Monday, 1/26/091 ATMO Class #8 Monday, January 26, 2009 Chapters 3,4 Temperature, Water

The daily temperature cycle Monday, 1/26/092

Temperatures above the surface: story of an inversion Monday, 1/26/093

Lapse rate name: Dry adiabatic lapse rate =10ºC/km Monday, 1/26/094

Absolute stability defined with the dry adiabatic lapse rate Monday, 1/26/095

Humidity measures how much water vapor is in the air Water vapor gets into the atmosphere by evaporation (and sublimation) Water vapor leaves the atmosphere by condensation (and deposition) Evaporation increases humidity and condensation decreases humidity Saturation occurs when rate of condensation = rate of evaporation Monday, 1/26/096

Saturating an air sample Monday, 1/26/097

Why measure water vapor content (humidity) in the air? Water changing phase is a source of energy for storms Water vapor is the source of clouds and precipitation The amount of water vapor (humidity) determines the rate of evaporation Water vapor is the most abundant greenhouse gas Monday, 1/26/098

One way to measure humidity: Water vapor pressure Water vapor pressure is the amount of the total atmospheric pressure that is due to the water vapor molecules Water vapor pressure is often called just vapor pressure Vapor pressure is measured in mb, just like total atmospheric pressure Saturation vapor pressure depends on T Monday, 1/26/099

Saturation vapor pressure depends only on Temperature Monday, 1/26/0910

Another way to measure humidity: the relative humidity Relative humidity is expressed as a percentage, where 100% is saturation Relative humidity can be defined in terms of the vapor pressure Relative humidity = 100% x vapor pressure ÷ saturation vapor pressure A low relative humidity allows a higher evaporation rate Monday, 1/26/0911

Relative humidity has disadvantages Relative humidity tells how the air is from saturation 0% relative humidity: No water vapor 100% relative humidity is saturated But air at one temperature with relative humidity of 50% may have more water vapor than air at a lower temperature with a relative humidity of 90% Monday, 1/26/0912

The saturation water vapor content varies greatly with T Monday, 1/26/0913

Relative humidity changes with T for the same air Monday, 1/26/0914

Another way to measure humidity: dew point Dew point is also called the dewpoint temperature, abbreviated as T d or T D Dew point is defined as the temperature to which air must be cooled (without changing the pressure) to become saturated Dew point does not exceed the temperature Monday, 1/26/0915

More on dewpoint When the dewpoint is below 0°C (32°F), it is called the frost point, because deposition (water vapor to ice) in the form of frost will occur when the air becomes saturated When air cools to the dewpoint, condensation occurs On surfaces, this condensation is called dew (or frost) Monday, 1/26/0916

Frozen dew When the dew point is above the freezing point of water and cooling occurs, dew forms when the temperature reaches the dew point Sometimes further cooling occurs after the dew forms If the temperature falls below 0°C, the dew can freeze. When liquid dew freezes, it is called frozen dew Monday, 1/26/0917

Frozen dew (continued) Frozen dew is also called “black ice” It is a major traffic hazard It also causes slips and falls for people on foot Frozen dew is hard to see Frozen dew frequently forms on roads where there is a significant slope, as well as bridges and overpasses Monday, 1/26/0918