National Weather Service United States Department of Commerce

Heavy Rain From the Central Plains to the Mid-Atlantic; Heat Wave Begins This Weekend

Strong to severe thunderstorms and heavy rain will impact portions of the central Plains into the Upper Midwest and the Mid-Atlantic through the weekend. A stretch of unusually hot and humid weather is expected across portions of the central U.S. and the Southeast beginning this weekend and continuing through next week. The heat may reach dangerous levels, particularly mid to late next week. Read More >

 

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Density Altitude

diagram Density Altitude Concept
diagram

On a hot, muggy day, the air becomes “thinner” or less dense, and its density at a pilot’s location is equivalent to a higher altitude in the standard atmosphere.  It is hazardous to aircraft during takeoff and climb because:

- Aircraft power is reduced. The engine will ingest less air to support combustion.

- Aircraft thrust is reduced.  A propeller will have less "grip" on the air, while a jet engine will will exhaust less mass.

- Lift is reduced.  The thinner air will exert less upward force on the airfoils.

As a result, a longer takeoff and landing roll is required, especially for a fully-loaded aircraft.  After takeoff, the aircraft will have a smaller rate of climb and will have a lower service ceiling.

Density altitude is usually reported at each AWOS/ASOS station in the Remarks section of the METAR, if the value is greater than 1000ft higher than the elevation of the airfield.  Here's an example:

METAR KMSY 291400Z AUTO 18006KT 10SM CLR 28/13 A3001 RMK SLP161 DENSITY ALT 1000FT

 

 

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