LECTURE 9 | DIABATIC FROST, FOG, DEW
• To get clouds or fog or dew, the air needs to be brought to saturation.
• This can be done by....
How do we achieve saturation?
• Add water vapor to the air
o water evaporated from rainfall
• Mix cold air with warm air
o contrails formed by jet aircraft
o “steam fog” or “mixing fog”
• Lower the temperature to the Dew-point
o Most clouds form this way
o there are several ways to do this...
Lowering Air Temperature
• Air temperature can be lowered by two classes of processes:
o diabatic processes
o adiabatic processes
• Diabatic Process-Energy added or removed from a system
• Adiabatic Process-Temperature of system changes but no heat is added or
• Energy is added or removed from a system
o pot of water over a stove
o air that passes over a warm (cold) surface gains (loses) energy
• Important for fog but not clouds
• Lapse Rate: rate of change in temperature with height
o lapse rate=temperature change/change in height
• We’ll look at two different lapse rates in determining atmospheric stability:
o lapse rate of the air parcel
o lapse rate of the surrounding atmosphere
• dry adiabatic lapse rate (DALR)
o unsaturated parcel (may still contain moisture!) o constant value of about 10 oC km-1
• saturated adiabatic lapse rate (SALR)
o Varies with temperature & moisture content (3-9 oC km-1)
o Always cools more slowly than DALR.