Chapter 16 Section 1: part 1
Water in the Air
Water can exist in three forms: solid, liquid, and gas. An example of water as a solid would be ice found in
clouds as snowflakes. As a liquid, water exists in clouds as water droplets. And water as a gas is water vapor.
Water in the air affects the weather. Weather is the condition of the atmosphere at a particular time and place.
Water Cycle
Water amount
Vs.
Temperature
Water in all three forms is constantly changed through a process
known as the water cycle. The water cycle is the continuous movement of
water from sources on the ground like lakes and oceans, into the air, onto and
into the ground, and back to the original water sources from evaporation,
condensation, precipitation, and runoff.
Humidity:
Humidity is the water vapor or moisture in the air. As water
evaporates from various sources, the amount of humidity increases.
But the air’s ability to hold water vapor depends on temperature. As
temperature increases, the air’s ability to hold water vapor also
increases. The summer is when we usually complain that is hot and
humid outside. See the chart at the left and/or figure 3 on page 423.
The relative humidity is the amount of moisture the air
contains compared with the amount it can hold at a particular
temperature. It is given as a percentage and is the one you have
been recording in your weather journal. When the air holds as much
as it can at a given temperature, the air is said to be saturated.
Saturated air has a relative humidity of 100%. How do you find the
R.H. of the air if the air is not saturated? If you know how much it
can hold, and how much it is presently holding, you can calculate
the R.H. Divide the amount of moisture currently in the air by
how much it can hold and then multiply that by 100 . See below.
R. H. formula = amt. of water at present
amt. of water at saturation
x 100%
= R.H. %
If you don’t know the moisture levels in the air, measuring the R.H. can be done with a devise called a
sling psychrometer. This instrument consists of two thermometers. One thermometer is called the dry-bulb, while
the other (with a wet cloth over it) is called the wet-bulb. The two thermometers are swung around in the air. As
air passes over the one with the wet cloth, the water in the cloth begins to evaporate. Evaporation causes cooling.
If there is very little humidity in the air, evaporation off the cloth happens readily. If the air is already humid, there
is less evaporation and less temperature change. A larger difference in the two readings indicates a lower humidity.
Along with the two temperature readings, a special chart is used to determine the relative humidity. To see an
actual psychrometer and chart, and learn how to use it, turn to page 424.
Condensation:
You have probably seen water droplets form on the outside of a glass of ice water. Where did the water
come from? It came from the air, and it formed because of condensation. Condensation is the process where a
gas, like water vapor, turns into a liquid. Before this can happen, however, the temperature of the air must be
cooled to a point beyond where it becomes saturated (R.H.=100%). The temperature beyond which saturation
occurs is called the dew point. The dew point is the temperature to which air must be cooled to be completely
saturated. With additional cooling, fog will develop or some form of precipitation will fall.
Chapter 16 Section 1-1:
Answer the following questions searching the reading assignment when appropriate.
__________1. The water vapor or moisture in the air is called ___ .
__________2. The ____ is the temperature to which air must be cooled to be completely saturated.
__________3. A devise to help measure the relative humidity is called a _____ .
__________4. When the air holds as much water vapor as it can, the air is said to be _____ .
__________5. The _______ is the continuous movement of water from the ground, to the air, and back to the
ground through the processes of evaporation, condensation, and precipitation.
__________6. The condition of the atmosphere at a particular time and place is called the _____ .
__________7. Name the three forms that water comes in.
__________8.
__________9.
_______%_10. If the air is holding as much water vapor as it can, what is the relative humidity?
__________11. Calculate the R.H. of the air if it is presently holding 5 g. of water per m3, but can hold 10 g.
Write out the formula, substitute the numbers into the formula, divide, and times by 100.
R.H. = ____________
_____________ x 100% = __________
Finish writing out the formula from the previous page
__________ x 100 % = ______ %
Substitute the numbers in and divide
Times by 100
__________12. In the summer is when it usually gets hot and ____ outside.
_______%_13. What is the R.H., using the chart below, for a dry-bulb of 600 and a wet-bulb of 500 ?
To determine relative humidity with the chart:
1 Subtract the wet-bulb temperature from the dry-bulb temperature.
2 Find that number (the difference in degrees) at the top of the chart and place your right index finger on it.
3 Find the dry-bulb temperature in the first column on the left. Place your left index finger on it.
4 Bring your fingers down the column and across the row.
The relative humidity (R.H.) percentage appears where the column and row intersects within the chart.
Chapter 16 Section 1: part 2
Clouds:
A cloud is made of millions of tiny water droplets or ice crystals. Clouds form as warm, moist air rises and
cools with altitude. At cooler temperatures, the air’s ability to hold moisture decreases. Cooling continues to a
certain temperature where the amount of moisture in the air reaches full capacity, called the dew point. At this
temperature, the invisible water vapor particles join and grow into a visible liquid or solid depending on the season
and the air temperature. At higher temperatures, the water vapor condenses to form droplets, clouds, and possibly
raindrops. At temperatures below freezing, the water vapor changes to ice crystals that eventually combine to make
snow flakes.
Clouds are classified by altitude and by shape. There are three cloud types based on altitude: low, middle,
and high. Low clouds are made up of water droplets and so are usually gray in color. The prefix strato- is
commonly used in the names of these clouds. Middle levels clouds can be made up of water droplets and ice
crystals and so can be gray and white as a result. The prefix alto- is commonly used when naming these clouds.
High clouds, because of the cold temperatures at high altitudes, are made up of ice crystals and are usually white in
color. The prefix cirro- is commonly used when naming these clouds.
There are also three cloud types based on shape. Cumulus is Latin for “heap”. Cumulus clouds are white,
puffy clouds. Usually, the weather is nice when these clouds are wondering around. Stratus is Latin for “spread
out”. Clouds that form in long layers and spread out over the entire sky are called stratus. This situation would
seem as though a dark, gray blanket has been pulled across the sky. You would not be able to locate the sun. When
a stratus cloud forms on the ground, it is fog. Cirrus is Latin for “curly”. Thin, feathery clouds are called cirrus
clouds. They appear wispy or have curled up ends because of strong winds blowing the ice crystals around. When
you see cirrus clouds, it won’t be long before the weather turns for the worse, however.
Cloud names are combinations of shape and altitude:
cirrus-thin, feathery wisps, cirrocumulus-small,high puffs,
cirrostratus-looks like a fine white veil pulled across the sky-can
still see the outline of the sun, altocumulus-small, higher level puffs,
altostratus-looks like a gray/white sheet pulled over the atmospherecan see where the sun might be, stratus-looks like a gray blanket
pulled over the atmosphere-not able to locate the sun,
stratocumulus-some breaks in the stratus clouds, nimbostratus-if
snowing or raining. When nimbus- or nimbo- is used in the name, it
is associated with precipitation falling.
A cumulus cloud looks like large, lower level white puffs. It
is also called “fair weather cumulus”. It can grow vertically into a
huge pile of clouds or “towering cumulus” or “thunderhead”,
however. When a thunderstorm is produced, the clouds making the
cold, pouring rain and lightning are called cumulonimbus.
Precipitation:
Water vapor, that condenses to form cloud droplets and clouds, can eventually grow larger and heavy
enough to fall to the ground as precipitation. Precipitation is water in solid or liquid form, that falls from the air to
the Earth. There are four major forms-rain, snow, sleet, and hail.
Rain is the most common form of precipitation. A cloud droplet must become 100 times its normal size to
become large enough the fall as precipitation. The most common form of solid precipitation is snow. Snow forms
when the temperature in the cloud is so cold that the moisture changes directly to a solid. Snow falls as individual
ice crystals or combine to form flakes. Sleet, also called freezing rain, forms when rain falls through a layer of air
below freezing. Hail is solid precipitation that falls as clumps or balls of ice after raindrops get thrown around and
into the upper part of a thunderhead. The drop freezes, falls, collects more moisture, and gets raised upward to
freeze again. If the upward movement within the cloud is strong enough, a hailstone can accumulate many layers
of ice. Eventually, it will become too heavy and fall to the ground.
Precipitation is measured in a device called a rain gauge. If the precipitation is in the form of snow, the
gauge would have to be heated to melt the snow into water first. See figure 13 on page 429.
Snow forms when the temperature in the cloud is so cold
that the moisture changes directly to a solid. Snow falls as
individual ice crystals or combine to form flakes.
Precipitation is measured in a device called a rain gauge.
If the precipitation is in the form of snow, the gauge would have
to be heated to melt the snow into water first.
A cloud droplet must become 100 times its normal size to become large enough
the fall as rain. If, before falling, the rain drops are lifted into the upper parts of the
thundercloud and freeze, it is called hail. Repeated lifting makes the hailstones bigger.
Chapter 16 Section 1-2:
Answer the following questions searching the reading on the previous page.
__________1. Cooling continues to a certain temperature where the amount of moisture in the air reaches
full capacity, called the ____ ____.
__________2. A ____ is made of millions of tiny water droplets or ice crystals.
__________3. Clouds are classified and by ____ and by shape.
__________4. There are ___ cloud types based on shape.
__________5. Thin, feathery clouds are called ____.
__________6. Clouds that spread out and form long layers covering the entire sky are called ____.
__________7. ____ clouds are white, puffy clouds.
__________8. When a stratus cloud forms on the ground, it is called ___.
__________9. During a thunderstorm, the clouds making the rain and lightning are called ____.
__________10. ______ is water falling in either solid or liquid form.
__________11. A cloud droplet must become ___ times its normal size to become large enough the fall.
__________12. The most common form of solid precipitation is ____.
__________13. Precipitation is measured in a device called a ____ gauge.
Chapter 16 Section 2:
Air Masses and Fronts
Air Masses:
Changes in the weather are caused by the movement of air masses. An air mass is a large
body of air with the same properties (temperature and pressure) as the area it formed over. These
areas are called source regions.
They are represented on maps with a two letter symbol. The first letter indicates the moisture
level of the air mass, and the second letter represents the temperature of the air mass. The map
at the left shows the source regions for the air masses that influence North America.
There are four source regions.
mT= Maritime Tropical-rainy, forms over the warm waters of the Tropics.
mP= Maritime Polar-snowy, forms over the cold waters of the polar regions.
cP = Continental Polar-dry, forms over land in the polar regions.
cT = Continental Tropical-hot and dry, forms over land in hot desert-like regions.
Cold Air Masses:
Most of our winter weather is caused by the polar air masses. Continental polar air from
over the middle of Canada, would cause extremely cold temperatures in the winter and cool, dry
conditions in the summer. Maritime polar air masses from over the Atlantic and Pacific, bring
snow in the winter and cooler, rainy weather in the summer.
Warm air masses:
Most of our summer time weather is influenced by the tropical air masses from the south.
Maritime tropical air masses from over the Gulf of Mexico, the Atlantic and Pacific Oceans, bring
warm temperatures, rain, and thunderstorms. The continental tropical air mass that forms over
land in Mexico and the deserts of the U.S., bring hot, dry conditions to the middle of the country.
Fronts:
Air masses do not usually mix. So, when two different air masses meet, a boundary is
formed. The boundary between two air masses is called a front. Weather at a front is usually
cloudy and stormy. There are four different fronts- Cold, Warm, Stationary, and Occluded.
When a cold air mass meets a warm air mass, a cold front forms. The cold air wedges under
and lifts the warm air upwards. Cold fronts can move fast, push the warm air up violently, and
produce thunderstorms. When a warm air mass meets and overrides a cold air mass, a warm front
forms. The warm air moves up and over the cold and generally brings drizzly precipitation. If a
fast-moving cold air mass overtakes a slower-moving warm front and then continues advancing and
catches another cold front, an occluded front forms. An occluded front has cool weather with a
lot of precipitation. When a cold air mass and a warm air mass parallel each other with little
horizontal movement, a stationary front forms. Weather is similar to a warm front, although it
may possibly last a longer time.
Alto
Cold air
A fast moving cold air mass sharply lifts warm air.
A cold front is formed.
Warm air overrides cold air in a warm front.
Warm front
Warm front
When warm and cold air show parallel or little
horizontal movement, a stationary front forms.
Occluded fronts occur when cold or cool air
catches up to a warm front.
Questions Section 16-2: Answer the following questions searching the reading on the previous page.
__________1. Changes in the weather are caused by the movement of ___ ____.
__________2. An air mass is a large body of air with the same _____and pressure as the area it formed over.
__________3. There are ___ source regions.
__________4. Continental ____air is formed over the middle of Canada.
__________5. Maritime _____ air masses bring warm temperatures, rain, and thunderstorms.
__________6. When a cold air mass collides with a warm air mass, a ___ front forms.
__________7. When a warm air mass overrides a cold air mass, a ___ front forms.
__________8. The symbol for a maritime polar air mass is ___ .
__________9. There are ___ different kinds of fronts.
__________10. The boundary between two air masses is called a ____.
Chapter 16 Section 3:
Severe Weather
Thunderstorms:
Weather changes from day to day. These changes result from the continual shifting of air masses (large
bodies of air having the same moisture and temperature properties as the area they come from). Sometimes severe
weather will develop along the front or leading edges of those air masses.
Thunderstorms are small, intense weather systems that produce strong winds, heavy rain, lightning, and
thunder. Thunderstorms usually occur along the leading edges of cold air masses or cold fronts. Thunderstorms
occur when warm, moist air rises rapidly. When the cooling nears the dew point temperature, cumulus clouds
develop. With further rising dark, cumulonimbus clouds form, and severe weather can result. Severe
thunderstorms produce high winds, hail, and even tornadoes.
Thunderstorms are very active electrically.
Lightning is a large electrical discharge that occurs
between two oppositely charged surfaces or areas.
Lightning within a cloud is most common, but strikes
also occur between clouds, and from clouds to the
ground.
Lightning occurs with all thunderstorms. When
lightning strikes, energy is released. This energy
causes the air around the bolt of light to rapidly expand
and send out shock waves. Thunder is the sound we
hear from the rapid expansion of air along a lightning
strike.
Tornadoes:
Tornadoes are produced in about 1% of all thunderstorms. A tornado is a small, violent, rotating column of
air that has high wind speeds and touches the ground. About 75% of the world’s tornadoes occur in the U.S., the
majority in the spring and early summer, and frequent
the midwestern part of the country. Wind speeds can
get to 300 mph, but average between 75 and 110 mph.
The spinning winds can cut a path of total destruction 5
miles long and 100 yards wide.
Tornadoes are produced when winds traveling is
two different directions cause a layer of air in between
to rotate like a roll of toilet paper. Next, the rotating
cylinder of air is turned upright into a vertical position
by strong updrafts of air within the cumulonimbus
cloud. Afterwards, the rotating column works its way
down to the bottom of the cumulonimbus cloud and
forms a funnel cloud. Finally, the funnel cloud is called
a tornado when it touches the ground.
Hurricanes:
A hurricane is a large, rotating tropical weather system with wind speeds from at least 74 mph to over 180
mph. Hurricanes are the most powerful storms on Earth because they cover vast areas of land, sometimes ranging
in size from 100-900 miles across. They are identified differently in other parts of the world, typhoons in the
western Pacific, and cyclones over the Indian Ocean. In the 1950’s, meteorologists began assigning them names.
Hurricanes generally form over the warm, tropical oceans between 5o and 20o north and south latitude.
They begin as a group of thunderstorms moving over tropical ocean waters. Winds traveling in two different
directions collide, causing the storm to rotate. Because the Coriolis Effect, storms rotate ccw in the northern
hemisphere, and cw in the southern hemisphere.
Hurricanes get their energy from the condensation of water vapor. Once formed, they are fueled through
contact with the warm ocean water. Moisture is added by evaporation. As the warm, moist air rises, the water
vapor condenses, releasing large amounts of energy. They continue to grow as long as they are over warm water.
As they move over cold water or over land, they die off.
At the center of a hurricane is the eye. The air in the
eye is calm. As the eye passes over, a small circle of clear
sky may be visible overhead for a short period of time. This
Eye
lull ends when torrential rains again fall around the outside of
Updrafts
Downdrafts
the eye, and winds rage, drawn in from hot air spiraling up
the wall of the eye. The eye wall is the strongest part of the
hurricane.
Hurricane winds can knock down trees and buildings.
They can also cause coastal flooding from on-shore ocean
Rainbands
tidal surges and heavy rains.
Eyewall
Questions Section 16-3: Answer the following questions searching the reading assignment above.
__________1. The leading edge of an air mass is called the ___ .
__________2. Small, intense weather systems that produce strong winds, rain, lightning, and thunder are ____ .
__________3. Large bodies of air with similar temperature and moisture levels as the area they come from are __.
__________4. A large electrical discharge that occurs between two oppositely charged surfaces or areas is ____.
__________5. Lightning occurs with all _____ .
__________6. When lightning strikes, ____ is released.
__________7. The sound we hear from the rapid expansion of air along a lightning strike is called ____.
__________8. A ____ is a small, violent, rotating column of air that has high wind speeds and touches the ground.
__________9. A rotating column of air that works its way down to the bottom of the cumulonimbus cloud is a ___.
__________10. About ___% of the world’s tornadoes occur in the U.S.
__________11. A ____ is a large, rotating tropical weather system with wind speeds of at least 74 mph.
__________12. The most powerful storm on Earth is the ____ because it covers such a large area.
__________13. Hurricanes generally form over a warm, tropical ____ .
__________14. At the center of a hurricane is the ___ .
__________15. Hurricanes get their energy from the ____ of water vapor.
Chapter 16 Section 4:
Forecasting the Weather
At some point in your life, you have been caught off guard by the weather. Weather can
affect what you wear and what you do outside, so it can be important to get the weather forecast.
A weather forecast is a prediction of future weather conditions over the next 3 to 5 days.
Meteorologists factor in recent weather as well as observe and collect the current conditions in
order to make the predictions. In order to collect the current conditions outside, meteorologists
make a number of measurements.
Temperature:
A thermometer is a tool used to measure the air temperature. A common type uses a liquid
sealed in a narrow glass tube. When the temperature increases, the liquid expands up the tube.
When the temperature decreases, the liquids shrinks and moves down the tube. Globally,
temperature is given in degrees Celsius. In the U.S., however, temperature is generally reported
on the TV and radio in degrees Fahrenheit.
Pressure:
A barometer is an instrument used to measure the air pressure. A mercury
filled barometer is the most accurate. A barometer consists of a glass tube open
at one end. It is placed upside down in a container of mercury. Some of the
mercury drains out of the tube, but the air pressure, pushing down on the
mercury in the container, causes much of the mercury to stop draining out. The
greater the air pressure pushing on the container of mercury, the more the
mercury will stay in the tube. Scientists measure the height of the mercury in
the tube. In the U.S, barometric pressure is reported in inches of mercury.
Wind direction:
Wind direction is measured using a wind vane or
wind sock. It is a cone-shaped cloth bag open at both ends
attached to a pole. The wind enters the bag through the wide end and leaves
through the narrow end. Therefore, the wide end points into the wind.
A wind vane is shaped like an arrow with a large tail at one end,
attached to a pole. The wind pushes the tail, spinning it on the pole until the
arrow points into the wind.
Barometer
Wind Speed:
Wind speed is measured with a device called an anemometer. It
consists of three or four cups connected by spikes to a pole. The wind
pushes on the hollow sides of the cups, causing them to rotate on the pole.
The faster the wind, the faster the rotation.
Wind sock
Measuring conditions in the upper atmosphere:
Studying the weather at higher altitudes requires more complicated equipment. Weather
balloons are sent up with equipment to measure and transmit conditions back to operators on the
ground. Radar is also used to find the location, type, movement, and intensity of precipitation and
storms. Satellites orbiting the Earth also provide images and can measure wind speeds, humidity,
and temperatures and various altitudes.
The National Weather Service (NWS) collects, analyzes,
and reports the conditions. They make maps based on
information gathered from over 1,000 weather stations across
the U.S. On these maps, each station is represented by a small
circle or station model. The circle includes a set of symbols and
numbers surrounding it, which represents its weather data.
Weather maps also include lines called isobars that connect
points on the map with equal air pressure. Isobar lines that form closed
loops represent areas of high and low pressure. These areas are usually
marked with a capital H or L . Fronts are also symbolized on the maps.
Questions Section 16-4: Answer the following questions searching the reading assignment above.
__________1. A weather _____ is a prediction of future weather conditions.
__________2. A ______ is an instrument used to measure the air pressure.
__________3. A ______ is a tool used to measure the air temperature.
__________4. Temperature is generally reported on the TV and radio in degrees ______.
__________5. Barometric pressure is reported in ____ of mercury.
__________6. The _____ the air pressure pushing on the container of mercury, the more the
mercury will stay in the barometer tube.
__________7. When the temperature increases, the liquid ____ up the thermometer tube.
__________8. Wind direction is measured using a wind vane or wind ____ .
__________9. The wind pushes the tail of the wind vane, spinning it on the pole until the arrow
points ___ the wind.
__________10. Lines called _____ connect points on the map with equal air pressure.
__________11. Wind speed is measured with a device called an _______.
__________12. _____ orbiting the Earth also provide images and can make measurements.