2194/02, Secondary three geography, elements of weather and climate, relative humidity and the types of rainfall,

1. Relative Humidity and Rain

2. Today’s objectives
• We will be able to explain the concepts
of relative humidity (rH) and how it
results in the formation of rain
• We will be able to explain and draw the
formation of relief and convectional
rain

3. What is relative humidity??

5. Relationship between relative
humidity and temperature

6. Relative Humidity
The proportion of water vapour
present in the air, in relation to the
maximum amount the air can hold
at a particular temperature
Expressed as a percentage
Saturation occurs when the air
carries the maximum amount of
water vapour it can hold at that
temperature
(i.e. relative humidity at 100%)
https://www.youtube.com/watch?
v=CL5cgXwKUXc

7. Relative Humidity
High relative humidity indicates rain is
likely to occur.
Thus, temperature and relative humidity
is closely related to precipitation.
Desert regions have low rainfall
and no water bodies  low
relative humidity.
High rainfall and many water bodies 
high relative humidity and the growth of
thick vegetation.
The temperature at whichThe temperature at which
saturation occurs issaturation occurs is
known as the dew pointknown as the dew point
temperaturetemperature

8. Condensation
• Condensation starts to occur at dew point
temperature.
• It is a process in which water vapour
changes to liquid form by cooling.

9. No two clouds are exactly alike, and they are always changing their shape. The
reason we have different types of clouds is that clouds formation takes place at
different heights and temperatures.

10. Precipitation
Water falling from the atmosphere onto the Earth’s
surface, i.e. rain, snow, frost or hail
Form of precipitation depends on the temperature
of the place where it falls
Precipitation in the form of hail.Precipitation in the form of rain.

11. Snow/ Sleet or Freezing Rain/
Rain

12. Relief Rain
Windward Side Leeward Side
• One side of the mountain (windward side) experience rain
• The other side (leeward side) experiences no rain

13. Relief Rain
• Relief rain occurs when moist air is forced
to rise over physical barriers such as a
mountain range.
• As the air rises over the windward side of
the mountain range, altitude increases and
temperature falls.
• Condensation occurs as the moist air
reaches past saturation point, hence
resulting in heavy rainfall along the
windward mountain ranges.

14. Relief Rain
• Over at the leeward side, the clouds
passing through have shed most of their
moisture.
• Hence, the leeward side receives little to
no rainfall.

15. Relief Rain

16. Difference between convectional
and relief rain
Convectional Rain Relief Rain
-Needs heat
-Air parcel rise due to instability
-Needs high relief (mountains)
-Air parcel rise due to prevailing wind
pushing it up the mountain

17. Activity
• Describe, with the aid of a diagram, the
formation of
– Convection Rain
– Relief Rain (DONE ALREADY)
• To be done on foolscap paper.

18. Convectional Rain
1. Occurs when the Earth’s surface is intensely heated
by the sun.
2. The air becomes unstable causing it to expand and
rise rapidly.
3. As it rises, it loses heat and cools. When it reaches
dew point temperature*, condensation occurs.
4. Cumulonimbus clouds are formed.
5. When the water droplets become too large and heavy,
they fall to the ground.
* The temperature when saturation point is reached.