Humidity is the amount of water vapor in the air.
Relative humidity is defined as the ratio of the
partial pressure of
water vapor in a
parcel of
air to the saturated
vapor pressure of water vapor at a
prescribed
temperature. Humidity may
also be expressed as specific humidity. Relative humidity is an
important
metric used in
forecasting weather. Humidity
indicates the likelihood of
precipitation,
dew, or
fog. High humidity makes
people feel hotter outside in the summer because it reduces the
effectiveness of
sweating to cool the body
by reducing the
evaporation of
perspiration from the skin. This effect is calculated in a
heat index table.
Types of humidity
Absolute humidity
Absolute humidity is the quantity of water in a particular
volume of air. The most common units are grams per cubic meter,
although any mass unit and any volume unit could be used. Pounds
per cubic foot is common in the U.S., and occasionally even other
units mixing the Imperial and metric systems are used.
If all the water in one
cubic meter of
air were condensed into a container, the container could be weighed
to determine absolute humidity. The amount of vapor in that cube of
air is the absolute humidity of that cubic meter of air. More
technically, absolute humidity is the mass of water vapor, m_w ,
per cubic meter of air, V_a :
- AH = {m_w \over V_a} .
Absolute humidity ranges from 0 grams per cubic meter in dry air to
30 grams per cubic meter (0.03 ounce per cubic foot) when the
vapour is saturated at 30 °C.
[8476](See also
Absolute Humidity table)
The absolute humidity changes as air
pressure changes. This is very inconvenient for
chemical engineering
calculations, e.g. for
dryer, where
temperature can vary considerably. As a
result, absolute humidity is generally defined in chemical
engineering as mass of water vapor per unit mass of dry air, also
known as the mass mixing ratio (see below), which is much more
rigorous for heat and mass balance calculations. Mass of water per
unit volume as in the equation above would then be defined as
volumetric humidity. Because of the potential
confusion,
British Standard BS 1339
(revised 2002) suggests avoiding the term "absolute humidity".
Units should always be carefully checked. Most humidity charts are
given in g/kg or kg/kg, but any mass units may be used.
The engineering of physical and thermodynamic properties of
gas-vapor mixtures is named
Psychrometrics.
Mixing ratio or humidity ratio
Mixing or
humidity ratio is expressed as a ratio
of water vapour mass, m_w , per kilogram of dry air, m_d , at a
given pressure. The colloquial term
moisture content is
also used instead of
mixing/humidity ratio. Humidity ratio
is a standard axis on psychrometric charts, and is a useful
parameter in
psychrometrics
calculations because it does not change with temperature except
when the air cools below dewpoint.
That ratio can be given as:
- MRi = {m_w \over m_d}
Mixing ratio can also be expressed with the
partial pressure of water vapor :
- MRi = \delta { p_w \over {p_a - p_w}}
where
- \delta = 0.62197 is the ratio of molecular weights of water
vapour and dry air
- p_w = partial pressure of water vapor in moist air
- p_a = atmospheric pressure of moist air
Technically speaking, this is a
dimensionless quantity as it is the
mass of water vapor to the mass of dry air. So it is expressed as
Kg/Kg. However, the mass of water vapor is much less than the value
of the mass of dry air and most commonly meteorologists use
g/Kg which is 10^{-3} Kg/Kg.
Relative humidity
Relative humidity is defined as the ratio of the partial
pressure of water vapor (in a
gaseous mixture of
air and water vapor) to the saturated vapor pressure of water at a
given temperature. Relative humidity is expressed as a percentage
and is calculated in the following manner:
RH = {p_{({\rm H_2O})} \over p^*_{({\rm H_2O)}}} \times 100%
where
- {p_{({\rm H_2O)}}} is the partial
pressure of water vapor in the gas mixture;
- {p^*_{({\rm H_2O)}}} is the saturation vapor pressure of water
at the temperature of the gas mixture; and
- RH_{\,_\,} is the relative humidity of the gas mixture being
considered.
Relative humidity is often mentioned in
weather forecasts and reports, as it is
an indicator of the likelihood of
precipitation, dew, or fog. In
hot summer
weather, it also increases the
apparent temperature to
humans (and other
animals) by
hindering the
evaporation of
perspiration from the skin as the relative
humidity rises. For example, if it was 80 degrees, with a relative
humidity of approximately 75%, it would feel about 84
degrees.
Specific humidity
Specific humidity is the ratio of water vapor to air
(including water vapor and dry air) in a particular mass. Specific
humidity ratio is expressed as a ratio of kilograms of water vapor,
m_w , per kilogram of air (including water vapor), m_t .
That ratio can be shown as:
- SH = {m_w \over m_t}
Specific humidity is related to mixing ratio (and vice versa)
by:
- SH = {MR \over 1+MR}
- MR = {SH \over 1-SH}
Humidity during rain
Humidity is a measure of the amount of water vapor dissolved in the
air, not including any liquid water or ice falling through the air.
For
clouds to form, and
rain to start, the air doesn't have to reach 100%
relative humidity at the Earth's surface, but only where the clouds
and raindrops form. This normally occurs when the air rises and
cools. Typically, rain falls into air with less than saturated
humidity. Some water from the rain may evaporate into the air as it
falls, increasing the humidity, but not necessarily enough to raise
the humidity to 100%. It is even possible for rain falling through
warm, humid air to be cold enough to lower the air temperature to
the
dew point, thus condensing water vapor
out of the air. Although that would indeed raise the
relative humidity to 100%, the water lost
from the air (as dew) would also
lower the
absolute humidity.
Dew point and frost point
Associated with relative humidity is
dew
point (If the dew point is below freezing, it is referred to as
the frost point). Dew point is the
temperature at which water vapor saturates from
an air mass into liquid or solid usually forming
rain,
snow,
frost, or
dew. Dew point normally
occurs when a mass of air has a relative humidity of 100%. This
happens in the
atmosphere as a
result of cooling through a number of different processes.
Measuring and regulating humidity
There are various devices used to measure and regulate humidity. A
device used to measure humidity is called a psychrometer or
hygrometer. A
humidistat is used to regulate the humidity of a
building with a de-humidifier. These can be analogous to a
thermometer and
thermostat for temperature control.
Humidity is also measured on a global scale using remotely placed
satellites. These satellites are able to
detect the concentration of water in the
troposphere at altitudes between 4 and 12
kilometers. Satellites that can measure water vapor have sensors
that are sensitive to
infrared radiation.
Water vapor specifically absorbs and re-radiates radiation in this
spectral band. Satellite water vapor imagery plays an important
role in monitoring climate conditions (like the formation of
thunderstorms) and in the development of future
weather forecasts.
Humidity and air density
Humid air is less dense than dry air because a molecule of water
(
M ≈ 18
u ) is less massive than a molecule of
nitrogen (M ≈ 28) and a molecule of
oxygen (M ≈ 32). About 78% of the molecules
in dry air are nitrogen (N
2). Another 21% of the
molecules in dry air are oxygen (O
2). The final 1% of
dry air is a mixture of other gases.For any gas, at a given
temperature and pressure, the number of molecules present is
constant for a particular volume - see
ideal gas law. So when water molecules (vapor)
are introduced to the dry air, the number of air molecules must
reduce by the same number in a given volume, without the pressure
or temperature increasing. Hence the mass per unit volume of the
gas (its density) decreases.
Isaac
Newton discovered this phenomenon and wrote about it in his
book
Opticks.
Effects on human body
The human body sheds heat by a combination of evaporation of
perspiration,
heat convection in the
surrounding air, and
thermal
radiation. Under conditions of high humidity, the evaporation
of sweat from the skin is decreased and the body's efforts to
maintain an acceptable body temperature may be significantly
impaired. Also, if the atmosphere is as warm as or warmer than the
skin during times of high humidity,
blood
brought to the body surface cannot shed heat by conduction to the
air, and a condition called
hyperpyrexia results. With so much blood going
to the external surface of the body, relatively less goes to the
active
muscles, the
brain, and other internal
organs.
Physical strength declines and
fatigue occurs sooner than it would
otherwise. Alertness and mental capacity also may be affected. This
resulting condition is called
heat stroke or
hyperthermia.
Recommendations for comfort
Humans control their body temperature mainly by
sweating and
shivering.
The
United
States Environmental Protection Agency cites the
ASHRAE Standard 55-1992
Thermal Environmental
Conditions for Human Occupancy, which recommends keeping
relative humidity between 30% and 60%, with below 50% preferred to
control
dust mites. At high humidity
sweating is lesseffective so we feel hotter; thus the desire to
remove humidity from air with
air
conditioning in the summer. In the winter, heating cold outdoor
air can decrease indoor relative humidity levels to below 30%,
leading to discomfort such as dry skin and excessive thirst.
Effects on electronics
Many electronic devices have humidity specifications, for example,
5 to 95%. At the top end of the range, moisture may increase the
conductivity of permeable
insulators leading to malfunction.
Too low humidity may make materials brittle. A particular danger to
electronic items, regardless of the stated operating humidity
range, is
condensation. When an
electronic item is moved from a cold place (eg garage, car, shed,
an air conditioned space in the tropics) to a warm humid place
(house, outside tropics), condensation may coat circuit boards and
other insulators, leading to
short
circuit inside the equipment. Such short circuits may cause
substantial permanent damage if the equipment is powered on before
the condensation has
evaporated. A
similar condensation effect can often be observed when a person
wearing glasses comes in from the cold. It is advisable to allow
electronic equipment to acclimatise for several hours, after being
brought in from the cold, before powering on. The inverse is also
true.
Low humidity also favors the buildup of
static electricity, which may result in
spontaneous shutdown of computers when discharges occur. Apart from
spurious erratic function, electrostatic discharges can cause
dielectric breakdown in solid state devices, resulting in
irreversible damage.
Data centers often
monitor relative humidity levels for these reasons.
Humidity in construction
Traditional building designs typically had weak
insulation, and this allowed air
moisture to flow freely between the interior and exterior. The
energy-efficient, heavily-sealed architecture introduced in the
20th century also sealed off the
movement of moisture, and this has resulted in a secondary problem
of
condensation forming in and around
walls, which encourages the development of mold and mildew.
Solutions for energy-efficient buildings that avoid condensation
are a current topic of architecture.
Most humid places on Earth
The most humid cities on earth are generally located closer to the
equator, near coastal regions.
Cities in South and Southeast Asia are among
the most humid, such as Kolkata
and those in
Kerala
in India
, the cities
of Manila
in the
Philippines
and Bangkok
in Thailand
: these
places experience extreme humidity during their rainy seasons
combined with warmth giving the feel of a lukewarm sauna.
Darwin, Australia experiences an extremely humid wet season from
December to April.
Kuala Lumpur
and Singapore
have very high humidity all year round because of
their proximity to water bodies and the equator and overcast weather; despite sunshine,
perfectly clear days are rare in these locations and it is often
misty. In cooler places such as Northern Tasmania,
Australia, high humidity is experienced all year due to the ocean
between mainland Australia and Tasmania. In the summer the hot dry
air is absorbed by this ocean and the temperature rarely climbs
above 35 degrees Celsius.
In the
United States the most humid cities, strictly in terms of relative humidity, are Forks
and Olympia, Washington
. This fact may come as a surprise to many,
as the climate in this region rarely exhibits the discomfort
usually associated with high humidity. Dew points are typically
much lower on the West Coast than on the East. Because high dew
points play a more significant role than relative humidity in the
discomfort created during humid days, the air in these western
cities usually does not feel "humid."
The
highest dew points are found in coastal Florida
and Texas
. When
comparing Key West and Houston, two of the most humid cities from
those states, coastal Florida seems to have the higher dew points
on average. But, as noted by Jack Williams of USA Today, Houston
lacks the coastal breeze present in Key West, and, as a much larger
city, it suffers from the
urban heat
island effect.
The US
city with the lowest annual humidity is Yuma
, Arizona
, averaging
under 50% for a high and 22% as a low. The next-lowest
humidity is Tucson,
Arizona
, average high humidity of 57% and a low of
26%. Lowest in the world is Antarctica
.
See also
References
External links