Transcript Moisture_Notes

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Table of Contents
Chapter
1
Introduction
7
From the Rocky Mountains to the
Pacific Ocean
8
Early September snowstorms
Thunderstorms
Canyon winds
Beautiful fall weather
Wind and rain
Chapter
2
Along the Pacific Coast
20
Rain, rain, and more rain
Storminess
Lightning, thunder and hail
Freezing weather for two weeks
A touch of spring
Chapter
3
From the Pacific Ocean to the Rocky
Mountains
43
Gusty winds
More rain
Warm temperatures
High water
Deep snow
Epilogue
70
Bibliography
72
Glossary
74
Index
76
City of Portland HYDRA Network
USGS Rainfall Network
Oregon Hydrometeorological
Networks
PRISM Maps
Oregon State University
Click Here

USA Annual Average = 29 inches
◦
◦
◦
◦
Oregon Average = 27 inches
Pennsylvania
= 40 inches
Texas
= 30 inches
Nevada
= 8.8 inches
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Coast -
up to 70 inches or more
◦ (as much as 168 inches in Coast Range)
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Eastern Oregon - 8 to 10 inches
So:
A Statistical Abstraction
Global Climate Animations
Department of Geography
University of Oregon
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Concepts of atmosperic moisture
Processes - precipitation
Variability over space and distance – maps
Variability in time – graphs
◦ Seasonal cycles
◦ Interannual cycles
◦ Glacial cycles
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Rain
Snow
Sleet
Hail
Fog Drip
……and everything else wet
Measures of Atmospheric Moisture
•absolute humidity
•specific humidity
•vapor pressure
•saturation vapor pressure
•relative humidity
•mixing ratio
•saturation mixing ratio
•wet-bulb temperature
•dew-point temperature
Adiabatic temperature change and stability
In "The Atmosphere" we discovered that air temperature usually decreases with an increase in elevation through the
troposphere. The decrease in temperature with elevation is called the environmental lapse rate of temperature or
normal lapse rate of temperature. Recall that the normal lapse rate of temperature is the average lapse rate of
temperature of .65o C / 100 meters. The environmental lapse rate of temperature is the actual vertical change in
temperature on any given day and can be greater or less than .65o C / 100 meters. Also recall that the decrease in
temperature with height is caused by increasing distance from the source of energy that heats the air, the Earth's surface.
Air is warmer near the surface because it's closer to its source of heat. The further away from the surface, the cooler the
air will be. It's like standing next to a fire, the closer you are the warmer you'll feel. Temperature change caused by an
exchange of heat between two bodies is called diabatic temperature change. There is another very important way to
change the temperature of air called adiabatic temperature change.
Adiabatic temperature change of air occurs without the addition or removal of energy. That is, there is no exchange of
heat with the surrounding environment to cause the cooling or heating of the air. The temperature change is due to work
done on a parcel of air by the external environment, or work done by a parcel of air on the air that surrounds it. What kind
of work can be done? The work that is done is the expansion or compression of air.
Imagine an isolated parcel of air that is moving vertically through the troposphere. We know that air pressure decreases
with increasing elevation. As the parcel of air moves upward the pressure exerted on the parcel decreases and the parcel
expands in volume as a result. In order to expand (i.e.. do work), the parcel must use its internal energy to do so. As the
air expands, the molecules spread out and ultimately collide less with one another. The work of expansion causes the
air's temperature to decrease. You might have had personal experience with this kind of cooling if you've let the air out of
an automobile or bicycle tire. Air inside the tire is under a great deal of pressure, and as it rushes outside it moves into a
lower pressure environment. In so doing, the parcel quickly expands against the outside environment air. By placing your
hand in front of the valve stem, you can feel the air cool as it expands. This is called adiabatic cooling.
As air descends through the troposphere it experiences increasing atmospheric pressure. This causes the parcel volume
to decrease in size, squeezing the air molecules closer together. In this case, work is being done on the parcel. As the
volume shrinks, air molecules bounce off one another more often ricocheting with greater speed. The increase in
molecular movement causes an increase in the temperature of the parcel. This process is referred to as adiabatic
warming.
The rate at which air cools or warms depends on the moisture status of the air. If the air is dry, the rate of temperature
change is 1oC/100 meters and is called the dry adiabatic rate (DAR). If the air is saturated, the rate of temperature
change is .6oC/100 meters and is called the saturated adiabatic rate (SAR). The DAR is a constant value, that is, it's
always 1oC/100 meters. The SAR varies somewhat with how much moisture is in the air, but we'll assume it to be a
constant value here. The reason for the difference in the two rates is due to the liberation of latent heat as a result of
condensation. As saturated air rises and cools, condensation takes place. Recall that as water vapor condenses, latent
heat is released. This heat is transferred into the other molecules of air inside the parcel causing a reduction in the rate of
cooling.
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Nature Notes – 1937
Climate of Crater Lake National Park
Annual Snowfall
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SNOTEL – Data and Products
SNOTEL Climate Anomaly Maps
Water Supply Outlook for the western U.S.
Long –Term Records
Precipitation
Temperature