By the end of the lesson…
We will be able to
► Explain the relationship between temperature
and relative humidity
► Explain the formation of convectional and relief
► Relative humidity (RH) is the ratio between
the actual amount of water vapour present in
the air and the maximum amount of water
vapour the air can hold at a given
► Expressed as a precentage (%)
Example PART 1:
► The actual amount of vapour in the air in the morning
was 10g/m3. Given that the air could contain a
maximum of 50g/m3, what was the relative humidity
of the air in the morning?
Example PART 2:
► As the temperature increased in the
afternoon, the maximum amount of vapour
that could be contained in the air now
increased to 75g/m3. Given that the actual
amount of vapour in the air remained the
same as before, what was the relative
humidity of the air now?
Measured using a sling psychrometer:
► Saturation Point
The point when the air can no longer
absorb any more water.
Relative humidity is at 100%
► Dew Point Temperature
The temperature when saturation point is
1) Describe and explain the relationship between
temperature and relative humidity.
When temperature increases, relative
This is because:
as the temperature rises, the surrounding
air will expand.
Thus, if the amount of water vapour in the
air remains the same, the maximum
capacity of the air to hold water vapour will
now increase, causing the relative humidity
level to drop.
Water converts to vapour as it evaporates and rises up
into the atmosphere.
Water vapour will condense on tiny particles in the air to
form water droplets known as condensation nuclei.
A cloud is essentially a huge mass of water droplets or
ice crystals suspended in the atmosphere.
Q: Are fogs and clouds the same?
There is no basic difference between a fog and a cloud. A fog is a cloud in
contact with the ground. They are caused by a cold current of air from
above striking down upon the warmer surface of the land or water.
Make your own cloud bottle!
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.
► High-level clouds: Cirrus clouds
Appear 6 kilometres in the sky and as
white lines or streaks.
Do not produce rain and are usually made
up of ice.
► Mid-level clouds: Altocumulus clouds
White puffy clouds with darker areas
Around 4 kilometres high and often
indicate an approaching storm.
► Low-level clouds: Stratus clouds
Often dense and dark or cottony white
clumps in blue skies.
Appears at around 2 kilometres high.
► Storm clouds: Cumulus clouds
Are large white and dark grey, extending in
height from 1 to 7 kilometres above the
1. Occurs when the Earth’s surface is intensely heated
by the sun.
2. The air becomes unstable causing it to expand and
3. As it expands, it loses heat and cools. When it
reaches dew point temperature*, condensation
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.
► 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.
► Over at the leeward side, the clouds
passing through have shed most of
► Hence, the leeward side receives little
to no rainfall.
4. Is convectional or relief rain more likely
to occur in SG? Explain your answer.
Ans: Convectional Rain
– Temperatures are regularly high
which allows for the intensive heating
of the earth’s surface and therefore
higher rates of evaporation.
– Also, there are no landforms such as
a mountain range which forces warm,
moist air to rise – a condition
necessary for the formation of relief
5. a) Describe the climate of Osaka
Questions to consider:
► Is the weather constant throughout
the year? Are there any variations?
► Are there any months whereby the
weather is cooler or warmer?
► Is the rainfall constant throughout?
Is there a co-relation between
rainfall and temperature?
► Is the relative humidity constant
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