Chapter 27 Evaporation, Condensation, and Precipitation

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
States of mater in which water can exist
Changing Forms of Water
Phase change
Gas Liquid Solid
Loses heat energy
SolidLiquid Gas
Gains heat energy
Latent Heat
Condensation is the process that changes water vapor to liquid
water. This process slows down the rate that air cools.
___________ – in the morning you find it on the ground
_______________ – a cloud on the ground
_____ – deposition when water vapor condenses as a
Evaporation- _____________________________________________
-This process takes energy out of the atmosphere
therefore evaporation is a COOLING process.
-The higher the temperature the faster the
-Air can hold some water vapor
Snow banks – become smaller due to sublimationchanging a solid to a gas (no liquid phase).
____________________________________________. The amount
always varies.
Dew point _____________________________________________.
Vapor pressure is the part of the total atmospheric pressure that is
caused by water vapor.
Absolute Humidity
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
____________. 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
If temp does not change and more moisture enters air
If moisture remains constant but temperature goes down
If temp increases but moisture remains constant
Reaching the Dew Point
_________________________. 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.
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.
It is an instrument that allows for the determination of relative
- 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
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.
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 -2 for several hours.
Adiabatic Cooling
Adiabatic Lapse Rate
The adiabatic lapse rate is the rate at which air cools as it rises.
Dry air cools as it rises; ____________________________________
Wet air cools as it rises; ____________________________________
Condensation Level
The condensation level ____________________________________
___________________________________________ .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_______________________
When the air temperature and the dew temperature are the
same condensation occurs and clouds form.
This occurs when two bodies of moist air mix
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
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 are classified by shape and altitude.
3Types of clouds are:
3 Altitudes are
Low – 0 to 2,000 Meters
Middle – 2,000 – 6,000 Meters
High – Above 6,000 Meters
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
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 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 valleys- it 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
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 – ________________________________________
Snow – _______________________________________
Sleet – ________________________________________
Freezing Rain – ___________________________________________
Hail – l___________________________________________________________
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.
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.
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
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
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
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.