6.1: Air Pressure on a Weather Map

What is Air Pressure?

While the air around us feels light, it still has mass. Because of gravity, this mass exerts a force called weight. Air pressure is the weight of the air above you. If you consider an imaginary column of air extending from sea level to the top of the atmosphere, it will weigh approximately 14.7 pounds. If you live close to sea level, the weight of the air above you is weighing down on every square inch of your body. Thankfully, our bodies exert an equal outward force, preventing us from being crushed by the weight of the air. Meanwhile, if you live in a high-altitude location like Denver, Colorado, which sits at 5280 feet above sea level, there is less air in the column above you, up to the top of the atmosphere. Thus, the air pressure in Denver will be significantly lower than at sea level. Meteorologists measure atmospheric pressure at weather stations using a device called a barometer. 

Sea-Level Pressure

There are two important considerations we need to make before plotting air pressure on a weather map: first, air pressure decreases with increasing elevation, so locations that are not at sea level will have lower pressure simply because of their elevation, rather than any meteorological reason. Second, topography varies greatly across the United States, so any surface map would, in theory, have "low pressure" on every mountain peak and "high pressure" in every valley. However, these areas of higher and lower pressure are not relevant to any actual weather patterns because they are simply caused by differences in elevation. To adjust for this, every weather station across the nation measures station air pressure and then calibrates it to sea-level air pressure, which is the pressure the location would have if it were at sea level. Rather than using station air pressure, weather maps use sea-level-calibrated pressure, which removes the effects of elevation on air pressure and allows us to focus on the meteorology. Sea-level air pressure values (expressed in millibars, the metric unit of air pressure) are then plotted on a weather map, as shown in Figure \(\PageIndex{1}\). 

Figure \(\PageIndex{1}\): A map of Sea Level Air Pressures over the United States.  (CC BY-NC 4.0; American Meteorological Society via Unidata) Alternative description of image. 

The average sea level pressure is 1013.25 mb. We use this number as a benchmark to identify areas of high and low pressure. 

1. Since there is more air above you at sea level than there would be in a city like Denver, sea level air pressures are generally _________ than station-measured air pressures. 
   1. higher
   2. lower

2. The highest pressure shown in Figure \(\PageIndex{1}\) is _______.
   1. 1016 mb
   2. 1020 mb
   3. 1024 mb
   4. 1032 mb. 

3. The lowest pressure present shown on the map is ________.
   1. 1012 mb
   2. 1004 mb
   3. 1000 mb
   4. 1008 mb. 

This map is a relatively simplified version of an air pressure map. The difference between two consecutive values displayed is called an interval. For this map, the interval is 4 mb. 

4. Based on the interval provided, this map should only display data for station pressures of:
   1. 1006 mb, 1008 mb, 1010 mb, 1012 mb, 1014 mb, 1016 mb, 1018 mb.
   2. 1004 mb, 1008 mb, 1012 mb, 1016 mb, 1020 mb, 1024 mb. 
   3. 1009 mb, 1012 mb, 1015 mb, 1018 mb, 1021 mb, 1024 mb.

Similar to a weather map showing temperatures, a map showing detailed pressure values across the country would be overcrowded. Meteorologists want to identify patterns quickly. Thus, they use isobars – lines of constant pressure – to present patterns visually in an easily identifiable manner. Let us attempt to draw isobars on Figure 6.1.1 and analyze whether they allow us to better examine the pressure patterns. Here are some guidelines for drawing isobars on a weather map:

• Drawing isobars is like connecting dots. Isobars should connect every location with the same air pressure. For example, the 1024 mb isobar should connect every location with a pressure of 1024 mb. 
• Draw isobars every 4 mb, only for multiples of 4. 
  • HINT: Should I draw an isobar for a particular pressure? Divide the pressure by 4. If it’s still an integer, then yes. If not, then no! 
• Isobars do not cross or branch.
• Isobars either extend to the edge of a map or close to form a full circle.
• If you're drawing on paper, use a pencil because mistakes happen.
• Label your isobars. 

5. Draw isobars on Figure 6.1.1, and save the final image.

6. On your map, you'll notice that the isobars created two bull's-eyes with centers located:
   1. one on the East Coast of the U.S. and another in California.
   2. one in Texas and another on the East Coast of the U.S.
   3. one in California and another in Canada.
   4. one in Utah and another in Kentucky. 

7. For one of the bull's-eyes, the air pressure increases as you approach the center. This represents a center of _________ pressure. 
   1. high
   2. low

8. For the other bull's-eye, the air pressure decreases as you approach the center. This represents a center of _________ pressure. 
   1. high
   2. low

While isobars are only drawn every 4th millibar, they provide an excellent indication of air pressure elsewhere. A location between two isobars will have an air pressure value between those two isobars. 

9. For example, on the isobar map in Figure 6.1.1, El Paso, in extreme western Texas, lacks a plotted sea-level pressure value, and no isobars cross it. However, based on the two closest isobars, it likely has a sea level pressure of approximately.
   1. 1016 mb
   2. 1018 mb
   3. 1020 mb
   4. 1022 mb