Weather & Climate

Humidity

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 rain.

Relative Humidity

► 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 temperature.

► Expressed as a precentage (%)

► Formula:

Relative Humidity

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?

50g

10g

Imagine:

Relative Humidity

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?

Relative Humidity

50g

10g

Imagine:

50g

10g

75g

RH: 20% RH: 13.33%

Relative Humidity

Instrument used:

Measured using a sling psychrometer:

Relative Humidity

Terms:

► 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 reached. Condensation occurs

Relative Humidity

50g

10g

Imagine:

RH: 20%

50g

10g

RH: 100%

PitStop 3Q1 and 2

1) Describe and explain the relationship between temperature and relative humidity.

When temperature increases, relative humidity decreases.

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.

Clouds

Clouds

Evaporation

Water converts to vapour as it evaporates and rises up into the atmosphere.

Clouds

Condensation

Water vapour will condense on tiny particles in the air to form water droplets known as condensation nuclei.

Clouds

Cloud formation:

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!

www.youtube.com/watch?v=ODImMpGFUa4

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.

Cumulonimbus

Cirrus

Nimbostratus

Clouds

► 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

underneath Around 4 kilometres high and often

indicate an approaching storm.

Clouds

► 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 Produces thunderstorms Are large white and dark grey,

extending in height from 1 to 7 kilometres above the Earth’s surface.

PitStop 3Question 3

Rainfall and Precipitation

Precipitation

► Refers to waster in any form that falls from the atmosphere.

► Includes hail, snow, sleet etc.

► Classification High rainfall: > 1,500mm Low rainfall: < 250mm

1. EVAPORATION

3. PRECIPITATION

5. STORAGE

2. CONDENSATION

4. RUNOFF

The Hydrological Cycle

1. TRANSPIRATION

Land

Land

Precipitation

Measured using a rain gauge:

Convectional Rain

Storm Clouds Gathering

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 expands, 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.

Relief Rain

Relief Rain

Key Terms:

► Windward side On the side facing the wind

► Leeward On or toward the side sheltered from

the wind

Relief Rain

Windward Side Leeward Side

Relief Rain

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.

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.

PitStop 3Q4, 5 and 6

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 rain.

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 throughout?