High Relative Humidity and Freezing Temp

[WannFly]

we are getting into a time in upper midwest where things have cooled down quite a bit. i am planning a XC to north shore (duluth, Grand Marais) this saturday - looks like the only day with some good flying WX and watching the weather like a hawk.

backup plan - drive or scrap the trip, i will have 2 pax.

checking the WX as on today (i know things can and probably will change between now and sat), i am seeing a large area in my route of flight having high relative humidity, near freezing or below freezing temp and this is giving me a pause.

low freezing level is not something new to me, it pretty much starts around this time and continues till about May around here and i am also aware that as long as i dont fly into visible moisture or cloud, i am covered. but the combination of high RH, low temp is making me think that those molecules of moisture in the air when disturbed or even on the ground would instantly stick to my air frame. to back up the theory, NOAA is reporting wide spread frost till about 9 AM and i see the RH dropping to high 40's if i take off say around 10 instead of 8.

am i on the right track? completely off track? somewhere in the middle?

on a diff note, i have been using WeatherSpork and its awesome. can i get all those info by scouring 5-7 different places? sure. but this app brings those information to me and i have cancelled a few flights based on the info i found on the app and all those info were spot on.

This is what i am seeing if i take off at 8 AM



if i take off at 10 AM, things improve quite a bit, the freeing level is lifting (i will still have to fly through it), but the RH improves quite a bit.



no precip predicted and we should have a wide area of high pressure in the area.

what does collective POA brain-trust say?

 

[Mr.T]

I’d look at the temperature / dew point spread. When it’s within a couple of degrees and temps at or near freezing, you might had reason to be a little more concerned.

But unless there’s visible moisture, I don’t think I’d be worried about a high humidity (well, maybe 97% would get my attention...)

 

[iamtheari]

Relative humidity is a function of the spread between the temperature and the dew point. See how the temperature goes up and the RH goes down? The dew point is probably constant throughout that period. Now think back to the written test, which covered the conditions in which frost forms... Frost forms when the temperature of the collecting surface is below the dew point and the dew point is below the freezing temperature. So if you put a -20C airplane into air with a dew point of -10C, you can form frost. But most of the time, the plane is at the same temperature as the surrounding air, so if the temperature of the plane is below the dew point then so is the air temperature, and you are in a cloud.

I did manage to get frost on a plane one time when there was some cold but thin fog that I taxied through on the way back to the hangar after a night cross-country lesson. So it is a concern. But unless you fly through a front or climb/descend through a temperature inversion, you shouldn’t pick up frost from clear air. And unless you are in freezing rain or cold clouds, you shouldn’t pick up any ice.

BTW I got to log some actual IMC time this week when I flew to Grand Forks at 5,000 feet. I picked that altitude because, even though it was going to be in and out of cloud bases all the way, it was also going to be no less than 36 or 37F all the way and 7,000 feet would have been less time in clouds but all of that time at risk of icing. And then today I got to log some bonus cross-country time and nearly finish my audiobook on the way home because of a 40-knot headwind at all altitudes. Fall is just a great time to fly because it forces you to think hard about the weather. Good luck!

 

[WannFly]

Relative humidity is a function of the spread between the temperature and the dew point. See how the temperature goes up and the RH goes down? The dew point is probably constant throughout that period. Now think back to the written test, which covered the conditions in which frost forms... Frost forms when the temperature of the collecting surface is below the dew point and the dew point is below the freezing temperature. So if you put a -20C airplane into air with a dew point of -10C, you can form frost. But most of the time, the plane is at the same temperature as the surrounding air, so if the temperature of the plane is below the dew point then so is the air temperature, and you are in a cloud.

I did manage to get frost on a plane one time when there was some cold but thin fog that I taxied through on the way back to the hangar after a night cross-country lesson. So it is a concern. But unless you fly through a front or climb/descend through a temperature inversion, you shouldn’t pick up frost from clear air. And unless you are in freezing rain or cold clouds, you shouldn’t pick up any ice.

BTW I got to log some actual IMC time this week when I flew to Grand Forks at 5,000 feet. I picked that altitude because, even though it was going to be in and out of cloud bases all the way, it was also going to be no less than 36 or 37F all the way and 7,000 feet would have been less time in clouds but all of that time at risk of icing. And then today I got to log some bonus cross-country time and nearly finish my audiobook on the way home because of a 40-knot headwind at all altitudes. Fall is just a great time to fly because it forces you to think hard about the weather. Good luck!

that makes a lot of sense. Thanks. Now hoping the wx stays as predicted!