Weather and Climate
Senior 1 · Geography
Weather and Climate
Lesson 3 of 6
25 min
In the rainy season in Kigali, mornings are often bright. By early afternoon towering clouds build over the hills, and by three o'clock a thunderstorm pours rain, sometimes with hail. By evening the sky is clear again. Where does that water come from, why does it fall in the afternoon, and how do we measure it?
Definition
Precipitation: water falling from the atmosphere to the ground in liquid or solid form: rain, drizzle, hail, sleet and snow. Dew and frost form on the ground, and fog and mist are clouds at ground level. Example: hail sometimes falls in storms over the highlands of Nyabihu and Musanze.
| Form | What it is | In Rwanda |
|---|---|---|
| Rain | Liquid drops over 0.5 mm across | The main form; heavy showers in the rainy seasons |
| Drizzle | Very small drops falling slowly | From low cloud on the highlands |
| Hail | Balls of ice formed in tall thunderstorm clouds | Occasional storms, damaging crops and roofs |
| Snow and sleet | Ice crystals, or a rain and ice mix | Very rare; only brief falls on the highest Virunga summits |
| Dew | Water droplets condensed on cool surfaces at night | Common on grass on clear mornings |
| Fog and mist | Cloud at ground level | Morning mist in valleys and on Nyungwe's ridges |
Air always holds some invisible water vapour. When air rises it expands and cools. Cool air cannot hold as much vapour, so some of it condenses onto tiny specks of dust into droplets: a cloud forms. When droplets join together and become too heavy to float, they fall as rain. So every kind of rainfall begins with rising air.
Figure 1. Rain forms whenever moist air rises and cools; these are the three ways air is made to rise.
Definition
Convectional rainfall: rain formed when the Sun heats the ground strongly, warm moist air rises in currents, cools and condenses, usually giving heavy afternoon thunderstorms. Example: most rainy-season afternoon storms in Rwanda.
Definition
Relief (orographic) rainfall: rain formed when moist air is forced to rise over high ground. The side facing the wind (windward) is wet; the far side (leeward) is drier and is called a rain shadow. Example: the Congo–Nile ridge and Nyungwe receive much more rain than the eastern lowlands.
Figure 2. Measuring rainfall. The narrow cylinder magnifies the small depth of water so it can be read to 0.1 mm.
Rainfall is given as the depth of water that would lie on flat ground if none soaked in, ran off or evaporated. 1 mm of rain on 1 m² of ground is 1 litre of water.
Worked example 1: Kigali's rainfall year
Kigali Airport's average rainfall (1991–2020) is 953.5 mm a year. March, April and May together receive 129.1 + 135.5 + 88.7 mm.
More than a third of the year's rain falls in these three months: the long rains. July, by contrast, averages only 10.2 mm.
Worked example 2: harvesting rain from a school roof
A classroom roof covers 120 m². How much water could it collect in an average April (135.5 mm)?
1 m³ = 1,000 litres, so that is about 16,300 litres, enough to fill more than 800 twenty-litre jerrycans. In practice gutters lose some, but rainwater tanks are a cheap way for schools and homes to store water for the dry season.
Figure 3. The water cycle. The same water moves endlessly between the lake, the air and the land.
Definition
Water cycle: the continuous movement of water between the oceans and lakes, the atmosphere and the land by evaporation, transpiration, condensation, precipitation, infiltration, surface run-off and groundwater flow. It is powered by the Sun's energy and by gravity.
| Process | What happens |
|---|---|
| Evaporation | The Sun's heat turns water from lakes, rivers, soil and oceans into water vapour |
| Transpiration | Plants give off water vapour through tiny pores in their leaves |
| Condensation | Rising vapour cools and becomes droplets, forming clouds |
| Precipitation | Droplets or ice grow heavy and fall as rain, hail or snow |
| Infiltration | Water soaks into the soil |
| Surface run-off | Water flows over the ground into streams and rivers |
| Groundwater flow | Water moves slowly through soil and rock, feeding springs, wells and lakes |
Important
Case study: Rwanda's water cycle
Much of Rwanda's rain comes from moist air carried from the Indian Ocean and from the Congo Basin, and a large part is recycled locally: water evaporates from Lake Kivu, the wetlands and the forests, and transpires from crops and trees, then falls again as afternoon storms. The rain that falls on the western highlands feeds two great river systems: about 80% of Rwanda drains to the Nile, through the Nyabarongo and Akagera, and 20% to the Congo, through Lake Kivu and the Rusizi. Forests such as Nyungwe act as sponges, soaking up storm rain and releasing it slowly to the rivers through the dry season.
Different types of rain, by the Met Office (3.5 min). Watch how each type of rainfall is started by air being forced to rise.
If a large area of forest on the Congo–Nile ridge were cleared, which parts of the water cycle would change, and what might this mean for farmers downstream in the dry season?
Tip
No rising air, no rain. Convectional = heated from below; relief = pushed up a hill; frontal = lifted over cold air. And "Every Cloud Promises Rain": Evaporation, Condensation, Precipitation, Run-off.
Practice
Remember
Sources and credits
The lesson text, diagrams, tables, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
