Monday, 9/13/10 1

ATMO 1300-006 Class #7

Monday, September 13, 2010Chapter 4, Water

continued

Saturation vapor pressure depends only on Temperature

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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

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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%

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The saturation water vapor content varies greatly with T

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Relative humidity changes with Temperature for the same air

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Another way to measure humidity: dew point

• Dew point is also called the dewpoint temperature, abbreviated as Td or TD

• 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

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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)

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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

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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

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Fig. 4-5, p. 91Monday, 9/13/10 11

Fig. 4-6, p. 92Monday, 9/13/10 12

Table 4-1, p. 92Monday, 9/13/10 13

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

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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

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Fig. 4-7, p. 9316Monday, 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

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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

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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

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Fog in a city

Fig. 4-8, p. 9520Monday, 9/13/10

Fig. 4-9, p. 9621Monday, 9/13/10

Radiation fog forms at night in valleys due to cooling

Fig. 4-10, p. 9722Monday, 9/13/10

Advection fog: warm moist air and a cool surface current

Fig. 4-11, p. 9723Monday, 9/13/10

Steam fog/evaporation fog: cold air and warm water

Fig. 4-12, p. 9824Monday, 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 freezeMonday, 9/13/10 25

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)

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Fig. 4-30, p. 11227Monday, 9/13/10

Fig. 4-35, p. 11728Monday, 9/13/10

How clouds form above the surface: lifting and cooling

Fig. 4-13, p. 9929Monday, 9/13/10