TEMPERATURE AND AIR MASS ADVECTION

Objectives:

Earth’s surfaces and atmosphere are unevenly heated by solar radiation. Averaged over a year, low latitudes receive more energy from the Sun than they lose directly to space as outgoing infrared radiation. High latitudes experience more outgoing than incoming radiation. These energy excesses and deficits are balanced by horizontal movements (called advection) of heat energy poleward by migrating air masses, storm circulations, and ocean currents. The transport of air from a region of relatively high temperatures to a region of lower temperatures by atmospheric motion (the wind) is referred to as warm air advection. Conversely, the transport of air from a region of relatively low temperatures to a region of higher temperatures by the wind is called cold air advection. Identification of areas of warm and cold air advection requires, in addition to information about winds, determination of the air temperature pattern made possible through the drawing of isotherms.

After completing this investigation, you should be able to:

• Draw lines of equal temperature (isotherms) to reveal the pattern of air temperatures across the nation at map time.
• Locate regions on a weather map where cold and warm air advection is occurring.
• Relate warm and cold air advection patterns to circulations of weather systems.

Introduction:

Temperature patterns are found on weather maps by drawing lines representing specific temperatures. These lines are called isotherms because every point on the same line has the same temperature value. The skills required to draw isotherms are much the same as those needed to draw isobars.

Tips on Drawing Isotherms:

a. Always draw an isotherm so that temperatures higher than its value are consistently to one side and lower temperatures are to the other side.

b. Assume a steady temperature change between neighboring stations when positioning isotherms; that is, use interpolation to place isotherms.

c. Adjacent isotherms tend to look alike. The isotherm you are drawing will often align in a general way with the curves of its neighbor because changes in air temperature from place to place are usually (but not always) gradual.

d. Continue drawing an isotherm until it reaches the boundary of plotted data or “closes” within the data field by making its way to its other end and completing a loop.

e. Isotherms can never be open ended within a data field and they never fork, touch, or cross one another.

f. Isotherms cannot be skipped if their values fall within the range of temperatures reported on the map. Isotherms must always appear in sequence; for example, when the isothermal interval is 10 degrees, there must be a 50 °F isotherm between the
40 °F and the 60 °F isotherms.

g. Always label isotherms.

The Figure 1 map segment shows temperatures in degrees Fahrenheit (°F) at various weather stations. Consider each temperature value to be located at the center of the plotted number. The 70- and 60-°F isotherms have been drawn and labeled. Complete the 50-, 40-, 30-, 20- 10-, and 0-°F isotherms. Be sure to label each isotherm at both ends or, if encircling an area, at a break in the line.

Figure 1. Central U.S. map segment of temperatures.

1. Isotherms are drawn at regular intervals; on this map, the interval between successive isotherms is [(5)(10)(20)(30)] Fahrenheit degrees.

2. Your temperature analysis reveals a pattern with coldest temperatures located to the [(northeast)(northwest)(southwest)(southeast)].

3. In Figure 2, a surface weather map of the contiguous U.S. for a different time shows a simplified view of a storm system in the eastern part of the country. Local wind data are displayed for several stations and three isotherms have been drawn on the map. The isotherm values (from lowest to highest) are [(50, 60)(60, 70)(50, 60, 70)] °F.

4. Hence, the interval between isotherms on the Figure 2 map is [(5)(10)(20)(30)] Fahrenheit degrees.

Figure 2. Simplified surface weather map showing a storm system in the eastern U.S.

Warm air advection occurs where the wind blows across isotherms from the higher (warmer) values to the lower (colder) values. That is, warmer air is being transported towards a cooler location by the horizontal winds. Based on this factor alone, one would expect temperatures at that location to be rising. Cold air advection occurs where the wind blows across the isotherms from the lower (colder) values to the higher (warmer) values. Then, colder air being transported by the wind to a warmer location produces falling temperatures.

Based on wind directions and the isotherm pattern on the Figure 2 map, determine the type of air advection (warm or cold) that would be occurring at each station.

5. Warm air advection was occurring at Station [(A)(B)(D)].

6. Cold air advection was occurring at Station [(B)(C)(D)].

7. Generalizing from this map depiction of wind and temperature patterns associated with weather systems, areas southeast of Lows can be expected to have [(warm)(cold)] air advection.

8. Meanwhile, areas to the west and southwest of Lows can be expected to have [(warm)(cold)] air advection.

9. Areas to the east of Highs would be expected to have [(warm)(cold)] air advection.

10. And while not shown here but relying on the hand-twist model of a High, areas to the west of Highs should have [(warm)(cold)] air advection.

On actual weather maps, the patterns of isotherms and winds vary greatly. The intensity of warm and cold air advection will depend on the wind speeds, the angle at which the wind crosses the isotherms, and the closeness of neighboring isotherms. In general, the faster the wind, the more perpendicular the angle, and the closer the isotherms, the stronger the advection will be.

11. Not surprisingly, behind cold fronts one can expect [(warm)(cold)] air advection.

12. And behind warm fronts one can expect [(warm)(cold)] air advection.

13. Where the horizontal wind blows parallel to isotherms, there is [(warm)(cold)(neither warm nor cold)] air advection.

14. Air temperatures are governed by a combination of warm or cold air advection and radiational controls. With no advection, the lowest temperature of the day is likely to occur around [(sunrise)(sunset)].

15. If a day’s highest temperature actually occurs just before midnight, then [(warm)(cold)] air advection likely occurred sometime from late afternoon until the time of highest temperature.

This episode examines a cold front marking the leading edge of cooler weather that advanced into the center of the contiguous U.S. The front caused a band of showers and thunderstorms to bisect the country. Incursions of cold and warm air that accompany fall storm systems can cause dramatic swings of temperature that also affect precipitation types as air masses travel. Here we consider how we detect those changes of temperature due to air mass movements.

Figure 3 is the map of plotted station models with reported surface weather conditions (Isotherms, Fronts, & Data) for 14Z 28 SEP 2013 (11 AM CDT, etc.). Note that on this Figure 3 map, the isopleth lines are isotherms (not isobars). The H and L centers and frontal positions are shown on the map at nearly the same time, as of 12Z.

Figure 3. Map of Isotherms, Fronts & Data for 14Z 28 SEP 2013.

At map time, the frontal system curved from International Falls, Minnesota, on the Canadian border southward to the Mexican border in west Texas. The front was shown as a stationary front from southeastern Nebraska northward to Canada and as a cold front southward to southwest Texas. Showers extended along the frontal region as indicated by the black dots, symbols for rain, from Fargo, ND to Kansas City, MO. The heavy red dashed and double dot curve in eastern Kansas and central Oklahoma denoted a squall line (SQLN), a line of heavy thunderstorms with high winds.

Relatively warm and humid air was found generally across the eastern half of the country to the east of the frontal system. To the west of the front, the air was generally cooler and less humid, i.e. lower dewpoints. Local low-pressure centers were along the frontal boundary and Highs were located in the Northeast and the Intermountain West.

16. The wind directions at stations east of the frontal system from south-central Texas northward to International Falls, MN, were generally from the [(south)(north or northwest)(east)].

17. San Antonio, TX, had a temperature of [(66)(79)(82)] degrees Fahrenheit.

18. San Antonio had a wind plotted as 10 knots generally from the [(south-southeast)(west-southwest)(north-northwest)(east-northeast)] at map time.

19. The isotherm drawn through the San Antonio area was the [(70)(80)(90)] degree Fahrenheit isotherm. (The isotherms are labeled by red numbers within breaks in the lines. This particular isotherm’s label was seen in northern Mexico and the eastern Gulf of Mexico.)

20. The wind at San Antonio exhibited a flow that was directed nearly perpendicular to the isotherm. This wind direction was from [(higher toward lower)(lower toward higher)] temperature regions. Stations northward to central Iowa generally displayed this same temperature and air flow pattern.

21. Therefore, the air flow shown by the wind across the isotherm at San Antonio demonstrated that [(cold)(warm)] air advection was occurring over this broad area.

22. This advection pattern was generally “ahead of” (to the east of) the advancing [(cold)(warm)] front.

23. One could conclude then that wind directions and temperature patterns preceding a cold front would display [(cold)(warm)] air advection. This type of advection was occurring, albeit weakly, over much of the east-central portion of the country at map time.

24. Now note the temperature and air flow pattern at Fargo, North Dakota. The [(40)(50)(60)] degree Fahrenheit isotherm was positioned in the Fargo area. The isotherm label was located in west-central Kansas.

25. The wind at Fargo was plotted as 10 knots from the [(southwest)(northwest)(northeast)(southeast)].

26. The wind at Fargo had a flow that was directed across the isotherm at a large angle. This wind direction was from [(higher toward lower)(lower toward higher)] temperature regions.

27. Therefore, the air flows shown by the winds across the isotherm near Fargo demonstrated that [(cold)(warm)] air advection was occurring in this region. This was “behind” (to the west of) the frontal system. Stations from North Dakota to northwestern Texas displayed this same temperature and air flow pattern.

In general, stronger winds, more direct flow across isotherms, and closer spacing of isotherms indicate stronger air advection patterns than where there are weaker winds and/or more widely spaced isotherms.

With an advance into the autumn season, successive pushes of colder air can be expected to provide more frequent cold-air advections. Cold air masses will likely become dramatically colder. The temperature contrasts of adjacent air masses will make for energetic fall storm systems! Conversely, the sequence is reversed moving into the spring season.

Suggestions for further activities: As weather systems cross your region, you might call up the “Isotherms, Fronts, & Data” map on the course website and identify patterns of cold or warm air advection associated with these passing systems. Warm and cold air advection patterns are often best seen in spring and fall when clashes of air masses make for dramatic weather episodes. You can lightly color regions of warm (red) and cold (blue) air advection on the map and relate them to the weather systems and to your daily temperature patterns.

Large temperature changes from one day to the next are likely to be the result of air mass advection. Sources for pinpointing these advection regions are maps of 24-hour temperature change provided by Intellicast: http://www.intellicast.com/National/Temperature/delta.aspx?location=default () and, by The Weather Channel: http://www.weather.com/maps/activity/achesandpains/us24hourtemperaturechange_large.html ().