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July-August 2012

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Weather Queries

Midland, Texas, where I grew up, always seems to be close to a dryline. What is it, how does it form, and why is it so often over Midland?

Lee Cassin

Aspen, Colorado

A dryline is a boundary between moist and dry air. It is not considered an atmospheric front because the temperature contrast across the dryline is minimal. At the surface, a trough of low pressure often occurs along the dryline, and a gradual wind shift is present across it. The dryline is well known in the Southern Plains of the United States, where it can extend for hundreds of miles, roughly north-south, across Texas. It marks the boundary between a shallow, moist air mass that moves north from the Gulf of Mexico and a warm, dry, sinking, continental air mass that moves northeast from the Mexican Plateau or east from the Southern Rocky Mountains. Tracking the dryline can be important because thunderstorms often fire in the moist air not far east of its position.

Figure 1. An east-west vertical cross section through a dryline. Surface elevation increases from sea level on the right (e.g., Gulf of Mexico) to 1.5 kilometers (e.g., Eastern New Mexico). The moist layer lies below the horizontal line. Warm, dry air lies above. The dryline is at the intersection of the ground surface and the top of the moist layer. Simple but representative soundings of temperature (solid line) and dewpoint (dashed line) are shown at points A (west of the dryline), B (just east of the dryline), and C (deep in the moist layer).

Figure 1. An east-west vertical cross section through a dryline. Surface elevation increases from sea level on the right (e.g., Gulf of Mexico) to 1.5 kilometers (e.g., Eastern New Mexico). The moist layer lies below the horizontal line. Warm, dry air lies above. The dryline is at the intersection of the ground surface and the top of the moist layer. Simple but representative soundings of temperature (solid line) and dewpoint (dashed line) are shown at points A (west of the dryline), B (just east of the dryline), and C (deep in the moist layer).

The upward-sloping topography from east to west plays a central role in the position of the dryline, particularly when weather conditions are benign. See Figure 1, from Howard Bluestein's book Synoptic-Dynamic Meteorology in Midlatitudes, Volume II: Observations and Theory of Weather Systems (p. 288). The horizontal line at 1.5-kilometer elevation marks the top of the moist layer. The moist layer is fairly deep near the Gulf Coast, but it thins toward the west. The intersection of the top of the moist layer with the ground marks the position of the dryline. Three idealized vertical profiles of temperature (solid lines) and dewpoint (dashed lines) are shown at the bottom of Figure 1, at locations marked by A, B, and C at the top of Figure 1. At A, west of the dryline, the lapse rate is steep, near the dry adiabatic value, which means a decrease in temperature of 1°C for each 100 meters increase in altitude. At B, just east of the dryline, the moist layer is capped by an inversion. Above the inversion is the same warm and dry air mass that lies west of the dryline at the surface. Well east of the dryline at C, the moist air is deeper, and the capping inversion lies well above the surface. Stratus or stratocumulus clouds are common well east of the dryline, which helps to suppress daytime heating of the ground by the sun. At and west of the dryline, the sky is usually clear. The air mass warms and dries by compression as it descends toward the northeast and east after coming off higher ground.

With quiet weather conditions, the dryline may move discontinuously eastward during the day and slide back westward at night. The climatological mean position of the dryline is not far from Midland, Texas, because that is where the western edge of moist southerly flow from the Gulf of Mexico tends to stall. Toward the western edge of the moist layer, near B, the inversion is shallow, and cloud cover is minimal. During the morning, the sun heats the ground, and the transfer of heat from ground to atmosphere destroys the inversion. Once this happens and the lapse rate near the ground reaches the dry adiabatic value, vertical mixing occurs freely, and the dry air initially above the inversion mixes downward into the moist air. The surface dewpoint drops, and the dryline appears to retreat rather suddenly to the east, where cloud cover is greater, solar heating is less, and the moist layer is deeper. At night, the ground cools more rapidly west of the dryline than east of it because the atmosphere is dry and clear. A surface inversion forms quickly in the dry air, quickly shutting off vertical mixing. Both moist air and clouds east of the dryline retard surface cooling after dark. The developing surface temperature gradient from west to east assists the dryline in oozing back westward during the night.

An upper-air disturbance (also called a trough) and an associated low-pressure system moving onto the central Great Plains, especially in spring, can bring strong west winds into Oklahoma and Texas, which sweep the dryline rapidly eastward. An example is shown in Figure 2, which is a surface weather map for 2:00 p.m. CST on February 20, 2012. Weather data are plotted next to each station circle. Circles partially or completely color-filled are reporting partly cloudy to overcast skies. The color indicates the height of the clouds: green-clouds are more than 3,000 feet above ground level (AGL); blue-clouds are between 1,000 and 3,000 feet AGL; red-clouds are between 500 and 1,000 feet AGL. The number to the upper left of the station circle is the temperature (°F), and the number to the lower left is the dewpoint (°F). The wind staff pointing out from the station circle extends into the wind. Barbs on the staff indicate wind speed. A small barb indicates five knots, and a large barb indicates ten knots. Add values of the barbs together to get total wind speed. At some locations, wind gust information lies off the end of the wind staff. For example, G35 means wind gusts to 35 knots.

Figure 2. A plot of surface observations, valid at 2:00 p.m. CST, February 20, 2012, illustrating a dynamic, fast-moving dryline. The dryline is the bold, black, dashed line. A cold front, entering the northern Texas Panhandle, is shown by the barbed blue line.

Figure 2. A plot of surface observations, valid at 2:00 p.m. CST, February 20, 2012, illustrating a dynamic, fast-moving dryline. The dryline is the bold, black, dashed line. A cold front, entering the northern Texas Panhandle, is shown by the barbed blue line.

The dryline is the black dashed line. It separates air with dewpoints in the 40s and 50s to the east from air with dewpoints in the 20s and much lower (Texas Panhandle and Eastern New Mexico). The transition from moist to dry air can sometimes be sharply defined across drylines, but it is gradual in this case. The wind shift across this dryline is also gradual. The air is a few degrees warmer west of the dryline, but the temperature contrast between west and east is minimal.

A cold front (blue curve with barbs) is just entering the Texas Panhandle, displacing the dry air and causing rapidly falling temperatures.

Figure 3 is a grayscale, visible image from the Geostationary Operational Environmental Satellite (GOES), in which clouds are easy to spot. Note the scattered clouds following behind the cold front in the northern Texas Panhandle and the scattered to broken clouds along and just east of the dryline in Western Oklahoma and Northern Texas. A few hours after acquisition of this image, thunderstorms, some severe, broke out within the latter cloud feature as it traversed Oklahoma, an event often associated with moving drylines in the spring. Finally, plumes of dust raised by the dry wind just south of the cold front are barely discernible as large swirls in Figure 3 (see white arrows).

Note the red label for Midland, Texas, in Figure 3. Here is the hourly sequence of Midland weather leading up to 3:00 p.m. CST, February 20, 2012.

Figure 3. A visible image of the Southern Great Plains from the GOES satellite, valid at 2:01 p.m. CST, February 20, 2012. The cold front and dryline are displayed as in Figure 2. Figures 2 and 3 both show that the extensive cloudiness is well east of the dryline.

Figure 3. A visible image of the Southern Great Plains from the GOES satellite, valid at 2:01 p.m. CST, February 20, 2012. The cold front and dryline are displayed as in Figure 2. Figures 2 and 3 both show that the extensive cloudiness is well east of the dryline.

Note in Table 1 the rise in temperature consistent with daytime heating in a dry air mass, the significant decrease in dewpoint as drier air moves in, and the gradual veering of the wind from 230° (SW) to 280° (WNW). As the data indicate, this dryline was diffuse at Midland. It was more sharply defined to the north and east.

Table: Table 1

Local Time (CST)

Temperature °F

Dewpoint °F

Wind Direction (degrees)

Wind Speed (knots)

7:00 a.m.

43

38

230

10

8:00 a.m.

45

34

230

13

9:00 a.m.

53

30

240

16

10:00 a.m.

59

28

250

22

11:00 a.m.

62

24

250

26

12:00 noon

63

20

260

23

1:00 p.m.

65

20

270

22

2:00 p.m.

68

14

270

21

3:00 p.m.

68

9

280

23

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