NATS 101 Lecture 9 Atmospheric Moisture. Hydrological Cycle. Ultimate source of all fresh water. 15%. 85%. 50%. 50%. Ahrens Fig. 4.1. 85% of water vapor in atmosphere evaporates from oceans.
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Ultimate source of all fresh water
Ahrens Fig. 4.1
85% of water vapor in atmosphere evaporates from oceans.
About 50% of precipitation that falls over land is runoff, and the other 50% is transpired/evaporated.
Water vapor molecules reside in atmosphere for one week.
SVP depends on temperature. As temperature increases, more molecules are energetic enough to escape into the air.
Concept applies to an ice surface. SVP over ice is lower because water molecules are bonded more tightly to ice.
For the temperatures of interest, some water molecules are energetic enough to escape into atmosphere and SVP>0.
SVP nearly doubles with a 10oC warming
SVP and T Graph
Supercooled water droplets can exist to temps of -40oC
For temps below 0oC, SVP runs 10%-30% lower over ice
Ahrens Fig. 4.5
Air with a RH=100% is said to besaturated.
RH depends on air temperature (SVP).
RH changes by either changing air’s water vapor content or the air’s temperature.
Ahrens Fig. 4.6
The RH for constant water vapor content can fluctuate greatly during the course of the day solely from the temperature changes
Consider air that is saturated at 0oC
0oC 6 mb 6 mb 100%
10oC 6 mb 12 mb 50%
20oC 6 mb 23 mb 28%
30oC 6 mb 42 mb 14%
40oC 6 mb 68 mb 9%
Vapor PressureDew Point
24 mb 20oC
12 mb 10oC
6 mb 0oC
3 mb -10oC
Wet Bulb Temp-Lowest temp to which air can be cooled by evaporation of water into it.
Warmer than dew point since moisture is being added to air which raises dew point.
Measured with slingpsychrometer.
Ahrens, Fig 4.9
Wet bulb temperature lies about 30% of the way from the dew point to the temperature
Application-Wet bulb temp gives maximum possible efficiency for a swamp cooler
MonthMAXDew PWet BulbJune 100oF 37oF 65oF July 100oF 63oF 75oF
Humidity reduces the rate at which sweat evaporates. Thus, the cooling rate is lowered.
Tucson Record MAX
Rocky Pt Summer
Ahrens, Fig 4.8