ATMO 170A1 1st Edition Lecture 13 Outline of Last Lecture I.Dew Point TemperatureII.Things to Remember About Humidity Outline of Current Lecture II.Two Ways to Achieve SaturationIII.Three Ways to Lower the Temperaturea.Dewb.Fogb.i.Radiation Fogb.ii.Advective Fogc.CloudsCurrent LectureTwo Ways to Achieve Saturation- There are two ways to achieve saturation:◦ Raise q (add water vapor)▪ In this case, water vapor is added to the air. An example of this is when you take a hot shower. The hot water from the shower evaporates, raising the water vaporcontent of the air in the shower room until saturation is reached. Condensation occurs first on the walls and mirrors. ◦ Lower qs (lower T to Td)- In this case, the temperature of the air is lowered until saturation is achieved. Lowering the temperature of the air (and thus the saturation humidity of the air) is probably the most important way to achieve saturation.Three Ways to Lower the Temperature◦ There are three ways to lower qs (lower temperature):◦ Reduce T to Td at the Earth's surface (dew)▪ At night, the surface cools faster than the air above it. If it reaches Td, there can be dew or frost (frost if the temperature is above 0°C).▪ T = Td at the surface▪ Not adding or removing water vapor: no condensation, no dew, did not reach Td◦ Reduce T to Td in the air layer above surface (fog)▪ Air near the ground also cools at night and forms fog – longer nights mean more probability of fog. ▪ Warm, moist air moving over a cold surface causes fog (San Francisco)▪ Radiation Fog: Cooling by radiative cooling of air above surface▪ Advective Fog: Moist air can be cooled by water → Can reach 100% RH and fog can occur◦ Reduce T to Td by lifting the air parcels (clouds)▪ As air rises, it expands and cools and can reach Td.- If the parcel is raised: ◦ Decrease P◦ Decrease R (air expands)◦ AND cools▪ If the parcel is lowered:- Increase P- Increase R (air contracts)- AND warms▪ Bottom line: Expanding air cools and air going under compression warms.▪ Less pressure, less density, less pressure2▪High pressure, high