Earth Science

Atmospheric Processes

Clouds

Study Tip
High altitude clouds are generally wispy, middle altitude clouds are more defined, and low altitude clouds are fluffy.

Humidity

  • The humidity at a location is the amount of water vapor in the air.
  • We use the term relative humidity to measure the amount of water vapor that an amount of air can hold relative to its maximum amount of water.
  • For instance, a relative humidity of 80% means that the air mass is holding 80% of the total amount of water it can hold.
Study Tip
High altitude clouds are generally wispy, middle altitude clouds are more defined, and low altitude clouds are fluffy.
  • If the humidity increases above 100%, the excess water condenses and forms precipitation.
  • The relative humidity can be changed by the temperature in the air because warm air can hold more vapor than cool air.
  • The dew point is the temperature at which the air becomes saturated with water.
  • Clouds form when air reaches its dew point, either when the humidity increases or the temperature decreases.
  • The air cools when it either comes into contact with a cold surface or when it rises.
  • Rising air creates clouds when it is pushed up over high altitudes, where the air goes over a mass of cold, dense air.

Clouds

  • Clouds can have a great impact on the weather.
  • Clouds can prevent solar radiation from reaching the ground, absorb the heat emitted from the ground, and cause precipitation.
  • When there are no clouds, the weather is either very hot or very cold due to the lack of insulation. When it is cloudy, the range in temperatures is much less.
  • Clouds are classified into four different groups, based on altitude.
  • High altitude clouds form from ice crystals when the air is extremely cold. They can hold only a little water vapor.
  • Middle altitude clouds are made of water droplets, ice crystals or both depending on the temperature.
  • Low altitude clouds are almost always made of water droplets.
  • Vertical clouds are clouds which grow vertically instead of horizontally, and are generally created when strong air currents are rising upward.
Relative humidity graph
The relative humidity is dependent on the temperature and the amount of water in the air

Clouds

  • Clouds can have a great impact on the weather.
  • Clouds can prevent solar radiation from reaching the ground, absorb the heat emitted from the ground, and cause precipitation.
  • When there are no clouds, the weather is either very hot or very cold due to the lack of insulation. When it is cloudy, the range in temperatures is much less.
  • Clouds are classified into four different groups, based on altitude.
  • High altitude clouds form from ice crystals when the air is extremely cold. They can hold only a little water vapor.
  • Middle altitude clouds are made of water droplets, ice crystals or both depending on the temperature.
  • Low altitude clouds are almost always made of water droplets.
  • Vertical clouds are clouds which grow vertically instead of horizontally, and are generally created when strong air currents are rising upward.

Earth Science

Fog

  • Clouds that are located at or near the ground are known as fog.
  • Fog is formed when humid air near the ground cools below its dew point.
  • There are different types of fog that form in different ways.
  • Radiation fog forms at night when the skies are clear and the relative humidity is high.
  • Advection fog is created when warm and moist air blows over cold water below its dew point.
  • Steam fog appears in the autumn when cool air moves over warm water.
  • Upslope fog is formed when warm, humid air travels up a hillside and cools down.
Concept Check
  • What does the relative humidity tell us?
  • What are the different types of clouds? Where can you expect to find each kind?
  • What are the different ways to form fog?