6.2: Air Pressure on a Weather Map
- Page ID
- 44401
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There are two important considerations that we need to make when discussing air pressure on a weather map: First, air pressure decreases with increasing elevation, so locations that are not at sea-level will have a 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 relevatn 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 located at sea level. Rather than using station air pressure, weather maps will use the sea-level calibrated pressure, because it removes the effects of elevation on air pressure, instead allowing us to focus on the meteorology. Sea level air pressure values (given in Millibars – The Metric unit for Air Pressure) are then plotted on a weather map, such Figure 6.2.1.
Average Sea Level pressure is 1013.25mb. We use this number as a benchmark to identify areas of high and low pressure.
- The highest pressure present on this map is _____________________
- 1027mb
- 1023mb
- 1028mb
- 1032mb.
- The lowest pressure present on this map is ______________________
- 1012mb
- 1004mb
- 1000mb
- 1008mb.
This map is a relatively simplified version of an air pressure map, so it’s a good map to get started with. On this map, air pressure values are only being displayed for every 4 mb. The difference between two consecutive values being displayed is called an interval, and for this map, the interval is 4mb.
- Based on the interval provided, this map should only display data for station pressures of:
- 1006mb, 1008mb, 1010mb, 1012mb, 1014mb, 1016mb, 1018mb.
- 1004mb, 1008mb, 1012mb, 1016mb, 1020mb, 1024mb.
- 1009mb, 1012mb, 1015mb, 1018mb, 1021mb, 1024mb.
While looking at a map with a bunch of numbers is okay, meteorologists want to be able to identify patterns quickly, and so isobars – lines of constant pressure – air plotted to take the raw data and present in a visual manner where patterns are easily identifiable.
- Draw isobars for every 4th millibar on figure 1 (a downloadable file of figure 1 is available on the assignment’s Canvas page). A full set of guideslines for drawing isobars can be found here: https://www.weather.gov/jetstream/ll_analyze_slp but here is a brief set of guidelines:
- Isobars are like connecting dots… they should connect every location with the same air pressure. For example, the 1024mb isobar should connect every location with a pressure value of 1024mb.
- Draw isobars for every 4 mb, and only for numbers that are divisible by 4.
- HINT: Should I draw an isobar? Divide the number by 4 and if it’s still an integer, then yes! If not, then no!
- Isobars do not cross with one another or branch off.
- Isobars either extend to the edge of a map, or close to form a full circle
- Use pencil when drawing isobars – mistakes happen.
- Label your isobars.
When you are finished drawing isobars on your map, scan/take a picture of it, and upload it on the investigation’s answer sheet.
- On your drawn map, you should notice three bullseyes. One of which, over South-central Canada (the provinces of Alberta and Saskatchewan) will have isobar values increasing as you approach the center of the bullseye. As a result, the bullseye center is a center of _______________________ pressure.
- High
- Low
- On the other hand, a bullseye in Central Utah, where air pressure values are decreasing as you approach the center represent a center of ____________________________ pressure.
- High
- Low
- On this map, two centers of high pressure are located:
- On the East Coast of the US and over Alberta and Saskatchewan
- Over Utah and Arizona
- Over Texas and the East Coast of the US
- Over California and over Alberta and Saskatchewan.
- On the other hand, a center of low pressure is located
- On the West coast of the US
- Over Utah
- In the Gulf of Mexico
- Over Alberta and Saskatchewan
While isobars are only drawn for every 4th millibar, they give us an excellent hint as to air pressure values elsewhere. A location in-between two isobars will have an air pressure value in-between those two isobars.
- For example, from the isobar map you plotted on Figure 1, Northern Louisianna, doesn’t have a plotted sea level pressure, nor do any isobars travel over it. However, based on the two closest isobars, it likely has a sea level pressure of approximately.
- 1015mb
- 1011mb
- 1019mb
- 1021mb