The Dryline•The dryline can be defined as the near surface convergence zone between moist air flowing off the Gulf of Mexico and dry air flowing off of semi-arid high plateau regions of Mexico and the southwestern United States:

Temperatures indegrees Celsius

Dry Line

• A surface boundary between warm, moist air and hot, dry air.

• Found in the western Great Plains in the United States

• Also found in China, India, and over Central West Africa

Dry Line

• Surface boundary between warm, moist air and hot, dry air.

New Mexico / Tx Panhandle

Abilene, TX

Houston, TX

Warm, Moist Air

Hot, Dry Air

Dry Line

• After sunrise, the sun will warm the surface which will warm the air near the ground.

• This air will mix with the air above the ground.• Since the air above the moist layer is dry (and

is much larger than the moist layer), the mixed air will dry out.

• The dry line boundary will progress toward the deeper moisture.

• The dryline tends to “jump” in discrete steps

Dry Line

Warm, Moist Air

Hot, Dry AirTop of moist layerbefore mixing

Boundary after mixing

Initial Positionof the Dry Line

Position of theDry Line after

mixing

New Mexico / Tx Panhandle

Abilene, TX

Houston, TX

Dryline Evolution and Movement

Here is the situation in the morning hours before surface heating creates thermals in the boundary layer:

New Mexico / Tx Panhandle

Abilene, TX

Houston, TX

Dryline Evolution and Movement

With time:

Dryline Evolution and Movement

With time, the dryline moves east as thermal turbulence mixes out the western part of the shallow moist layer.

Dry Line• After sunset, a nocturnal inversion forms and the

winds in the moist air respond to surface pressure features.

• The dry line may progress back toward the west – in sharp contrast to fronts (i.e. the dryline advances eastward in the day and retreats back toward the west at night)

• Dry line can act as a focal point for surface convergence and severe weather formation.

• When radar cells exist within 200 miles either side of the dryline, the first echoes usually develop within 10 miles of it

Southern Plains

Dry Line

Temperatures indegrees Celsius

©1993 Oxford University Press

-- From: Bluestein, Synoptic-Dynamic

Meteorology in Midlatitudes,

Volume II

Dry Line

Dry Line -- Southern Plains USA

Behind the Dry Line

Westerly winds (often strong)

Clear Skies

Warm Temperatures

Low Moisture

In Front of the Dry Line

S or SE winds

Hazy or Cloudy Skies

Warm Temperatures

High Moisture

Dryline Example

DFWABQ

Sounding West of the

Dryline

NCAR

Sounding East of the

Dryline

The Dryline•The dryline is observed generally from April through June about 40% of the time

•It’s observed from the southern Great Plains up into the Dakotas – from the Rockies eastward to about 96 degrees W longitude.

•It is easiest to identify with a moisture variable such as mixing ratio:

From Shaefer (86)

Storm Initiation Along the Dryline

Dryline Bulges

•Somewhat common

•80-100 km in scale

•Preferred locations for convective initiation

Shaefer (86)