Unit 2: The atmosphere                                                     1   2   3   4   5

4. Atmospheric phenomena

Atmospheric phenomena occur in the troposphere. They are responsible for changes in the weather.

The wind and water vapour in the air can cause these phenomena.

4.1. Atmospheric phenomena caused by the wind

     - Hurricanes

       These are violent tropical storms that form over the ocean.

       They consist of a central area (the eye), around which clouds

       and winds revolve at great speeds (200 km/h).

     - Whirlwinds (or dust storms)

       They occur on sunny days in dry open spaces.

       The ground heats up and it heats up the air.

       The air rises in a spiral and collects sand and dust particles.

       Whirlwinds only last a few minutes and can be 100 m high.

     - Tornadoes

       They start inside large storm clouds and have an inverted cone shape

       when they leave the clouds and reach the ground or the sea.

       These winds can reach speeds of 480 km/h and demolish everything

       in their path.

4.2. Atmospheric phenomena caused by humidity

a) Due to condensation 

   - Clouds form when air that is charged with water vapour, rises, cools,

     and condenses around tiny dust particles, salt, or ash.

   - Fog consists of low clouds that form close to the ground.

     This occurs when the air on the ground cools down but does not go

     below 0°C.

   - Frost forms when the temperature of the Earth’s surface is below 0°C

     and water vapour freezes.

   - Dew is condensed water vapour. This occurs during the night

     when temperatures drop.

b) Due to precipitation

- Rain occurs when the water droplets in a cloud join together.

  When they are big enough, gravity makes them fall.

  - Snow occurs when the temperature inside a cloud reaches 0°C.

    The water droplets freeze. They fall from the cloud when they are

    heavy enough.

  - Hail occurs when there are strong rising air currents inside the cloud

    which deep freeze the water droplets (-50 °C).

4.3. Electrical phenomena

Thunder and lightning

4.1. Look at the following pictures and indicate if it is an atmospheric

        phenomena caused by wind, by condensation of water vapour,

        or by precipitation

4.2. Read and identify what atmospheric phenomenon is described:

       a. They form when wet air rises and water vapour condenses………………

       b. It forms when water vapour freezes on the Earth’s surface…………..…

       c. They are large whirlpools and have cone invert shape………………………

       d. It forms when water vapour condenses during night…………………………