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10.2: Air Pressure Changes in Frontal Passages

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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 an area 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 (7am 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.

    Weather map for the continental United States. Details in caption.
    Figure \(\PageIndex{1}\): Surface Weather Conditions for the United States at 1200 UTC on July 28, 2019. (CC BY-NC 4.0; American Meteorological Society via Unidata). Alternative description of image. 
    1. Based on the direction the “spikes” are pointing towards, this front is traveling:
      1. Towards the North
      2. Towards the East/Southeast
      3. Towards the West
    2. 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.
      1. North
      2. Southeast
      3. Southwest
      4. West

    Figure 10.2.2 is a map of Surface Conditions across the United States for 0000 UTC on July 29, 2019, 12 hours later than Figure 10.2.1.

    Weather map showing pressure, frontal systems, precipitation, and reported weather information for the continental United States. Details in caption.

    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. 

    1. By 0000 UTC on July 29th, the front _____________ passed Bismarck.
      1. Had   
      2. Had Not Yet
    2. Bismarck’s Wind Direction
      1. Shifted to the Northwest
      2. Remained similar as 1200 UTC
      3. Shifted to due 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.

    Meteogram for KBIS on July 28, 2019, showing temperature changes, precipitation, changing visibility, wind, cloud cover, and pressure trends. Details in caption.
    Figure \(\PageIndex{3}\): Meteogram of Surface Weather Data for Bismarck International Airport for the period between 0700 UTC on July 28th and 0700 UTC on July 29th. (CC-BY-NC 4.0; Plymouth State). Alternative description of image. 
    1. Air Temperature was ___________ at the end of the period than the beginning of the period.
      1. Higher   
      2. The Same   
      3. Lower
    2. Between 0700 UTC and 0900 UTC on July 28th, Air Pressure     
      1. Increased
      2. Decreased
      3. Remained Generally Steady

    After the shift in pressure mentioned above, air pressure stabilized until approximately 1800 UTC.

    1. After 1800 UTC, Air Pressure
      1. Began Rising
      2. Began Dropping
    2. A significant change in wind direction, from South/Southwest to Northwest occurred __________________________ air pressure began rising.
      1. About the same time as   
      2. A few hours before   
      3. A few hours after
    3. A major decrease in dew point occurred ____________________ air pressure began rising.
      1. About the same time as   
      2. A few hours before   
      3. A few hours after

    Therefore, despite little change in air temperature, Changes in Air Pressure, Dew Point, and Wind Direction does 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, the change in conditions tends to be much more subtle and gradual than that of 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.

    In the next section, we will examine the weather systems responsible for most fronts more holistically: Mid-latitude cyclones.

    10.2: Air Pressure Changes in Frontal Passages is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts.