Dry line

A dry line (also called a dew point line, or Marfa front, after Marfa, Texas) is an imaginary line across a continent that separates moist air from an eastern body of water and dry desert air from the west. One of the most prominent examples of such a separation occurs in central North America, especially Texas, Oklahoma, and Kansas, where the moist air from the Gulf of Mexico meets dry air from the desert south-western states. 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.

Near the surface, warm dry air is more dense than warm moist air of lesser or similar temperature, and thus the warm dry air wedges under the moist 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 55 °F (13 °C) 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.

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