Chapter 23
Weather and Climate
Cloud Forming Processes
• A cloud with a volume of 1 cubic km
contains about a million liters of water.
• You see a white cloud when you are
between the cloud and the sun, since you
see reflected sunlight from the cloud.
• The same cloud appears to be gray if it is
between you and the sun, since it filters
the sunlight coming to you.
Cumulus clouds that will soon lead to rain.
Grey Clouds-The clouds are between you and the sun.
The clouds filter the sunlight.
White clouds-When you are
between the sun and the
Clouds-You see reflected
sunlight from the cloud.
Precipitation
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Water that returns to the surface of the earth, either in
the liquid or solid form, is called precipitation.
• Dew and frost are not classified as precipitation
because they form directly on the surface of the earth
and don’t fall through the air.
• Precipitation forms in clouds by one of two processes:
1. The coalescence of cloud droplets. This takes place in
warm cumulus clouds that form near the ocean in the
tropics, since these clouds contain giant salt
condensation nuclei and have been observed to
produce rain within about 20 min after forming.
2. The growth of ice crystals. Occurs with clouds away
form the ocean. These clouds extend up in the
atmosphere and are above the freezing point of water.
They form as conglomerates of ice crystals and fall to
the ground. During the summer they reach the ground
as raindrops, during the winter as snow.
Precipitation and Clouds
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The basic form of a cloud has meaning about
the general type of precipitation that can occur
and also the types of weather:
1. Cumulus clouds usually produce showers or
thunderstorms that last only brief periods of
time.
2. Stratus clouds lead to longer periods of drizzle,
rain, or snow.
3. Cirrus clouds do not produce precipitation of
any kind but may have meaning about the
coming weather.
Stratus Clouds
Cumulus Clouds
Cirrus Clouds
Types of Clouds
• According to shape:
1. Cumulus clouds are heaped and puffy. They can be of
low or middle altitudes in the atmosphere.
2. Stratus clouds are layered. They can be of low or
middle altitude in the atmosphere.
3. Cirrus clouds are wispy. These clouds are of high
altitudes in the atmosphere.
• According to altitude: Note: the prefix nimbo or the suffix
nimbus imply that precipitation is falling from the clouds.
1. Low altitudes (below 2000 m): Stratus, Nimbostratus,
Cumulus, Stratocumulus, Cumulonimbus
2. Middle altitudes (2000-6000 m): Altocumulus,
Altostratus
3. High altitudes (above 6000 m): Cirrus Cirrocumulus
Cirrostratus
Weather Producers
• The region between 10oN and 10oS of the equator receives more
direct radiation on the average than other regions of the earth’s
surface.
• The air over this region is heated more, expands, and becomes less
dense as a consequence of the heating.
• This less dense air is buoyed up by convection to heights up to 12
mi and is then cooled to less than -110oF.
• The accumulating mass of cooled, dry air spreads north and south
towards both poles, then sinks back toward the surface about 30oN
and 30oS.
• The descending air is warmed and is warm and dry by the time it
reaches the surface.
• Part of the sinking air then moves back toward the equator across
the surface completing a large convective cell.
• This giant cell has a low pressure belt over the equator and a high
pressure belt over the subtropics near latitudes of 30oN and 30oS.
The sinking air moves pole ward across the surface, producing belts
of westerly winds in both hemispheres to latitudes of about 60o , and
also trade winds moving south.
Weather Producers
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Changing weather is attributed to:
– Movement of air masses
– Fronts of air masses
– High and low pressure systems
Air Masses
• An air mass is a large, horizontally uniform body of air
with nearly the same temperature and moisture
conditions.
• An air mass forms when a large body of air remains over
a large area of land or water for an extended period of
time. It acquires the temperature and moisture
characteristics of the land or water over which it is.
• A large body of air that remains over the cold, dry, snow
covered surface of Siberia for some time will become
cold and dry.
• A large body of air that remains over a warm tropical
ocean will become warm and moist.
• When the air mass breaks away it maintains those same
characteristics and moves long distances sometimes.
Air Masses
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3.
4.
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Classifications of air masses according to their
temperature and moisture conditions where they
originate: These are shown on weather maps with the
abbreviations.
Continental polar (cP)-Dry, cold air
Maritime polar (mP) -Moist, cold air
Continental tropical (cT) -Dry, warm air
Maritime tropical (mT) -Warm, moist air
Once an air mass leaves its source region it can move
at speeds of up to 500 mi per day while mostly
retaining the temperature and moisture characteristics
of the source region.
Polar air mass moving southeast over the southern US. Clouds form over the warmer
waters of the Gulf of Mexico and the Atlantic Ocean, showing the state of atmospheric
Instability from the temperature differences.
Weather Fronts
• A front is the boundary between air masses of different
temperatures.
• It is a thin transition zone between two air masses that
range between about 3-20 mi. and the air masses do not
mix.
• The density differences between the two air masses
prevent any general mixing, since the warm, less dense
air mass is forced upwards by the cooler, more dense air
moving under it.
• On weather maps the fronts are represented with a line
bulging in the direction of air mass movement.
• A cold air mass is like a huge, flattened bubble of air that
moves across the land. The line on a weather map
represents the place where the leading edge of this
bubble of air touches the surface of the earth.
Fronts-Shown in Weather Map
Fronts – boundary between air masses
This weather map of the US shows two fronts with associated low pressure areas and
Five areas with high pressure.
Cold Fronts
• Cold Fronts-Cold air mass moves into warmer
air, displacing it in the process.
• If the warm air is moist it is quickly cooled as it
rises to the dew point temperature and large,
towering cumulus clouds result along the front
which produce intense thunderstorms.
• The thunderstorms are over quickly and are
followed by a rapid drop in temperature from the
cold air mass moving. There is also an increase
in the barometric pressure due to the colder air,
which is more dense.
Cold Front
Weather producers
Cold Front – cold air moving into a region of warmer air.
Cold Front
Weather producers
1) Warm air forced to rise
2) Air saturates
3) Cumulus clouds & precipitation.
Warm Fronts
• Warm fronts-Forms when a warm air mass
advances over a mass of cooler air.
• The warm air is less dense than the cooler air
and forms a long, sloping front.
• The warm overriding warm air may form clouds
far in advance of the front. This may produce
high cirrus clouds a day or more in advance of
the front followed by thicker and lower stratus
clouds as the front advances.
• These stratus clouds cause a broad band of
drizzle, fog, and continuous light rain. Which
may last for days.
Warm Front
Warm Front – warm air advances over cooler air.
Stationary Fronts
• When a warm or cold front stops
advancing a stream of cold air moves
along the north side of the front. And a
stream of warm air moves along the south
side in an opposite direction.
• This is called a stationary front because
the edge of the front is not advancing.
• It represents an unstable situation that can
result in a major atmospheric storm.
Major Storms
• A thunderstorm is a brief but intense storm with
rain, lightning and thunder, gusty and often
strong winds and sometimes hail.
• Thunderstorms usually develop in warm, very
moist and unstable air.
• Many thunderstorms occur during warm, humid
afternoons and especially during the month of
July in the northern hemisphere.
• Damage from a thunderstorm is usually caused
from associated lightning, strong winds, or hail.
Different parts of a thunderstorm cloud develop centers of electric charge. Lightning
Is a giant electric spark that discharges the accumulated charges.
Thunderstorms
Lightning heats, expands atmosphere creating
large pressure wave - thunder.
This hailstones fell from a thunderstorm in Iowa, damaging automobiles, structures,
And crops.
Other Major Storms
• Tornadoes-The smallest, most violent weather
disturbance that occurs on the earth.
• Tornadoes occur with intense thunderstorms.
• They resemble long, narrow funnels or ropelike
structures that drop down from a thundercloud and may
or may not touch the ground.
• The ropelike structures are rapidly whirling columns of
air about 330-1300 ft in diameter. It will usually travel 4-5
mi on the ground, lasting only a few minutes and moving
at about 30 mi/h.
• The speed of the whirling wind in the column is
estimated to be up to about 300 mi/h, with most having
winds of less than about 112 mi/h.
• The destruction is produced by the powerful winds.
• On an average several hundred tornadoes are reported
in the US every year and they occur mostly during spring
and early summer afternoons over the Great Plains
states: Texas, Oklahoma, Kansas, and Iowa are called
the tornado alley.
A tornado moving across an open road struck Dallas, Texas.
Tornadoes
Tornadoes – smallest, most violent
weather disturbance on Earth.
Major storms
Tornadoes
Tornado alley
Hurricanes
• A tropical depression, a tropical storm and a
hurricane are all storms with strong upward
atmospheric motions and a cyclonic surface
wind circulation.
• They are born over tropical or subtropical waters
and are not associated with a weather front.
• The varieties of storm intensities are classified
according to the speed of the maximum
sustained surface winds.
Hurricanes
• A tropical depression is an area of low pressure around
which the winds are generally moving 35 mi/h or less. It
may dissolve into nothing or it might develop into a more
intense disturbance.
• At tropical storm is a more intense low pressure area
with winds between 35-75 mi/h.
• A hurricane is a very intense low pressure area with
winds greater than 75 mi/h.
• It is only called a hurricane if it occurs over the Atlantic
Ocean or the Pacific Ocean east of the international date
line.
• It is called a typhoon if it occurs over the North Pacific
Ocean west of the international date line.
Hurricanes
• A fully developed hurricane has heavy bands of clouds,
showers, and thunderstorms that rapidly rotate around a
relatively clear, calm eye.
• As the hurricane approaches a location the air seems
unusually calm as a few clouds appear, then thicken as
the wind begins to gust and rain showers occur
accompanied by thunderstorms and perhaps tornadoes.
• The winds die down right after the strongest winds occur
as the eye passes by. The eye is an average of 6-9 mi
and takes about an hour to cross a location.
• After the eye has passed the intense rain showers and
thunderstorms begin again, this time blowing from the
opposite direction.
• Hurricanes usually move at a rate of 10-30 mi/h.
Fig. 23.20
Hurricane John-Counterclockwise motion
Climate
• Changes in the atmospheric conditions over a brief
period of time are referred to as changes in the weather.
• Weather changes follow a yearly pattern of seasons that
are referred to as climate.
• Climate determines what type of vegetation grow in a
region.
• Oranges, grapefruits and palm trees grow in a region
that has a climate with warm monthly temperatures
throughout the year.
• Blueberries, aspen, and birch trees grow in a region that
has cool temperature patterns throughout the year.
• Climate determines what types of plants and animals live
in a location and the types of houses people build and
the lifestyles of the people.
• Climate also influences the processes that shape the
landscape and the type of soils that form.
Major Climate Groups
• Organization of the earth according to latitudes:
1. Low latitudes-Those that some time during the year
receive solar radiation at noon. They receive a high
amount of incoming solar radiation that varies little
during the year. Temperatures are high throughout the
year and vary little from month to month. This is the
tropical climate zone. It includes latitudes between
the Tropic of Cancer (23.5oN) and the Tropic of
Capricorn (23.5oS).
This zone is near the equator and receive the greatest
amount of sunlight throughout the year, so the overall
climate is hot. The average monthly temperatures are
above 64oF.
Major Climate Zones
2. High altitudes- Those that some time during the year
receive no solar radiation at noon. They receive a
maximum amount of radiation during one part of the
year and none during another part. Overall
temperatures are low, and there is a large range of
annual temperatures. This is called the polar climate
zone and it occurs north of latitude 66.5oN or south of
latitude 66.5oS.
The sun never sets here during some summer days and
never rises during some winter days. The overall
climate is cold and the average monthly temperature
stays below 50oF even during the warmest month of
the year.
Major Climate Zones
3. Middle Latitudes-Between the low and the high
latitudes. They receive a higher amount of
incoming radiation during one part of the year
and a lower amount during the other part.
Overall temperatures are cooler than in the low
latitudes and have a wide seasonal variation.
This is the temperate climate zone and is
between 23.5oN and 66.5oN and between
23.5OS and 66.5OS.
The temperature is neither very cold or very hot,
the average monthly temperature stays between
50oF and 64oF throughout the year.
Precipitation and Latitudes
• Air is forced up around the equator because it is heated most by the
sun.
• It is forced upwards, expands, cools to the dew point, and produces
abundant rainfall during the year.
• Air then sinks over 30oN and 30oS, warms up and becomes warmer
and dry as it is compressed.
• Most of the great deserts of the world are near 30oN and 30oS.
• The air that sinks at 30oN and 30oS moves to a great extent towards
the equator, but some of it moves north or south towards 60oN and
60oS. It is met here by polar air from the north or south. This air will
be warmer than the polar air and will rise, since it is less dense. As it
rises it cools and can hold less water vapor, so as a result
precipitation occurs.
• Therefore, there are wet zones near the 60oN and 60oS latitudes.
• The air will then move north and south from 60oN and 60oS and that
which moves north and south towards the poles will be dry air.
• There dry zones near the poles.
• The wet and dry zones shift north and south during the year with
the changing seasons.
Global patterns of precipitation
Regional Climates
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4.
Four factors affecting regional climate:
Altitude
Mountains
Large bodies of water
Ocean currents
Regional Climates
1. Altitude:
• The atmosphere is warmed mostly by the
greenhouse effect from the surface upward and
air at higher altitudes increasingly radiates
more and more of its energy to space.
• Average air temperatures decrease with
altitude. Tops of mountains are often covered
with snow.
• St. Louis and Denver at similar latitudes but the
altitude in Denver is 5280 ft whereas St. Louis
is 465 ft. The average temperature in Denver is
about 50oF and in St. Louis is about 57oF.
Regional Climates
2. Mountains:
• Mountains affect the condition of passing air
mass.
• The western US has mountainous regions
along the coast. When a moist air mass from
the Pacific meets these mountains it is forced
upwards and cools.
• The western slopes of the mountains are moist
and have forests of spruce, redwood and fir
trees. The eastern slopes are dry and have
grassland or desert vegetation.
Climate
Regional climate groups
Rain Shadows, regions of little or no precipitation.
Climate
Regional climate groups
Lake Tahoe, Nevada; west of
the Sierra Nevada
mountains.
Carson City, Nevada; east of
the Sierra Nevada
mountains.
Regional Climates
3. Large bodies of water:
• Temperatures are relatively even from day to night and
from summer to winter.
• San Diego and Dallas are about the same latitude but
San Diego is a t a seacoast and Dallas is inland. San
Diego has an average summer temperature about 13oF
cooler and an average winter temperature about 9oF
warmer than the average temperature in Dallas.
• The northern hemisphere is about 39% land and 61%
water and has an average yearly temperature range of
about 25oF.
• The southern hemisphere is about 19% land and 81%
water and has an average yearly temperature of about
13oF.
• It can be seen that nearness to large bodies of water
influences climate.
Climate
Regional climate groups
San Diego, California
Dallas, Texas
Oceans (or other large bodies of water) minimize large
temperature fluctuations.
San Diego’s summer is 13°F cooler and winter is 9°F
warmer than Dallas.
Climate
Regional climate groups
Northern hemispheric temperature range is 25°F while
the Southern is 13°F.
Regional Climates
4. Ocean Currents:
• Currents in the ocean can bring water that has
different temperature than the land.
• Currents can move warm water northward or
they can move cool water southward. This can
influence the temperatures of air masses that
move from the water to the land and thus the
temperatures of the land.
• The North Pacific current brings warm waters to
the western coast of North America which results
in warmer temperatures for cities near the
western coast of US like Seattle.
Ocean currents affect local climates. U.S. eastern coast, warm ocean currents
travel northward. U.S. western coast, cold currents travel southward.
Describing Climates
• There are no sharp boundaries that exist
naturally between two adjacent regions
with different climates.
• The climates blend gradually into one
another and no two places on earth have
exactly the same climate.
Major Climate Zones
1. Marine Climate:
• Area near the ocean that is influenced by air
masses from the ocean.
• Marine climates have mild winters and cool
summers compared to areas further inland.
• They have abundant precipitation.
• The western coast of Canada and the
northwest coasts of Washington and Oregon
and northern California have marine climates.
• They are covered with forests of spruce, fir,
and other conifers.
Major Climate Zones
2. Continental Climate:
• The temperatures are not even. The
summers are hot and the winters are cold.
3. Other ways to describe climate:
• Arid means dry, humid means moist. In
between is semiarid and they are all
based on the amount of yearly
precipitation.
Climate
Describing climates
• Microclimates – other factors affecting
local climate.
– Cities (heat island effect)
– Smog
– Trees
– Etc.
Review Exercises
• P. 565-566 Applying the Concepts:
# 1, 2, 3, 4, 5, 10, 12, 13, 14, 15, 17, 18, 19,
20, 21, 22
New Book: p. 622-625 # 1, 2, 3, 4, 5, 6, 7, 8,
13, 14, 15, 18, 19, 22, 27, 29, 30, 31, 32,
35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,
47.
Summary Chapter 23
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The water cycle: water evaporates due to greater kinetic energy of
particles with latent heat of vaporization. Remaining water gets cooler.
The water that evaporates cools as it rises in the atmosphere due to
expansion and the air becomes saturated and condensation occurs.
Clouds are formed. Eventually the water returns to the earth as
precipitation.
The hydrologic cycle: More water evaporates from oceans than is
returned as rain. Deficit is from streams and rivers flowing into the
ocean.
After tiny droplets condense in the atmosphere and form clouds
condensation nuclei are needed before larger droplets can form.
White clouds-You are between the sun and the clouds. Grey cloudsThe clouds are between you and the sun.
Dew and frost-not precipitation. Rain, hail, snow-precipitation.
Near ocean-clouds form water droplets around large salt crystals and
precipitation occurs. Away from oceans clouds are higher and ice
crystals grow; for these precipitation is liquid in summer, could be solid
if winter.
Cumulus clouds (puffy)-shorter rain showers; stratus clouds (layered)longer rain drizzle or snow; cirrus clouds (wispy, high altitudes)-no rain
showers.
Cumulus and stratus clouds are of low and middle altitudes. The prefix
nimbo or the suffix nimbus means rain. Prefix alto means middle
altitudes.
Close to the equator the earth receives more sunlight and the air is
warmer, rises, then sinks back towards the surface at around 30oN and
S. Part of the air then comes back towards areas close to the equator
and part towards the poles. Low pressure over equator, high pressure
over 30oN and S.
Changing weather is attributed to: movement of air masses, fronts of
air masses, high and low pressure systems.
Air masses-Continental polar (cold, dry), continental tropical (warm,
dry), maritime polar (moist, cold), continental tropical (dry, warm).
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A front is the boundary between air masses of different temperatures
and they don’t mix-density of cold air higher than warm air. Shown as a
bulging line on weather map. Bulge is in the direction of movement of
front.
Cold fronts produce high P systems and result in cumulus clouds and
thunderstorms. Warm fronts produce low sloping fronts and cirrus
clouds followed by stratus clouds which produce long periods of fog,
drizzle and rain.
Stationary fronts produce major storms. Cold and warm fronts moving
in opposite directions.
Thunderstorms develop in very warm, moist and unstable air. Often
July afternoons. Thunder, lightning, strong winds, rain, hail generated.
Different parts of a thunderstorm cloud develop centers of electric
charge. Lightning is a giant electric spark that discharges the
accumulated charges.
Tornadoes are the most powerful weather disturbances on earth.
Occur in great plains states-tornado alley.
A tropical depression (<35mph winds), a tropical storm(>35,<75mph
winds) and a hurricane(>75mph winds) are all storms with cyclonic
surface wind circulation. Occur over tropical or subtropical waters and
are not associated with a weather front. Storm intensities according to
the speed of the maximum sustained surface winds.
Called a hurricane if over the Atlantic Ocean or the Pacific Ocean east
of the international date line. Called a typhoon if it occurs over the
North Pacific Ocean west of the international date line.
Climate-weather following a yearly pattern of seasons. Low latitudestropical, middle altitudes-(between tropic of cancer and arctic circle or
tropic of capricorn and antarctic circle)-temperate, high altitudes-polar.
Dry climate 30oN and S and poles, wet climates equator and 60oN and
S.
Four factors affecting regional climate: altitude (T decreases),
mountains (W of Rockies-wet, E of Rockies-dry), large bodies of water
(Southern hemisphere, more water, less T changes), ocean currents
(warmer water upwards, cooler water downwards, moderate
temperatures near the coast) .
Ocean currents give rise to marine (warmer winters and cooler
summers) vs. continental climates (colder winters, hotter summers).
Arid, humid and semiarid are based on amounts of precipitation.