Download Background: What is Weather?

8.10B: Weather Maps
Earth and Space
Background: What is Weather?
Weather is the day-to-day state of the atmosphere. The interaction of three important factors
result in weather systems: air temperature, air pressure, and the amount of moisture in the air
(relative humidity.) All of the other things that we associate with “weather conditions,” like how
much and the type of precipitation that occurs, wind direction and speed, and cloud cover, are
caused by the interactions between temperature, pressure, and humidity in the atmosphere.
In the Engage activity, you were introduced to instruments used by a meteorologist (a scientist
who studies weather) as shown on the chart below:
Weather Factor
Instrument Used for
Measurement
Air Temperature
Thermometer
Air Pressure
Barometer
Relative Humidity
Sling Psychrometer
Wind Speed and
Direction
Anemometer and Wind
Sock or Weather Vane
Precipitation
Rain Gauge
Pictures of Instruments
Of course more sophisticated digital instrumentation can be used to measure these weather
factors. Weather satellites and balloons, airplanes, and ocean buoys are examples of
instrumentation used to record weather data from all around the globe. Data collected for
temperature, pressure, humidity, wind speed, and the type and amount of precipitation is used
to study weather systems and make predictions about weather conditions. These factors are
different all around the world, but they cause world-wide patterns that can affect the local
weather. In these Explore activities, you will learn about factors that determine the weather.
You will also see how they are demonstrated using a weather map.
Complete the Background questions in your Student Journal.
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8.10B: Weather Maps
Earth and Space
Part I: Air Lift
You have learned that radiant energy is absorbed by Earth’s waters and landmasses. The
absorption of the radiant energy will not occur equally because of the varying surfaces on
Earth. This causes global changes in temperature patterns. The equatorial region receives
more direct radiant energy as compared to other areas on the planet, so convention currents
result. These convection currents cause global wind patterns with the warmer air rising and
cooler air sinking. This recurring pattern of uneven heating will happen at other latitudes
causing convection in these locations. The global winds formed from this influence weather all
over Earth. Differences in air temperatures will cause differences in air pressure resulting in the
formation of wind. The pressure differences can cause local to global winds.
Air is matter. It is made of atoms, has mass, and has
volume. Therefore, the matter in the atmosphere is actually
pulled down on Earth’s surface due to the force of gravity
and causes pressure. The more matter present in an area,
the higher the air pressure. The less matter present in an
area, the lower the air pressure. Human bodies are able to
handle air pressure although the only time you might notice it
is when your ears “pop” as they adjust to a change in
atmospheric pressure. This can occur when you go to higher
elevations where you have less air pressure.
Your body self-regulates to the different “weight” of the air above you. Weather throughout
Earth also changes, depending on the atmospheric pressure.
In this activity you investigate the meaning of air pressure.
1.  Take a flat board, and lay the board on a table so that 16 inches of the
board sticks out from the table.
2.  Lay a large sheet of chart paper, open and flat, on top of the part of the
board on the table. Smooth out the paper, and make it as flat as
possible.
3.  Predict what will happen if you hit the part of the board sticking out
from the table. Write your prediction in your Student Journal.
4.  Hit the board, and record the results in your Student Journal.
5.  Use a yardstick to find the surface area of the chart paper. Use the
value to complete Part I of your Student Journal.
6.  Lastly, observe the current air pressure on the classroom barometer,
and record it in your Student Journal.
Complete Part I in your Student Journal.
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8.10B: Weather Maps
Earth and Space
Part II: High and Low Air Pressure Systems
In the last activity, you learned that air pressure results from the force of gravity that pulls the
atmosphere toward Earth’s surface. A HIGH pressure area has more air pressure because the
atmosphere above is denser. Dense, high pressure systems are represented on weather maps
with a capital H. A LOW pressure area has lower air pressure because the atmosphere above
is less dense. Less dense, low pressure systems are represented on weather maps with a
capital L.
Wind Movement and Conditions in Low Pressure Areas
Air is pulled into a LOW pressure area and lifts higher into the atmosphere. When the air rises
to higher altitudes it cools. Low pressure systems in the Northern Hemisphere have a
counterclockwise circulation pattern. The cooling causes any moisture in the air to condense
and form clouds with possible precipitation.
Low pressure areas are associated with stormy weather conditions. The more moisture in the
air that spirals upwards in a low pressure system, the more likely it is that precipitation will
occur as rain, sleet, or snow.
Please continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part II: High and Low Air Pressure Systems, continued
Wind Movement and Conditions in High Pressure Areas
Air sinks in HIGH pressure areas and spreads out slowly. The spreading air tends to move
toward neighboring areas of lower pressures.
High pressure areas are associated with fair weather and mostly clear skies. Most high
pressure systems are large and move slowly. High pressure systems in the Northern
Hemisphere have a clockwise rotation.
L
H
Complete Part II in your Student Journal.
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8.10B: Weather Maps
Earth and Space
Part II: High and Low Air Pressure Systems, continued
Look at the map set below. On the left is a satellite image of Texas that shows cloud cover.
The center of the high pressure system and the low pressure system are indicated on the
image and the corresponding outline map with L and H.
L
L
H
H
Map View of Texas
Satellite View of Texas
Part III: Air Masses and Fronts
Wind is a result of temperature and pressure differences. Wind is generated within a low
pressure system as the air spirals upward and within a high pressure system as denser air
sinks. Wind also results when air moves from a HIGH pressure area toward a LOW pressure
area. Molecules of air move directionally from areas of HIGH to LOW pressure, just as thermal
energy always transfers from substances of high temperatures to low temperatures, never in
the opposite direction.
Complete Part II in your Student Journal.
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8.10B: Weather Maps
Earth and Space
Part III: Air Masses and Fronts
Wind is a result of temperature and pressure differences. Wind is generated within a low
pressure system as the air spirals upward and within a high pressure system as denser air
sinks. Wind also results when air moves from a HIGH pressure area toward a LOW pressure
area. Molecules of air move directionally from areas of HIGH to LOW pressure, just as thermal
energy always transfers from substances of high temperatures to low temperatures, never in
the opposite direction.
Global High and Low Pressure Areas
Photo courtesy of NOAA
Because Earth's shape is spherical, not flat, the surface heats unevenly and causes global wind
belts. The area near the equator receives the most direct rays from the Sun. This area gets
warmer and causes surface air to rise and form a low-pressure zone. At about 30° north and
south, the air sinks toward the surface and forms a set of subtropical highs which are large
bands of high surface pressure. The Coriolis effect forces these convection cells of sinking air
to deflect in both hemispheres causing two bands of global wind movement called the northeast
and southeast Trade Winds (convection cell 1 on the image). Another set of convection cells
begins at about 90° north and south, where surface air rises, then moves toward and sinks at
about about 30° north and south. The Coriolis effect forces these sets of sinking air in the
convection cells to deflect in both hemispheres causing two bands of global wind movement
called Westerlies (convection cell 2 on the the image). A final set of convection cells begin at
about 90° north and south where warmer air rises, then cools and sinks as it moves toward the
poles. The Coriolis effect forces these sets of sinking air in the convection cells to deflect in
both hemispheres causing two bands of global wind movement called Polar Easterlies
(convection cell 3 in the image). These three sets of convective wind movements are semipermanent and strongly influence the climate and daily weather in the regions in which they
blow.
Continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part III: Air Masses and Fronts, continued
What is an air mass?
An air mass can be huge, as big as the size of a continent, and these can be thought of as
almost being on a global level. Temperature and moisture content are nearly the same
throughout the entire air mass. Air masses tend to form in high pressure areas, where stable
conditions allows the air mass to remain in one location long enough to take on the
characteristics of the surface below. For example, when an air mass forms over an ocean,
water evaporates into the atmosphere, and the air becomes moist. When an air mass forms
over land, the air is typically dry. Air masses that form over the warm surfaces of tropical
latitudes are warmer than those that form over surfaces nearer to the poles. So, air masses
can take on a combination of characteristics: cool or warm, dry or moist. Typical air masses
that form and affect weather patterns in the United States are shown on the map below. Air
masses that form over ocean surfaces are labeled as “Maritime,” and those that form over land
surfaces are labeled “Continental” on the map.
Continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part III: Air Masses and Fronts, continued
These large air masses move slowly away from where they formed. Global wind patterns and
the jet stream (narrow bands of very fast moving wind located in the highest part of the
atmosphere) affect the movement of these air masses. An air mass has nearly uniform
moisture content and temperature throughout, so when it moves to a new area, it brings along
its moisture and temperature characteristics. Over time, as the air mass moves over new
surfaces and blends with other air masses, the original characteristics can change.
Boundaries between air masses are called fronts; they are zones of transition between the
very different characteristics of the two air masses. Fronts extend from low pressure systems
because air flows into a low pressure area. Satellite images show the cloud cover that forms
around the low pressure systems and along the edge where the two air masses meet. Weather
conditions can get interesting at fronts due to interactions between different temperatures,
pressure changes, and moisture content of the air masses.
There are four kinds of fronts:
1. A warm air mass pushes a cooler air mass – a warm front
2. A cool air mass pushes a warmer air mass – a cold front
3. A warm air mass and a cool air mass both push against each other – a stationary front
4. A warm air mass can get trapped between two cooler air masses – an occluded front
Your teacher will demonstrate interactions between fluids with different temperature
characteristics. Although you will view liquids, gases in the atmosphere behave the same way
based on temperature differences. Complete the Part III questions based on the demonstration
of front boundaries in the Student Journal before moving on to Part IV: Four Types of Fronts.
Part IV: Four Types of Fronts
As you saw in the demonstration, the interaction at frontal boundaries, or where two air masses
meet, can be turbulent. The characteristics of the two air masses determine actual weather
conditions; however, there are four types of front boundaries that produce typical weather
conditions.
Complete Part III in your Student Journal. Continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part VI: Four Types of Fronts, continued
Cold Front
When a denser, cooler air mass pushes a warmer air mass,
the boundary is called a cold front. Cold fronts are usually
fast moving and push the warmer, less dense air up quickly to
higher altitudes where it cools, and the moisture condenses,
producing heavy cloud cover and short, but intense
precipitation until the front passes. Notice the heavy cloud
band line in the satellite image and the corresponding map
symbol line on the outline map. The map symbol is a line with
triangles, and the triangles point toward the direction of the
front movement.
Map View of Texas
Satellite View of Texas
Continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part VI: Four Types of Fronts, continued
Warm Front
When a warm air mass pushes a dense, cooler air mass, the boundary is called a warm front.
A warm front moves more slowly than a cold front. At the boundary, the warm air slides slowly
up and over the dense cool air to higher altitudes where it cools, and the moisture present
condenses, often producing light scattered clouds and showers. Weather conditions may
remain cloudy with scattered precipitation for several days due to these slow moving fronts.
Notice the scattered cloud bands in the satellite image and the corresponding map symbol line
on the outline map. The map symbol is a line with half circles, and the circles point toward the
direction of the front movement.
Satellite View of Texas
Map View of Texas
Continue to the next page.
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8.10B: Weather Maps
Earth and Space
Part VI: Four Types of Fronts, continued
Stationary Front
As the name implies, these fronts typically do not move one way or the other. The two air
masses stall, and at the boundary line where the warm and cool air meet, moisture in the warm
air mass condenses causing precipitation and cloud cover. These conditions can remain in
place for several days until one of the two air masses begins to move forward.
The map symbol is a line with alternating half circles and triangles on the opposite side of the
line.
Occluded Front
An occluded front is the most complex of the four types of
boundaries. Warm, moist air becomes trapped between two
cooler air masses. Since the warm air is less dense, it rises,
and the two cooler masses move in underneath. The moisture
in the warm air mass cools and condenses at higher altitudes,
producing clouds and precipitation.
The map symbol is a line with alternating half circles
and triangles on the same side of the line, and they
point toward the direction of the front movement.
Complete Part VI and the Reflections and Conclusions in your Student Journal.
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