Temp/Dew Point Spread

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HAPPYDAN
Posts: 390
Joined: Mon Mar 16, 2015 11:49 am

Temp/Dew Point Spread

Post by HAPPYDAN »

Calling all Weather Wizards! I was told or read somewhere, that if the temperature-dew point spread was less than 2 degrees Celsius, you could expect low clouds or fog with possible precip to form within the hour. Well, I just checked the 1353Z METAR for KTIW (Tacoma Narrows) and the spread is 1 degree, 97% humidity. Looked out the window, bright blue sky. Hmmm. Since most of our weather comes from the southwest, I checked the METAR and TAF for KOLM (Olympia). Same deal, but no fog or precip forecast for the next 12 hours. Hmmm. What am I missing here?
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MrMorden
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Location: Athens, GA

Re: Temp/Dew Point Spread

Post by MrMorden »

HAPPYDAN wrote:Calling all Weather Wizards! I was told or read somewhere, that if the temperature-dew point spread was less than 2 degrees Celsius, you could expect low clouds or fog with possible precip to form within the hour. Well, I just checked the 1353Z METAR for KTIW (Tacoma Narrows) and the spread is 1 degree, 97% humidity. Looked out the window, bright blue sky. Hmmm. Since most of our weather comes from the southwest, I checked the METAR and TAF for KOLM (Olympia). Same deal, but no fog or precip forecast for the next 12 hours. Hmmm. What am I missing here?
I think the temp might also have to be low enough for the moisture to stick around. Above a certain temp the moisture in the air vaporizes too quickly and doesn't form fog or mist. It will almost certainly be hazy though up to some altitude.

DISCLAIMER: I AM NOT A METEOROLOGIST, SO ALL OF THE ABOVE IS PROBABLY BS. :D
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA
Cluemeister
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Re: Temp/Dew Point Spread

Post by Cluemeister »

When I head to the airport in the morning, quite frequently the numbers are the same. Then as the morning progresses, the spread gets larger.

My memory is when the spread starts to close is when you need to look out.

Same disclaimer as Andy, what I just told you is probably wrong. :)
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CharlieTango
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Location: Mammoth Lakes, California

Re: Temp/Dew Point Spread

Post by CharlieTango »

Is there ever dew on the grass in the morning of a clear blue day?
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FastEddieB
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Re: Temp/Dew Point Spread

Post by FastEddieB »

CharlieTango wrote:Is there ever dew on the grass in the morning of a clear blue day?
Often?
Fast Eddie B.
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Flocker
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Re: Temp/Dew Point Spread

Post by Flocker »

HAPPYDAN wrote:Calling all Weather Wizards! I was told or read somewhere, that if the temperature-dew point spread was less than 2 degrees Celsius, you could expect low clouds or fog with possible precip to form within the hour.
Not necessarily. Season, time of day, terrain, wind, etc. all have an effect on the temperature/dew point relationship.

Take a look at Chapter 2 of "Weather Flying" by Robert Buck.
Aviation Real Estate Broker
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designrs
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Re: Temp/Dew Point Spread

Post by designrs »

This is where knowing your local weather systems is very valuable.

Agree, it's a narrowing trend of the dew point spread that is of concern, usually occurring around sunset / evening. This is especially true in coastal areas.

Many locals on Long Island want to see at least a 5 point dew point spread against the fog rolling in towards evening.
- Richard
Sport Pilot / Ground Instructor
Previous Owner: 2011 SportCruiser
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