School Science Lessons
37.1 Weather science, Coriolis effect, greenhouse effect, layers of the atmosphere,
solar ultraviolet radiation, weather maps
2012-05-05c SPw
Please send comments to: J.Elfick@uq.edu.au
Table of contents
37.39.0 Weather science
3.32.1 Composition of the atmosphere and greenhouse gases
24.4.3 Condensation nuclei, supersaturation (atmosphere)
37.48 Dew point
37.41 Plug hole experiments, [Coriolis effect, (Coriolis force)]
37.40 Trade winds and weather rotations, [Coriolis force, (Coriolis effect)]
37.40.1 Trade winds, easterlies and westerlies
37.43 Greenhouse effect in a model greenhouse
37.43.1 Global warming and climate change
37.47 Hot air rising
37.43.2 Ocean Iron Fertilization (OIF)
37.39.2 Inversion layers
37.39.1 Layers of the atmosphere, lapse rate
31.7.4.0 Lightning, sparks
37.44 Navigation data used by a ship at sea
13.1.31 Ozone, O3
37.45 Ship's compass
37.34 Solar ultraviolet radiation
37.42 Weather maps (synoptic charts), Buys Ballots law, geostrophic wind and
gradient wind
37.46 Weather sayings
History
3.32.1 Composition of the atmosphere and greenhouse gases
Gas and percentage volume in dry air: N2 78.08%, O2 20.95%, Ar 0.93%,
CO2 0.03%, Ne 0.0018%, He 0.00052%, Kr 0.00011%, Xe 0.000009%,
Rn 6 × 10-18%.
The average molecular mass air molecules is 28.8 (80% of 28 + 20% of 32).
The apparent molar mass is 28.96 g / mol.
The main greenhouse gases are as follows:
1. Water vapour from evaporation of water or sublimation of ice
2. Carbon dioxide, an acidic oxide, from burning of fossil fuels, wood and chemical
reactions. However, plants remove carbon dioxide from the atmosphere, sequester,
during photosynthesis so concentration drops during the Northern hemisphere
growing season. Carbon dioxide transmits visible light but absorbs infrared radiation.
3. Methane, CH4, from volcanoes, coal, natural gas, oil, digestion by herbivores and
anaerobic decay of plants in rice paddy and solid waste landfills.
4. Nitrous oxide, N2O from combustion of fossil fuels and solid wastes and from
chemical reactions and agricultural activities including emission by tropical soils.
5. Ozone, O3, concentrated in the ozone layer of the atmosphere, shields the earth
from excess high frequency ultraviolet light. However, it harms the respiratory system.
6. Chlorofluorocarbons, CFCs, contain C, Cl and C, and hydrofluorocarbons, HCFCs,
that also contain H, e.g. tetrafluoroethane (CH2FCF3, R-134a) perfluorocarbons,
e.g. tetrafluoromethane (CF4, carbon tetrafluoride, R14) and sulfur hexafluoride
(SF6) from chemical reactions. They have high global warming potential but they are
not ozone-depleting as are CFCs, e.g. dichlorodifluoromethane (CCl2F2, R-12,
"Freon-12") HCFCs, e.g. difluoromonochloromethane (CHClF2, "Freon 22") and
halons, e.g. bromochlorodifluoromethane (CF2ClBr, "Halon 1211").
37.39.0 Weather science
Make weather instruments and a weather station
Study of weather is a topic that is close to the life of every student. Even at the lowest
levels of primary instruction, you may make observations of the weather from day to
day. At the intermediate levels you may construct a simple weather station. At the
level of general science and later, you may study the causes of weather phenomena.
At all stages of the work it is an advantage to represent readings and observations in
graphical form.
Weather systems usually come from the west, hence the saying: "A red sky in the
morning gives a shepherd warning. A red sky at night gives a shepherd delight."
Similarly in the Bible, Matthew 16:3 (King James version): "And in the morning,
It will be foul weather today: for the sky is red and lowering. O ye hypocrites, ye
can discern the face of the sky; but can ye not discern the signs of the times?"
37.39.1 Layers of the atmosphere, lapse rate
See diagram 37.39.1: Atmosphere divided into vertical divisions
1. Troposphere
The upper limit, the tropopause, varies from a height of 28 km in the tropics and 7 km
in polar regions. In this layer most of the components of weather occur including winds,
water vapour, clouds, rainfall and lightning. Temperature decreases with height, lapse
rate (environmental lapse rate, ELR), by about 6.5oC (5oC to 10oC) per km.
2. Stratosphere
The upper limit, the stratopause, is about 50 km, when the temperature ceases to rise.
The absorption of ultraviolet radiation from the sun in the stratosphere causes a rise in
temperature resulting in it being a stable layer.
3. Mesosphere
In the lower part, the isothermal layer, the temperature hardly changes, then
temperature decreases with height, down to -95oC, at about 80 km above the earth,
the mesopause.
4. Thermosphere
This layer of rising temperature above the mesopause may extend for 400 to 500 km
depending on the activity of the sun. Ultraviolet rays and X-rays from the sun break
molecules into atoms and ions. 37.39.5 The exosphere
This level contains a very low density of neutral atoms and molecules, and electrically
charged particles. This level merges with the interplanetary region.
5. Ionosphere (the upper mesosphere and the thermosphere)
This region of concentration of ions, the ionosphere, acts as a reflector of radio waves,
sky waves,. Reflection properties change with height, from 50 to 150 km level, and
change diurnally and with different solar activity. Television waves are have shorter
wavelengths than longer wavelength radio waves and are not reflected by the
ionosphere. The wavelengths used by satellites must be shorter than radio waves so
that they can penetrate the ionosphere.
37.39.2 Inversion layers
Most of the atmosphere, and nearly all of the water vapour, is contained in the
troposphere, a layer up to 20 km deep in the tropics and 8 km deep at the poles.
Weather occurs in the troposphere. Between the troposphere and the stratosphere is
the tropopause where the temperature no longer decreases with altitude but starts to
increase. The temperature, density and pressure of the atmosphere decreases with
height. The drop in temperature, the lapse rate, is about 6.5oC per km increase of
height. During the day, radiation from the sun heats the ground much faster than it
heats the air. The ground then heats the air in direct contact with it and this warm air
rises. As the volumes of ground-heated air rise, they expand to match the lower
density of the air around them, like a hot air balloon. Expansion of an air volume
causes it to cool at about 10oC per km height. However, this rising volume of air may
remain warmer than the air surrounding it. So it can continue to rise, causing unstable

turbulent conditions when the warmer and cooler air mix.
On dry cloudless nights, the ground cools faster than the air due to radiation of heat
out to space. The ground cools the air in contact with it so the temperature of the
atmosphere increases with height to produce a temperature inversion that traps
pollutants in this lower layer, e.g. smoke from fires and exhaust gases from motor cars.
The boundary where the switch of temperature change occurs can be clearly seen
from above the inversion layer, like a hill. In the morning, the sun heats the ground. The
ground then heats the air in direct contact with it and this warm air rises. Unstable
conditions begin, the inversion layer is broken, and the usual cycle starts again.
Use a transparent square tank with an immersion heater at the bottom. Fill the tank
half full of cold water. Carefully pour hot water on top to form a separate layer. Shine
a strong torch through the water onto a screen. Note the sharp boundary because of
the difference in refractive index of the water at different temperatures if no mixing
between the two layers occurs. Turn on the heater at the bottom of the tank. Note the
warm water rising through the cold layer as a turbulent swirling shadow on the screen.
When the rising water reaches the hot water layer, it rises no further and is trapped. In
the atmosphere, the inversion layer prevents mixing between the troposphere and
higher atmosphere. Water does not move through and no clouds occur beyond the
stratosphere, otherwise the Earth would eventually lose all its water. Material that does
get into the stratosphere layer stays there for years and can take part in chemical
reactions, e.g. reactions with ozone and oxygen atoms. Radiation at around 265 nm is
most dangerous to living things, including plants. Ozone prevents radiation below 290
nm from reaching the ground. Ozone also stops great deal of radiation in the 290 nm
to 320 nm range. This radiation causes skin cancer. Concentrations of ozone in the
stratosphere fluctuate with natural changes in rates of production and destruction. In
any one year, the maximum concentrations in the spring can be half as high again as
the minimum in the autumn. While the rates of ozone production appear to be out of
control, the compounds added to the atmosphere will affect the destruction. The oxides
of nitrogen, both natural and from car pollution, account for perhaps two thirds of the
destruction.
37.40 Trade winds and weather rotations, [Coriolis force, (Coriolis effect)]
1. The circumference of the Earth at the equator is larger than near the north pole or
south pole and the Earth rotates once every 24 hours, so the surface of the Earth at
the equator must move faster at the equator than near the north pole or south pole. A
super missile fired from the north pole and aimed at the south pole would to be
deflected to the right in the Northern hemisphere and deflected to the left in the
Southern hemisphere. The Coriolis force does not only operate on objects travelling
in a north south direction. The size of the Coriolis force is independent of the direction
in which something is moving. The missile goes similarly off track after being fired in an
easterly or westerly direction. Some people regard "Coriolis force" as a fictional force
used to account for movement of air and water over the spinning Earth and "Coriolis
acceleration" refers to the apparent tendency of a moving body to swing to one side
when its motion is defined by rotating axes. Other people do not use the term "Coriolis
effect" because it is too vague. They say that in a rotating co-ordinate system there is a
Coriolis force that causes a mass to be accelerated. The Coriolis force does no work
but that does not disqualify from being called a force.
2. The rotation of the Earth does influence the direction of rotation of large weather
systems and large vortices in the oceans. These long-lived phenomena allow the very
weak Coriolis force to produce a significant effect, given enough time. The Coriolis
 force causes the air to rotate around a low pressure centre in a cyclonic direction,
i.e. air or water rotates in the same direction as the Earth. The air flowing around a
cyclone (hurricane, typhoon) spins anticlockwise in the Northern hemisphere, and
clockwise in the Southern hemisphere. If the Earth did not rotate, the air would flow
directly in towards the low pressure centre.
3. The Coriolis force, operating on its own causes a moving object to experience a
force to the right of its path in the Northern hemisphere and to the left of its path in the
Southern hemisphere. In a geophysical flow in the atmosphere or oceans, there is
always another force operating, e.g. the pressure gradient force that cause material to
start to move. The direction of any rotation depends upon the net force where the
Coriolis force and other forces are present. Around a high atmospheric pressure area,
the pressure gradient force points radially outward. Around a low atmospheric
pressure area, the pressure gradient force points radially inward.
4. If a body of air moves horizontally at constant speed where friction with uneven
terrain is negligible, the two horizontal forces on it are the pressure gradient force and
the Coriolis force. If the magnitudes of these forces are equal then the Coriolis force
does not cause a deflection to left or right. The Coriolis force may be larger or smaller
than the pressure gradient force, depending on the wind speed. If the pressure gradient
force is greater than the Coriolis force, the flow will be curved around a low pressure
area.
In the Northern hemisphere the flow of the gradient wind is anticlockwise around the
low pressure areas because the Coriolis force acts to the right.
In the Southern hemisphere the flow of the gradient wind is clockwise around the low
pressure areas because the Coriolis force acts to the left.
Study the direction of rotation winds around Highs and Lows from weather charts in
the newspaper or on television.
It is unlikely that the construction of wind farms affects the rotation of the earth. The
relative forces are not comparable. Some people have suggested that half the wind
farms could face east and the other half face west to counteract any effect on the
rotation of the earth!
37.40.1 Trade winds, easterlies and westerlies
A wind in the Southern hemisphere originally heading due north towards the equator
will reach the equator to the west of its original point of aim. So at the equator an
observer will observe more easterly winds called the trade winds that blow from the
southeast in the southern hemisphere and from the northeast in the Northern
hemisphere. Trade winds from north and south meet near the equator so the air rises,
cools and forms rain to create tropical conditions. Similarly a wind heading towards
the south pole from the equator will arrive east of the original point of aim to produce
westerlies across the middle latitudes, e.g. the roaring forties that blast around the
Southern hemisphere at 40o to 50o latitude. Both trade winds and westerlies blow
away from 30o latitude region.
37.41 Plug hole experiments, Coriolis effect (Coriolis force)
Some people think that when water goes down a bath plug hole its direction of spin is
determined by the Coriolis effect. These people say that if you leave the water still in
the bath for a long time then pull out the plug it will spin anticlockwise in USA and spin
clockwise in Australia. Other people say that the Coriolis effect is too small to affect
the small amount of water in a bath tub and they cannot produce the effect by
experiment in a bath. The Coriolis force is very small compared with common
rotations, e.g. water down a plug hole, because the rotation of the Earth is only one
 rotation per day. The direction of rotation of water down a plug hole depends on the
way it was filled or by vortices due to washing action. If you want to obtain the
rotation down a plug hole that is always cyclonic, use a 1 m smooth pan with a very
small hole in the centre and a stopper that can be removed from below. Leave the
water undisturbed for weeks before removing the stopper so that the water takes
hours to drains through the hole. As a fluid parcel moves towards a wall it will be
deflected and turn. It is this rotary motion that is accentuated when the water converges
towards the drain. Similarly, some people report that if you put a flat round dish full of
still water in a refrigerator, the water heaps up as it freezes to form a roughly
north-south ridge because of the Coriolis force.
37.42 Weather maps (synoptic charts), Buys Ballots law, geostrophic wind and
gradient wind
See diagram 37.149.1: Geostrophic wind flow
See diagram 37.149.2: Gradient wind flow
See diagram 37.149.3: Veering and backing wind
1. Study laminar and turbulent flow from weather maps. Daily weather maps show
large scale fluid dynamics. The usual weather map is a mean sea level synoptic chart.
Synoptic means overall view. Lines on a weather map joining places of equal
atmospheric pressure are called isobars. An anticyclone may be shown by a group of
isobars labelled high or H and the central pressure in hectopascals. Anticyclones, highs,
are much larger than lows, cyclones, so the pressure gradients of highs is more gradual
and so generate lighter winds than lows. Anticyclones that do not move for long
periods, blocking highs, cause the weather pattern to remain constant. The winds
blowing from the centre of highs towards lows blow anticlockwise around the high in
the Southern hemisphere and clockwise in the Northern hemisphere under the influence
of the Coriolis effect.
Buys Ballot was a Dutch meteorologist who described the relationship between wind
direction and pressure as shown by isobars. The Buys Ballots law states that if an
observer stands with the back to the wind, in the Northern hemisphere the lower
pressure is to the observer's left and in the Southern hemisphere the lower pressure is
to the observer's right. So if an observer standing in the Southern hemisphere feels the
wind blowing into the face, the pressure on the observer's right is higher and on the
observer's left is lower. If an observer facing south feels a southerly on the face then
feels a wind on the right side of the face, the wind has veered from a southerly wind to
an easterly wind. If the wind then moves back to a southerly wind, the wind has
backed. So the direction of a veering wind moves clockwise, to the right, and the
direction of a backing wind moves anticlockwise, to the left.
2. An air mass moving horizontally at constant speed with no friction with the
surroundings has two forces on it, 2.1 the pressure gradient force from high pressure
to low pressure and 2.2 the Coriolis force. If 2.1 and 2.2 are exactly equal and
opposite, the air mass continues moving as a geostrophic flow horizontally in a great
circle around the world, i.e. in a straight line on a synoptic weather chart. For any
given latitude, at a certain wind speed, called the geostrophic speed 2.1 = (b). As no
Coriolis force exists at the equator, air masses there move in the direction of the
pressure gradient force from high pressure to low pressure. Similarly, no geostrophic
flow occurs between 15o north and 15o south because the Coriolis force is too weak.
3. If 2.1 is not equal to (b), the air mass moves to the left or right tangential to the
isobar as a gradient wind, i.e. along the curved isobars on a synoptic weather chart. If
the pressure gradient force is greater than the Coriolis force, the air mass moves in a
curve around a low pressure area, anticlockwise in the Northern hemisphere and
clockwise in the Southern hemisphere. This curved motion is called cyclonic flow and
is in the direction of the Earth's rotation. Remember that an observer above the north
pole would observe the Earth spinning anticlockwise and an observer below the south
pole would observe the Earth spinning clockwise. Regions of high pressure,
anticyclones are found mainly over the poles and around the globe at 30o of latitude
each side of the equator. Regions of low pressure, troughs, are found mainly near the
equator and between 30o of latitude and the poles.
37. If the pressure gradient force is less than the Coriolis force, then the movements of
the air mass is the opposite to the movements as in 3.
5. Measure surface wind at a standard level of 10 m above the Earth's surface where
forces of friction with the rough surface of the Earth decrease the geostrophic wind
speed and cause the wind to move across the isobars at an angle of about 30o over
land and 10o over sea. At about 1 km above the ground the friction force is zero.
6. Horizontal convergence refers to a gain of air mass above a place causing increased
atmospheric pressure. Horizontal divergence refers to a loss of air mass above a place
causing decreased atmospheric pressure. Low pressure at X causes air to move
towards X due to the pressure gradient force, followed by slow upward movement of
air. If the upward moving air contains moisture, cloud will form at X1. Low pressure
areas are associated with a low, low pressure centre, depression or cyclone and wet
weather. A trough is an elongated area of low pressure. High pressure at Y causes air
to move away from Y, followed by a slow downward movement of air. So clouds do
not form at Y1. High pressure areas are associated with a high or anticyclone and fine
weather with light winds. A ridge is an elongated area of high pressure. Horizontal
winds, advection winds, are always much greater than vertical winds, convection
winds.
37.43 Greenhouse effect in a model greenhouse, global warming
See diagram 37.43.1: Average global air temperature
A blanket on a bed reduces heat loss by reducing conduction. Air, a poor conductor
of heat is trapped within the threads of the blanket and between the blanket and the
person on the bed. Similarly the glass roof of a greenhouse or cucumber frame traps
air and so reduces heat loss by conduction and by convection. However, the
greenhouse effect reduces heat loss by radiation.
The sun emits light and short wavelength infrared radiation that can pass through gases
in the atmosphere and the glass roof of a greenhouse to heat the Earth and the
contents of the greenhouse that in turn emit longer wavelength infrared radiation as
their temperature rises. Some longer wavelength radiation emitted by the Earth is
absorbed by "greenhouse gases" in the atmosphere that have three or more atoms in
each molecule e.g. carbon dioxide, methane, nitrogen oxides, ozone and water
vapour, which in turn emit about half of this radiation to be absorbed by the lower
atmosphere and the Earth to give an average temperature of +18oC instead of -18oC
if the greenhouse gases did not exist.. However, the longer wavelength radiation cannot
pass through glass so the contents of the greenhouse get hotter than if outside the
greenhouse. If you place a sheet of glass between a red hot fire and your hand, you
cannot feel any heat from this longer length radiation. So the "greenhouse effect" is a
natural process accentuated in the last two hundred years by industrial and agricultural
development causing increases in the concentration of "greenhouse gases" in the
atmosphere. This increase has probably caused "global warming" the steady increase
in average temperature now being experienced. Some people say that "greenhouse
effect" is a misnomer because the main function of a greenhouse is to stop loss of heat
by convection yet allow plants to receive the radiation necessary for photosynthesis.
If that is true then the above experiment is not a good simulation of the greenhouse
effect.
A plant greenhouse traps insolation. The glass roof and sides transmits most radiation
wavelengths except the infra-red and ultraviolet wavelengths. The radiation that passes
through the glass is absorbed by the plants which then get warmer and radiate
infra-red radiation that cannot pass through the glass. So a greenhouse is a "hot house".
1. Use a thermometer to read the ambient temperature in the shade. Leave a closed
bottle in direct sunlight for some time. Put the bottle in the shade, open it and read the
inside temperature with the thermometer.
2. Hold a sheet of glass between your hand and the sun. You can feel the increase in
temperature due to the radiant heat passing through the glass. Hold the sheet of glass
between a fire and your hand. You cannot feel any temperature change due to the
radiant heat from the fire passing through the glass.
3. Line a household bowl with aluminium foil. Put a piece of food, e.g. cheese, on the
end of a tooth pick and fix it in the centre of the bowl. Cover the bowl with clear food
wrap and leave the bowl in the sun. The bowl acts as a greenhouse and the cheese
melts.
37. Use two small identical packets or cardboard boxes. Cut identical square holes in
the upper surface of each box. Punch a hole in the side of each box and insert a
thermometer through the hole. Find a piece of window glass to cover the square hole
of one box or make a glass roof with microscope slides. The other box has no cover
over the square hole in the roof. The initial readings of the two thermometers should be
the same. Take the boxes out of the room and put them in the direct sunlight for 20
minutes. Read the thermometers and record the temperature. The box with the glass
covering the square hole is a model greenhouse. It absorbs radiant energy though the
glass roof. The temperature in the model greenhouse box is greater than the
temperature in the other box.
37.43.1 Global warming and climate change
It is estimated that while the average sea level has risen 222 mm since 1875, i.e. 1.7
mm per year, during the period 1993 to 2009 it rose 3 mm per year. This much faster
increase was possibly caused by increased average annual temperatures leading to
expansion of sea water, 30%, ice melting in glaciers and ice sheets in Greenland and
Antarctica, 55%, and increased wetlands drainage, 15%. This evidence of global
warming may be called the enhanced greenhouse effect. The problem is how to
distinguish this global warming from patterns of climate change in the past that included
long periods of warming and cooling, e.g. the ice ages.
37.43.2 Ocean Iron Fertilization (OIF)
An unproved theory that iron is a limiting nutrient in oceans and the supplementary
application of iron would beneficially stimulate phytoplankton populations to act as a
biological carbon pump to sequester carbon dioxide.
37.44 Navigation data used by a ship at sea
Position: 10.23 UTC (Co-ordinated Universal Time (UTC) replaced Greenwich
Mean Time (GMT) as the World standard for time in 1986. It is based on atomic
measurements rather than the earth's rotation. Greenwich Mean Time (GMT) is still
the standard time zone for the Prime Meridian (Zero Longitude).
20o57.05' S
039o52.82' W
Course: 32o
Speed: 18.8 Kts (knots)
Relative wind: 55 Km \ h
Depth of sea: 47 metres (154 feet)
N | | NE | | E
Ships time: 07.29
Water temperature: 25oC
Air temperature: 29oC
Conditions: Cloudy sky
Air pressure hPa
Beaufort Wind Scale 3 (Beaufort number 0 --> 12)
Wind direction: 8 km / h from south
Barometer: 1015 mb, 761 mm Hg, 30.00 inch
Tendency: Slowly increasing
37.45 Ship's compass
The ship's compass is used to steer a ship on a preselected course and to take
bearings of visible objects to fix a ship's position on a chart.
1. The magnetic compass with the north mark pointing to the magnetic North Pole.
2. The gyroscopic compass (gyro) with the north mark pointing to the true North
Pole. The marine gyrocompass has a perfectly balanced gyroscope rotor wheel that
spins symmetrically at high speed about an axis. A gyroscope rotor maintains the
direction of its plane of rotation unless a strong force is applied to overcome its inertia.
37.46 Weather sayings
1. "Clear moon, frost soon"
With no clouds to impede heat lost by radiation the earth cools quicker and frost is
more likely.
2. Rainbow in the morning gives a fair warning
Rainbows appear in the side of the sky opposite the sun so a rainbow in the western
sky indicates rain as the system moves from west to east.
3. "Red sky at night, shepherd's delight, red sky in morning, shepherd's warning"
Weather systems usually move from west to east. Dust and moisture in the air makes
the sky red towards sundown. High concentration of particles in the air means that
the atmosphere is stable, air pressure is high and weather next day will be fine.
However, in the morning, high concentration of particles in the air
may indicate that a storm cell is approaching.
4. "Ring around the moon, rain or snow soon"
The ring is caused by the ice crystals in cirrus clouds that indicates that low pressure
is approaching, associated with a warm front. The brighter the ring the more likely the
incidence of rain or snow. A similar ring may form around the sun.
5. "Year of snow, year of plenty"
More stable cold weather during winter is better for crop plants that a mixture of
warmer and cooler weather. that may cause "false springs" and the untimely flowering
of plants.
37.47 Hot air rising
When air is heated it expands as the air particles move faster and consequently the air
becomes less dense. However, the heated air will not rise by itself. Cooler air around
it is less dense, weighs more, and squeezes the lighter less dense air up. So the rising
of the hot air is caused by the falling of the cooler air. The rising is controlled by gravity.
37.48 Dew point
The atmosphere usually contains some water vapour, less than required for saturation
or partially saturated. The saturation vapour pressure decreases as the temperature
decreases, so the amount of water vapour required for saturation decreases as the
temperature is lowered. If the temperature of the air is lowered, the mass of water
vapour in the air does not change and a temperature will be reached where the mass of
water vapour in the air is sufficient to saturate the air at this lower temperature. At this
temperature moisture is deposited and that temperature is the dew point. The dew
point is the atmospheric temperature at which dew is deposited, depending on
pressure and humidity, i.e. at which the pressure of atmospheric water vapour
becomes equal to the saturation vapour pressure. So relative humidity =
saturation vapour pressure at the dew point / saturation vapour pressure at the room
temperature. For every 1 °C difference in the dew point and dry bulb temperatures,
the relative humidity decreases by 5%, starting with relative humidity = 800px when
the dew point equals the dry bulb temperature.
The dew point is the temperature to which a given parcel of humid air must be cooled,
at constant barometric pressure, for water vapour to condense into liquid water. The
condensed water is called dew when it forms on a solid surface. The dew point is a
saturation temperature. The dew point is associated with relative humidity. A high
relative humidity indicates that the dew point is closer to the current air temperature.
Relative humidity of 800px indicates the dew point is equal to the current temperature
and the air is saturated with water. When the dew point remains constant and
temperature increases, relative humidity will decrease.
The dew point is derived from dry bulb and wet bulb temperatures, with a correction
for elevation above sea level.
Relative humidity = mass of water vapour in a given volume of the atmosphere / mass
of water vapour required to saturate the same volume at that temperature
Relative humidity = vapour pressure of the water vapour in the air / saturation vapour
pressure at the same temperature × 800px
Relative humidity = saturation vapour pressure at the dew point / saturation vapour
pressure at room temperature
If the dry bulb temperature is the same as the dew point, the air is said to be saturated
and the relative humidity is 800px. Relative humidity is also approximately the ratio of
the actual to the saturation vapour pressure.
Actual vapour pressure is a measurement of the amount of water vapour in a volume
of air and increases as the amount of water vapour increases. Air that attains its
saturation vapour pressure has established an equilibrium with a flat surface of water.
That means, an equal number of water molecules are evaporating from the surface of
the water into the air as are condensing from the air back into the water.
History of experiments in this document
This version of selections from the New UNESCO source book for science teaching,
Third impression 1979, ISBN 92-3-101058-1, was edited by Dr J. Elfick, School of
Education, University of Queensland, Brisbane, Australia, assisted by Mr R. Smith,
Central Queensland University, Australia, working under UNESCO contract
8347201, 2001-12-15