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Lecture 9

GEOG 1112 Lecture 9: Diabatic Frost, Fog, Dew

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GEOG 1112
James Shepherd

LECTURE 9 | DIABATIC FROST, FOG, DEW Saturation • 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 Shower 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 removed. Diabatic Process • Energy is added or removed from a system • Examples: 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 Rates • 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. Environmental Laps
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