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9.19: Clouds

Difficulty Level: At Grade Created by: CK-12
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Have you ever looked at the sky and found shapes in the clouds?

Clouds have a great effect on the weather and climate, but they can also be lovely (if they're not pouring rain on you). It's fun to sit and watch the clouds go by.


Humidity is the amount of water vapor in the air in a particular spot. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, it means that the air contains 80% of the total amount of water it can hold at that temperature. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation.

Since warm air can hold more water vapor than cool air, raising or lowering temperature can change air's relative humidity (Figure below). The temperature at which air becomes saturated with water is called the air's dew point. This term makes sense, because water condenses from the air as dew if the air cools down overnight and reaches 100% humidity.

This diagram shows the amount of water air can hold at different temperatures. The temperatures are given in degrees Celsius.


Water vapor is not visible unless it condenses to become a cloud. Water vapor condenses around a nucleus, such as dust, smoke, or a salt crystal. This forms a tiny liquid droplet. Billions of these water droplets together make a cloud.


Clouds form when air reaches its dew point. This can happen in two ways: (1) Air temperature stays the same but humidity increases. This is common in locations that are warm and humid. (2) Humidity remains the same, but temperature decreases. When the air cools enough to reach 100% humidity, water droplets form. Air cools when it comes into contact with a cold surface or when it rises.

Rising air creates clouds when it has been warmed at or near the ground level and then is pushed up over a mountain or mountain range or is thrust over a mass of cold, dense air.

Effects on Weather

Clouds have a big influence on weather:

  • by preventing solar radiation from reaching the ground.
  • by absorbing warmth that is re-emitted from the ground.
  • as the source of precipitation.

When there are no clouds, there is less insulation. As a result, cloudless days can be extremely hot, and cloudless nights can be very cold. For this reason, cloudy days tend to have a lower range of temperatures than clear days.

Types of Clouds

Clouds are classified in several ways. The most common classification used today divides clouds into four separate cloud groups, which are determined by their altitude (Figure below).

The four cloud types and where they are found in the atmosphere.

  • High clouds form from ice crystals where the air is extremely cold and can hold little water vapor. Cirrus, cirrostratus, and cirrocumulus are all names of high clouds.
  • Middle clouds, including altocumulus and altostratus clouds, may be made of water droplets, ice crystals or both, depending on the air temperatures. Thick and broad altostratus clouds are gray or blue-gray. They often cover the entire sky and usually mean a large storm, bearing a lot of precipitation, is coming.
  • Low clouds are nearly all water droplets. Stratus, stratocumulus, and nimbostratus clouds are common low clouds. Nimbostratus clouds are thick and dark. They bring steady rain or snow.
  • Vertical clouds, clouds with the prefix "cumulo-," grow vertically instead of horizontally and have their bases at low altitude and their tops at high or middle altitude. Clouds grow vertically when strong air currents are rising upward.

An online guide to cloud development and different cloud types from the University of Illinois is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/home.rxml.


Fog (Figure below) is a cloud located at or near the ground . When humid air near the ground cools below its dew point, fog is formed. Each type of fog forms in a different way.

  • Radiation fog forms at night when skies are clear and the relative humidity is high. As the ground cools, the bottom layer of air cools below its dew point. Tule fog is an extreme form of radiation fog found in some regions.
  • San Francisco, California, is famous for its summertime advection fog. Warm, moist Pacific Ocean air blows over the cold California current and cools below its dew point. Sea breezes bring the fog onshore.
  • Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface and condenses as it cools, appearing like steam.
  • Warm humid air travels up a hillside and cools below its dew point to create upslope fog.

(a) Tule fog in the Central Valley of California. (b) Advection fog in San Francisco. (c) Steam fog over a lake. (d) Upslope fog around the peak of Mt. Lushan in China.

Fog levels are declining along the California coast as climate warms. The change in fog may have big ecological changes for the state.

Learn more at http://www.kqed.org/quest/television/science-on-the-spot-science-of-fog.


  • Air reaches its dew point when humidity increases or temperature decreases. Water droplets form when the air reaches 100% humidity.
  • Clouds block solar radiation, absorb heat from the ground and are the source of snow and rain.
  • Fog forms when there is a difference in temperature between the land and the air.

Making Connections


Use this resource to answer the questions that follow.


1. What is a cloud?

2. What causes clouds to form?

3. What is the Foehn effect?

4. What causes a front to form?

5. Explain what causes fog.


1. Imagine a place with a daytime temperature of 45 degrees F. How will the nighttime temperature change if the sky is cloudy? How will it change if the sky is clear?

2. What set of conditions causes tule fog?

3. The low temperature a few degrees above freezing last night. Why is your car covered with frost this morning?

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cloud Tiny water or ice particles that are grouped together in the atmosphere.
dew point The temperature at which air is saturated with water vapor; when it has 100% humidity.
fog Air condensed below its dew point that is near the ground like a cloud.
relative humidity The amount of water vapor in the air relative to the maximum amount of water vapor that the air could contain at that temperature.

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Date Created:
Feb 24, 2012
Last Modified:
Dec 22, 2016
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