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Unit 6 Chapter 20
Water in the Atmosphere
Section 1 Atmospheric Moisture
Water is always in motion, even though we can’t see the
molecules moving.
States of mater in which water can exist
Solid- ice at temperatures below 0o
Liquid-water at temperatures between 0o and 100o C
Gas- water vapor at temperatures at 100o C or above
Changing Forms of Water
Phase change
Gas Liquid Solid
Loses heat energy
SolidLiquid Gas
Gains heat energy
Latent Heat
Latent Heat is the energy that is absorbed or released during a phase
change.
Condensation is the process that changes water vapor to liquid water.
This process slows down the rate that air cools.
Dew – in the morning you find it on the ground
Fog – a cloud on the ground
Frost – deposition when water vapor condenses as a solid.
Evaporation
Evaporation- process of water liquid changing into water gas.
This process takes energy out of the atmosphere therefore evaporation
is a COOLING process.
The higher the temperature the faster the evaporation.
Air can hold some water vapor
Sublimation
Snow banks – become smaller due to sublimationchanging a solid to a gas (no liquid phase).
Humidity
Humidity is the amount of water vapor present in the air. The amount always varies.
Dew point is the equilibrium between condensation and evaporation.
Vapor pressure is the part of the total atmospheric pressure that is caused by water
vapor.
Absolute Humidity
It is the mass of the water vapor contained in a given amount of air.
It is expressed as the number of grams of water vapor per kilograms of air. The
amount changes with temperature. The warmer the air, the more water vapor it
can hold. The capacity roughly doubles for every rise in temperature of about
11o C. When the air is holding as much water as it can hold it is said to be
saturated.
Relative Humidity
Relative Humidity is the how close to the dew point the temperature of the air is.
It is the ratio of the actual amount of water vapor in the air to the airs
capacity to hold water at a certain temperature and pressure. It is the
percent(%) of saturation. Saturated air has a relative humidity of 100%.
Changes to RH
Increase:
If temp does not change and more moisture enters air
If moisture remains constant but temperature goes down
Decrease:
If temp increases but moisture remains constant
Dew Point
The Dew Point is the temperature at which air becomes saturated with water
and condensation occurs. It is a measure of the amount of water vapor in the
air. The more water vapor the air contains, the less the air has to cool in order
for it to condense. As the air temperature drops during the night the airs
ability to hold water decreases. When the air temperature reaches the dew
point temperature the air is saturated and dew forms (frost if the
temperatures are below freezing).
Dew Point and Relative Humidity changes are related. When the air is saturated
100% (relative humidity), the Dew Point is usually high.
As the air warms its ability to hold water rises therefore the dew point
temperature raises, Relative Humidity drops.
Relative Humidity is usually lowest in the afternoon
Measuring Humidity
Meteorologists use a variety of instruments to measure the humidity in the
air.
Hygrometer
This device measures relative humidity by using the expansion and
contraction of substance due to changes in the water vapor content of
the air. Leonardo da Vinci built the first crude hygrometer in the 1400s.
Francesco Folli invented a more practical hygrometer in 1664.
Psychrometer
It is an instrument that allows for the determination of relative humidity.
There are two thermometers are mounted on a stick
One thermometer is normal-the dry bulb
The other has a cotton sock on it- the wet bulb
The sock gets wet, the psychrometer spins evaporation occurs on the wet bulb
cooling it.
The drop in temperature is dependent on the amount of moisture in the air.
Using the dry bulb temperature and the difference between the wet and dry
bulb a chart can be used to determine the relative humidity and the dew
point.
Other Methods for Measuring Humidity
Dew Cell method.
It is a ceramic cylinder with electrodes attached to it and treated with
lithium chloride (LiCl). As water is absorbed, the cell creates electricity.
Hair Hygrometer
As a piece of hair curls it indicates a decrease in relative humidity. As the
hair becomes longer, it indicates that it has an increase in relative humidity.
Measuring Humidity at High Altitudes
Meteorologists use weather balloons to send the instruments into the
atmosphere to record numerous weather variables.
Section 2 Clouds and Fog
A cloud is a collection of water droplets or ice crystals in the
air.
Dew-condensation caused by the drop in air temperature
Fog- condensation that creates a cloud @ ground level
Cloud Formation
Cooling & Condensation
Two conditions must be met for water vapor to condense.
Air must be cooled below dew point and it needs something to condense on
to.
Condensation nuclei are tiny particles suspended in the air. Very small like
dust, ash etc. (one puff of smoke contains millions of particles). For ice to
form you need ice nuclei, tiny bacteria or clay particles work best.
Air must cool in order to have condensation. It can accomplish this by
hitting a cooler surface, mixing with cooler air, or expanding as the air
rises. Air must be saturated to condense
Dew forms when air is cooled to the dew point temperature as water vapor
hits any colder object for temperatures above 0o.
Frost forms when temperatures fall below 0o. A killing frost will occur when
ground temperatures go below -2o for several hours.
Adiabatic Cooling
This is the decrease in air mass temperature as it rises and cools.
Adiabatic Lapse Rate
The adiabatic lapse rate is the rate at which air cools as it rises.
Dry air cools as it rises; -1o for every 100 meters up
Wet air cools as it rises; between .5o and .9o for every 100 meters up
Condensation Level
The condensation level is the altitude at which condensation may occur. It is
marked by the base of a cloud. Knowing the condensation levels is
important for forecasting the severity of a storm.
Condensation Levels
Dew point decreases 2o/KM
When the air temperature and the dew temperature are the same
condensation occurs and clouds form.
continued
Mixing
This occurs when two bodies of moist air mix
Lifting
This is when air is lifted upward due to the terrain. The air is forced to
cool and condense.
This can also occur when a cold air mass comes into an area with a
warmer one. It will push the warmer one up and condensation occurs.
Advective Cooling
This occurs when the temperature of an air mass cools as it moves over a
colder surface like land or the ocean. The cold surface will absorb the
heat from the air and if it cools below the dew point, condensation will
occur.
Classification of clouds
Clouds continued
Clouds are classified by shape and altitude.
3Types of clouds are:
Stratus, Cirrus and Cumulus
3 Altitudes are
Low – 0 to 2,000 Meters
Middle – 2,000 – 6,000 Meters
High – Above 6,000 Meters
Stratus clouds
The have flat, uniform bases that begins to form at low altitudes. They are
sheet like. They form when a layer of warm, moist air lies above a layer of
cool air. They cover large areas, and often black out the sun. They contain
very little rain.
Nimbo Stratus – Rain clouds, dark and heavy
Alto Stratus – Middle Altitude, little rain
Cumulus Clouds
They are low altitude billowy clouds that often look like cotton balls with a dark
base. Hot humid days form the highest clouds.
Cumulonimbus – high dark storm clouds. Thunderheads.
Stratocumulus – a combination of stratus and cumulus
formed at low levels.
Cirrus Clouds
They form in the highest level and are feathery clouds composed of ice
crystals.
Cirrocumulus – high altitude billowy clouds usually appearing before a snowfall
or rain storm.
Cirrostratus – Long thin clouds that look like transparent veils. A halo can look
like it appears around the sun or moon due to the light bending through the
water vapor.
Fog
Fog forms when a cold surface cools the warmer moist air above it. As
the water vapor condenses, tiny droplets of water form around dust
(condensation nuclei) – only light winds are needed to keep these tiny
particles in the air (if it is very cold, you can get ice crystals).
Radiation Fog
Radiation FogWhen the night sky is clear and the ground loses heat rapidly through
radiation. A stirring of the cool ground and warm air occur. The fog at
ground level is colder than the air above it. Common in humid valleysit can be very thick in the mornings.
Other Types of Fog
Advection FogWarm moist air blows over a cool surface. Northern USA and
Canada are prime areas. Forms when warm, moist southerly winds
blow over snow-covered ground.
Upslope fog
Caused by lifting and cooling air by a slop
Steam fog
This is a shallow layer of fog that forms when cool air moves over an
inland body of water.
Section 3 Precipitation
Precipitation is water falling from a cloud. This will occur at
different areas for different reasons and at different
temperatures.
Forms of Precipitation
Rain – liquid water
Snow – crystallized water
Sleet – frozen rain droplets
Freezing Rain – super cooled water droplets that don’t freeze until it
touches something.
Hail – large frozen water droplets formed inside thunder clouds, it
moves up and down inside the clouds and grows until it gets too
heavy and falls.
How ice crystals form:
Most clouds contain temperatures usually below freezing (except
shallow clouds in warm tropics). When super cooled water
evaporates, it becomes deposited on ice crystals
Causes of Precipitation
In order for water to fall, it needs to be big enough for gravity to pull it
down. It needs to become bigger. This occurs by two processes.
Coalescence
This process is when droplets formed by condensation bump into
another droplet capturing it and combining it together to form a
bigger droplet. The more time in a cloud, the bigger it grows. Mixing
of air from different parts of a cloud also form variable sizes of water
droplets.
Supercooling
This is a condition in which a substance is cooled below its freezing
point, condensation point or sublimation point without going through
a change of state. In other words, the water does not have enough
freezing nuclei to become a solid. Most of the water will usually
evaporate, however some water will condense on the ice crystals
that were formed and falls as snow. In summer it falls as rain drops.
Measuring Precipitation
Meteorologists use various methods and instruments to measure
precipitation.
Amount of Precipitation
Meteorologists report rain fall in 100ths of an inch. A rain gauge is
used, but the rain has to be measured before it can be evaporated,
run off or soaked into the ground.
Snow is measured with a stick. Rain equivalent is done after the snow
is melted. Dry snow is usually deeper than wet snow. Most times
the wet snow equivalent is approximately 10:1, and dry snow can be
approximately 20:1.
Doppler Radar
Doppler radar works by bouncing radio waves of rain or snow droplets
and timing how long it takes the signal to return. Meteorologists can
determine location, direction of movement and intensity
Weather Modifications
Cloud seeding is one way to improve chances of getting rain to areas of
the world that need the precipitation.
Methods of Cloud Seeding
Some scientists try to create conditions for rain by dropping frozen pellets
of CO2 or silver-iodide ice crystals. You still need a cloud to do this.
Improving Cloud Seeding
Because there are so many variables to consider, cloud seeding does not
always produce the conditions that are needed (rain).
Some scientists are trying to prevent hail and eliminate fog at air ports by
seeding extra large clouds but without any real success.
Research is still pending.