DEW POINT HYGROMETER

Notice the RH for the polar area is 100% but for the desert only 21% Notice the RH for the polar area is 100% but for the desert only 21%. However the desert air with a dew point of 100C (500F) contains much more water vapor that the polar area which has a dew point of -20C!

TYPE MEASURED IN Vapor Pressure Millibars kilopascals pascals 100 Pa = 1mb = 0.kPa Absolute Humidity # of grams of water vapor in a parcel of air. Specific Humidity # grams of water vapor per kilogram (relative to total mass) Mixing ratio # grams of water vapor per kilogram (relative to dry mass (total-vapor) Relative humidity % - specific humidity/saturation specific humidity X 100 Dew point Degrees of temperature – temperature are which air reaches saturation

Temperature dropped below freezing after FROZEN DEW Temperature dropped below freezing after the dew formed.

Dew point is below freezing FROST Dew point is below freezing

ΔH = p(Δα) + cv(ΔT) ΔH= change in heat P= air pressure Δα = change in volume (+ for expansion, - for contraction) cv = specific heat for air (assuming a constant volume) and ΔT = change in temperature

The adiabatic process is a case where there is no change in heat so 0 = p(Δα) + cv(ΔT) Or p(Δα) = - cv(ΔT) or - p(Δα) = cv(ΔT) If no heat is added or removed from the system, work performed by the air (expansion or contraction) will result in the raising or lowering of the temperature OR Expanding air cools, compressing air warms.

RADIATION FOG THE BEST CHANCE FOR FORMATION is a clear night (no clouds to reflect IR radiation back to the ground) when there is a shallow layer of moist air near the ground and a layer of warmer air above it (SURFACE INVERSION) and cooler air below the warmer. The lower level chills from contact (conduction) with the ground and reaches the dew point (saturation point) and fog forms. Remember this is caused radiation, inversion and conduction

From CAT PEOPLE (1942) Val Lewton and Jacques Tourneur

RADIATION FOG AROUND THE SPACE NEEDLE IN SEATTLE

Ground Fog in Lincoln Nebraska

ADVECTION FOG This occurs when warm moist air moving over a cold surface. It can be cold land or water. If the surface is cold enough to bring the warm air to its dew point it will condense and produce fog. Remember advection has to do with horizontal movement.

ADVECTION FOG AT THE GOLDEN GATE BRIDGE

Labrador Current meets the Gulf Stream

OCEAN CURRENTS

PATH OF THE GULF STREAM AND LABRADOR CURRENT ADVECTION FOG OCCURS WHEN WARM GULF STREAM AIR CROSSES LABRADOR CURRENT WHICH COOLS IT TO THE DEW POINT

UPSLOPE FOG

STEAM FOG

STEAM FOG RISING FROM THE OCEAN

Frontal fog is generally associated with warm fronts and forms when precipitation evaporates into the atmosphere. Not only does this increase the moisture content of the surrounding atmosphere – it cools the atmosphere due to a process known as evaporative cooling. As this continues to occur, the temperature decreases and the dewpoint rises, eventually converging at the “wet bulb” temperature. If additional moisture is added to the air after the wet bulb temperature has been reached, frontal fog results. FRONTAL FOG

PRECIPTIATION FOG

PARCEL A PARCEL B T= 200C -100C w = ? ? Td = 150C -150C ws= ? ?

PARCEL A PARCEL B T= 200C -100C w = 10.8 g/kg 1.2 g/kg Td = 150C -150C ws= 15.0 g/kg 1.8 g/kg

PARCEL A PARCEL B T= 200C -100C w = 10.8 g/kg 1.2 g/kg Td = 150C -150C ws= 15.0 g/kg 1.8 g/kg

PARCEL A PARCEL B T= 200C -100C w = 10.8 g/kg 1.2 g/kg Td = 150C -150C ws= 15.0 g/kg 1.8 g/kg FOR PARCEL A RH= 10.8 /15 = 72% FOR PARCEL B RH = 1.2/1.8 = 67%

PARCEL A PARCEL B T= 200C -100C w = 10.8 g/kg 1.2 g/kg Td = 150C -150C ws= 10.8 g/kg 1.8 g/kg FOR PARCEL A RH= 10.8 /15 = 72% FOR PARCEL B RH = 1.2/1.8 = 67% FOR JOINT PARCEL Average temperature = (20) + (-10) = 50C Total Water vapor = Water vapor A + water vapor B 10.8 g/kg + 1.2 g/kg = 12g/kg/2 = 6.0 g/kg From the chart the saturation For 50C is 5.5 g/kg RH = 109% super-saturated!

Different Fog Types Commonly Found in North America