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Physical Geography > 2.3 Weather processes and phenomena > Flashcards

2.3 Weather processes and phenomena Flashcards

1
Q

What is evaporation?

A

Water changes from a liquid to a gas

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2
Q

Why does evaporation occur?

A
  • Water vapour in the atmosphere exerts pressure on surrounding bodies of water/air- known as vapour presssure
  • Occurs when vapour pressure of a water surface exceeds pressure in the atmosphere
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3
Q

3 factors affecting evaporation

A
  • Humidity - if air is dry => strong evaporation
  • Supply of heat - hotter the air more evaporation as vapour pressure increases
  • Wind strength - calmer conditions => more evapo.
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4
Q

What is condesation?

A

Gas to liquid

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5
Q

Why does condensation occur?

A
  • Air mass becomes fully saturated (rare) or the temperature decreases so that dew point is reached
  • Dew point is the point at which air is saturated
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6
Q

3 ways condensation can occur

A
  • Radiation cooling of air (radiation given off to space)
  • Cooling of air when it rises
  • Contact cooling of air when it rests over a cold surface
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7
Q

What is required for condensation to occur?

A

Condensation nuclei or particles

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8
Q

Define absolute humidity

A

Amount of water vapour in the atmosphere

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9
Q

Define relative humidity

A

Amount of water vapour present in the atmosphere expressed as a % of the maximum amount of water the air can hold at that temperature

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10
Q

Why does relative humidity change?

A

As air is warmed, the amount of moisture it can hold increases, so if new moisture is added to a pocket of air as it warms, its relative humidity will decrease

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11
Q

Define precipitation

A

Refers to all forms of deposition of moisture from the atmosphere in either solid or liquid states

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12
Q

How is precipitation formed?

A

Water vapour in the air is condensed into tiny water droplets forming clouds. If these droplets coalesce to form larger droplets => heavier => heavy enough to overcome ascending air currents => precipitation

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13
Q

Define lapse rate

A

The rate of temperature decrease with altitude. It does vary according to height, time of the year and over different surfaces

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14
Q

Define condensation level

A

The altitude at which the dew point is reached (m/ft). It marks the base of the cloud

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15
Q

Define air parcel

A

Imaginary volume of air used to conceptualise the thermodynamic fluid motions of the atmosphere

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16
Q

What is the Environmental Lapse Rate (ELR)?

A

The lapse rate for mixed air. It is 6°C/km

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17
Q

What is the adiabatic process?

A

Process relates to the rising and sinking of air. This means that the temperature of the air is changed internally without any other influence. e.g. it is the rising and the sinking of air that causes it to change temperature

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18
Q

What is the adiabatic lapse rate?

A

When parcels of air (rising and sinking) move up through the atmosphere, they cool at a different rate to the surrounding air

19
Q

What is the DALR?

A

Parcels of dry (unsaturated) air cool at a higher rate than the surrounding air. Approximately 10°C /km

20
Q

What is SALR?

A
  • Parcels of saturated air cool at a slower rate than the dry air because condensation takes place so that air releases heat offsetting the cooling process - Approximately 4°C -9°C /km
21
Q

When does the parcel of air stop rising?

A
  • Air will continue to rise and cool until it reaches the sam temp as the surrounding air
  • This marks the top of the cloud development
22
Q

What is the Fohn effect?

A
  • When air passes over a mountain
    1. Rising air cools before condensation at DALR
    2. Precipitation removes moisture from the air, cloud and rain on windward slopes
    3. On the lee ward slope, air is no longer saturated and begins to warm at 10°C /km
23
Q

What is atmospheric instability?

A

Relates to the atmospheric conditions associated with rising air, low pressure conditions, cloud formation, rain and wind

24
Q

What is atmospheric stability?

A

Relates to the atmospheric conditions associated with dry, descending air, which is characterised by calm conditions and relatively clear skies

25
How does atmospheric instability occur?
- When a rising parcel of air cools more slowly than the air surrounding it. If air is displaced upwards, it continues to expand because the air is warmer and lighter - ELR \> DALR - Occurs on very hot days when the ground layers are heated considerably
26
How does atmospheric stability occur?
- When a rising parcel of air cools more quickly than the surrounding air. If air is displaced upwards, it immediately gets cooled, becomes denser and sinks - ELR \> DALR/SALR
27
How does conditional instability occur?
- When the ELR lies between the SALR and the DALR (SALR \< ELR \< DALR) - Conditional instability in the atmosphere exist when air parcels are stable if they are dry and unstable if they are saturated.
28
Weather phenomena associated with atmospheric stability
* On a global scale it produces the great high pressure belts * On a local scale, stability can lead to the formation of fog, mist and frost * Under clear skies, temperatures may drop enough to form frost. Where there is moisture present, the cooling of air at night may be sufficient to produce mist and fog
29
Weather phenomena associated with instability
Produces clouds and is the mechanisms for the formation of rain and precipitation
30
Define cloud
A cloud is a visible mass of condensed water vapour or ice particles floating in the atmosphere
31
3 criteria that classify clouds
- Altitude - Shape - Whether they bring precipitation
32
How can clouds be classified in terms of altitude?
- Below 2000m they are called low level (made of water) - Between 2000-6000m the are called mid level and is given the prefix alto (mixture of ice and water) - Above 6000m they are called high level and is given the prefix cirro (ice crystals)
33
How can clouds be classified in terms of shape?
- Cumulus which are heavy, lumpy, fluffy cloud - Cirrus which look like wispy locks of hair - Stratus which means 'spread out' or 'layer'
34
Name of clouds that bring precipitation
Nimbus
35
What are the 3 types of precipitation?
- Convectional - Frontal or cyclonic - Orographic or relief
36
How does convectional rainfall work?
- Land becomes hot and it heats the air above it - Air expands and rises - As it rises, it cools and condensation takes place - Precipitation - Common in tropical areas
37
How does orographic/relief rainfall work?
- Air is forced to rise over a barrier such as mountain - As it rises, it cools condenses and forms rain - Often there is a rain shadow effect where the leeward slope receives very little rain
38
How does frontal/cyclonic rainfall work?
- Warm air meets cold air - Warm air slowly rises above the cold air, forming a low pressure zone. As the rising warm air reaches higher elevations, the liquid in it condenses and forms clouds and rain. Cool air rushes in to fill the low pressure zone, pushing more warm air up and creating a cycle that can result in high winds and storms
39
What is hail?
Frozen pellets of rain
40
How is hail formed?
- Raindrops being carried up and down in the vertical air currents in large cumulonimbus clouds - Freezing and partial melting may occur several times before the pellet is large enough to escape from the cloud
41
How is snow formed?
- When temp is below the freezing point and water vapour is converted into a solid (sublimation) - Every level of the troposphere must be below the freezing point so the snow does not melt on its way down - Usually happens with warm moist air forced over high mountains or comes into contact with cold air at the front (cold air holds less moisture)
42
What is frost?
Deposit of fine ice crystals onto the ground/vegetation
43
How is frost formed?
Occurs on cloud-free nights when there has been radiation cooling to below freezing point. Water vapour condenses directly onto these surfaces by sublimation

Physical Geography (12 decks)

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