Chapter 18: Water, Clouds, and Precipitation

Water in the Atmosphere

The amount of water vapor in the air can vary from 0-4% by volume depending on location.

However, its importance can't be overstated. Water is responsible for clouds, rainfall, snow,

and for moderating Earth's temperatures.

Water Vapor in the Air

Changes of State in Water

Earth is unique in that water can exist in three different states at normal temperatures.

When water is a solid (ice) and it melts into a liquid, it absorbs heat while it is melting.

The amount of heat that is required to change a state of water is called latent heat, because while the water is changing a state, it's temperature does not change.

Changes of State in Water

When liquid water changes into a gas by evaporation, it requires latent heat to change the liquid into a gas.

So, to clarify, if you set out a piece of ice at room temperature, the ice and the melt water that is produced by the ice would all stay at 0 Celsius until all of the ice melted.

If you boiled water in a pan, the water would not get any hotter than 100 Celsius until all of the water evaporated completely.

Latent Heat and Water

Other Important Changes of State in Water

If a gas changes into a liquid, this process is referred to as condensation.

If a solid turns directly into a gas without changing into a liquid first, this process is referred to as sublimation.

If a gas turns directly into a solid without changing into a liquid first, this process is referred to as deposition.

Changes in State of Water

Changes in State of Water

Humidity vs. Relative Humidity

The word humidity very general and can have many different meanings.

Sometimes we may say “the humidity is high”, or “it's really humid today”.

But this is a general term that has no real quantitative value associated with it.

Meteorologists use relative humidity to measure how much water vapor is in the air.

Relative Humidity

To understand relative humidity, we have to first explain the concept of percent water saturation in the air.

First of all, warm air always holds more water vapor than cold air.

If a parcel of air contains the maximum amount of water it can hold at a specific temperature, we say it is saturated.

Warm Air holds more moisture than Cold Air

Relative Humidity

Relative Humidity is a ratio or percentage of how much water vapor is in the air compared to how much water vapor could be in the air to make the air completely saturated.

There are basically two different ways to change the relative humidity of air.

The first way is, you could simply add or remove moisture from the air.

Relative Humidity

Relative Humidity and Dew Point

The second way you can change the relative humidity of a parcel of air is to change the temperature of the air.

If you change the temperature of air, you change the capacity of the air to hold moisture.

For example, If the temperature of air is lowered, the relative humidity increases.

Once air temperature drops to a temperature where the air becomes saturated, this temperature is referred to as the dew point temperature.

Dew Point

Dew Point and Fog

Relative Humidity and Dew Point

So for example, if a parcel of air at 20 Celsius had a relative humidity of 50%, and the air temperature dropped to 10 Celsius, the relative humidity would become 100%.

So in this example, 10 Celsius would be the dew point temperature.

Relative humidity can be measured with a hygrometer or a sling psychrometer.

Hygrometer and Sling Psychrometer

Adiabatic Temperature Changes

• Have you ever pumped up a bike tire and felt the pump get warmer. Or let air out of an air compressor and felt the cold air being released.

• This is because the increase in pressure causes the molecules in the air to collide more frequently, creating thermal energy or heat.

• As altitude increases, air pressure decreases, and temperature decreases.

Adiabatic Temperature Changes

• These changes in air temperature caused by changes in altitude and pressure are called adiabatic temperature changes.

• When air is unsaturated, it cools about 10 Celsius every 1km increase in elevation.

• This is called the dry adiabatic rate.

• When air is saturated, it cools about 5-9 Celsius, every 1km increase in elevation.

• This is called the wet adiabatic rate.

Adiabatic Temperature Changes

The Formation of Clouds

• When air rises and cools, it eventually reaches its dew point.

• When air reaches its dew point, condensation occurs, and this forms clouds.

• There are four different mechanisms that cause air to rise and form clouds.

• They are orographic lifting, frontal wedging, convergence, and localized convective lifting.

Orographic Lifting

• Orographic lifting occurs when air rises as it ascends over mountains, forming clouds.

Orographic Lifting

Frontal Wedging

• Frontal wedging occurs when a parcel of warm air encounters a parcel of cold air.

• When this occurs, the warm air ascends over the cold air, creating a wedge-shaped front.

• As the ascending warm air rises, it cools to its dew point and forms clouds.

Frontal Wedging

Convergence

• When air masses converge and collide, like in the case of areas of low pressure, then the resulting air masses rise.

• This mechanism of rising air and cloud formation is called convergence.

Convergence

Localized Convective Lifting

• Because the surface of the ground is not equal, some places heat up more quickly than other places.

• For example an area with bare rock is more likely to heat up than an area with vegetation.

• In places that heat up more quickly, a rising pocket of hot air results causing localized convective lifting.

Localized Convective Lifting

Stability of Air

• When meteorologists refer to stability of air, what they are referring to is how rapidly air is rising or falling.

• Hot air is more buoyant than cold air. This causes hot air to rise.

• If air has very little difference in temperature with height, the density will be close to the same and the air will be stable.

Stability of Air

Stability of Air

• Unstable air that rises quickly, forms clouds.

• Air that gets colder gradually as elevation increases is stable.

• The most stable air is when air gets warmer with increasing elevation.

• This is referred to as a temperature inversion.

Temperature Inversion

Condensation of Water Vapor

• When air becomes saturated with water vapor, condensation occurs.

• When condensation occurs, there has to be some surface for the water to condense on.

• In the air, the water usually condenses on small particles called condensation nuclei.

• Condensation nuclei may include dust, smoke, volcanic ash, pollen, ions etc.

Types of Clouds

• Cirrus – the name means “a curl of hair”. Clouds are high, white, and thin. They have a delicate wispy or feathery appearance.

• Cumulus – the name means “a pile”. Clouds look like rounded cloud masses. They have flat bases with the appearance of rising domes or towers. Frequently described as having a cauliflower appearance.

Cirrus Clouds

Cumulus Clouds

Types of Clouds

• Stratus – name means “a layer”. Clouds appear as sheets or layers that cover most or all of the sky. No distinct separation.

• Types of clouds can also be subdivided by their elevation above the ground.

• High clouds – Different types of cirrus clouds are found at these elevations. Usually associated with good weather.

Stratus Clouds

Types of Clouds

• Middle Clouds – Clouds that are found about 2-6 km (about 1-4 miles) above the ground are classified as middle clouds. These clouds are usually named with the prefix alto- before them.

• Low Clouds – Clouds below 2 km (1 mile).

• Nimbus is a Latin word that is used to describe a rainy cloud.

Types of Clouds

• By joining the different names of clouds we can classify many different types of clouds.

• For example, for high clouds we can have cirrus, cirrostratus, and cirrocumulus.

• For middle clouds we have examples like altocumulus and altostratus.

• For low clouds we have examples like stratocumulus, stratus, nimbostratus, cumulus and cumulonimbus.

Types of Clouds

Types of Clouds

Types of Clouds

Fog, Hail, and Sleet

• Fog is a cloud that forms close to the ground.

• One way fog can form is when a parcel of air reaches its dew point.

• Another way that fog can form is when cool air moves over warmer water and evaporation from the water forms fog.

• Hail and sleet form when liquid water falls from a warm pocket of air and then goes through a layer of air at freezing temperatures.

Fog

Hail and Sleet