Hello blonder-ga,
The short answer to your question is changes in temperature cause
humidity in the air to rise and drop. The most commonly used measure
of humidity is the ?relative humidity?. A brief overview can be found
at HowStuffWorks.com.
HowStuffWorks.com
What is relative humidity and how does it affect how I feel outside?
http://science.howstuffworks.com/question651.htm
?Humidity can be measured in several ways, but relative humidity is
the most common. In order to understand relative humidity, it is
helpful to first understand absolute humidity.
Absolute humidity is the mass of water vapor divided by the mass of
dry air in a volume of air at a given temperature. The hotter the air
is, the more water it can contain.
Relative humidity is the ratio of the current absolute humidity to the
highest possible absolute humidity (which depends on the current air
temperature). A reading of 100 percent relative humidity means that
the air is totally saturated with water vapor and cannot hold any
more, creating the possibility of rain. This doesn't mean that the
relative humidity must be 100 percent in order for it to rain -- it
must be 100 percent where the clouds are forming, but the relative
humidity near the ground could be much less.?
The Environmental Literacy Council provides the following explanation:
Humidity and Precipitation
http://www.enviroliteracy.org/article.php/686.php
?Measurements of humidity are generally given in one of two ways.
Weather reports commonly refer to relative humidity, which is given as
a percentage and refers to the amount of moisture in the air relative
to the air's moisture capacity. 100 percent relative humidity means
that, at the current temperature, the air is at the saturation point.
Since warmer air has a higher moisture capacity than cooler air, a
temperature increase will cause relative humidity to decrease if
moisture content remains the same. On warm days, for example, the
highest relative humidity is usually in the morning. As the day warms,
the relative humidity goes down even if the actual moisture in the air
stays the same. This can be confusing - humidity measures going down
even as the humidity itself remains the same. However, there is a
different measure of humidity - dew point - that does not change as
temperature changes. In contrast to relative humidity, dew point is a
measure of absolute humidity - the actual moisture content in the air,
and so it will not change merely because the temperature changes. Dew
point is expressed as the temperature at which the current moisture
content would be at saturation.?
USAToday.com
What relative humidity means
http://www.usatoday.com/weather/wrelhum.htm
?The warmer air is, the more water vapor it can "hold." Dew point is a
measure of how much water vapor is actually in the air. Relative
humidity is a measure of the amount of water in the air compared with
the amount of water the air can hold at the temperature it happens to
be when you measure it.?
=======================================
Another way for the relative humidity to change it for there to be a
change in the amount of water vapor in the air.
How Humidity is Expressed
Palomar College
http://daphne.palomar.edu/jthorngren/measures.htm
?Relative humidity depends on two factors: the amount of moisture
available, and on the temperature. So you can have a change in
relative humidity in one of two ways:
1) Change the amount of water vapor available; if there is liquid
water present, for instance, a lake, you can have an increase in
relative humidity by evaporation from the surface of the lake. This is
pretty obvious. You?re adding water vapor, so the humidity increases.
2) The other way is to change the temperature of the air, while
holding the water vapor constant. Even though there is no water
source, and no water vapor is added, a lowering of air temperature
results in a rise of relative humidity. This is automatic. The amount
of water vapor that could be present at saturation is less at the
lower temperature, so the existing amount of water vapor represents a
higher percentage of the saturation level of the air. Similarly, a
rise in temperature results in a decrease in relative humidity, even
though no water vapor has been taken away.
Key point to remember: Given that the amount of water vapor is held
constant, then if you
--reduce the temperature, the relative humidity goes up
--increase the temperature, the relative humidity goes down.?
envirochex.com
humidity
http://www.envirochex.com/Humidity.htm
?Changes in Relative Humidity
Mother nature establishes our humidity and depending on geography,
weather and seasons; outdoor levels are constantly changing.
Outdoor air creates the baseline for indoor humidity levels and we
typically heat or cool this air to maintain our desired temperature
range. In the process of changing the temperature of air, we are
altering the indoor relative humidity as explained above.
Numerous activities add moisture to indoor air: occupancy (people
exhale moisture and perspire), cooking, cleaning, etc. In addition,
we can intentionally increase moisture levels using humidifiers.
(Humidification is rarely required in the Dallas/Ft Worth area due to
the natural ambient levels.)?
=======================================
The references listed above assume that the pressure is constant.
However, changes in pressure affect the amount of water vapor in the
air and therefore can also affect the humidity.
Meteorological Service of Canada
http://www.qc.ec.gc.ca/meteo/Documentation/Humidite_e.html
?The amount of water vapour present in the air varies considerably,
and its saturation point depends only on the temperature-pressure
ratio. Under constant pressure, the moisture content will rise if the
temperature falls, and fall if the temperature rises.
At a given temperature, the quantity of water vapour is inversely
proportional to changes in pressure: falling pressure will increase
the moisture content.?
---
?Finally, relative humidity is the ratio of the actual water vapour
content to the quantity necessary to reach saturation.?
=======================================
Changes in humidity in a particular area may also be a result of
changes in the weather due to high and low pressure systems.
USATODAY.com
What's happening inside highs and lows
http://www.usatoday.com/weather/tg/whighlow/whighlow.htm
?As the name says, a "high" is an area where they air's pressure is
higher than the pressure of the surrounding air. A "low' is where it's
lower. Meteorologists don't have any particular number that divides
high from low pressure; it's the relative differences that count.
The pressure is high at the surface where air is slowly descending -
much to slowly to feel. And, this is going on over a large area, maybe
a few hundred square miles. As air descends, it warms, which inhibits
the formation of clouds. This is why high pressure is generally - but
not quite always - associated with good weather.
The air that descends in high pressure areas, has to get to high
altitudes in some way, and that way is by rising in areas where the
pressure at the surface is low.
As air rises it cools. As the air cools, the humidity in it begins to
condense into tiny drops of water, or if it's cold enough, into tiny
ice crystals. If there is enough water, or ice, rain or snow begin to
fall. This is why low pressure is associated with bad weather.?
=======================================
Differences in the climate of a region also affect the humidity levels.
Idaho Museum of Natural History (Google cache)
http://216.239.57.104/search?q=cache:4kqwwJ4u4gsJ:imnh.isu.edu/digitalatlas/clima/imaging/humid.htm+%22changes+in+relative+humidity%22&hl=en&ie=UTF-8
?As one traverses 30o of latitude across the continental United
States, one notices a distinct difference between the climate of
Arizona and that of Alabama. The air masses that prevail over the
western deserts of the U.S. come from the Pacific Ocean. The Pacific
Ocean is normally very cool. Air over the Pacific is also cool. As
westerly winds move this air over land, its temperature increases but
its water vapor content does not. Hence the air over the western
deserts has a high temperature but a low dew point and very low
relative humidity. The Gulf of Mexico, on the other hand, is quite a
bit warmer as is the air over it. When this air moves over the
southeastern U.S. it warms only a little while maintaining the same
water vapor content. The air and dew point temperatures are very close
and the relative humidity is consequently quite high. This is why the
southwest U.S. is normally hot and dry while the southeast is hot and
muggy.?
=======================================
Other references:
University of Wisconsin-Stevens Point
Moisture in the Atmosphere
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/atmospheric_moisture/humidity.html
HyperPhysics
Georgia State University
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html
Met-office.gov.uk
Measuring moisture in the atmosphere
http://www.met-office.gov.uk/education/curriculum/leaflets/moisture.html
=======================================
I hope you have found this information helpful. If you have any
questions, please request clarification prior to rating the answer.
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