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Meteo 3: Chapter 4 Water Vapor and Clouds Read Chapter 4 Who Cares About Water Anyways? Phase changes of water important for energy transport in atmosphere Clouds! Precipitation! Overview Cloud: Collection of liquid water drops or ice crystals Clouds form as

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Meteo 3: Chapter 4

Water Vapor and Clouds

Read Chapter 4


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Who Cares About Water Anyways?

  • Phase changes of water important for energy transport in atmosphere

  • Clouds!

  • Precipitation!


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Overview

  • Cloud: Collection of liquid water drops or ice crystals

  • Clouds form as

    • 1) Water vapor condenses onto small particles known as condensation nuclei to form liquid water drops, or

    • 2) Water vapor deposits onto small particles known as ice nuclei that allow for ice crystal formation

  • In a cloud, water can be present in all three phases at the same time


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Hydrologic Cycle

  • Global precipitation = Global evaporation


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Terms

  • Evaporation: liquid water molecules break bonds with other molecules to escape to gaseous phase

  • Condensation: Water vapor returns to liquid state

  • Sublimation: Ice changes directly to water vapor

  • Deposition: Water vapor changes directly to ice

  • Transpiration: Plants releasing water vapor into air


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Temperature and Evaporation

  • Evaporation occurs when liquid water molecules gain enough kinetic energy to break bonds

    • The higher the temperature of water, the higher the kinetic energy of its molecules, thus the higher the evaporation rate

  • Evaporation is a cooling process


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Balance of Evaporation & Condensation

  • Net Condensation: Condensation exceeds evaporation

  • Net Evaporation: Evaporation exceeds condensation

  • Vapor Pressure: Water vapor’s contribution to total pressure



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Birth of Clouds

  • Relative Humidity: (vapor pressure / equilibrium vapor pressure) * 100

    • At saturation (rate of condensation = rate of evaporation), RH = 100%

    • Clouds form when RH exceeds 100% by a few tenths of a percent

    • Water vapor condenses onto CCN…some hygroscopic, meaning they attract water vapor

    • In summer, when RH exceeds 80%, net condensation occurs on some particles (pollution), leading to haze…associated with poor air quality


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Mechanisms to induce cloud formation

  • For clouds to form, there must be net condensation

  • We can get this by cooling the air

    • As temperature lowers, molecular speeds decrease, and water vapor gathers near CCN

    • The amount of cooling needed is inversely proportional to the amount of water vapor in the air



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Air pressure (density) decreases with height

Rising air parcels expand, cooling as they do work on environment

If vapor pressure = equilibrium vapor pressure => condensation

Cooling via lifting



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Orographic Lifting: Lifting by Terrain

  • Windward side of mountain, facing prevailing wind, is extremely wet

  • Leeward side, sheltered from wind, very dry…known as rain shadow





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Assessing Air’s Moisture Content

  • Problems with RH because denominator depends on temperature

    • Cold, dry air masses can have a high RH, even if they hold little water vapor

    • Relative humidity varies with time of day

    • http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.html


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Dew Point: Absolute measure of water vapor

  • Dew Point: Temperature air must be cooled (at constant pressure) to reach saturation

    • Less than or equal to temperature

    • Higher the dew point, more water vapor in air

    • Frost point if air temperature below 32ºF

    • Measured with a hygrometer or sling psychrometer

    • Changes by evaporating water into air, mixing drier air from above, wind blowing in moist or dry air from another region (air dries behind cold front, moistens before cold front)


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Applying Dew Point to Weather Forecasting

  • 1) Cloud Base Height

    • Temperature of rising air decreases faster than dew point…has a decent chance of eventually reaching dew point

  • 2) First-Guess Low Temperature

  • 3) Severe Weather

    • High dew points indicate enhanced risk


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