Class #3 July 9, 2010 1

Class #3: Humidity, condensation, and clouds

Chapters 4 and 5

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Atmospheric Humidity

Chapter 4

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Circulation of Water in the Atmosphere

• A general definition of humidity is the amount of water vapor in the air.

• Remember, humidity is not constant through time or space, there is constant circulation of water through the hydrologic cycle.

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Class #3 July 9, 2010 5Stepped Art

Fig. 4-1, p. 90

Class #3 July 9, 2010 6Fig. 4-2, p. 91

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The Many Phases of Water

• Phase is related to molecular motion, an increase or decrease in motion creates a phase change.

• Ice is the coolest/slowest phase• Water vapor is the warmest/fastest phase

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Evaporation, Condensation, & Saturation

• Evaporation is the change of liquid into a gas a requires heat.

• Condensation is the change of a gas into a liquid and releases heat.– Condensation nuclei

• Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses.

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Humidity

• Any of a number of ways of specifying the amount of water vapor in the air.

• Absolute humidity: mass of water vapor/volume of air– Water vapor density– Not commonly used due to frequent change of

volume

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Humidity

• Specific Humidity: mass of water vapor/mass of air

• Mixing ratio: mass of water vapor/mass of dry air

• Neither measurement changes with volume, must add or subtract water vapor.

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Humidity

• Vapor pressure: the pressure exerted by water vapor molecules in an air parcel (Dalton’s Law of Partial Pressure)– Fraction of total vapor pressure (1% or so)– More water molecules = high vapor pressure

• Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature

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Humidity

• Special Topic: Vapor Pressure & Boiling– Once water boils it requires more energy to

increase temperature.– Water boils at a low temperature in the mountains

and thus needs more energy and time to cook items as compared to sea level.

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Humidity

• Relative Humidity: (actual water vapor/saturation water vapor)*100– RH can be changed two ways:• Change vapor content• Change saturation

– Decrease temperature causes an increase in relative humidity (inverse relationship).

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Humidity

• Relative Humidity and Dew Point– Dew point is the temperature at which saturation

occurs– Cool air parcel to dew point and liquid water

condenses– A good measure of actual water vapor content– Relative humidity indicates how close to

saturation, dew point indicates the amount of water vapor

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Class #3 July 9, 2010 25Fig. 4-13, p. 98

Class #3 July 9, 2010 26Fig. 4-13, p. 98

Class #3 July 9, 2010 27Fig. 4-13, p. 98

Class #3 July 9, 2010 28Fig. 4-13, p. 98

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Class #3 July 9, 2010 33Fig. 4-15a, p. 100

Class #3 July 9, 2010 34Fig. 4-15b, p. 100

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Humidity

• Relative Humidity in the Home– Due to an increase in temperature in a heated

home there is a decrease in relative humidity, causing more evaporation from body, plants, etc• Humidifier, chapped lips

– Swamp cooler

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Humidity

• Relative humidity & human comfort– “It’s not the heat, it’s the humidity.”– High relative humidity equates to less evaporative

cooling.– Sweat cannot evaporate and cool the body– Wet bulb temperature– Heat Index

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Humidity

• Special Topic: Heavier humid air– Due to the molecular weight of water as

compared to nitrogen, humid air is lighter than dry air.

– Baseball announcers are incorrect.

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Humidity

• Measuring humidity– Sling psychrometer– Hygrometer

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Condensation: DEW, Fog, & clouds

Chapter 5

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The Formation of Dew & Frost

• Dew forms on objects near the ground surface when they cool below the dew point temperature.– More likely on clear nights due to increased

radiative cooling• White frost forms when temperature cools

below the dew point and the dew point is below 0°C

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Condensation Nuclei

• Particles suspended in the air that around which water condenses or freezes.– Hydrophobic/hygroscopic

Class #3 July 9, 2010 47Table 5-1, p. 113

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Haze

• Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze.

• Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating grayish or white haze.

Class #3 July 9, 2010 51Fig. 5-4, p. 114

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Fog

• Saturation reached condensation forms a cloud near the ground

• Radiation fog: ground cools through conduction and radiation; ground fog– Valley fog created by cold air drainage– High inversion fog

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Fog

• Advection Fog: warm moist fog moves horizontally (advects) over a cool surface.– Summer fog on the Pacific coast

• Observation: Headlands– Air converges and rises over headlands forming

fog as compared to lower elevation beaches.

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Fog

• Upslope Fog: moist air flows up an orographic barrier– East side of the Rockies

• Evaporation Fog:– Warm moist surface provides enough moisture to

saturate a dry air parcel; short lived• Steam fog• Breath in winter

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Foggy Weather

• In general fog not common for most location in the US. However several areas do exist with a high frequency of fog. Two causes:– Elevation– Ocean currents

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Foggy Weather

• Environmental Issue: Fog dispersal– Mix air with air craft or fans– Introduce large particle into air to reduce total

number of cloud droplets.– Use dry ice to lower temperature below freezing.

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Clouds

• Classification of clouds: use Latin words to describe height and appearance.

• Factors described– Height: low, mid, high, vertical– Appearance: shape, density, color

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Some Unusual Clouds

• Not all clouds can be placed into the ten basic cloud forms.

• Unique atmospheric processes and environmental conditions create dramatic and exotic clouds.

• Unusual clouds and weather balloons often cause of UFO reports.

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Class #3 July 9, 2010 82Fig. 5-26, p. 130

Class #3 July 9, 2010 83Fig. 5-27, p. 130

Class #3 July 9, 2010 84Fig. 5-28, p. 130

Class #3 July 9, 2010 85Fig. 5-29, p. 130

Class #3 July 9, 2010 86Fig. 5-30, p. 131

Class #3 July 9, 2010 87Fig. 5-31, p. 131

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Cloud Observations

• Sky conditions: cloud coverage divided into eighths and each amount associated with term such as scattered clouds.

• Observations: cloud ceilings– Ceilometer used at airports to determine height

from clouds by light or laser striking clouds and then amount and speed of reflected light recorded.

Class #3 July 9, 2010 89Fig. 5, p. 132

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Cloud Observations

• Satellite Observations– Geostationary, polar orbiting– Visible light provides a black and white picture of

clouds– Infrared approximates cloud temperature which

infers height– Satellites measure many other variables: sea

surface temperatures, ozone, upper level features, snow cover, land cover

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