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Over 70% of the planet is covered by water |
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Water is unique in that it can simultaneously
exist in all three states (solid, liquid, gas) at the same temperature |
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Water is able to shift between states very
easily |
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The hydrologic cycle refers to the regular cycle
of water through the earth-atmosphere system |
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Liquification of water occurs frequently at
normal Earth temperatures |
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Occurs when air is saturated with respect to
water vapor |
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The addition of water vapor, or the lowering of
temperature, in saturated air will lead to condensation |
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Latent heat is the heat released or absorbed per
unit mass when water changes phase. |
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Latent heating is an efficient way of
transferring energy globally and is an important energy source for Earth’s
weather and climate. |
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Evaporation: the process whereby molecules break
free of the liquid volume. |
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Condensation: water vapor molecules randomly
collide with the water surface and bond with adjacent molecules. |
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by mass |
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by vapor pressure |
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by temperature è Dew Point Temperature |
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Specific Humidity: How many grams of water vapor
in one kilogram of air (in unit of gm/kg). |
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Relative Humidity: The percentage of current
moisture content to the saturated moisture amount (in unit of %). |
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Clouds form when the relative humidity reaches
100%. |
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The
air’s content of moisture can be measured by the pressure exerted by the
water vapor in the air. |
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The total pressure inside an air parcel is equal
to the sum of pressures of the individual gases. |
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In the left figure, the total pressure of the
air parcel is equal to sum of vapor pressure plus the pressures exerted by
Nitrogen and Oxygen. |
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High vapor pressure indicates large numbers of
water vapor molecules. |
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Unit of vapor pressure is usually in mb. |
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Saturation vapor pressure describes how much
water vapor is needed to make the air saturated at any given temperature. |
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Saturation vapor pressure depends primarily on
the air temperature in the following way: |
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è |
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Saturation pressure
increases exponentially with air temperature. |
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Three ways: |
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Increase (inject more) water vapor to the air (Aà B). |
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Reduce the temperature of the air (A à C). |
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Mix cold air with warm, moist air. |
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If a planet has a very high temperature that the
air can never reach a saturation point |
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Water vapor can be added into the atmosphere. |
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More water vapor traps more heat (a greenhouse
effect) |
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The planet’s temperature increases furthermore |
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Ever
more water evaporated into the atmosphere |
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More
greenhouse effect |
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More
warming |
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More
water vapor |
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….. |
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Dew point temperature is another measurement of
air moisture. |
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Dew point temperature is defined as the
temperature to which moist air must be cool to become saturated without
changing the pressure. |
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The close the dew point temperature is to the
air temperature, the closer the air is to saturation. |
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Dew points can be only equal or less than air
temperatures. |
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When air
reaches saturation at temperatures below freezing the term frost point is
used. |
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The easiest way to measure humidity is through
use of a sling psychrometer - A pair of thermometers one of which has a
wetted cotton wick attached to the bulb. |
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The two thermometers measure the wet and dry
bulb temperature. |
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Swinging the psychrometer causes air to
circulate about the bulbs. |
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When air is unsaturated, evaporation occurs from
the wet bulb which cools the bulb. |
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Once evaporation occurs, the wet bulb
temperature stabilizes allowing for comparison with the dry bulb
temperature. |
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The wet bulb depression is found with a greater
depression indicative of a dry atmosphere. |
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Charts gauge the amount of atmospheric humidity. |
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Aspirated and hair hygrometers are alternatives. |
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Notes