1. Humidity Clouds Rainfall – formation and types WEATHER AND CLIMATE

2. Humidity Humidity is the state of the atmosphere with respect to its water vapour content/It is the amount of water vapour in the air. the water vapour comes from evaporation from water bodies and transpiration of vegetation amount of water vapour the air can hold depends on the temperature. Warm air holds more water vapour than cold air Air becomes saturated when it contains all the water vapour it is able to hold at a particular temperature The temperature at which the saturation of air occurs is called dew point

3. Relative Humidity Ratio of the actual amount of water vapour in the air to the maximum amount of water vapour that the air can hold at a particular temperature. A RH of 90% and more shows that the air is holding a lot of moisture at that particular temperature RH of saturated air is 100% Hygrometer is used to measure relative humidity

4. Hygrometer dry bulb thermometer wet bulb thermometer distilled water wick muslin

5. Hygrometer the dry bulb thermometer measures the temperature of the surrounding air the wet bulb thermometer has its bulb wrapped with a muslin which is kept moist by a wick dipped into a container of distilled water if air is not saturated, evaporation from the wet muslin occurs

6. Hygrometer as latent heat is used up during evaporation, cooling occurs which causes the wet bulb thermometer to register a lower reading the difference between the dry bulb and wet bulb temperature readings is called the wet bulb depression Refer to the chart to get the RH value

7. RH chart if the wet bulb depression is large, the RH is low if the wet bulb depression is low, the RH is high

8. Formation of Clouds

9. Types of Clouds As warm air cools, its ability to hold moisture decreases. It becomes saturated at dew point further cooling after saturation causes water vapour to condense into water droplets which join together to form clouds the base of the clouds indicates the level at which condensation starts

10. Types of clouds 1. Cirrus clouds (7 - 13 km above sea level) highest clouds, detached, thin and wispy associated with fine weather, bringing little or no rain 2. Stratus clouds (below 2 km above sea level) grey, featureless sheets of clouds that can cover the whole sky often bring drizzle that may last for several hours

11. Types of Clouds 3. Cumulus clouds (2- 7 km above sea level) cauliflower-shaped clouds flat base is grey but the parts the parts illuminated by the sun are brilliant small and white cumulus clouds indicates fine weather larger cumulus clouds may develop into cumulonimbus clouds brining heavy showers

12. Types of Clouds 4. Cumulonimbus clouds (3 - 11 km above sea level) dark grey cauliflower-shaped clouds spread out at the top to form anvil-looking tops associated with heavy rain, lightning, thunder and strong winds On hot mornings in tropical areas, they grow in extent and by afternoon they bring heavy rain, accompanied by lightning and thunder

13. Rainfall Rainfall refers to the amount of rain that an area receives over a period of time It is measured in millimetres (mm) or centimetres (cm) The rain gauge is an instrument used to measure rainfall It consists of a funnel that collects and channels rainwater into a container It should be placed in an open area where there are no obstructions to block the rain(grass area, not concrete)

14. Standard Rain Gauge The main features of the gauge are: the sharp inner edge of the funnel which allows it to have an accurate 5" internal diameter; http://www.metoffice.gov.uk/education/secondary/students/observing_rainfall.html

15. Standard Rain Gauge the whole gauge is set into the ground to keep it secure and upright with the rim 30 cm above the surrounding short grass or gravel, this height being chosen so that no rain splashes from the surroundings into the funnel; the inner can is provided so that the glass jar can be lowered gently into the gauge and can also hold the water if the jar overflows or cracks in cold weather; the funnel has a narrow spout so that there is little exposure of the water in the jar to the air, to reduce evaporation. http://www.metoffice.gov.uk/education/secondary/students/observing_rainfall.html

16. Calculating Rainfall Daily rainfall refers to the amount of rain that falls over 24 hours Monthly rainfall refers to the total amount of rainwater collected throughout the month Annual rainfall is the total amount of rainwater collected throughout the year

17. Calculating Rainfall A rainfall map shows the distribution of annual rainfall in a place for a period of time It makes use of isohyets which are lines joining points of equal rainfall to show the distribution of rainfall in a place

18. Types of Rainfall: Convectional Rain

19. Convectional Rainfall – Formation 1.Convectional rain (most common in Singapore) Commonly occurring in warmed/heated regions (tropical regions/continental interior in summer). Air layer near the ground/sea surface is heated up by conduction because of intense insolation. Warm air expands and becomes unstable and rises – convection uplift. Rising air cools to its dew point at its level of condensation.

20. Convectional Rainfall – Formation 2. Air that has been lifted cools. 3. Saturation takes place at dew point. 4. Air is cooled further as it continues to rise - water vapour condenses into water droplets around atmospheric particles (like dust). 5. Water droplet coalesce to form clouds. 6. Rain falls when water droplets in the air becomes too heavy to be suspended.

21. Convectional Rainfall – Formation 1.Convectional rain Warm moist air becomes saturated at dew point temperature. Relative humidity reaches 100%. Condensation takes place upon further cooling to form cumulonimbus clouds. When water vapour in the air becomes too heavy, convectional rain falls.

22. Convectional Rainfall – Formation 1.Convectional rain Convectional rain is usually associated with tropical depressions (typhoons) and thunderstorms, sometimes with lightning. Almost a daily occurrence in tropical areas. Less frequent but still occur in tropical desert regions and continental interiors in summer. Usually in the afternoon.

23. Convectional Rainfall – Formation 1.Convectional rain It brings intense, but short-lived showers and is localised. Rain drops are usually large; heavy downpour

24. (Source: geographyfieldwork.com/ReliefRain3.gif) Types of Rainfall: Relief Rain

25. Formation of Relief Rain As moisture bearing wind from the sea reaches large obstacles such as hills and mountains, the air is forced to rise over the top. As it rises, it cools, condenses and forms clouds. When the water droplets are large and heavy enough, rain falls This side which receives rain is called the windward side As the air moves over the high ground, it may warm up and descend over the hills. Moisture may evaporate to give generally dry weather conditions on the other side of the mountain. This dry side is called the leeward side (rain shadow)

26. References Lau, L. (1998) Physical Geography for Distinction, Hong Kong: Greenwood Press.