hygroscopic condensation nuclei, such as dust, smoke, and salt particles.
- Conditions for condensation:
- When the air temperature is reduced to the dew point with its volume remaining constant.
- When both the volume and temperature are reduced.
- When moisture is added to the air through evaporation.
Changing States of Water: Diagram showing transitions between solid, liquid, and gaseous states through processes like melting, evaporation, and condensation. - Forms of Condensation
- Dew: Moisture deposited in the form of water droplets on cooler surfaces, such as stones or plant leaves, when the dew point is above freezing.
- Frost: Forms when condensation occurs below freezing (0°C), depositing moisture as ice crystals instead of water droplets.
- Fog and Mist: When air cools suddenly, condensation occurs around fine dust particles, forming fog. Mist contains more moisture than fog. Fog is a cloud near the ground, often reducing visibility to near zero.
- Smog: A combination of fog and smoke, prevalent in urban and industrial areas due to the presence of nuclei from smoke particles.
Forms of Condensation: Explanation of condensation forms such as dew, frost, fog, and clouds, and the conditions necessary for their development. - Clouds
- Clouds are masses of minute water droplets or tiny ice crystals formed by condensation at considerable altitudes.
- Types of clouds based on height, expanse, density, and transparency:
- Cirrus: High-altitude clouds (8,000 - 12,000 m), thin, feathery, and always white.
- Cumulus: Cotton-wool-like clouds formed at 4,000 - 7,000 m, scattered in patches with flat bases.
- Stratus: Layered clouds covering large portions of the sky, often due to heat loss or air mass mixing.
- Nimbus: Dark, dense, and opaque clouds formed at middle or low altitudes, often bringing precipitation.
Common Types of Clouds: Representation of cloud types including cirrus, cumulus, and stratus, highlighting their distinct features and weather implications. - Cloud combinations:
- High clouds: Cirrus, Cirrostratus, Cirrocumulus
- Middle clouds: Altostratus, Altocumulus
- Low clouds: Stratocumulus, Nimbostratus
- Clouds with extensive vertical development: Cumulus, Cumulonimbus
- Precipitation
- The release of moisture after condensation is known as precipitation, which can occur in liquid or solid form.
- Rainfall: Precipitation in liquid form when the temperature is above 0°C.
- Snowfall: Precipitation in the form of fine flakes of snow when the temperature is below 0°C.
- Sleet: Frozen raindrops formed when raindrops encounter a cold layer of air below freezing.
- Hailstones: Solidified raindrops that pass through colder layers, forming rounded pellets of ice with concentric layers.
Forms of Precipitation: Illustration of various forms of precipitation, including rain, snow, sleet, and hail, and their occurrence.
Types of rainfall: Convectional, Orographic & Cyclonic Rainfall
- Types of Rainfall
- Convectional Rain
- The air becomes light when heated, rises in convection currents, and cools as it expands, causing condensation and the formation of cumulus clouds.
- Thunder and lightning accompany heavy rainfall, which is typically short-lived.
- This type of rain is common in the summer and in the hotter parts of the day.
- It is very common in the equatorial regions and interior parts of continents, particularly in the northern hemisphere.
- Orographic (Relief) Rain
- When a saturated air mass encounters a mountain, it is forced to ascend. As it rises, it cools, and moisture condenses, causing rainfall.
- The windward slopes receive greater rainfall, while the leeward slopes remain dry and are in a rain-shadow area.
- The rain-shadow area is the dry region on the leeward side of the mountain, where rainfall is minimal.
- Cyclonic Rain
- Cyclonic rain occurs due to extra-tropical cyclones. Please refer to Chapter 10 for more details on cyclonic rainfall.
Types of Rainfall: Diagram explaining convectional, orographic, and cyclonic rainfall with their formation processes and examples. - Convectional Rain
- World Distribution of Rainfall
- Different places on Earth receive varying amounts of rainfall throughout the year and during different seasons.
- Generally, rainfall decreases as we move from the equator to the poles.
- Coastal areas receive more rainfall than the interior of continents, and oceans receive more rainfall than landmasses.
- Between latitudes 35° and 40° N and S of the equator, rainfall is heavier on the eastern coasts and decreases towards the west.
- Between latitudes 45° and 65° N and S, rainfall is first received on the western margins of continents and decreases towards the east due to the westerlies.
- Mountains parallel to the coast cause heavier rainfall on the windward side and less on the leeward side.
- Major Precipitation Regimes of the World
- Heavy Rainfall (over 200 cm per annum): The equatorial belt, windward slopes of mountains along western coasts in the cool temperate zone, and coastal areas of monsoon lands.
- Moderate Rainfall (100 - 200 cm per annum): Interior continental areas and coastal areas of continents.
- Low Rainfall (50 - 100 cm per annum): Central parts of tropical lands and eastern and interior parts of temperate lands.
- Very Low Rainfall (less than 50 cm per annum): Areas in the rain-shadow zones of interior continents and high latitudes.
- Seasonal Distribution of Rainfall
- In some regions, rainfall is distributed evenly throughout the year, such as in the equatorial belt and western parts of the cool temperate regions.
