Dew, Frost and Fog

RECAP Hydrological cycle: transport of water and energy. Humidity: absolute humidity, specific humidity, water mixing ratio, relative humidity. Saturation pressure: the maximum vapor pressure at a given T. Dew point: the temperature to which air would have to be cooled in order to get saturated. The effect of large water bodies on the humidity of a climate (Florida/California). The effect of cooling/heating systems on the humidity in the house. Heat index: what the air temperature “feels like” to the average person for different combinations of temperature and relative humidity

Condensation As air cools, it becomes first saturated and then supersaturated -> condensation/deposition Condensation requires some initiator ♦ surface (dew, frost) ♦ nuclei (haze, fog, clouds) Classification of condensation nuclei according to ♦ size (not important) ♦ properties hygroscopic: “water- seeking”, condensation at RH<100% hydrophobic: “water- repelling”, condensation at RH>100%. unsaturated Super- saturated saturated

Dew Condensation of water vapor on cool surfaces. Usually forms at ground level (air is coldest there). Frozen dew: air temperature drops below zero after dew has formed Atmospheric conditions: clear, calm nights. ♦ Clear air: the ground emits IR radiation to space. There are no clouds to absorb and reradiate the IR energy. ♦ Calm air (no winds): The wind enhances evaporation and inhibits condensation. It also enhances mixing in the air, so in the absence of wind, the ground gets colder (recall radiation inversion).

Frost Deposition of water vapor on cool surfaces. Atmospheric conditions: cold, clear, calm nights. ♦ Clear air: the ground emits IR radiation to space. There are no clouds to reabsorb IR energy. ♦ Cold: the temperature of the ground is below freezing. ♦ Calm air: no winds- wind enhances evaporation and inhibits condensation. It also enhances mixing in the air, so if there is no wind, the ground gets colder (recall radiation inversion).

Frost on Windows Cold, outside temperatures cool the window. The water vapor in the room is cooled down to below freezing temperatures and is deposited on the inside of the window, forming delicate ice crystals.

Haze A layer of small nuclei (dust, aerosols, salt particles). ♦ Dry haze: at low relative humidity ♦ Wet haze: at relative humidity 75%-100%. The presence of hygroscopic (water-seeking) nuclei facilitates condensation at RH<100%. Haze is more visible in the morning and in the evening (higher relative humidity) because the wet particles are bigger. Book sez: Dry haze appears bluish on dark background and yellowish on light background (why? Wait for the answer until Chapter 19).

Haze on Titan Cassini spacecraft imaging system

Fog Wet haze near RH=100% is called fog. Fog is a cloud resting near the ground. Wet haze or fog? International definition: fog, if visibility < 1km. The individual water particles are bigger than the haze particles and they become visible to the eye. Fog scatters light at all wavelengths (geometric scattering) and as a result it appears white/gray. Fog is most likely to form in the morning or evening when the RH is highest.

Making Fog

Radiation (Ground) Fog: Preconditions – local radiative processes dominate. Formation. ♦ The ground cools down due to thermal (IR) radiation; ♦ The layer next to the ground cools down; ♦ The RH is increasing and the air saturates forming fog close to the ground. When does it occur? ♦ Late night hours when the ground temperature is minimum. ♦ Late fall and winter (long nights) When does it dissipate? ♦ As the sun comes up it heats up the ground. The ground air is heated by conduction and the fog droplets evaporate. ♦ The fog dissipates from the ground up.

Advection Fog: Preconditions. ♦ Air mass movements (advection, weak wind) over a moisture source. ♦ Temperature difference between the water surface and the land. Formation. ♦ Warm land and cold water: warm air mass moves over to the water. It cools down, the RH rises, the air saturates; ♦ Cold land and warm water: cold air mass moves over the water. It mixes up with warmer moist air from above the water surface. The resulting mixed air is saturated. ♦ Cold water and warm water: two ocean currents with different temperatures flow next to each other. Warm air moving over the cold current produces fog.

Upslope Fog Preconditions. ♦ Weak winds ♦ Presence of topography Formation. ♦ Moist air moved up the slope of a hill; ♦ The air mass cools down and the RH increases; ♦ The saturated air forms fog

Evaporation Fog Preconditions: ♦ Cool air over a body of warm water (no need for advection). Formation: ♦ Water evaporates from the warm lake (pool). ♦ The cool air moistens and saturates. ♦ It is a local effect. When does it occur? ♦ Often during the fall and early winter (water is warmer than the land) ♦ After a rainshower as the sun heats up the ground

Foggy weather in the US

The nuisance of foggy weather The limited visibility leads to ♦ Poor driving conditions Use fog lights Do not use high beams ♦ Poor sailing conditions Lighthouses ♦ Poor flying conditions Airport closure Railways also are affected