Atmospheric Moisture

1
Atmospheric Moisture
This chapter discusses 1. The role of water in
the atmosphere 2. Terms and definitions for
atmospheric moisture 3. The impact of moisture
on comfort
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Structure of Water
Figure 5.1
Figure 5.2
Water's unique molecular structure and hydrogen
bonds enable all 3 phases to exist in earth's
atmosphere. Sublimation deposition describe
the non-incremental changes between solid and
vapor phases.
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Evaporating Water into Air
Liquid water experiences dynamic departures of
water molecules from its surface, called
evaporation, together with arrivals of molecules
from adjacent vapor, called condensation. When
air is saturated, evaporation and condensation
are in equilibrium.
Figure 5.3A
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Condensation onto Nuclei
Condensation of water vapor into liquid water is
more likely to occur when the vapor cools and
slows, and attaches to nuclei. Heated vapor
moves so fast that it bounces away from
condensation nuclei
Figure 5.4A
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Closing the Hydrologic Cycle
Figure 5.5
Evaporation, and transpiration by plants,
transports liquid water into atmospheric vapor.
Condensation converts this gas back to a liquid
droplet, which may then fall as precipitation to
ground or surface water supplies.
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Descriping Atmospheric Moisture
Atmospheric water vapor has been defined several
different ways. These terms include absolute
humidity, specific humidity, mixing ratio, vapor
pressure, and relative humidity.
Figure 5.6
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Absolute Specific Humidity
For a given mass of water vapor in an air parcel,
the absolute humidity changes as the parcel
volume changes (e.g., lifts or descends).
Figure 5.7
Specific humidity is concerned with the mass of
vapor to mass of air, and is not affected by
changes in parcel volume.
Figure 5.8
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Specific Humidity vs. Saturation
Warm air can absorb more vapor than cold air, so
for a given parcel of air, specific humidity
declines from its highest in the tropics to its
lowest in the colder poles. Desert air, at 30,
however, is not more saturated than polar air.
Figure 5.9
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Determining Vapor Pressure
Average atmospheric pressure of 1013 mb is
comprised in part by the weight of vapor
molecules. Warmer air can absorb more vapor
than cooler air before it saturates, and can have
higher saturated vapor pressures.
Figure 5.10
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Relative Humidity Trends
Figure 5.11
Relative humidity (RH) indicates air parcel
proximity to saturation. Saturation can be
achieved, or RH increased, by adding more water
or dropping the air temperature. Dew point is
the temperature at which saturation occurs.
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Seasonal Dew Point Maps
Figure 5.12B
Figure 5.12A
January, July, dew point
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Dew Point vs. Relative Humidity
Dew point is the temperature for saturation, and
used with a vapor pressure curve reveals the mass
of vapor in the air. While relative humidity
may be higher in polar air, more water is
actually absorbed in desert air.
Figure 5.13A
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Relative and Specific Humidity
Figure 5.14
Relative humidity (RH) as an indicator of
saturation reveals that desert air is far from
saturated, and that cold polar air nears
saturation. Graphs of RH contrast with specific
humidity in the deserts and poles.
Figure 5.9
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Sources of Moisture
Figure 5.15
Patterns of US humidity are strongly governed by
wind direction and ocean temperatures. Cooler
Pacific waters create lower humidities in the
west, while warmer Gulf waters generate high
humidity along the southeast and east coast.
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Relative Humidity and Comfort
Unsaturated air may absorb more water from the
evaporation of human sweat. The departure of
fast moving, and by definition higher
temperature, water molecules into the vapor phase
cools the human skin. In winter, this process
can make a dry house extra chilly.
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Heat Index Safety
Human perception of temperature is distinct from
measured air temperature, and is particularly
different at higher humidities when the human
body is less efficient at sweating and
self-cooling. On hot days, fans that move
saturated air away from the skin help humans
avoid unwanted heat syndromes.
Figure 5.16
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Sling Psychrometer
Figure 5.17
Wet bulb temperature indicates how cool a surface
will become by evaporating water into the air,
and when compared with the dry bulb, or regular,
air temperature it indicates relative humidity.
These two temperatures are measured by this
instrument.
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Hair Other Hygrometers
Figure 5.18
Human and horse hair becomes roughly 2.5 shorter
as relative humidity drops from 100 to 0, which
is the principle operating the hair hygrometer.
Other hygrometers are based on electrical
resistance, infrared absorption, and dew point
condensation.