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
- A mixture of snow and rain.
- Usually formed by snow melting as it falls.
- 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.
- 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
- 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.
- 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.
- is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects.
- Often seen on trees.
- 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.
- Very fine rain.
- Usually falls from layers of cloud that have very limited vertical development e.g stratus clouds.
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