Name ___________________________________
Partner_____________________
Period _______
Due Date: ___________
Due Date
Under Pressure and Isobars
OBJECTIVE: To understand how a barometer is used to measure air pressure.
PURPOSE:
1.
2.
3.
4.
5.
6.
To name two types of barometers.
To tell what barometric readings indicate on a barometer.
To tell what an isobar is on a weather map.
To tell what barometric readings indicate about the weather.
To describe the effects of temperature, humidity, and altitude on air pressure.
To understand a HIGH and a LOW pressure system.
BACKGROUND:
An airplane trip to a different country, putting air in a basketball, drinking soda through a straw all relate to air
pressure. Air has weight and occupies space. The same as the air inside a basketball pushes out to make a basketball
firm, air in the atmosphere pushes down on the Earth’s surface.
The diagram below shows the pressure that pushes down on the Earth’s surface. An average force of 14.7 pounds
of air is pressing one each square inch of surface.
What would be the total force on a surface of 10 square inches?
Answer: _______________ lbs of pressure
AIR PRESSURE
Tell if the air pressure would be (more, less, or the same) as that of the air pressure at sea level (14.7 lb/in2) at
the following locations.
a. At the top of Mt. Everest: _______________ b. Atlantic Ocean Seashore: ________________
Adapted from K. Phillips; Modified 01/2012
1
BACKGROUND:
In 1643, the Italian physicist, Torricelli discovered that air has weight. He invented the Mercury
Barometer. This instrument showed that atmosphere could support a column of mercury approximately 30
inches high.
In 1656, it was discovered that the height of the mercury in the barometer
tube varies with weather. It was found that moist (humid) air was “lighter”
(less dense) than dry air. Therefore, the height of the mercury in the barometer
rises in the tube in the dry air and falls when the air becomes moist. Also, because
cold air is “heavier” (more dense) warm air, it has higher pressure than warm air.
Study the Mercury Barometers below and answer the following questions.
1. At what time was the air pressure the highest? ____________________
2. At what time was the air pressure the lowest? ____________________
3. At what time was the air temperature the coolest? _________________
It was also noted that at low altitude the height of mercury rises in the tube. It gradually falls as the
barometer is taken to higher altitudes. At sea level, the barometer should read 29.92 inches of Mercury or
1013.2 millibars of Mercury. Sometimes you may see 76 cm of Mercury. They all mean the same thing.
It became clear to scientists that the Mercury rises with an increase in atmosphere pressure and falls with
a decrease in atmospheric pressure. We now know that a column of Mercury falls one inch every 1000 feet rise
in elevation.
The Torricelli Mercury Barometer is not always convenient to use. Therefore, a new barometer called
an ANEROID BAROMETER was made. It consists of a small circular disk with all the air inside removed.
This makes a vacuum inside the disk. It contains no liquid to spill. As the air pressure decreases or increases, a
spring at the top of the disk moves a needle up or down. The scale on the outside is calibrated to the Mercury
Barometer.
Use the Earth Science Reference Table chart to help you convert the following air pressure readings:
1. Convert from inches to millibars of Mercury.
a. 29.70 inches of Mercury = __________________ mb
b. 30.14 inches of Mercury = __________________ mb
c. 29.37 inches of Mercury = __________________ mb
d. 30.70 inches of Mercury = __________________ mb
2. Convert from millibars to inches of Mercury.
a. 1004.5 mb = __________________ inches of Mercury
b. 1017.5 mb = __________________ inches of Mercury
c. 987.6 mb = __________________ inches of Mercury
d. 972.0 mb = __________________ inches of Mercury
Adapted from K. Phillips; Modified 01/2012
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USING THE BAROMETER TO FORECAST WEATHER
Reading a barometer can help you forecast the weather of a short period of time. Look at the chart
below and then answer the questions that follow:
BAROMETER READING
30.10 – 30.20 rising rapidly
30.10 – 30.20 steady
30.10 – 30.20 falling slowly
30.10 – 30.20 falling faster
30.00 or below – rising slowly
30.00 or below – falling slowly
30.00 or below – falling rapidly
29.80 or below – falling rapidly
WEATHER FORECAST
Fair, rain within 48 hours
Fair, little chance of rain
Rain within 24 hours
Rain within 12 hours
Clearing
Rain for 24 to 48 hours
Rain with winds
Storm with strong winds
1. The TV weather person announces that the barometer reading is 30.10 inches and falling slowly.
What does this mean?
_______________________________________________________________________________________
2. What would be the approximate barometer reading during a severe thunderstorm?
_______________________________________________________________________________________
3. What type of weather will generally follow a rising barometer reading?
_______________________________________________________________________________________
READING A BAROGRAM
A BAROGRAPH is an instrument that measures and records air pressure. A pen in the instrument records the
air pressure in inches of Mercury on a special sheet of paper for a period of seven days. The sheet of paper with
the record on it is called a BAROGRAM.
Use the Barogram on the next page to answer the following questions.
1. What is the lowest recorded pressure shown on the barogram? _________________________________
2. At what hour and on what day did this occur?
Hour = ___________________________
Day = ____________________________
3. Pressure reading BELOW 30 inches is considered LOW. What type of weather probably occurred
when the pressure was low? ____________________________________________________________
____________________________________________________________________________________
4. What is the highest recorded pressure sown on the barogram? __________________________________
5. Pressure reading ABOVE 30 inches is considered HIGH. What type of weather probably occurred
when the pressure was high? ____________________________________________________________
____________________________________________________________________________________
6. Between noon on Thursday and noon on Friday, how much did the pressure drop? _________________
Adapted from K. Phillips; Modified 01/2012
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Adapted from K. Phillips; Modified 01/2012
4
INTERPRETING AN AIR PRESSURE MAP
The following map represents three air pressure systems in the United States. The connected lines that
make up the air pressure system are called ISOBARS. Isobars connect equal areas of pressure. If the value
of the isobars increases towards the center of the pressure system, the pressure system is considered to be a
HIGH pressure system. If the value of the isobars decreases toward the center of the pressure system, the
pressure system is considered to be a LOW pressure system.
Identify the type of air pressure system (High or Low) at each of the following locations 1, 2, and 3.
PRESSURE SYSTEM 1 (left)
____________________
A
PRESSURE SYSTEM 2 (middle)
____________________
PRESSURE SYSTEM 3 (right)
____________________
C
B
D
1. a. What do the numbers on the map represent? ______________________________________________
b. In what units are these numbers recorded? ______________________________________________
2. a. What are the lines called that connect the numbers? _______________________________________
b. What do the lines on the map represent? ________________________________________________
3. Look at each pressure system. Label the HIGH pressure system(s) with an H. Label the LOW pressure
system(s) with an L.
4. Suppose air movement occurred between PRESSURE SYSTEM 2 and PRESSURE SYSTEM 3.
Draw a large arrow on the map to show the direction of air movement.
5. a. What is the air pressure at point A? ___________________________
b. What is the air pressure at point B? ___________________________
c. What is the air pressure at point C? ___________________________
d. What is the air pressure at point D? ___________________________
Adapted from K. Phillips; Modified 01/2012
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RULES FOR DRAWING ISOBARS
There are several rules for drawing ISOBARS. Each time an isobar is drawn, every rule listed below
must be followed.
RULES for ISOBARS:
1. Only connect points of the same value
2. Lines should NEVER TOUCH OR CROSS
3. Intervals/spacing always EVEN between lines
a. Only by 4’s – isobar of 1016, 1020, 1024, 1028…
4. Always make SMOOTH Curves
5. Line should divide higher and lower values from the isobar
6. Isobars either:
a. Touch edge of the map/page
b. Form closed loops/rings
Note the isobars drawn in the above diagram. The isobars drawn indicate the locations that currently
have a pressure reading for 1020.0 mb. Take note of the following: (a) the line only connects values of 1020.0
mb (b) the 1020.0 mb isobar separates values higher and lower than 1020.0 mb (c) the isobars are
loops/smooth line
Complete the following steps to draw the 1024.0 mb isobar.
Step 1: Locate and circle all 1024.0 values in the diagram.
*Hint: There should be 12 values circled.
Step 2: Determine if the values are increasing/decreasing towards the center of each pressure system.
Step 3: Connect all of the 1024.0 values making sure to follow all of the RULES for ISOBARS.
*Hint: You will have to draw two 1024.0 isobars.
Refer to the pressure system over the Western United States.
Knowing that the closer the isobars are together, the greater the wind speed, what direction from the center of
the system would the wind be at its greatest velocity? _________________
Adapted from K. Phillips; Modified 01/2012
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