Chapter 18 WATER IN THE ATMOSPHERE
Section 1: Humidity and Condensation
Objectives:
1. Describe the three ways (states) in which water can
exist in the atmosphere
2. Explain how air temperature affects the amount of water
vapor the air can hold (contain)
3. Analyze how condensation occurs in the atmosphere
I.
Characteristics of Water
1. Water is the only substance that commonly exists in all three
states of matter (depending on temperature)
A. Water is a solid at 0 C or below
B. Water is liquid between 0 – 100 degrees C
C. Water is a gas (water vapor) above 100 degrees C
2. Water vapor is invisible but can be FELT as humidity
3. CONDENSATION: The change from water vapor to liquid
water.
A. The process of condensation releases heat!
B. Condensation slows down the rate at which air cools
C. Forms of condensation are: Dew, fog, clouds.
4. EVAPORATION: The change from liquid water to water
vapor
A. The process of evaporation absorbs heat
B. Evaporation speeds up the cooling process by absorbing
energy from the surroundings
C. Form of evaporation is sublimation
!DIAGRAM ON PAGE 391!
II.
Humidity
1. Specific Humidity: The actual amount of water vapor in the
air at a given time and place. Expressed as grams per
kilogram of air.
2. Humidity is limited to air temperature. Temperature and
water vapor in the atmosphere are positively correlated.
A. As temperature increases the amount of water vapor the
air can hold increases, as temperature decreases, the
amount of water vapor the air can hold decreases.
3. Relative Humidity:
A. How close the air is to capacity for holding water vapor
(%)
B. To calculate the relative humidity of a kilogram of air,
divide the specific humidity by the capacity
C. WORKSHEETS!!!!
4. Measuring Humidity
A. Measured with a psychrometer
a. wet bulb and dry bulb: evaporation = cooling therefore
the difference in the temperature between the wet bulb
reading and the dry bulb reading when compared on a
chart will give the relative humidity of the air.
b. CHART ON PAGE 393!
III Condensation:
1. Cooling and Condensation
A. In order for condensation to occur, the air must be cooled
to it’s dew point, AND there needs to be something onto
which the water vapor can condense (CONDENSATION
NUCLEI)
B. Air becomes supersaturate if the water vapor has nothing
on which to cool
C. Air cools in several ways:
a. contact with a colder substance/surface
(DEW/FROST)
b. radiation of heat (radiational cooling) (DEW/FROST)
c. mixing with colder air
(FOG)
d. expansion as it rises (ADAIBATIC LAPSE RATE)
2. Formation of Dew and Frost
A. When air cools to it’s dew point through contact with a
colder surface, water vapor will condense directly on to
that surface.
a. Ground and surfaces near the ground become cooler
more quickly because they lose heat more rapidly than
air does.
b. Dew = above 0 degrees C
c. Frost = DEPOSITION of water vapor when
atmospheric temperatures are below 0 degrees C
3. Formation of Fog
A. When a cold surface cools the warm, moist air directly
above it
B. Radiation Fog: Radiational cooling causes the air directly
above the surface to cool. Light winds mix this cool air
with the warmer air above it cooling it to it’s condensation
(dew) point
a. the fog at ground level is cooler than the warmer air
directly above it so a temperature inversion takes place
b. common near lakes and rivers and most frequent in
late fall and winter.
C. Advection Fog: When warm moist air blows over a colder
surface.
a. When warm air blows over a snow covered area.
Chapter 18: WATER IN THE ATMOSPHERE
Section 2: Clouds
Objectives:
1. Describe the three basic forms of clouds
2. Explain how the shape of a cloud shows how the air is
moving through it
I.
Types of Clouds
1. There are 4 types of clouds: low, middle, high, and clouds
with vertical development
2. Clouds are classifies according to their HEIGHT and
SHAPE
A. HEIGHT: low, middle, high
B. SHAPE: Vertical – layer or stratiform clouds
Horizontal – great puffs, or cumuliform clouds
3. Cloud distances are measured above the ground, not above
sea level.
4. Cloud names are formed from one or more of the same five
words or word parts:
A. Stratus/ Strato = layer clouds
B. Cumulus/ Cumulo = vertical clouds
C. Cirrus/ Cirro = feathery clouds
D. Alto = clouds that are between 2000 and 7000 meters
E. Nimbus/ Nimbo = dark rain clouds
II.
Cloud Formation
1. Background of cloud formation:
A. The shape of a cloud shows how air moves through it.
B. Where (at what altitude) the cloud forms tells you where
the condensation level is, or where air reaches it’s
saturation point and the water vapor condenses as liquid
water.
2. Dry and Moist Adiabatic Lapse Rates
A. The adiabatic lapse rate is the rate at which air cools as it
rises
a. Unsaturated (dry) air cools more quickly than saturated
air. This occurs because condensation of water from
water vapor causes/releases heat.
b. Air cools as it rises because the air is expanding due to
decreased pressure.
c. The Dry Adiabatic Lapse Rate = 10 degrees per
kilometer while the Moist Adiabatic Lapse Rate = 5-9
degrees per kilometer.
3. Cumulonimbus Clouds (vertical rain clouds)
A. Cumulonimbus clouds begin forming when moist air rises
and cools to it’s dew point, the cloud base will form at the
condensation level.
B. THE SUMMARIZATION OF THIS CONCEPT IS SHOWN
IN THE PICTURE ON PAGE 399!!!!! (if you understand
the picture and the captions than you understand the
major concept of the formation of cumulonimbus clouds)
4. Layer Clouds
A. Form in stable air because it does not easily move up or
down it will spread out vertically
B. Clouds form in stable air in two ways:
a. Air can be forced slowly upward to it’s condensation
level (when it moves up a mountain, or when it moves
over a layer of colder, denser air)
b. By radiating heat into the atmosphere or mixing with
cooler air which causes it to cool to it’s dew point and
form clouds.
5. Predicting the Condensation Level
A. Predicting the condensation level of an air mass involves
two things: cooling and dew point.
a. As the warm air rises, both it’s temperature and dew
point will decrease! Therefore if you have the
temperature and dew point of an air mass at ground
level you can predict the condensation level by
considering two laws of nature: 1. Temperature of a
dry air mass will drop 10 degrees per kilometer gain of
altitude (dry adiabatic lapse rate) 2. The dew point will
drop 2 degrees for every kilometer gained in altitude.
WHEN THE TEMPERATURE OF THE AIR MASS
AND THE DEW POINT ARE THE SAME, THAT IS
THE ALTITUDE AT WHICH CONDENSATION WILL
OCCUR!
b. This information helps meteorologists predict if storms
will occur as well as how severe the storm may be
Chapter 18: WATER IN THE ATMOSPHERE
Section 3: Precipitation
Objectives:
1. Compare and contrast how precipitation forms in warm
clouds and cold clouds
2. Describe how rising air produces precipitation
I.
How Precipitation Forms
1. Growth of water droplets
A. By bumping into one another and combining
B. The longer a droplet has been in a cloud, the bigger it will
be due to more time to combine with other droplets
C. Droplets will also range in size due to the condensation
nuclei on which they form
2. Growth of ice crystals
A. Ice crystals present in the upper, colder portion of clouds.
Ice crystals grow when they come in contact with other
ice crystals or through deposition of ice onto the ice
crystals from the water vapor in the clouds. (super-cooled
water droplets)
3. Kinds of Precipitation
A. Drizzle
B. Rain
C. Snow
D. Sleet
E. Freezing rain
F. Hail
II.
Measuring Precipitation
1. Rain is reported to the hundredths of an inch
2. Snow is reported by melting it then reporting
III.
Where Does Precipitation Occur?
1. The warmer the air before it rises, the more moisture it can
contain, the higher the air rises, the more moisture it can
release
2. Kinds of areas where precipitation is common:
A. Equator
B. In storm areas of all kinds
C. Mountain ranges
a. moist air is pushed up a mountain range from wind, it
cools and condenses in the form of rain or snow.
D. (USUALLY ANYWHERE THE AIR IS FORCED TO RISE)
E. It doesn’t rain anywhere the air sinks
IV.
Weather Modification
1. Cloud “seeding”
SUMMARY OF KEY IDEAS
18-1
1. Water exists in the atmosphere as a solid liquid and gas
2. Evaporating water absorbs heat from it’s surroundings which
become cooler
3. Condensing water releases heat to it’s surroundings
4. Warmer air can contain more water vapor than cooler air
5. Fog and clouds form when air is cooled to it’s dew point and
water vapor in the air condenses around condensation nuclei
6. Dew or frost forms if contact with cold ground cools air to it’s
dew point
18-2
1. Clouds are classified by their height above ground and their
shape
2. Cumuliform clouds are formed by rising air and stratiform
clouds form in horizontal layers
3. Rising air cools at the dry adiabatic lapse rate with no
condensation and the moist adiabatic lapse rate with
condensation
4. Heat released through condensation within a cloud can
cause the air within the cloud to rise to great heights.
18-3
1. Water droplets in clouds grow by colliding with each other
2. Ice crystals in clouds grow through collisions and by using
water vapor that evaporated from super-cooled drops.
3. Precipitation’s type depends on the conditions as it forms
and falls.