Water Vapour in the Atmosphere

Water vapour is the gas form of water found in the air. Even though we can’t see it, it plays a big role in the weather.

• Water vapour makes up 0 to 4% of the atmosphere, depending on the region (more over oceans, less over deserts).
• The amount of water vapour in the air affects things like humidity, cloud formation, and rain.

Water gets into the atmosphere through two main processes:

• Evaporation: This is when water from oceans, lakes, rivers, and soil turns into water vapour and rises into the air.
• Transpiration: This is when plants release water into the atmosphere through their leaves.

Humidity

Humidity is the amount of water vapour in the air. It affects how hot or cold it feels, how easily you can sweat, and whether it’s likely to rain.

Absolute Humidity

• Absolute humidity tells us how much water vapour is in the air by weight, usually in grams per cubic meter.
• The warmer the air, the more water vapour it can hold.

Relative Humidity

• Relative humidity tells us how much water vapour is in the air compared to how much it can hold at a certain temperature, shown as a percentage.
• If the relative humidity is 100%, the air is fully saturated (it can’t hold any more water), which often leads to rain or dew.
• If it’s 50%, the air is holding half the water vapour it can.

Dew Point

• The dew point is the temperature at which the air is fully saturated with water vapour, and the excess water begins to condense into droplets, forming dew, fog, or clouds.
• When the temperature drops to the dew point, condensation starts.

Evaporation and Condensation

These are the two main processes that move water between its gas, liquid, and solid states in the atmosphere.

Evaporation

• Evaporation happens when liquid water (from oceans, lakes, etc.) turns into water vapour (a gas) and goes into the atmosphere.
• It requires heat to change water from liquid to gas. This is why it mostly happens in warm areas, like the equator.

Condensation

• Condensation happens when water vapour cools down and turns back into liquid water.
• This is how clouds, dew, and fog form.
• For example, when warm air rises, it cools, and when it cools enough, the water vapour in it condenses into tiny water droplets or ice crystals, which eventually form clouds.

Forms of Condensation

When water vapour condenses, it can take different forms depending on the temperature and location.

Dew

• Dew forms when water vapour in the air condenses into droplets on cool surfaces, like grass or car windows.
• It happens when the air cools overnight and reaches its dew point. Dew usually forms in the early morning when the temperature is cooler.

Frost

• Frost happens when water vapour turns directly into ice crystals instead of water droplets. This occurs when the temperature drops below freezing (0°C).
• Frost forms on cold surfaces like grass, windows, or cars, creating a thin layer of ice.

Fog and Mist

• Fog is like a cloud on the ground. It forms when warm, moist air cools down close to the surface, and the water vapour condenses into tiny droplets that float in the air, reducing visibility.
• Mist is similar to fog but contains more moisture and is usually lighter.

Clouds

• Clouds are made up of tiny droplets of water or ice crystals that form when warm air rises, cools down, and water vapour condenses at high altitudes.

There are different types of clouds, depending on their height and shape:

• Cirrus Clouds: Thin, wispy clouds that form high in the sky. These are usually made of ice crystals.
• Cumulus Clouds: Fluffy, cotton-like clouds that often signal good weather but can also turn into storm clouds.
• Stratus Clouds: Low, flat clouds that cover the sky like a blanket and often bring drizzle or light rain.
• Nimbus Clouds: Thick, dark clouds that usually bring heavy rain or storms.

Precipitation

Precipitation happens when water in the atmosphere falls back to the Earth’s surface in various forms, like rain, snow, sleet, or hail.

Rainfall

• Rain forms when water droplets in clouds grow large and heavy enough to fall due to gravity.
• It’s the most common type of precipitation and happens when air rises, cools, and the water vapour condenses into droplets.

Snowfall

• Snow forms when water vapour in the clouds turns directly into ice crystals without becoming liquid first. This happens when the temperature is below freezing.
• Snowflakes are collections of these ice crystals.

Sleet

• Sleet happens when raindrops freeze into small ice pellets before hitting the ground.
• It occurs when air temperatures are just below freezing.

Hail

• Hail is formed when strong winds in thunderstorms carry water droplets up into very cold parts of the clouds, where they freeze into ice.
• As the droplets get pushed up and down by the wind, they grow larger before eventually falling to the ground as hailstones.

Types of Rainfall

Rainfall can happen in different ways depending on how the air is lifted into the atmosphere.

Convectional Rainfall

• This happens when the sun heats the ground, and the warm air rises. As the air rises, it cools and condenses into rain.
• Convectional rainfall is common in tropical regions, especially during the afternoon when the ground is the warmest.

Orographic (Relief) Rainfall

• This type of rain happens when moist air is forced to rise over a mountain. As the air rises, it cools and causes rain on the side of the mountain that faces the wind (the windward side).
• Once the air moves over the mountain, it loses most of its moisture, so the other side (the leeward side) stays dry. This area is called a rain shadow.

Cyclonic (Frontal) Rainfall

• This happens when a warm air mass meets a cold air mass. The warm air is lighter, so it gets pushed up over the cold air. As it rises, the warm air cools, condenses, and forms rain.
• This type of rainfall is common in temperate regions where cold and warm air masses meet.

World Distribution of Rainfall

Rainfall is not evenly spread across the world. Different regions receive different amounts of rainfall depending on their location and climate.

Equatorial regions (like the Amazon Rainforest) get the most rainfall, often more than 200 cm per year because of frequent convectional rain.

Deserts (like the Sahara and Arabian Desert) receive very little rainfall, often less than 25 cm per year, due to the dry air in subtropical high-pressure zones.

Temperate regions (like Europe and the Eastern US) get moderate rainfall because of cyclonic weather patterns.

Summary

• Water vapour in the atmosphere plays a major role in weather, leading to clouds, rain, snow, and fog.
• Humidity measures how much water is in the air, and when the air can’t hold more water vapour, it condenses into liquid or ice.
• Precipitation falls in different forms like rain, snow, sleet, and hail, depending on temperature.
• Types of rainfall include convectional, orographic, and cyclonic rainfall, each caused by different movements of air.
• Global rainfall varies, with wet equatorial regions, dry deserts, and moderate rainfall in temperate areas.