Causes of Precipitation

Question 1

What does The Bergeron Theory suggest?

Answer 1

• for rain to form, water + ice must be present in clouds at T°Cs below 0°C (up to -40°C)
  
  • sublimes to form water + ice droplets
• ice crystals aggregate into larger snowflakes, heavy enough to fall
• when falling, ice passes into warm air layers near ground so may melt into rain

Question 2

What are the causes of pptn?

Answer 2

• uplift of air:
  
  • convectional
  • frontal
  • orographic
• radiation cooling

Question 3

How does convectional uplift of air lead to pptn?

Answer 3

• when land is hot, air heated directly above it expands (so less dense) + rises
• as air rises, cooling + condensation occurs, forming clouds + pptn falls.
• common in tropical areas

Question 4

How does frontal uplift of air lead to pptn?

Answer 4

• warm air meets cold air
• less dense warm air, is forced to rise over cold, denser air
• as it rises, it cools + condenses forming rain
• common in middle + high latitudes where warm tropical air + cold polar air meet

Question 5

How does orographic uplift of air lead to pptn?

Answer 5

• pressure forces air to rise over a barrier (e.g. a mountain)
• as air rises, it cools, condenses + reaches dew point so clouds form + pptn falls
• leeward slope often has a rainshadow effect so receives less rain due to unstable air causing continued rising

Question 6

How does radiation cooling lead to pptn?

Answer 6

• cloudless night so ground loses heat rapidly by returning radiation to space
• little wind so air in contact w valley sides cool by conduction + sinks to bottom of valley
• ground T°C inversion occurs in morning, causing fog + ice to form