Weather elements: Relative humidity, clouds, rainfall

Question 1

Define relative humidity

Answer 1

• ratio of actual amt of water vapour in air
• compared with max amt of water vapour a unit of air can hold at that given temp
• Saturated air: 100% humidity
• knowing how near air is to being saturated is impt in forecasting chances of precipitation

Question 2

Relative humidity eqn

Answer 2

actual amt of water vapour in air(g/m3)/max amt of water vapour air can hold(g/m3) x 100%

Question 3

Relationship between humidity and temp

Answer 3

• relative humidity decreases as temp increases
• warm air can hold more water vapour than cool air
• temp increase, amt of water vapour stays same (assumption)
• rise in temp makes air ability to hold water vapour to increase (air expands) , RH falls
• temp affects humidity (not other way round)

Question 4

Define saturation

Answer 4

• occurs when air is carrying max amt of water vapour it can hold at the given temp
• RH 100%

Question 5

Define dew pt temp

Answer 5

• temp at which saturation occurs

Question 6

Define condensation

Answer 6

• process in which cooled water vapour changes into water droplets (change of state)
• more water vapour in air, more condensation occur, clouds form, more likely to rain

Question 7

Define clouds

Answer 7

• visible mass of water droplets/ice crystals suspended in atmosphere
• water droplets eventually grow large enough to fall onto earth’s surface as precipitation

Question 8

How are clouds formed?

Answer 8

• earth’s surface heated up, water converted into water vapour via evaporation
• vapour rises, starts to cool
• water vapour cools to dew pt temp , condensation occurs if there are condensation nuclei for the water vapour to condense on (e.g. dust particles)
• water droplets (and dust particles) in air bump into each other, become larger thru process called coalescense
• form clouds

Question 9

How are clouds formed?

Answer 9

• earth’s surface heated up, water converted into water vapour via evaporation
• vapour rises, starts to cool
• water vapour cools to dew pt temp , condensation occurs if there are condensation nuclei for the water vapour to condense on (e.g. dust particles)
• water droplets (and dust particles) in air bump into each other, become larger thru process called coalescence
• form clouds

Question 10

Define precipitation

Answer 10

• refers to water in any form that falls frm atmosphere to surface of earth
• e.g. rain, snow, sleet (mixture of both), hail

Question 11

Air instability

Answer 11

• tendency of air to rise rather than stay in its original position
• cooler air, denser, sinks and remains near ground
• thus cool parcel of air is stable
• air heated, expands, less dense than surrounding air, rises
• thus this heated parcel of air is unstable

Question 12

Types of rainfall

Answer 12

1. Convectional rainfall

2. Relief rainfall

Question 13

Convectional rainfall characteristics

Answer 13

• occurs when there is intense heating of earth surface
• falls over small area (few square kilometres)
• usually heavy, short lived
• accompanied by lightning and thunder
• late afternoon , early evening

Question 14

How is convectional rainfall formed?

Answer 14

1. air parcels near surface heated up by sun
2. air becomes unstable, expands and rises rapidly
3. air parcels cool as they rise, rching dew pt temp (saturation pt: 100% RH)
4. condense at dew pt temp , rain-bearing clouds formed
5. droplets get large and heavy, falls as convectional rain

Question 15

Relief rain characteristics

Answer 15

• occurs mainly in plcs where moist air frm sea forced to rise above/ over landforms
• e.g. mountain range

Question 16

How is relief rain formed?

Answer 16

1. moist warm air frm ocean rises up windward side of mountain, forced to cool
2. temp of air rches dew pt, condensation occur, clouds form
3. water droplets large and heavy enough, fall as rain on windward side of mountain where air has risen up
4. other side of mountain range is leeward side, dry as most of moisture fallen at windward side
5. descending air warmed up by heat near earth surface, becomes dry wind
6. dry condition of air on leeward side called “rain shadow” effect e.g. Death Valley, California (Sierra Nevada mountain range)

Question 17

How is relative humidity measured?

Answer 17

• sling psychrometer (dry, wet bulb thermometer)
• dry bulb: normal thermometer giving air temp
• wet bulb: fine muslin cloth wrapped,container of water beneath (keep bulb moist)
• air not saturated, water evaporate from muslin (vice versa)
• evaporation cause bulb to cool, mercury contract, temp of thermometer drop

Question 18

How to interpret sling phychrometer readings

Answer 18

• calculate diff of dry and wet bulb readings (wet bulb depression)
• refer to table

Question 19

Anomaly in calculating RH

Answer 19

-air conditioner (sucks moisture out frm air)

Question 20

What does RH data indicate

Answer 20

• larger wet bulb depression, RH low
• inverse relationship
• no wet bulb depression/diff, RH is 100% (saturated, evaporation didn’t occur)