10.1: Fronts

A front is a boundary between two air masses, which are large bodies of air with similar temperatures. These air masses originate either near the poles (cold air masses) or near the tropics (warm air masses), and interact with each other. The United States, located in the mid-latitudes, is an excellent battle ground for different air masses because it lies approximately half way between the equator and the poles (hence it's located in the mid-latitudes). 

Along a warm front, cold air is retreating making room for warm, often moist air to take its place. Figure 10.1.1 is a 3-dimensional cross-section of a warm front, including the location of both warm and cold air along it.

Figure \(\PageIndex{1}\): A warm front. The front is traveling in the direction of the wind. Along the front, warm air gently overrides cold air, producing fog, stratus, and nimbostratus clouds. (CC-BY-SA 4.0; via Wikimedia Commons). Alternative description of image. 

Weather conditions ahead of a warm front are typically cold and unsettled, with warming and clearing conditions behind the warm front, as the warm air mass moves over. Because warm air only rises gently over the cold air mass, clouds that form are typically shallow, and precipitation, if any, is typically light. Because warm air is generally lighter, it travels more slowly than cold air, and warm fronts generally travel more slowly than cold fronts. 

On the other hand, cold fronts are more abrupt. Along a cold front, a cold, dense air mass rapidly pushes warm air away, leading to more abrupt cooling. Furthermore, as seen in the cross-section of a cold front in Figure 10.1.2, cold air forces warm air up at a large angle (almost straight up), generating tall cumulus and cumulonimbus clouds, often leading to severe weather (such as the severe weather the author of this investigation was hoping to see in the introduction). 

Figure \(\PageIndex{2}\): A cold front. The front is traveling in the direction of the wind. Along the front, warm air is forced up at a high angle over cold air, producing tall cumulonimbus clouds and stormy weather. (CC-BY-SA 4.0; via Wikimedia Commons). Alternative description of image.

Weather conditions ahead of a cold front are typically warm, with increasing cloudiness. Rapidly cooling and stormy conditions immediately follow the cold front, and cool, generally clearing conditions prevail after the front passes. Cold fronts produce some of the stormiest and most unsettled weather in the United States. Now we are going to delve deeper into the conditions before, during, and after a cold frontal passage.

Figure 10.1.3 is a Meteogram showing 24 hour weather conditions taken at Kerrville-Schriener Municipal Airport for the 24 hours beginning at 0000 UTC on December 27th (6pm Local Time on 12/26) and ending at 0000 UTC on December 28th (6pm Local Time on 12/27).

Figure \(\PageIndex{3}\): A meteogram showing temperature and dew point (first panel), precipitation, visibility, and wind barbs (second panel), cloud height (third panel), and air pressure tendency (fourth panel) for Kerrville-Schriener Municipal Airport for 24 hours beginning at 6 pm on 12/26/2015 and ending at 6 pm on 12/27/2015. (CC-BY-NC 4.0; Plymouth State). Alternative description of image. 

For this investigation, we will focus on air temperature (the warmer line in the first panel), dew point temperature (the cooler line in the first panel), and wind direction/speed (in the second panel). 

 

1. Air temperature (the warmer of the two black lines on the first panel) was approximately _________ at the beginning of the period.
   1. 73°F    
   2. 60°F    
   3. 54°F    
   4. 35°F
2. The biggest drop in both air temperature (the top black line) and dew point temperature (the bottom of the two black lines) occurred between      on December 27th (the numbers on the x-axis of the bottom panel represent time in UTC).
   1. 0100 and 0200 UTC    
   2. 0900 and 1000 UTC
   3. 1200 and 1300 UTC    
   4. 1500 and 1600 UTC.
3. After the sudden drop in air temperature, both temperature and dew point ____________________ until the end of the period.
   1. Recovered substantially        
   2. Leveled off
   3. Continued to decline steadily 
   4. Began rising steadily
4. Other conditions also experienced substantial changes. Before the temperature drop from question 2, winds were primarily coming from the:
   1. North/Northwest    
   2. West/Southwest    
   3. South/Southeast
5. After the temperature drop, winds shifted to the:
   1. North/Northwest    
   2. West/Southwest    
   3. South/Southeast