secondary three geography: elements-relative humidity and rain

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Relative Humidity and Rain

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Relative Humidity and Rain

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 measure rH using the sling psychrometer

• We will be able to measure rainfall using the rain gauge

• We will be able to explain and draw the formation of relief and convectional rain

Relative HumidityThe 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 percentageSaturation 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

Relative HumidityHigh 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.

Measuring instrument: Hygrometer

• One wet bulb and one dry bulb

• Evaporation from wet bulb causes the temperature reading to drop, causing it to have a lower reading

• Compare temperatures with the relative humidity table

Sling Psychrometer

Sling Psychrometer

1.Make sure the wet bulb wick is fully saturated with clear water. Do not hold the wick with your hands.

2. Stand in a shady spot.

3. Keep the psychrometer far enough from your body and face into the wind.

Sling Psychrometer: Things to note

• Fill the compartment with water (distilled preferable)

• Face the wind (don’t block the wind, it will affect the readings)– Make sure there is ample space around you

(safety for the instrument)

• Swirl it for 1minute– Swirl at 2 rounds/sec

Sling Psychrometer

4. Take the dry bulb reading

5. Swirl it again for 1min

6. Take the wet bulb reading– (wet bulb has a wick)

Sling Psychrometer

• Take the dry-bulb temperature minus the wet-bulb temperature

• The difference is called the “Depression of Wet Bulb”

• Read off from the chart the point of intersection of the “dry bulb reading” and the “depression of wet bulb”

Sling Psychrometer

• RH of Staff Lounge is 88%

Relationship between relative humidity and temperature

PrecipitationWater falling from the atmosphere onto the Earth’s

surface, i.e. rain, snow, frost or hailForm of precipitation depends on the temperature

of the place where it falls

Precipitation in the form of hail.Precipitation in the form of rain.

Precipitation: Convectional Rain• Surface heated up

– Evaporation – Parcel of air instable

• Parcel of hot air rises– Temperature drops as the air rise– RH increase as temperature drops

• Condenses at height of dew-point temperature– Requires condensation nuclei to

condense– Cloud builds up as more

condensation occurs

Cloud/ Rain: Convectional Rain

• Condensation process alone too slow to produce rain– Takes about several days

• Collision and Coalescence process– Big droplet falls– Collides with other droplets

Precipitation: Convectional Rain

• Collision and Coalescence process– Big droplet falls– Collides with other droplets– Splits– Droplets grow as they fall– Process repeated until droplets large and

heavy enough to fall to earth as rain

• All can happen in an afternoon

Snow/ Sleet/ Freezing Rain/ Rain

Relief Rain

• Relief (land) means the physical shape or profile of the land.

• Relief Rain– Rain caused by the relief

• Formation– Moist air from sea blown to land– If mountain or high relief is present, it gets blown up as it

passes over it– Temperature drops as the air rise– RH increase as temperature drops– Condenses into cloud at dew-point temperature– Fall as rain when water droplet becomes big enough

Relief Rain• Consequence

– One side of the mountain (windward side) experience rain

• As seen by vegetation

– The other side (leeward side) experiences no rain

• Lack of vegetation

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

Extra: Measuring Rainfall• Measured using a rain

gauge• Rain falls through a

funnel and is collected in the glass bottle

• Excess rain that overflows is collected in the copper cylinder

• Amount of rain collected is poured into a measuring cylinder (mm or cm)

• Usually placed Usually placed in the in the open open space free space free from from obstructionsobstructions

• Should not be Should not be placed under a placed under a tree or an tree or an overhanging overhanging roofroof

Extra: Measuring Rainfall

copper cylinder

funnel

glass bottle

outer cylinder

• 1/3 sunk into ground to prevent it from toppling over

Extra: Measuring Rainfall

Extra: Rainfall map

• Shows annual rainfall

• The lines on the map are called isohyets.

• An isohyet is a line joining all the places receiving the same amount of rainfall.

Activity

• Describe, with the aid of a diagram, the formation of– Convection Rain– Relief Rain

• To be done on foolscap paper.

Conclusion

• Relative humidity– Properties affecting RH– Sling Psychrometer/Hygrometer

• Precipitation– Convectional Rain– Relief Rain– Rain gauge