Monday, 9/13/101 ATMO Class #7 Monday, September 13, 2010 Chapter 4, Water continued

Saturation vapor pressure depends only on Temperature Monday, 9/13/102

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, 9/13/103

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 a high temperature with relative humidity of 50% may have more water vapor than air at a lower temperature with a relative humidity of 90% Monday, 9/13/104

The saturation water vapor content varies greatly with T Monday, 9/13/105

Relative humidity changes with Temperature for the same air Monday, 9/13/106

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, 9/13/107

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, 9/13/108

Frozen Dew Occurs in two steps First, condensation occurs and the temperature is above freezing (32ºF); that is, the dew point is above freezing The condensation is dew Second, the temperature falls below freezing. The dew freezes to frozen drops This ice is called frozen dew or black ice Monday, 9/13/109

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, 9/13/1010

Fig. 4-5, p. 91 Monday, 9/13/1011

Fig. 4-6, p. 92 Monday, 9/13/1012

Table 4-1, p. 92 Monday, 9/13/1013

Condensation in air (not on a surface like frost or dew) In a lab with perfectly clean air, saturation requires a relative humidity of more than 200%. RH > 100% is supersaturation Condensation is inhibited by the curvature effect Small, very curved droplets have molecules with few neighbors, and are quick to evaporate Monday, 9/13/1014

Condensation In the lab with perfectly clean air (no aerosol) takes a relative humidity of more than 200%. RH > 100% is supersaturation Condensation is inhibited by the curvature effect Small, very curved droplets have molecules with few neighbors, and are quick to evaporate Monday, 9/13/1015

Fig. 4-7, p Monday, 9/13/10

Condensation in the atmosphere Is inhibited by the curvature effect Is enhanced by the solute effect Some aerosol, salt particles for example, dissolve and have the ability to hold on to water molecules and suppress evaporation Other aerosol particles form nuclei, or small surfaces for condensation Monday, 9/13/1017

Condensation in the atmosphere (continued) A cloud nucleus gives water molecules more neighbors, by acting like a small flat surface There are always abundant cloud condensation nuclei in the atmosphere— dust, salt, pollen, pollutants The solute effect permits condensation at RH < 100%. This is called haze Monday, 9/13/1018

Condensation in the atmosphere (continued) Supersaturation (RH > 100%) does not occur in the atmosphere. The solute effect cancels the curvature effect. When the relative humidity reaches 100%, cloud particles form. Cloud at the surface is called fog. Fog reduces visibility to less than 1 km or 0.6 miles Heavy fog is a travel hazard Monday, 9/13/1019

Fog in a city Fig. 4-8, p Monday, 9/13/10

Fig. 4-9, p Monday, 9/13/10

Radiation fog forms at night in valleys due to cooling Fig. 4-10, p Monday, 9/13/10

Advection fog: warm moist air and a cool surface current Fig. 4-11, p Monday, 9/13/10

Steam fog/evaporation fog: cold air and warm water Fig. 4-12, p Monday, 9/13/10

Ice in clouds Deposits (deposition) on small particles called ice nuclei (clay minerals, tiny ice crystals) There is a scarcity of ice nuclei at high subfreezing temperatures near but < 32ºF Many water droplets do not freeze at subfreezing temperatures, called supercooling. Below -40ºC (or F), all water drops freeze Monday, 9/13/1025

Ice in clouds (continued) Ice takes on different crystal shapes in clouds, depending on temperature and supersaturation Clouds are saturated with respect to water, supersaturated with respect to ice The saturation vapor pressure over ice is less than that over water There can be ice fog (inland Alaska) Monday, 9/13/1026

Fig. 4-30, p Monday, 9/13/10

Fig. 4-35, p Monday, 9/13/10

How clouds form above the surface: lifting and cooling Fig. 4-13, p Monday, 9/13/10