All about Humidity
What is humidity?
Humidity is water vapor in the air (water in gaseous form). When you boil water, it is quickly converted into water vapor. When you set a saucer of water on your countertop, the water is slowly being converted into water vapor, and eventually disappears. Likewise, water vapor enters the atmosphere in similar manners.
The amount of water vapor in the atmosphere is referred to as relative humidity (RH). If the RH reaches 100%, that means the air cannot hold any more water and some water condenses. Warmer air has a higher capacity for water; therefore, there is more water vapor in 80 degree air at 100% RH than 50 degree air at 100% RH. An example of this mechanism is dew. At night, when the air cools, it cannot hold as much water vapor. If the air started with enough water vapor, when it cools it might reach 100% RH (the dew point) and water will condense out (as dew on grass and other surfaces).
What natural mechanisms cause humidity to increase?
Water vapor is added to the atmosphere by two primary natural mechanisms:
Transpiration - Plants take in water in their root systems and release it into the atmosphere through their leaves.
Evaporation - Water is converted to vapor phase from the ground or a body of water and mixes with the atmosphere.
Evaporation accounts for approximately 90% of water vapor, and transpiration accounts for 10% of water vapor.
Does the amount of water vapor in the atmosphere vary?
The amount of water vapor in the atmosphere can vary significantly by location. The diagram below illustrates this. The red regions have greater than 60 mm of water in a column of atmosphere, while the blue and purple regions have less than 25 mm.
http://www.wmo.ch/web/sat/en/ap8-06.htm
Following are some of the reasons which explain the extremes in the diagram above, listed from most important to least important.
Solar
energy from the sun is the key driver for evaporation. By adding
heat to a body of water, its temperature will increase and some water will
evaporate. Because solar energy is more intense at the equator,
there tends to be more water vapor in the atmosphere near the equator.
It is of course necessary to have a body of water in order to have evaporation;
this is why there is a lot of water vapor over the oceans, but less over land.
Movement of frontal systems from
bodies of water is a key cause of high humidity over land. For example, air
masses from the Gulf of Mexico can bring moist air to the southern and eastern
United States. In fact, about 90% of the water vapor in all of the
atmosphere comes from the oceans. Additionally, cold fronts typically bring dry
air near the ground which can accelerate evaporation from the
ground.
Higher winds can move water vapor away from its
source, causing the air immediately above the water to become drier, increasing the rate of evaporation.
What natural
mechanisms cause humidity to decrease?
There are two mechanisms which cause water vapor to be removed from the atmosphere.
Precipitation -
Warm, humid air cools as it rises, and if it is humid enough, will reach 100% RH
and condense to form clouds in the sky. If the air is humid enough, the water vapor
concentration in the clouds will be high enough to form water droplets and
fall back to the earth.
Condensation - If the amount of water
vapor in the atmosphere near the ground reaches 100% RH, fog will appear
and water will condense out of the atmosphere onto
surfaces as dew.
Also, it should be noted that humid air can be
transported away from a region and replaced by dry air, thereby decreasing the
local humidity.
How does humidity affect temperature, evaporation, cloud formation, and precipitation?
Temperature
Humidity can affect ground temperature by absorbing heat energy. A study conducted by Nessler et al.1
suggests that increasing local humidity in the lower troposphere (where
combustion would release water vapor) from 30% - 100% caused a 0.3% reduction
in albedo (reflectivity). As a result, the atmosphere absorbs more
heat. This doesn't seem like a lot. This is because most of
the effect of higher humidity near the ground is increased scattering, which
doesn't affect the net absorption of heat. Therefore, the likely
overall impact directly on temperature is small, regardless of how much water
vapor is added to the atmosphere. However, see another very important
possible temperature impact under "cloud formation", and see the effect of high
altitude humidity on the Climate Change page.
Evaporation
The rate of evaporation is dependent upon humidity. The lower the
humidity, the drier the air above, and the faster the evaporation of water.
If the humidity is very low, ground can get very dry because it is quickly
evaporating any water that it contains. On the contrary, if the humidity
is very high, evaporation is inhibited and the ground will stay wet. This could cause build-up of surface water, and more
mosquitoes and other water-borne pests.
Cloud
Formation
Clouds form when water nucleates on aerosols (tiny airborne particles).
Assuming no change in the availability of aerosols, higher humidity could
produce more water droplets and/or larger water droplets. This could
result in an increase in the number of clouds. If low or mid altitude
clouds are created, these typically block sunlight from reaching the ground and
can cool the surface (decrease the temperature). If high altitude cirrus
clouds are created, they can allow sunlight through but block infrared radiation
reflected, and can warm the surface (increase the temperature). The
overall consensus is that the net effect is global cooling due to the effects of
reflection.
Precipitation
If enough
water is contained in a cloud, the droplets will coalesce and become large
enough to fall to the earth as precipitation. An increase in humidity can
increase the probability that water droplets will get big enough to reach the
earth as rain drops. Increased precipitation could act to increase the
amount of water in the local area, causing wetter ground, higher lakes and
streams, and even damage from flash flooding. Additionally, rain
acts to cool the surface (decrease the temperature).
Now that you have the basics of humidity, let's investigate whether water vapor
from combustion affects sea levels!
1 Nessler R, Weingartner E, Baltensperger U Title: Effect of humidity on aerosol light absorption and its implications for extinction and the single scattering albedo illustrated for a site in the lower free troposphere Source: JOURNAL OF AEROSOL SCIENCE 36 (8): 958-972 AUG 2005
Some information on this page obtained from http://ga.water.usgs.gov/edu/watercycleevaporation.html