go with the BBC presentation for a few days because my usual sources keep
changing things up all the time. It's grey as anything but I am going out for my
morning walk to the food shops anyway...
Also found a very useful text from Météo France about relative humidity. Should have a
translation up later today.
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source: Météo France
translation : doxa-louise
Humidity
Definition and Units of Measure
The humidity parameter im Meteorology refers to the quantity of water vapor in the air.
Water vapor is one of the forms that water - necessary to life - can take on earth. The
others are liquid form (oceans, lakes, rain...) and solid form (glaciers, pack ice, snow...)
Water vapor is utterly transparent and invisible. Let us recall that the atmosphere is
essentially made up of nitrogen (78%) and oxygen (21%) leaving roughly 1% on
average for other gases. Thus the proportion of water vapor varies between 0,1 %
to 5 % at the very most in the air which surrounds us! Yet it is its presence in the
atmosphere which creates the conditions for the presence or absence of clouds,
precipitation, fog. Moreover, ambient is humid even when it doesn’t ‘wet’! Take a glass
of iced tea out of the refrigerator, quickly, the sides will exhibit water droplets. This
water comes from the condensation of water vapor in the air.
The air contains, as a function of its temperature, a variable maximum quantity of
water vapor. In effect, the gas ‘water vapor’ condenses when temperature goes down
and goes from gaseous to liquid form: this is saturation. Beyond the saturation point,
water molecules, until then independent, will form deposits on all supporting materials
(the tea cup, vegetation, or dust particles higher up) and form droplets visible to the
naked eye. This is condensation. At high altitude, these droplets will form clouds: on
the ground, one will notice dew (or white frost in the case of negative temperatures) or,
if there is a bit of wind, mist if not fog.
In Meteorology, one quantifies humidity in two ways: absolute humidity and relative
humidity.
-Absolute Humidity
Absolute humidity refers to the quantity of water vapor contained in air. One is dealing
with grams of water per cubic meter of air (g/m3). This quantity doesn’t vary, it does
not respond to changes in temperature. In effect, air is a mixture of dry air and water
vapor and a cubic meter of air always contains a few grams of water vapor. This mass
of water vapor will not change even if the temperature of the given volume of air
changes ( assuming that there is no condensation involved, the transformation of
part of the water vapor to liquid form).
-Relative Humidity
Relative humidity is the relation of the quantity of water vapor contained in the air to
the possible maximum. It is this quantity which meteorologists measure. Relative
humidity is a percentage. 100% corresponds to air which is saturated in water vapor
(risk of cloud, rain, fog, dew or frost), 0% to perfectly dry air (this value for relative
humidity is not found in a natural setting, even in deserts).
The hotter the air is, the more water vapor it can contain. Inversely, when air cools,
vapor condenses and forms water droplets: one expresses this saying the threshold of
saturation goes up with temperature.
The relative humidity pf an air mass will thus vary with the temperature of the air. When
temperature goes up, during the day, relative humidity goes down, although the water
content of the air mass - absolute humidity - remains unchanged.
A level of humidity between 10 and 20% corresponds to very dry air, too dry for comfort
for an organism. Inversely, when temperature goes down, during the night, relative humidity
goes up and can reach saturation, that is, relative humidity at 100%. At
30°C, the air can contain up to 30g/m3 of water as vapor, hence 10 times more than at -5°C.
This explains in part why rains are much more abundant in tropical regions than in temperate ones and that in the middle of Antartica, snowfall is rare with little intensity.
Reading the table, at 30°C an air particle whose relative humidity is 50% contains
15g/m3 of water vapor, thus half as much as it could contain at maximum.
Being efficient with aeration or how not to trap humidity
At the end of the vacation period, you assiduously clean your cottage, boat or motor
home, before closing things down for the winter. If this aeration occurs on a hot day,
worse on a stormy and humid one, the air boxed up in the closets and cushions will
contain a great deal of water vapor. At the slightest cooling, this vapor will condense
and create mold. Inversely, aeration performed in the dead of winter will imprison air
poor in water, which will become increasingly dry with rising temperatures.
Instrument and Measure
Air humidity can be measured with a hygrometer or a psychrometer. Meteorologists
apply norms defines by the World Meteorological Organization to make the measures
comparable. The instruments, or probes in the case of telemetry readings, must be placed
in a well-aired shelter to avoid direct sunlight or rain.
Meteorologists are also interested in air humidity at high altitude. Thus, during
atmospheric radio probes, the humidity profile is measured - as is that of temperature,
pressure or again wind. Knowing these parameters permits to foresee fog, clouds,
thunderstorms...Meteorological satellites also carry along instruments making possible
estimates as to the relative humidity of air.
Hygrometer
In order to measure the humidity of air, or hygrometry, we use electronic probes containing a condenser whose capacity will vary with the surrounding air mass.
Beginning in the middle of the XVth century, the first hygrometers relied on the properties
of certain organic substances (wool, rope, sponge, wheat awns, hair, leather...) which on
absorbing water vapor change in mass, form, length or color. In 1781, Horace Bénédict de
Saussure developed the first hair sensitive hygrometer in history. Simple, not expensive and
relatively accurate, it will be used until the XXth century. The length of hairs varies as a
function of relative humidity and move the pen which inscribes the humidity reading on
a metered piece of paper surrounding a cylinder. This instrument was used in meteorological
stations at the end of the XIXth century up to the 1970s. It was then kept in the stations as
back-up until the years 1980-1990s.
Psychrometry
A psychrometer is made up of two thermometers. The first, the ’dry thermometer’,
measures air temperature. The reservoir of the second, the ‘wet thermometer’, is kept
wet by a cloth kept permanently soaked. Water evaporation cools the wet thermometer.
The dryer the air, the more active the evaporation, the more the temperature on the wet
thermometer goes down and the greater the difference between the two thermometers
becomes. If humidity is at 100%, the temperature of the two thermometers will be
identical.This instrument, invented by Gay Lussac in 1825 and carried on by August
in 1831, was in use between 1860 up to the 1970s. Then, kept in meteorological shelters
as back-up, its usage was abandoned after 1994.
The humid regions of Metropolitan France
Coastal regions, especially so if the sea is cold and winds coming from the open sea
dominate, are more humid than continental regions. This is true notably on the coasts
of the English Channel and the Atlantic. the Mediterranean coast is dryer because of a
warmer sea and the predominance of winds from the interior ( mistral and tramontane).
In the interior, it is more difficult to delineate dry and humid zones. The kind of
ground and hence its capacity to retain rain water, exposure to wind and sun for mountainous
regions, will have much influence on the humidity of air.
Humidity is greater on average in December, and least in July and August. For
Metropolitan France, air humidity is rarely seen below 20%. Taking Marignane as an
example, one of the dries cities in France, this value was reached on the average 5 times
per year in the period 1990-2009.
Heat and Humidity
The more humid it gets, the less the air can take in additional water. Thus in times of high temperature and strong humidity, the human body has trouble cooling itself: it becomes
impossible for sweat to evaporate.
http://www.meteofrance.fr/prevoir-le-temps/observer-le-temps/parametres-observes/humidite
* * *
https://scijinks.gov/pressure/
http://www.meteofrance.fr/climat-passe-et-futur/comprendre-le-climat-mondial/le-systeme-climatique
nfrared radiation (IR) is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore generally invisible to the human eye (although IR at wavelengths up to 1050 nm from specially pulsed lasers can be seen by humans under certain conditions [1][2][3][4]). It is sometimes called infrared light. IR wavelengths extend from the nominal rededge of the visible spectrum at 700 nanometers (frequency 430 THz), to 1 millimeter (300 GHz)[5] Most of the thermal radiationemitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon.
source: Wikipedia
http://www.meteofrance.fr/prevoir-le-temps/observer-le-temps/parametres-observes/humidite
* * *
https://scijinks.gov/pressure/
http://www.meteofrance.fr/climat-passe-et-futur/comprendre-le-climat-mondial/le-systeme-climatique
nfrared radiation (IR) is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore generally invisible to the human eye (although IR at wavelengths up to 1050 nm from specially pulsed lasers can be seen by humans under certain conditions [1][2][3][4]). It is sometimes called infrared light. IR wavelengths extend from the nominal rededge of the visible spectrum at 700 nanometers (frequency 430 THz), to 1 millimeter (300 GHz)[5] Most of the thermal radiationemitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon.
source: Wikipedia
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