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Geography Lecture 5 (1. Introduction (Hadley cell is the summary of a…
Geography Lecture 5
1. Introduction
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As we move further away from the equator, there is a clearer wet and dry season as places are dominated by the STHP. Total precipitation also decreases
5. Atmospheric stability
- Absolute stability occurs when the temperature of the environment is higher than the air parcel. This causes the air parcel to tend to shrink and fall, thus forming clouds with no precipitation. (ELR always steeper than DALR and SALR)
- Absolute instability occurs when the temperature of the environment is lower than the air parcel. This causes the air parcel to tend to expand and rise, thus forming cumulonimbus clouds with precipitation. (ELR always gentler than DALR and SALR)
- Conditional stability/instability occurs when both stable and instable conditions are met at some point, and the ELR crosses the DALR/SALR at some point close to the condensation level
10. World precipitation
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Equatorial
Equatorial regions have the most precipitation, largely due to the warmer air being able to hold large amounts of moisture, and the surrounding oceans act as a source of water
Most precipitation is convectional, with some areas having monsoon seasons
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2. Air masses
Air masses are defined as large bodies of air with fairly uniform moisture and temperature
Air masses get their characteristics from the places where they originate
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Additional info
- Adiabatic means that an air parcel experiences changes in temperature without gaining or losing heat
- Some air parcels never reach the height/temperature required for ice to appear, hence, the type of precipitation formed is usually due to dust nuclei
4. Lapse rates
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Lapse rates include:
- Environmental Lapse Rate (ELR)
- Dry Adiabatic Lapse Rate (DALR)
- Saturated Adiabatic Lapse Rate (SALR)
(Refer to additional info 1)
This refers to the rate at which the temperature of the surrounding falls, with increasing altitude
On average, the ELR is about 6.4 degrees per km
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This is the rate at which the temperature of the air mass falls before it reaches condensation point, due to adiabatic expansion
On average, DALR is about 9.8 degrees per km
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This is the rate at which the temperature of the air mass falls with increasing altitude, after it has passed condensation point
As the air parcel condenses, it releases latent heat. This latent heat sows down the cooling of the air parcel, so the SALR is usually lower than the DALR
On average, the SALR is about 5 degrees per km
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6. Formation of rainfall
Clouds are formed by a collection of water droplets that are formed when water vapour condenses. There are 2 theories to suggest how raindrops fall.
- Collision-Coalescense Theory
This is usually used to explain rainfall in warm regions, where convectional activity is strong
As convectional activity is strong, winds bring clouds upwards, into the atmosphere
As more water droplets are brought up, they collide and join together to form larger water droplets
When the water droplets are big enough and heavy enough to overcome the updraft, they fall as rain
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- Hygroscopic Nuclei
(Refer to additional info 2)
For this process to occur, the following must be met:
- Air mass must be saturated
- There must be hygroscopic particles present
As the air cools, it shrinks and is unable to hold as much moisture. Thus, condensation occurs
When the air mass reaches dew point, condensation concentrates around large hygroscopic particles, such as dirt, dust or ice crystals. (They grow by electrical attraction)
