Thermodynamics

Question 1

(Vapour Pressure)

What is vapour pressure?

What is specific vapour pressure?

Answer 1

Vapour pressure (e)

• the partial pressure exerted by the water vapour in the atmosphere
• measure of the amount of water vapour in the atmosphere

Specific vapour pressure (eS)
- w.r.t water or ice, it is the pressure when water vapour is in equilibrium with liquid water or ice

When e > eS, condensation occurs

Question 2

(Humidity)

What is relative humidity? (H)

What is specific humidity? (q)

Answer 2

Relative humidity (H)

• ratio of vapour pressure to specific vapour pressure
• at the same temperature and pressure

H = e / eS * 100%

Specific humidity (q)
- ratio of density of water vapour over density of moist air

q = ρv / ρm

Question 3

(Condensation)

What causes condensation to occur?

Answer 3

1. Cooling at constant specific humidity and pressure
2. Mixing of unsaturated air
   - supersaturation (eS decreases exponentially as T drops, but internal energy of e decreases linearly)
3. Adiabatic ascent of moist air
   - as air ascents, T falls
   - e does not change much but eS decreases exponentially

Question 4

(Precipitation)

What are the different types of precipitation?

How is rainfall measured?

Answer 4

Type of precipitation

• rain
• snow
• hail
• graupel
• sleet

How to measure rainfall

• intensity
• area coverage
• duration
• frequency (days per month)

Question 5

Where are the rainy regions in the world?

Where are the deserts in the world?

Answer 5

Rainy regions

• converging air e.g. tropics, mid-latitude storm tracks where cyclonic activity is high
• windward slopes of mountains e.g. (Himalayan foothills in SW monsoon)

Deserts

• diverging air e.g. subtropics
• high-latitude regions e.g. Antarctica, Siberia
• high-altitude regions e.g. Tibet