Weather and Climate
Senior 1 · Geography
Weather and Climate
Lesson 5 of 6
25 min
Wet washing hung out in Kigali in July is dry by lunchtime; in April the same shirt can stay damp all day. The temperature is almost the same in both months. The difference is how much water vapour is already in the air, its humidity. Humid air takes up little extra water; dry air soaks it up fast.
Definition
Humidity: the amount of water vapour in the air. Relative humidity (RH) compares the vapour the air holds with the most it could hold at that temperature, as a percentage: at 100% the air is saturated and cannot take any more. Example: Kigali's average relative humidity is about 81% in April but about 55% in July.
Figure 1. A wet- and dry-bulb hygrometer. The drier the air, the faster water evaporates from the wick and the bigger the difference.
Water evaporating from the wet bulb takes heat from it, so the wet bulb reads lower than the dry bulb. In dry air evaporation is fast and the difference is large; in saturated air there is no evaporation and both read the same. The relative humidity is then found from a table.
| Dry bulb (°C) | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|---|
| 16 | 100 | 90 | 80 | 71 | 62 | 54 | 45 |
| 18 | 100 | 91 | 81 | 73 | 64 | 56 | 48 |
| 20 | 100 | 91 | 82 | 74 | 66 | 58 | 51 |
| 22 | 100 | 91 | 83 | 75 | 68 | 60 | 53 |
| 24 | 100 | 92 | 84 | 76 | 69 | 62 | 55 |
| 26 | 100 | 92 | 85 | 77 | 70 | 64 | 57 |
| 28 | 100 | 92 | 85 | 78 | 72 | 65 | 59 |
Relative humidity (%) for each difference between the dry- and wet-bulb readings (°C, top row). Values calculated by Ganzaa with the standard psychrometer formula at sea-level pressure and rounded; at Kigali's altitude they are 1 to 3% higher.
Worked example 1: reading the humidity table
In Figure 1 the dry bulb reads 24 °C and the wet bulb 20 °C.
Step 1 Difference .
Step 2 Find the row for 24 °C and the column for 4: the table gives 69%.
Interpretation. The air holds about 69% of the water vapour it could hold at 24 °C: fairly humid, typical of a rainy-season afternoon before a storm.
Important
Dew point: if air is cooled, its relative humidity rises until it reaches 100% at the dew point. Then vapour condenses as dew on grass, as mist in valleys, or as cloud in rising air. That is why Rwanda's valleys are often misty at dawn after a clear, cool night.
Clouds are masses of tiny water droplets or ice crystals formed when rising air cools to its dew point. They are named by their shape (cirrus = curl of hair; cumulus = heap; stratus = layer; nimbus = rain) and their height.
Figure 2. The main cloud types and their usual heights above the ground.
| Cloud | Height | Appearance | Weather |
|---|---|---|---|
| Cirrus | High | Thin, white, wispy streaks of ice | Fair; may show change coming |
| Cirrostratus | High | Thin white veil; halo round the Sun or Moon | Often rain within a day |
| Altocumulus | Middle | Rows or patches of rounded white or grey clouds | Usually fair |
| Altostratus | Middle | Grey sheet; Sun seen dimly | Rain may follow |
| Stratus | Low | Flat grey layer, like fog above the ground | Drizzle, dull weather |
| Nimbostratus | Low to middle | Thick, dark grey layer | Steady, long-lasting rain |
| Cumulus | Low (grows upward) | White heaps with flat bases | Fair; may grow into storms |
| Cumulonimbus | All levels | Huge, towering, dark base, anvil top | Heavy rain, thunder, lightning, hail |
Figure 3. Cloud cover is estimated by eye in oktas: eighths of the sky covered.
Definition
Okta: one-eighth of the sky. Cloud cover is estimated by eye on a scale from 0 oktas (clear sky) to 8 oktas (completely overcast).
The number of hours of bright sunshine is measured with a Campbell–Stokes sunshine recorder: a glass ball focuses the Sun's rays, like a magnifying glass, onto a card marked in hours. When the Sun shines, the focused rays burn a line in the card; when cloud hides the Sun, the burning stops. At the end of the day the observer adds up the length of the burnt trace.
Worked example 2: how sunny is Kigali?
Kigali has about 2,974 hours of bright sunshine in an average year (an estimate from the Weather.Directory website). On average, how many hours a day is that, and what share of the daylight?
Near the Equator there are about 12 hours of daylight every day, so
On an average day the Sun shines for about two-thirds of the daylight hours; the rest is lost to cloud, mostly in the rainy-season afternoons.
Cloud spotting guide, by the Met Office (3 min). Watch the sky afterwards: try to name the clouds over your school and estimate the cover in oktas.
A farmer wants to dry harvested beans before storing them. Using what you know about humidity, clouds and sunshine, which month in Kigali would be best for drying, and why? (Hint: compare 55% humidity in July with 81% in April.)
Tip
Big wet–dry difference = dry air; no difference = saturated air. Cloud names: cirro- high, alto- middle, nimbo- or -nimbus rain.
Practice
Remember
Sources and credits
The lesson text, hygrometer, cloud and okta diagrams, tables (humidity values calculated by Ganzaa), worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
