Water exists in the atmosphere in all three states: as invisible gas (water vapour), as liquid (cloud droplets and rain) and as solid (ice crystals, snow and hail). The constant cycling of water between the atmosphere, the oceans and the land is called the hydrological cycle or water cycle. The atmosphere is the essential link in this cycle, carrying moisture from the oceans to the land through the processes of evaporation, condensation and precipitation.
The amount of water vapour in the air, called humidity, determines weather conditions. When moist air cools, water vapour condenses to form clouds, fog and dew, and when the water droplets become heavy enough they fall as precipitation, which includes rain, snow and hail. In this chapter we study humidity, evaporation, condensation, different types of clouds, and the various forms of precipitation. This knowledge explains how and where rain, snow and storms develop, forming the basis of climate studies.
Humidity is the amount of water vapour present in the atmosphere. It is expressed in two main ways: - Absolute humidity: The actual amount of water vapour present in a given volume of air, expressed in grams per cubic metre. - Relative humidity: The percentage ratio of the actual amount of water vapour present in the air to the maximum amount it can hold at that temperature. Relative humidity is the most commonly used measure and determines whether evaporation can take place and how comfortable we feel.
Warm air can hold more water vapour than cold air. As temperature rises, the capacity of air to hold moisture increases, so relative humidity falls if no new moisture is added. Conversely, when temperature falls, relative humidity rises, and when air is cooled to the dew point, it becomes saturated (relative humidity 100%) and condensation begins.
The dew point is the temperature at which air becomes saturated and condensation begins. Cooling air to the dew point can occur in several ways: by contact with a colder surface (as in the formation of dew and frost), by adiabatic cooling of rising air (as in cloud formation), and by the mixing of warm and cold air masses.
Evaporation is the process by which liquid water changes into water vapour. It depends on temperature, the amount of water vapour already in the air, the area of the water surface, and wind. Higher temperatures, dry air, larger surface area and stronger winds all increase evaporation. The oceans are the main source of atmospheric moisture.
Condensation is the process by which water vapour changes back into liquid (or solid) water. It occurs when air is cooled to its dew point and there are condensation nuclei (tiny particles of dust, smoke or salt) on which water can condense. The products of condensation include dew, frost, fog and clouds.
Clouds are classified on the basis of their height and appearance: 1. High clouds (above 6,000 metres): Cirrus (thin, wispy, composed of ice crystals), Cirrocumulus and Cirrostratus. They are white, feathery clouds that indicate fair weather or the approach of a storm. 2. Middle clouds (2,000 to 6,000 metres): Altocumulus and Altostratus. They are made of water droplets and sometimes ice crystals. 3. Low clouds (below 2,000 metres): Stratus (layered, grey clouds that bring drizzle), Stratocumulus and Nimbostratus (rain-bearing clouds). 4. Clouds with vertical development: Cumulus (fluffy, white, fair-weather clouds) and Cumulonimbus (towering clouds that produce thunderstorms, heavy rain, lightning and hail).
Precipitation is the falling of water (liquid or solid) from the atmosphere to the Earth's surface. The main types are: - Rain: Drops of water larger than about 0.5 mm in diameter. - Drizzle: Fine droplets smaller than 0.5 mm. - Snow: Ice crystals that form when the air temperature is below freezing throughout the atmosphere. - Sleet: A mixture of rain and snow, or frozen raindrops. - Hail: Hard, irregular lumps of ice formed in cumulonimbus clouds by the freezing of water on ice pellets during strong updrafts.
On the basis of how air is uplifted, rainfall is of three types: 1. Convectional rainfall: Occurs when air heated at the ground rises, expands, cools and condenses. It is common in the equatorial regions and in the afternoon in summer. It is often accompanied by thunder and lightning. 2. Orographic (relief) rainfall: Occurs when moist air is forced to rise over a mountain barrier. The windward side receives heavy rain (rain-shadow effect leaves the leeward side dry). Example: heavy rainfall on the windward side of the Western Ghats. 3. Cyclonic (frontal) rainfall: Occurs when warm moist air meets cold air and is lifted along a front, as in temperate cyclones. The precipitation is generally steady and prolonged.
Rainfall distribution is shown on maps using isohyets (lines joining places of equal rainfall). The wettest regions are the equatorial belt, the windward slopes of tropical mountains and the monsoon regions. The driest regions are the subtropical deserts, polar regions and rain-shadow areas. The annual average rainfall of the world is about 100 cm.
| Form | Condition | Description |
|---|---|---|
| Dew | Surface cools below dew point, above freezing | Water droplets on cool surfaces |
| Frost | Dew point below freezing | Ice crystals by deposition |
| Fog | Near-ground air cooled to dew point | Visibility below 1 km |
| Mist | Same as fog, less dense | Visibility 1-2 km |
| Clouds | Adiabatic cooling of rising air | Droplets or ice crystals aloft |
| Cloud Group | Height | Examples |
|---|---|---|
| High clouds | Above 6,000 m | Cirrus, Cirrocumulus, Cirrostratus |
| Middle clouds | 2,000-6,000 m | Altocumulus, Altostratus |
| Low clouds | Below 2,000 m | Stratus, Nimbostratus, Stratocumulus |
| Vertical development | From low to high | Cumulus, Cumulonimbus |
| Type | Cause | Example Region |
|---|---|---|
| Convectional | Heating and rising of air | Equatorial regions, afternoon summer rain |
| Orographic | Air forced over mountains | Windward side of Western Ghats |
| Cyclonic | Lifting along fronts | Temperate latitudes |
Water in the atmosphere is the source of all precipitation and is central to the hydrological cycle. Humidity, the measure of water vapour in the air, is expressed as absolute and relative humidity, and condensation begins at the dew point. Cooling of moist air produces dew, frost, fog and clouds, which are classified into high, middle, low and vertically developed types. When cloud droplets grow large enough, precipitation falls as rain, drizzle, snow, sleet or hail, and rainfall is classified as convectional, orographic or cyclonic depending on how the air is lifted. The study of atmospheric water explains the distribution of rainfall, the formation of storms and the moisture conditions that shape climates around the world.
Keywords: Humidity, relative humidity, dew point, condensation, fog, clouds, cirrus, cumulonimbus, precipitation, convectional, orographic, cyclonic.