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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.

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chapter 23

Chapter 23

Weather and Climate

cloud forming processes
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.

White clouds-When you are

between the sun and the

Clouds-You see reflected

sunlight from the cloud.

  • 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:
  • 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.
  • 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
Precipitation and Clouds
  • The basic form of a cloud has meaning about the general type of precipitation that can occur and also the types of weather:
  • Cumulus clouds usually produce showers or thunderstorms that last only brief periods of time.
  • Stratus clouds lead to longer periods of drizzle, rain, or snow.
  • 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
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
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 producers12
Weather Producers
  • Changing weather is attributed to:
    • Movement of air masses
    • Fronts of air masses
    • High and low pressure systems
air masses
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 masses14
Air Masses
  • 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

3. Continental tropical (cT) -Dry, warm air

4. 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
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 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

  • Warm air forced to rise
  • Air saturates
  • Cumulus clouds & precipitation.
warm fronts
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
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
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.



Lightning heats, expands atmosphere creating large pressure wave - thunder.

other major storms
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.


Tornadoes – smallest, most violent weather disturbance on Earth.


Major storms


Tornado alley

  • 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.
  • 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.
  • 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.
  • 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
Major Climate Groups
  • Organization of the earth according to latitudes:
  • 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
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 zones44
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
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.
regional climates
Regional Climates
  • Four factors affecting regional climate:
  • Altitude
  • Mountains
  • Large bodies of water
  • Ocean currents
regional climates50
Regional Climates
  • 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 climates51
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.


Regional climate groups

Rain Shadows, regions of little or no precipitation.



Regional climate groups

Carson City, Nevada; east of the Sierra Nevada mountains.

Lake Tahoe, Nevada; west of the Sierra Nevada mountains.

regional climates54
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.


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.



Regional climate groups

Northern hemispheric temperature range is 25°F while the Southern is 13°F.

regional climates57
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
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 zones60
Major Climate Zones
  • 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 zones61
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.


Describing climates

  • Microclimates – other factors affecting local climate.
    • Cities (heat island effect)
    • Smog
    • Trees
    • Etc.
review exercises
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
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 clouds-The 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).

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 latitudes-tropical, 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.

Summary Chapter 23