An answer to a friends question.


Gyroplane CFI
Oct 30, 2003
Givens Predator
Total Flight Time
2400+ in rotorcraft
My friend John emailed me requesting information about my minimum temperature/dew point spread of 4 degrees C for takeoff from the Santa Maria Public Airport (KSMX) in my VFR gyroplane that I have often written about.

In response to my email John who flies his American Ranger gyroplane out of Virginia wrote; “One morning last week it was VFR when I got to the airport, ambient temp and dew point were the same and half hour later it was 200 ft overcast.”

“Probably a good topic for you to post on.”

Below is my response to John’s question.

For the non-pilots here in the USA a temperature/dew point spread is usually a part of aviation weather.

Dew point is the temperature where moisture in the air become visible; AKA fog.

I wish more people would ask that question John.

Many people imagine that fog rolls in or it may get blown in.

Fog (visible moisture) forms when the temperature and the dew point are the same and what you see is actually cold air rolling in or even forming around you.

It may happen quite suddenly.

My personal minimum at my airport on the California Coast is four degrees.

We have a lot of microclimates here and the temperature and dew point available are what it is at the airport and may be very different just a few miles away.

My experience flying here has led to a personal minimum of four degrees.

Any less and I am likely to encounter fog so I don’t takeoff.

Around here the warm ground heats the air and it rises sucking the cooler air over the ocean inland.

Oceans, Lakes and marshes can cause visibility to change very quickly.

My personal limit is six degrees at night because if there are no lights I can't see the fog ahead.

The stars may be blocked by a high overcast.

You need to develop your own minimums based on the local weather patterns.

Chapter 12 of the Pilot's Handbook of aeronautical knowledge has a lot of useful weather information:

When I am in an unfamiliar area I like to see a minimum of six degrees C of temperature/dew point spread.


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