Dew, frost and fog

Question 1

What is dew and when does it form?

Answer 1

• clear calm nights -> radiational cooling -> surface temp < dew point temp -> condensation occurs

Question 2

What is frost and when does it form?

Answer 2

• when temp dew point < 0 C

- water vapour cools directly to ice

Question 3

What is condensation?

Answer 3

• forms when a deep layer of air cools, RH increases

- occurs on condensation nuclei

Question 4

As relative humidity increases to 100% what happens

Answer 4

• more condensation
• tiny droplets grow and become visible
• FOG happens

Question 5

How does fog form: 2 ways

Answer 5

Cooling

1. Temp air < saturation pt (temp dew pt)
2. Evaporation and mixing

Question 6

Cooling: a) radiation fog

Answer 6

• IR emitted at night, surface cools to temp dew pt
• clear/calm skies
• light winds to mix

Question 7

Cooling: b) Advection fog

Answer 7

• warm moist air moves over cool surface, air cooled to temp dew point and becomes saturated
• day or night

Question 8

Cooling: c) Upslope Fog

Answer 8

• moist air pushed up slope
• expands and cools temp dew pt and becomes saturated
• day or night

Question 9

Evaporation and mixing fog

Answer 9

• Water vapour added by evaporation, then mixes with dry air
• temp dew point increases