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Lecture 10

GEOG 1112 Lecture 10: Humidity
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Department
Geography
Course
GEOG 1112
Professor
James Shepherd
Semester
Spring

Description
LECTURE 10 | HUMIDITY The Hydrologic Cycle • Earth has been called the “water planet” as over 3⁄4 of the Earth’s surface is covered by water. • Water is unique in that it can simultaneously exist in all three states (solid, liquid, gas) at the same temperature and shift between states easily. • The hydrologic cycle refers to the cycle of water through the earth and atmosphere. Distribution of Water Vapor • The hydrologic cycle refers to the cycle of water through the earth and atmosphere. Atmospheric Humidity • Humidity: A generic term for the amount of water vapor present in the air. Ways to Express Humidity • Absolute Humidity: o mass WV in air (g) / volume of air (m3) • Specific Humidity: o mass of WV in air (g) / total mass of air (kg) • Mixing Ratio: o mass of WV in air (g) / mass of dry air (kg) Vapor Pressure • The amount of atmospheric pressure exerted by water vapor molecules. • We can measure vapor pressure in the same way as with total pressure, in millibars... Saturation • Saturation: the maximum amount of water that can exist in the atmosphere as a vapor at a given temperature. • This occurs when evaporation = condensation • We can look at saturation values for each of the measures of atmospheric humidity: o Saturation Vapor Pressure o Saturation Specific Humidity o Saturation Mixing Ratio Processes That Cause Saturation • Air may become saturated through 3 ways: o the addition of water vapor o mixing cold air with warm air o moist air—by cooling the air to dew point • If enough vapor is added to the air to saturate it, a precipitation fog forms beneath the cloud (#1 listed above) Relative Humidity • This is probably the most familiar measure of water vapor content. • It is the amount of water vapor present relative to its saturation point (e.g. the maximum possible at the current temperature). • RH = actual amount of WV/ capacity that can be held for a given temperature • We can plug in specific humidity for “actual” and saturation specific humidity for “saturation” or “capacity” value. • RH = (specific humidity / saturation specific humidity)*100% • Relative Humidity does not uniquely determine the
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