Sport Pilot Talk

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?

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.

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.

Is there ever dew on the grass in the morning of a clear blue day?

CharlieTango wrote:Is there ever dew on the grass in the morning of a clear blue day?

Often?

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.

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.