Rainfall

Rainfall. This is the form of precipitation formed as a result of evaporation and condensation.

Or   It refers to the water droplets falling from above (sky) to the earth’s surface with in a given area in a given time.

The amount of rainfall received in a given area at a given time is recorded and measured using an instrument called rain gauge.

Terms related to rainfall

  • Rainfall distribution; this refers to how many times or seasons rain is received in a area.

OR    The spread of rainfall through the rainy season.

Forms of / Types of rainfall Distribution

  • Monomodal / Unimodal: this is a type of rainfall distribution where there is a single long rainy season in the middle of the year.It is suitable for drought resistant crops e.g. millet, sorghum etc.
  • Bi-modal rainfall distribution: this is the type of rainfall distribution where the rainfall seasons are received over the year e.g. March – June, August- December. It is suitable for perennial crops e.g. coffee, tea, bananas etc.

Effects of rainfall distribution

It determines the types of crops to be grown.

It influences the time of planting.

It affects the availability of water for pasture growth and water for livestock.

Rainfall reliability

it refers to the chances of receiving an amount of rainfall for a given period of time adequate for plant growth.

OR    It refers to how sure one can be that the rains will come when they are expected to.

Effects if rainfall reliability

It helps the farmer to plant crops at the right time.

It determines when to irrigate the crops.

Reliable rainfall ensures high yields since crops are not destroyed by drought.

Rainfall intensity; this refers to the heaviness of rainfall received in an area for given period of time.

Rainfall effectiveness; this is the ability of rainfall received to satisfy the crop needs.

It is the amount of rainfall which is enough to support or sustain the crop from planting to harvesting.

Formation of rainfall

Rainfall is formed when air containing moisture cools.

Cooling occurs when the moistened air rises and reaches the cool layers of the atmosphere.

When the air cools, it becomes saturated with water vapour. Tiny droplets coalesce or join together to form larger drops; which become heavy and fall to the earth as rain. The amount of rain formed depends on the relative humidity and cooling extent of the air.

Types of rainfall

Relief rainfall (orographic rainfall)

Convectional rainfall

Frontal rainfall (cyclonic rainfall)

water cycle / The hydrological cycle

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This is a presentation of the process which shows the movement or circulation of water from the water bodies, earth or land, vegetation and living organisms through the atmosphere and back to those different sources.

Rainfall from the clouds falls to the ground after condensation.

The rain water runs into the rivers, swamps and lakes some of the water sinks into the ground.

Evaporation of water from the soil and open water bodies, animal excreta (urine, sweat, dug) and transpiration from plants occur.

The water vapour rises and reaches the condensation level and condenses into clouds.

Explanation

When it rains,water collects in water bodies and other water sink into the soil through percolation to form springs and wells.This water by capillary pull moves upwards and used up by plants.When the sun heats the earth,evaporation takes place from water bodies and vegetation.

Water vapour in the atmosphere rises upwards and meets the cold air region which condense to form clouds.Clouds descend due to weght and form rainfall which result into water runoff and percolation