10.2: Air Pressure Changes in Frontal Passages
- Page ID
- 45365
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Not shown on the meteogram for Kerrville, but also of importance, is air pressure. Air Pressure Data was not available for this particular day in Kerrville. Both warm and cold fronts behave like areas of low pressure, with air pressure steadily declining as the front approaches and increasing once the front passes. In fact, especially in summer months when differences between warm and cold air masses are small (hence frontal conditions are not as well pronounced), the decreasing air pressure followed by a recovery in air pressure is a telltale sign of a frontal passage. Let’s investigate a more detailed example of this. Figure 10.2.1 is a map of surface conditions across the United States at 1200 UTC (7 am Central Daylight Time) on July 28, 2019, focusing on a cold front stretching from central Canada through western North Dakota, down to a low-pressure system in Wyoming.
- Based on the direction the 'spikes' on the cold front are pointing towards, this front is traveling:
- Towards the north
- Towards the east-southeast
- Towards the west
- At 1200 UTC, Bismarck, in central North Dakota, is reporting a temperature 68°F, a dew point of 65°F, and winds coming from the _______ at 5 knots.
- north
- southeast
- southwest
- west
Figure 10.2.2 is a map of surface conditions across the United States at 0000 UTC on July 29, 2019, 12 hours after Figure 10.2.1.
Figure \(\PageIndex{2}\): Surface Weather Conditions for the United States at 0000 UTC on July 29, 2019. (CC BY-NC 4.0; American Meteorological Society via Unidata). Alternative description of image.
- By 0000 UTC on July 29th, the front _______ passed Bismarck.
- had
- had not yet
- Bismarck’s wind direction:
- Shifted to the northwest
- Remained similar to 1200 UTC
- Shifted to the east
Figure 10.2.3 is a Meteogram for Bismarck International Airport during the 24 hours between 0700 UTC (12 am Central Daylight Time) on July 28th and 0700 UTC (12 am Central Daylight Time) on July 29th.
- Bismark's air temperature was _______ at the end of the period in Figure Figure \(\PageIndex{3}\) than the beginning of the period.
- higher
- the same
- lower
- Between 0700 UTC and 0900 UTC on July 28th, air pressure:
- Increased
- Decreased
- Remained steady
After the shift in pressure mentioned above, air pressure stabilized until approximately 1800 UTC.
- After 1800 UTC, the air pressure
- Began rising
- Began dropping
- A significant change in wind direction, from south-southwest to northwest occurred __________ air pressure began rising.
- at the same time as
- a few hours before
- a few hours after
- The dew point began decreasing ___________ air pressure began rising.
- at the same time as
- a few hours before
- a few hours after
Therefore, despite little change in air temperature, changes in air pressure, dew point, and wind direction confirm the passage of a cold front during the period between 1200 UTC and 0000 UTC, as suggested by Figures 10.2.1 and 10.2.2.
Detection of warm and occluded frontal passages is also possible using surface observations. However, changes in conditions tend to be much subtler and more gradual than those associated with cold frontal passages. A few things to note is that in every frontal passage, changes in air pressure are similar, while changes in wind direction, dew point and air temperature are very different, while this may sound counterintuitive, the passage of a cold front does not necessarily mean that it’s going to get colder suddenly, so it’s necessary to look at all of the factors at play when analyzing the presence and movement of a frontal boundary.