What Is A Dry Line?

A dry line is a boundary between moist and dry air masses. Unlike a cold or warm front, one airmass is not rapidly overtaking the other. Additionally, the temperature on either side of a dry line will be similar, meaning there isn't a large temperature gradient.

The biggest difference in the air masses lies in the moisture content.

Lifting Action Created By Air Mass Differences

The reason dry lines create thunderstorms is due to the difference in density of the air masses.

The point where the two air masses and winds converge is the dry line, and that's where the convection starts.

The dry dense air acts like a wedge, lifting the less dense moist air. This behavior creates lifting action, but it is typically weaker than a cold front.

However, with enough wind shear aloft, significant lifting action can occur, forming severe weather.

Where Dry Lines Form

A dry line will typically form north to south in the southern and central plains of the United States. This line represents the boundary between the moist air that is drawn up from the Gulf of Mexico, and the drier air blowing across the deserts of the southwestern United States and northern Mexico.

You'll sometimes hear a dry line referred to as a Marfa front, after the town of Marfa, Texas. This area commonly sees dry lines during spring and early summer. Dry lines typically form in tornado alley, which is marked in red on the diagram below:

A dry line typically advances east during the day, and then retreats west at night. This is due to moist air aloft mixing down to the surface. The point where severe weather is most likely to form along the dry line happens as it moves to the east in the late afternoon.

How To Identify A Dry Line

The easiest way to identify a dry line is with a surface analysis chart. An orange scalloped line means a dry line is present.

You can also identify a dry line by looking at a graphical depiction of the dewpoint.

Another way to identify a dry line is with the weather associated with the east and west sides. You can expect a dry line to have cumuliform-type clouds with extensive vertical development on the east side. On the west side you might see clear skies, or with an unusually strong dry line, you might see dust storms that are caught in the dry line's updrafts.

How Does It Affect Aviation?

Because a dry line doesn't have a powerful temperature gradient like a cold front, the chances of developing severe weather are reduced, but not eliminated. While a cold front can develop severe weather like squall lines, if a dry line develops convective activity, it's typically in the form of individual cells. However, thunderstorm cells from a dry line can still mature into supercells, given enough wind shear.

Here's a video of a line of storms that formed from a dry line in west Texas. Check it out:

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