2.3 weather processes n phenomena

Question 1

what accounts for the vertical transfer of heat?

Answer 1

evaporation, condensation, convection and rainfall

Question 2

Evaporation

Answer 2

Change of state of a liquid into a gas as a result of heating.

Heat absorbed into water as latent heat. Takes 600 calories of heat to change 1g of water from liquid to a vapour.

Cools surrounding air - 1kg of air cooled by 2.5 degrees.

Question 3

Condensation

Answer 3

Change of state of gas to a liquid as a result of cooling.

Latent heat locked in water vapour is released

Warms surrounding air.

Question 4

Sublimation

Answer 4

Change of state of a solid to a gas (ice crystals melt to vapour) without going through the liquid phase. It can also work in reverse.

Heat absorbed.

Cools surrounding air.

Question 5

Freezing

Answer 5

Liquid changes state to solid e.g. water to ice when temperatures are below 0 degrees C.

Heat is released.

Warms surrounding air.

Question 6

Melting

Answer 6

Solid ice turns to liquid water when temp rises above 0 degrees.

Heat is absorbed.

Cools surrounding air.

Question 7

Deposition

Answer 7

Gas turns to solid e.g. formation of rime at high latitudes and altitudes as fog freezes onto cold surface or moist air condenses on cold surface to form dew but as temps are below freezing, the water vapour is deposited as hoar frost.

Heat is released.

Warms surrounding air.

Question 8

Factors increasing rates of evaporation

Answer 8

• Humidity of the air is low and air is very dry
• Temperatures rise - the average energy of molecules in the liquid is governed by temperature. Some molecules may become so energetic that they escape the bonds holding them in the liquid and evaporate. Warm air can hold more moisture.
• Winds stronger - calm conditions when molecules aren’t blown away will become more saturated more quickly so will reduce evaporation over time. Higher wind speed will blow moisture away so air can hold more.
• There is a plentiful supply of moisture e.g. lake, sea.

Question 9

When does condensation occur?

Answer 9

a) When enough water vapour is evaporated into the air so it becomes saturated (rare)

b) When the temperature drops so that the dew point temperature (at which air becomes saturated) is reached.
The air will cool and contract and not be able to hold as many molecules. These will now condense from water vapour back to liquid.

Question 10

Dew point

Answer 10

The dew point is the temperature to which the air must be cooled at constant pressure in order for it to become saturated. i.e. the relative humidity becomes 100%

Question 11

Precipitation

Answer 11

all forms of deposition of moisture from the atmosphere onto the earth’s surface in either solid or liquid states e.g. rain, hail, sleet, snow, dew.

Question 12

Formation of clouds

Answer 12

1. Air that contains water vapour which has evaporated/transpired is forced to rise.
2. This may be due to convection (less dense, rising currents of air), forced over higher relief or if a body of air meets another body of air and they are different temperatures so don’t mix. This creates a weather ‘front’ which will see warm air rise above cold air or cold air undercut the warm air and force it to rise.
3. Minute droplets of water are condensed from water vapour as temperatures cool with an increase in altitude (adiabatic cooling. cools at the Environmental Lapse Rate which is 0.6C for every 100m) and reach dew point temperature (saturation) at the condensation level.
4. This is aided by hygroscopic nuclei eg dust/smoke particles which have an affinity to water and act as a nuclei around which condensation of water vapour will occur. Reduces the saturation point required to 80%.
5. These droplets kept afloat in the atmosphere as clouds by ascending air currents.
6. If the droplets coalesce, they can form larger droplets that may become heavy enough to overcome gravity and the ascending current keeping them afloat. If successful, they fall as rain.

THEREFORE, cloud droplets must get larger to form rain.

Question 13

Collision theory

Answer 13

Different sized water droplets have different falling rates and are carried in rising and falling air currents within cumulonimbus clouds. The droplets collide with others and join together to form a larger drop. Three processes occur:

• Coalescence: two water droplets collide and join together. This is the main mechanism for the formation of rainfall.
• Aggregation - two ice crystals collide and join together to form snow
• Accretion - an ice crystal collects a water droplet, leading to the formation of hail.

Question 14

Stratus clouds

Answer 14

low lying clouds with uniform colour

Question 15

Cumulus clouds

Answer 15

cauliflower-shaped clouds in fair conditions

Question 16

Cirrus clouds

Answer 16

high level wispy clouds, sheet-like

Question 17

Rain

Answer 17

Rain and drizzle are found when temperature is above 0 degrees C.

Liquid drops of water with diameter of 0.5mm-5mm.
Heavy enough to fall to the ground (drizzle has a diameter <0.5mm)

There must be vertical uplift of air high enough for temperatures to drop to the dew point in order for clouds to form.

Question 18

Frontal rainfall

Answer 18

(MARK SCHEME)

• Uplift of air caused by meeting of two air masses
• Warm air rises over colder air at a warm front
• Cold air undercutting warm air at a cold front
• The less dense warm air cools as it rises, condenses (dew point), leads to water droplets which, when of sufficient size, fall as rain

Question 19

Orographic rainfall (relief)

Answer 19

• A barrier e.g. a mountain forces air to rise at the windward slope
• Forms banner clouds (small clouds only at summit) which disappears on leeward side as air sinks
• As it rises, air cools adiabatically and reaches dew point temperature, leading to condensation and rain on the high land.

Question 20

Convectional rainfall

Answer 20

• Common where land becomes very hot e.g. in tropical areas + summer in temperate areas (e.g. SE UK)
• Happens at ITCZ

1. The Earth’s hot surface heats the air above it
2. The heated air rises and expands and cools, condensation takes place
3. Further ascent causes more expansion and more cooling, rain takes place. Latent heat is released, which adds to rising.
4. Cool air descends and replaces the warm air.

Question 21

Hail

Answer 21

Alternate concentric rings of clear and opaque ice. Diameter greater than 5mm.

Question 22

Hail formation

Answer 22

Within cumulonimbus clouds (1), updraught and falling of water particles forming ice (1). Super-cooled water droplets collide with and freeze around the ice (1). When the droplets are too heavy, they will fall as hail (1).

Question 23

Snow

Answer 23

Snow is frozen precipitation. Formed when temp is below 0C and water vapour is converted into a solid.

Question 24

Snow formation

Answer 24

As very cold air contains very little moisture, snow is more likely to form when warm, moist air is forced over mountains or when it comes into contact with very cold air at a weather front.

Collision theory - aggregation - 2x ice crystals come together.

Question 25

Dew

Answer 25

Dew is the direct deposition of water droplets onto a surface.

Question 26

Dew formation

Answer 26

radiation cooling occurs under cool, clear skies and in stable, high pressure conditions. Cooling occurs until dew point temperature is reached and then condensation occurs. This leads to dew forming on the low-lying surface.

Question 27

Radiation cooling

Answer 27

When the ground cools as a result of outgoing LW radiation from radiation that has been absorbed in the day.

Question 28

Fog

Answer 28

Fog is cloud at ground level.

Question 29

Fog formation

Answer 29

1. Radiation Fog - In HP conditions with clear, cloudless night skies, heat radiates from the Earth’s surface so it cools to a dew point temperature and reaches saturation near the Earth’s surface and leads to condensation and fog.
2. Temperature Inversion Fog - This occurs where cold temperatures are found closer to the Earth’s surface with warmer temperatures above them. This is common in autumn in HP conditions and leads to cooling of the air at the ground to dew point temperature.
3. Advection fog - As warm, moist air passes over cooler oceans or land surfaces and is cooled to its dew point - it becomes saturated and condenses to form fog. This links to cold ocean currents and desert formation.