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| Relative humidity (RH) - A ratio of the amount of water vapor that is in the air to the amount of water vapor the air can have, which depends on temperature. | |
| Mixing ratio (w)- A measurement of the amount of water vapor in the air of a given sized quantity of dry air; grams of vapor per kilogram of dry air. | |
| Saturations mixing ratio (ws)- The capacity the atmosphere to hold water vapor, dependent upon temperature. i.e. how much "w" (mixing ratio) can there be? More precisely, it is the amount of vapor that will be present in the air when the rate of condensation and evaporation achieve a dynamic equilibrium. | |
| The warmer the temperature, the more water vapor that will be present. The mixing ratio defines how much water vapor actually exists (NOT dependent on temperature). The saturation mixing ratio defines the capacity, or potential, of how much water vapor can be present, dependent on temperature. The greater the temperature, the greater the saturation mixing ratio. Relative Humidity is the ratio between how much water vapor exists, and how much water vapor can exist given the temperature. If the two are equal, the RH must be 100%. 100% Humidity means the air has as much water vapor as it can have. | |
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| Relative humidity (RH)- A ratio of the amount of water vapor that the air is holding to the amount of water vapor the air can hold, which depends on temperature. | |
| Vapor Pressure (e)- The contribution to the overall atmospheric pressure made by water vapor | |
| Saturation vapor pressure (es)- The maximum amount of pressure that can be exerted by vapor. The wamer the temperature, the more water vapor that can be present, thus the more pressure that can result from water vapor. Dependent upon temperature. | |
| The mixing ratio and vapor pressure act the same way. The higher w is, the higher e is. Both e and w allow one to know the exact amount of water vapor in the air. Saturation mixing ratio and saturation vapor pressure are both tools to show one how much water vapor can be present (the capacity for the air to hold moisture) which is dependent upon temperature. The warmer the temperature, the higher the saturation mixing ratio and saturation vapor pressure. | |
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Dewpoint and Wet Bulb Temperatures |
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| Dew point (Td) - The temperature to which air must cool at constant pressure in order for air to reach saturation (commonly dew to form); indicates moisture content. | |
Wet bulb temperature
(Tw) - An easily measurable quantity (using a sling
psychrometer) which indicates the
effect of evaporative cooling on temperature; used to determine relative
humidity. The Wet Bulb Temperature is the temperature to which air would
cool if all the moisture were evaporated out of it.
| The temperature of the air allows one to
know how much water vapor CAN be in the air. The dewpoint tells
you the exact degree, given a constant pressure, that you could cool the
air down to where the capacity and the actual water vapor content are
equal. 50 degrees has a potential to hold a certain amount of water vapor
and 60 degrees has a potential to hold more than that. If the temperature
were 60 degrees and the dewpoint 50, this would allow you to know that the
amount of water vapor in the air is as much as the air could hold at 50
degrees. If the temperature cools down to 50, the air will saturate. The
closer Td is to the temperature (T), the higher the Relative Humidity.
This graphic may help you
understand dewpoint, mixing ratios and humidity more. | The wet bulb temperature has the same concept as dewpoint. A wet bulb temperature of 50 degrees (F) denotes that if you cooled the temperature down to 50 (at a constant pressure), the water would no longer evaporate, b/c the air is saturated (full, 100% RH, etc). The actual wet bulb is just a thermometer with a wet end to it. The faster the water from the wet bulb evaporates, the cooler the wet-bulb temperature is, much like when you get out of a swimming pool on a dry day. |
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