Water in the AtmosphereClass 11 Fundamentals of Physical Geography Notes

Water in the Atmosphere

You might know that the air around us isn't just a mix of gases like oxygen and nitrogen; it also contains water. This water, in the form of water vapour, can make up from zero to four per cent of the atmosphere by volume and is a major player in creating our weather. Water exists in the air in three different states:

• Gaseous (water vapour)
• Liquid (water droplets)
• Solid (ice crystals)

This moisture gets into the atmosphere primarily from two sources: evaporation from oceans, lakes, and rivers, and transpiration from plants. This creates a continuous cycle where water moves between the atmosphere, oceans, and land through these processes, along with condensation and precipitation.

Humidity: Measuring Moisture in the Air

When we talk about the water vapour present in the air, we are talking about humidity. There are a few ways to measure it.

• Absolute Humidity: This is the actual amount of water vapour present in the air. It's measured as the weight of water vapour in a specific volume of air, usually in grams per cubic metre. The absolute humidity can be very different from one place to another.
• Relative Humidity: This is a comparison. It’s the percentage of moisture the air is holding compared to the maximum amount it could hold at that specific temperature. The air's ability to hold water vapour is completely dependent on its temperature—warmer air can hold more moisture.

Because temperature affects the air's capacity, a change in temperature will also change the relative humidity. Generally, relative humidity is highest over the oceans and lowest over continents.

Saturated Air and Dew Point

When a parcel of air is holding the maximum amount of moisture it possibly can at a given temperature, we say it is saturated. At this point, it cannot absorb any more water vapour.

The temperature at which the air becomes saturated is known as the dew point.

Evaporation and Condensation

The amount of water vapour in the atmosphere is a balance between two key processes: evaporation, which adds moisture, and condensation, which removes it.

• Evaporation is the process where liquid water transforms into a gaseous state (water vapour). The main driver for this is heat. The energy required for water to start evaporating is called the latent heat of vapourisation.
• Condensation is the opposite process: the transformation of water vapour back into liquid water. This happens because of a loss of heat, or cooling.

Several factors influence the rate of evaporation:

• Temperature: An increase in temperature gives the air a greater capacity to absorb and hold water, speeding up evaporation.
• Existing Moisture: If the air is already very humid, it has less potential to absorb more moisture, so evaporation is slower.
• Air Movement: Wind replaces the layer of saturated air right above a water surface with a drier, unsaturated layer, which encourages more evaporation. The greater the wind, the greater the evaporation.

Condensation occurs when moist air cools to the point where it can no longer hold all its water vapour. The excess vapour then turns into liquid. In the open air, this condensation happens around tiny particles called hygroscopic condensation nuclei.

Condensation can happen under several conditions:

• When the air temperature is reduced to its dew point while its volume stays the same.
• When both the volume and temperature of the air are reduced.
• When more moisture is added to the air through evaporation until it becomes saturated.

However, the most common and favourable condition for condensation is a decrease in air temperature.

Forms of Condensation

After condensation, the moisture in the atmosphere can appear in several different forms, depending on temperature and location.

Dew

Dew forms when moisture is deposited as water droplets on cool surfaces of solid objects like stones, grass, or plant leaves. This happens on the ground, not on nuclei high up in the air. The ideal conditions for dew formation are:

• Clear sky
• Calm air
• High relative humidity
• Cold and long nights For dew to form, the dew point must be above the freezing point (0°C).

Frost

Frost forms on cold surfaces when condensation occurs below the freezing point (0°C). In this case, the excess moisture is deposited directly as minute ice crystals instead of water droplets. The ideal conditions for frost are the same as for dew, but the air temperature must be at or below freezing.

Fog and Mist

When an air mass with a lot of water vapour cools suddenly, condensation can happen within the air itself on fine dust particles. This creates fog, which is essentially a cloud with its base at or very near the ground. Both fog and mist can reduce visibility to near zero.

• In urban and industrial areas, smoke provides a large number of nuclei, helping fog and mist to form. When fog mixes with smoke, it's called smog.
• The main difference between mist and fog is that mist contains more moisture. In mist, each nucleus is coated with a thicker layer of water.
• Mists are common in mountainous areas where warm, rising air meets a cold surface. Fogs are drier and often occur where warm air currents meet cold currents.

Clouds

A cloud is a mass of tiny water droplets or ice crystals formed by the condensation of water vapour high up in the atmosphere. Because they form at different heights and have different characteristics, clouds are classified into four main types based on their height, density, and appearance.

Cirrus

• Altitude: High altitudes (8,000 - 12,000 m).
• Appearance: Thin, detached clouds that look feathery.
• Color: Always white.

Cumulus

• Altitude: Generally formed at a height of 4,000 - 7,000 m.
• Appearance: Look like cotton wool, with a flat base. They often appear in scattered patches.

Stratus

• Appearance: As their name suggests, these are layered (stratified) clouds that cover large portions of the sky.
• Formation: Usually form due to loss of heat or the mixing of air masses with different temperatures.

Nimbus

• Color: Black or dark gray.
• Altitude: Form at middle levels or very near the Earth's surface.
• Appearance: Extremely dense and opaque, blocking sunlight. They are shapeless masses of thick vapour and can be so low they seem to touch the ground.

Precipitation

As condensation continues, the tiny water droplets or ice crystals in a cloud grow larger. Eventually, they become so heavy that the air can no longer hold them up against the force of gravity, and they fall to the Earth's surface. This release of moisture from the atmosphere is called precipitation. It can be in liquid or solid form.

• Rainfall: Precipitation in the form of liquid water.
• Snowfall: When the temperature is below 0°C, precipitation occurs as fine flakes of snow, which are hexagonal ice crystals.
• Sleet: This is essentially frozen raindrops. It forms when raindrops fall from a warmer layer of air through a subfreezing layer of air near the ground, causing them to solidify into small ice pellets.
• Hailstones: These are small, rounded pieces of ice that fall to the ground. Hailstones form when raindrops are carried up and down through colder layers of a cloud, freezing in concentric layers, one over the other, until they become heavy enough to fall.

Types of Rainfall

Based on how it originates, rainfall can be classified into three main types.

Convectional Rain

1. The air near the ground gets heated, becomes light, and rises in convection currents.
2. As this warm, moist air rises, it expands and cools.
3. Cooling leads to condensation, forming cumulus clouds.
4. This results in heavy rainfall, often accompanied by thunder and lightning. This type of rain usually doesn't last long.

Orographic Rain

1. A saturated air mass encounters a mountain barrier and is forced to rise.
2. As the air ascends, it expands and cools, causing condensation and rainfall.
3. The side of the mountain facing the wind, called the windward slope, receives heavy rainfall.
4. After releasing its moisture, the air descends on the other side, known as the leeward slope. As it descends, it warms up, and its capacity to hold moisture increases.
5. This makes the leeward side dry and rainless. This area is known as the rain-shadow area. This type of rainfall is also called relief rain.

Cyclonic Rain

This type of rainfall is associated with cyclones, which you can learn more about in the chapter on extra-tropical cyclones.

World Distribution of Rainfall

The amount of rainfall received varies greatly across the Earth's surface and through different seasons. Here are some general patterns:

• Latitude: In general, rainfall decreases as you move from the equator towards the poles.
• Proximity to Water: Coastal areas receive more rainfall than the interior of continents. Oceans receive more rainfall than landmasses because they are the primary source of water vapour.
• Prevailing Winds and Latitude:
  • Between 35° and 40° N and S latitudes, rain is heavier on the eastern coasts and decreases towards the west.
  • Between 45° and 65° N and S latitudes, the westerlies bring rain first to the western margins of continents, and it decreases towards the east.
• Mountains: When mountains run parallel to a coast, the coastal plain on the windward side gets heavy rain, while the leeward side gets very little.

Major Precipitation Regimes

Based on the total annual amount, the world's precipitation can be categorized:

• Heavy Rainfall (over 200 cm per year): Found in the equatorial belt, the windward slopes of mountains in cool temperate zones, and coastal areas of monsoon lands.
• Moderate Rainfall (100-200 cm per year): Occurs in interior continental areas and the coastal areas of continents.
• Low Rainfall (50-100 cm per year): Found in the central parts of tropical lands and the eastern and interior parts of temperate lands.
• Very Low Rainfall (less than 50 cm per year): Occurs in the rain-shadow zones in the interior of continents and in high-latitude regions.