2.3 weather processes and phenomena

Question 1

explain convectional rainfall

Answer 1

1. land and air heat
2. convection, adiabatic cooling, CCN
3. clouds not extensive: relatively short over a certain area
4. ITCZ: most common reason for rainfall, temperate/mid-lat: more in summer, associated with cold fronts

Question 2

explain intense convectional rainfall

Answer 2

• unstable air masses - rapid cloud formation- intense precipitation
• common in equatorial, polar areas
  1. updraft by rapid evap of warm water lifts air mass
  2. thunder and lightning (try to balance instability)
  3. large: cloud may develop until tropopause : anvil shape
  4. downdraughts by rain drag air back down (lift stronger than updraft:dissipation)
  5. sequence of drafts and cycling of air: thunderstorm cell

Question 3

explain frontal/cyclonic rainfall

Answer 3

1. two air masses of different densities meet
2. warmer forced to rise-cools-condensation at saturation point-precipitation
3. most common in mid/high latitudes:interface between warm equ and cool polar air
4. often form nimbostratus clouds
5. cold displacing warm: short and intense, warm displacing: prolonged drizzle

Question 4

explain orographic relief

Answer 4

1. orographic uplift - adiabatic cooling - ppt falls as rising clouds ascend windward side
2. leeward: spillover:rain shadow
3. areas with consistent winds (trade winds, southwesterlies)
4. barrier forces air to rise
5. dew point condensation

Question 5

list the type of clouds of vertical development

Answer 5

cumulonimbus, cumulus,

Question 6

list the low clouds

Answer 6

strato-cumulus, stratus, numbostratus

Question 7

list the medium clouds

Answer 7

altostratus, altocumulus

Question 8

list the high clouds

Answer 8

cirrostratus, cirrus, cirrocumulus

Question 9

explain the formation of rain

Answer 9

• atmospheric water vapour droplets, heavy, pulled down by gravity
  -condensation around CCN

Question 10

explain the formation of dew

Answer 10

• as temperatures drop and objects cool down. If the object becomes cool enough, the air around the object will also cool. Colder air is less able to hold water vapour than warm air. This forces water vapour in the air around cooling objects to condense
• dew point T: The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%

Question 11

explain the formation of hail

Answer 11

• ice crystals and snow pellets collide in turbulence and convection of cumulonimbus clouds
• updraughts catch - grow - heavy - drop
• sumblimation

Question 12

explain the formation of snow

Answer 12

• T below freezing snow crystal forms
• little moisture in cold air: orographic or frontal

Question 13

explain the formation of radiation fog

Answer 13

• surface cools by radiation heat loss, cooling the air above
• often air gets stuck in low-lying pockets:dew point
  -strong wind:lift
  -sun:disperses fog

Question 14

explain the formation of advection fog

Answer 14

-warmer ai moves horizontally over cooler ground surface
- air cooled close to surface, saturation leads to condensation, water droplets remained suspended, CCN allow to coalesce and grow

Question 15

explain the formation of tropical storms

Answer 15

• an area of low pressure with winds moving in a spiral around a central calm point
• needed: water T min 25, min 50 m deep, trade winds, start in 5-15 degrees lat
  1. evap - updraft - cumulonimbus clouds, latent heat, anvil shape as diverges at tropopause
  2. removal of air in upper parts causes increased convergence in lower - Coriolis effect and cross winds cause vapour to spin around an area of low pressure
  3. converging spiral draw thunderclouds - pattern of rainbands around centre, eye wall:strongest, eye pulls down cooler high pressure air