Dry line

A dry line (also called dew point line, or Marfa front) separates moist air from the Gulf of Mexico (to the east) and dry desert air from the south-western states (to the west). The dry line is an important factor in severe weather frequency in the Great Plains of North America. It typically lies north-south across the High Plains states in the warm sector of an extratropical cyclone and stretches into the Canadian Prairies during the spring and early summer. The dry line is also important for severe convective storms in other regions of the world, such as northern India.

Characteristics

Near the surface, warm moist air is denser than dry air of greater temperature, and thus the warm moist air wedges under the drier air like a cold front. At higher altitudes, the warm moist air is less dense than the cooler, drier air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front. The dry line is most common in the spring. Its location is close to the location of the isodrosotherm, or line of equal dewpoint. The location of the dryline may not be marked with a surface pressure trough or shift of the wind direction. It bulges more to the east underneath the location of the highest winds within the jet stream. While dry lines are most common in the Great Plains, northern India also witnesses a similar moisture boundary. In northeast India, it occurs mainly before the onset of their summer monsoon, while northwest India experiences it during the monsoon season. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley or Texas/Louisiana border, regardless of the time of day. Stronger dry line passages result in a sharp drop in dew point, clearing skies, and a wind shift from south or south-easterly to west or south-westerly. Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward during the evening and nighttime hours. Severe and sometimes tornadic thunderstorms often develop along the slope reversal zone east of the surface dry line, especially when it begins moving eastward.

Associated weather

In the dry sector west of the dry line, clear skies are the rule due to the dryness of the air mass sweeping off the Rocky mountains in North America, and the Aravalli range in India. If winds are strong enough, dust storms can develop. The moist sector is normally capped with a lid of an elevated mixed dry layer, which can extend well downwind and poleward of their source region, which are Earth's deserts. Clouds such as altocumulus or altocumulus castellanus can develop at the vertical boundary between the low-level moisture and the dry mixed layer above the low-level moist air mass east of the dry line.

See also

Surface weather analysis

Weather front

References

Category:Severe weather and convection Category:Weather fronts