Rainfall. Image via the Herald.

Rainfall. Image via the Herald.

ZIMSEC O Level Geography Notes: Rainfall.

  • Rainfall is just one form of precipitation.
  • Precipitation occurs when warm moist air rises or is forced to rise.
  • Water vapor in the air parcel cools and condenses to form clouds.
  • This is because temperature falls with increase in altitude.
  • When condensation occurs it produces minute droplets of less than 0.05mm in diameter.
  • When these droplets join together and grow to fall of their own weight it is called rain.
  • There are other forms of precipitation besides rain such as sleet, hail, dew, hoar frost, fog and rime and snow.

Types of rain

  • There are various types of rain depeding on how the air rises/ is forced to rise these are:

Other types of precipitation

1. Sleet

Sleet falling image by Pixshark

Sleet falling image by Pixshark

  • A mixture of snow and rain.
  • Usually formed by snow melting as it falls.

2. Hail

A hailstorm. Image via Riverfronttimes.

A hailstorm. Image via Riverfronttimes.

  • A solid form of precipitation.
  • It consists of balls of irregular lumps of ice falling from the sky each of which is called a hailstone.
  • When large chunks of ice falling from the sky it is referred to as a hailstorm.

3. Dew

Dew in the morning. Image by UK Met Office

Dew in the morning. Image by UK Met Office

  • Dew is the small droplets of water that appear on objects such as leaves or grass in the morning or evening due to processes of condensation.
  • Dew forms when the temperature of a surface cools down to a temperature that is cooler than the dew point of the air next to it.
  • The temperature at which droplets form is called the dew point.

4. Hoar Frost

Hoar Frost. Image via DevianArt.

Hoar Frost. Image via DevianArt.

  • Frost is the coating or deposit of ice that may form in humid air in cold conditions, usually overnight.
  • Is common in places like Nyanga, Marondera and some parts of Harare in winter.

5. Fog/Mist

Fog. Image via MediaWiki.

Fog. Image via MediaWiki.

  • It is a visible mass consisting of cloud water droplets or ice crystals suspended in the air at or near the Earth’s surface.
  • Fog can be considered a type of low-lying cloud, and is heavily influenced by nearby bodies of water, topography, wind conditions, and even human activities.
  • Fog reduces visibility to less than 1 km while in a mist visibility is reduced to between 1 and 2 km.

6. Rime

Rime. Image by MediaWiki

Rime. Image by MediaWiki

  • is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects.
  • Often seen on trees.

7. Snow

Snow. Image by Crossvillenews.

Snow. Image by Crossvillenews.

  • is precipitation in the form of flakes of ice particles that fall from the clouds.
  • Snow does not occur in Zimbabwe although it is common in countries like the UK during winter.

8. Drizzle

Drizzle. Image by Gothamgirlchronicles.

Drizzle. Image by Gothamgirlchronicles.

  • Very fine rain.
  • Usually falls from layers of cloud that have very limited vertical development e.g stratus clouds.

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