.................................................................................................................................................................................................................................................................................................................................................................................................................................
Sper Scientific Sling Psychrometer to Measure Humidity Top is wet bulb -- Bottom is dry bulb |
What
is humidity?
How is it measured?
edinformatics.com
What is Humidity?
Easily stated
humidity is simply the amount of water vapor held in the air.
Water vapor is the
gaseous state of water.
As the temperature
of the air increases more water vapor can be held since the movement of
molecules at higher temperatures prevents condensation from occurring.
There are three
main measurements of humidity: relative, absolute and specific.
Absolute humidity (units are grams of water vapor per
cubic meter volume of air) is a measure of the actual amount of water vapor in
the air, regardless of the air's temperature.
The higher the
amount of water vapor, the higher the absolute humidity.
For example, a maximum
of about 30 grams of water vapor can exist in a cubic meter volume of air with
a temperature in the middle 80s.
Relative humidity, expressed as a percent, is a
measure of the amount of water vapor that air is holding compared the amount it
can hold at a specific temperature.
Warm air can
possess more water vapor (moisture) than cold air, so with the same amount of
absolute/specific humidity, air will have a higher relative humidity.
A relative humidity
of 50% means the air holds on that day (specific temperature) holds 50% of
water needed for the air to be saturated.
Saturated air has a
relative humidity of 100%.
The relative
humidity of an air-water mixture is also defined as the ratio of the partial
pressure of water vapor in the mixture to the saturated vapor pressure of water
at a given temperature (See what is vapor pressure).
Thus the relative
humidity of air is a function of both water content and temperature.
Specific humidity refers to the weight of water vapor
contained in a unit weight (amount) of air (expressed as grams of water vapor
per kilogram of air).
Absolute and
specific humidity are quite similar in concept.
What is Dew Point?
Dew Point is the temperature at which air is
saturated with water and condensation begins. The higher the dew point rises,
the greater the amount of moisture in the air.
What is the relationship between Dew Point and Relative
Humidity?
Compared to relative
humidity, dew point is frequently cited as a more accurate way of measuring the
humidity and comfort of the air, since it is an absolute measurement (unlike
relative humidity).
The relative
humidity is 100 percent when the dew point and the temperature are the same.
If the temperature
drops any further, condensation will result, and liquid water will begin to
form.
If the relative
humidity is 100 percent (i.e., dew point temperature and actual air temperature
are the same), this does not necessarily mean that precipitation will occur.
It simply means
that the maximum amount of moisture is in the air at the particular temperature
the air is at.
Saturation may
result in fog on the ground and clouds aloft (which consist of tiny water
droplets suspended in the air).
While dew point
gives one a quick idea of moisture content in the air, relative humidity does
not since the humidity is relative to the air temperature.
In other words,
relative humidity cannot be determined from knowing the dew point alone, the
actual air temperature must also be known.
Relative humidity
is also approximately the ratio of the actual to the saturation vapor pressure.
RH = (Actual Vapor
Pressure) / (Saturation Vapor Pressure) X 100%
Where actual vapor
pressure is a measurement of the amount of water vapor in a volume of air, and
increases as the amount of water vapor increases.
Saturated vapor
pressure is the maximum VP that can exist at any given temperature.
Air which is at
100% relative humidity (RH) contains water vapor whose VP is its SVP at the
given temperature. This corresponds to air which is in equilibrium with liquid
water.
RH is the ratio
VP/SVP expressed as a percentage. 'Dry' air will contain water vapor with a VP
which is less than the SVP at the given temperature.
How is humidity measured?
A device to measure
relative humidity is called a hygrometer.
The simplest
hygrometer - a sling psychrometer - consists of two thermometers mounted
together with a handle attached on a chain.
One thermometer is
ordinary. The other has a cloth wick over its bulb and is called a wet-bulb
thermometer.
How does a psychrometer measure relative humidity?
A psychrometer also
called a sling psychrometer has two thermometers attached.
One is dry (often
called the dry bulb thermometer) and measures the actual air temperature.
The other called
the wet bulb thermometer has a wet cloth at the tip.
As water molecules
evaporate from the surface of the wet bulb they will take heat with them
lowering the reading on the thermometer.
The rate of
evaporation depends on the vapor pressure or amount of water vapor in the air.
At 100% relative
humidity no water will evaporate from the wet bulb and the readings on both
thermometers will be the same.
Comparing the two temperatures in a chart will give the relative humidity.
ABOUT
THE EDINFORMATICS SCIENCE CHALLENGE
Intermediate
school is designed to provide a strong foundation for students in science in
order to prepare them for the high school science sequence consisting of
biology, chemistry and physics. Although there is some controversy that
students are being over-tested on a yearly basis, an eighth grade science test
should remain a cornerstone in science assessment.
https://www.edinformatics.com/math_science/what-is-humidity.html
You might also like:
CLICK HERE . . . to view . . .
https://puricare.blogspot.com/2020/01/humidity-makes-hair-frizzy-when-air-is.html
............................................................................................................................................................................................................................................................................................
CLICK HERE . . . to view . . .
https://puricare.blogspot.com/2018/07/relative-humidity-relative-humidity-is.html
............................................................................................................................................................................................................................................................................................
CLICK HERE . . . to view . . .
https://puricare.blogspot.com/2017/05/old-injuries-and-joint-aches-why-do-old.html
...........................................................................................................................................................................................................................................................................................
Water Vapor Saturation
CLICK HERE . . . to view . . .
https://puricarechronicles.blogspot.com/2019/10/water-vapor-saturation-air-in-our.html
............................................................................................................................................................................................................................................................................................
CLICK HERE . . . to view . . .
https://puricare.blogspot.com/2019/08/ideal-office-temperatures-for.html
............................................................................................................................................................................................................................................................................................
Maintaining Indoor
Air Quality
CLICK HERE . . . to view . . .
https://puricare.blogspot.com/2019/02/maintaining-indoor-air-quality-balanced.html
No comments:
Post a Comment