Aviation Weather-Temperature Dewpoint Spread Why does it matter when flying helicopters or aircraft?
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- Опубліковано 30 вер 2024
- The temperature-dewpoint spread is the difference between the temperature and the dewpoint. Great, so what is the dewpoint exactly and why does this matter?
The dewpoint of a parcel of air is the temperature at which an air parcel would need to be cooled to become completely saturated with moisture (a cloud would form, or in more proper terms, the air can no longer hold the moisture). Remember that the warmer the air, the more moisture it can hold in gas form (as we cool the air without removing any moisture from it, the air becomes saturated with moisture).
Bottom line, if the temperature and dewpoint are far apart, the air has to be cooled a whole lot before it becomes saturated (meaning there isn’t much water or moisture in the air in the first place). If the temperature and dewpoint are close together, then the air has lots of moisture in it. If the temperature and dewpoint are the same, then you likely have fog, low visibility, lots of clouds, etc. since the air is saturated and now the moisture that was a gas is becoming a liquid (and visible).
How is moisture added to air?
The air gains moisture through two processes, evaporation and sublimation. Evaporation is when water goes from a liquid to a gas, such as a puddle on your driveway on a hot summer day that “evaporates”. Sublimation is when water goes from a solid to a gas, such as when snow or ice “sublimates” and adds moisture to the air without going through the liquid phase.
The Scenario
You depart on a clear VFR late afternoon for a destination that is forecast to be VFR at your time of arrival (your time of arrival is just after sunset). You note that the current METAR shows only a 3-degree difference between the temperature and dewpoint, but that means nothing to you because you had not yet taken this course. As you approach your destination airport you listen to the AWOS only to find visibility has dropped greatly and there is fog around the airport. This fog appeared due to the moisture in the air (and the air being nearly saturated already with that close temperature dewpoint spread) and the ground radiating its heat, cooling itself as well as the air just above ground level quickly as the sun disappeared for the day. You now will now have to divert to another airport as landing with a low layer of fog on the runway (even if you can see through it from above) is extremely dangerous and would likely result in a significant reduction of forward visibility and an exceedance of VFR minimums upon entering the fog. @Fly8MA
The glass of water analogy was great, thank you for this
I don’t get the part where “the aircraft radiates heat away and so it cools down below the ambient air”. I think that’s wrong.
I believe the correct explanation is that the aircraft which was at a much colder temperature when in flight has relatively quickly been brought into a warmer (high humidity) environment and so while the external surface of the landed aircraft may be at TOAT temperature (39F) momentarily after landing, the internal structure is far cooler (e.g. 20F) as it hasn’t had a chance to equalize with the outside air on ground. This produces a temperature gradient between the innermost/coldest part of the aircraft (at 20F) and the ambient air (at 39F) and if this gradient is sharp enough I.e. the aircraft has descended relatively quickly to warmer/high humidity air after a long cruise flight in colder air, it will quickly cool the skin back down to at or below freezing (32F) while the aircrafts sits unpowered in the parking area.
The reason why horizontal surfaces ice up first is because that’s where the fuel tanks are and the fuel at 20F will take much longer to equalize to outside air therefore the wings have a greater potential for such icing. On the other hand, the air inside the cabin is usually at much higher temperatures (e.g.70F) and so the gradient is usually reversed on most vertical surfaces (non-wing surfaces, more correctly) and so there’s zero possibility of icing at those areas.
Bottom line is if the ambient is assuredly above freezing, there is absolutely no way frozen moisture will stay frozen on the skin given enough time has elapsed for the aircraft reach equilibrium with its surrounding. Furthermore, if the ambient is above dew point temperature, even the thawed out moisture will evaporate back into the air after enough time has elapsed. I made a point to write this comment because I found the explanation as well the conclusion that in such occurrences, one has to wait until morning for higher TOAT before they could depart quite misleading. Depending on amount of fuel and the coldest temp. the aircraft has seen in cruise, the wait would mostly likely be less than a couple of hours for skin frost to disappear.
It also happens on cars. I get frost on my car (parked) when it gets down to 40F (36F dew point) but only on the roof and not the doors. The roof radiates heat away into the sky but the doors receive heat from walls and objects.
I also did not understood it initially, yet the video is right, its radiation the one creating the frost, remember objects can cool down on 3 ways, 1. Conduction, 2. Convection, 3. Radiation, you can look up the first 2 but basically its due to molecule to molecule heat transference, on the other hand Radiation cooling happens due to electromagnetic radiation, what this means is that any surface of the plane that is looking into the sky (specially on a cloudless night) is releasing thermal energy trough electromagnetic radiation into the space, this type of radiation usually wont interfere with surrounding water or air molecules ( it depends on the wavelength, with what type of material will interact this electromagnetic waves) but it will go directly into space, so to make it easier:
The upper surface of the wings are radiating heat trough electromagnetic radiation into space and only the space (not interacting with the surrounding air) so they cool down yet they absorb trough conduction temperature from the surrounding air and as a result the surrounding air cools down even more dropping its already low temperature below freezing temp and creates that frost over the wing, if the plane is left outside long enough the process will have time for it to create frost
If the air is moving or the plane its moved to a new location the process fo cooling the surrounding air will start again, so technically I believe the guy on the video could have started to fly and not worry about ice over the wings as the process would have been interrupted (however by regulation you must never operate an airplane that hast snow or ice on its surfaces) yet if the air is so cold at ground level is really likely it would be even colder up at flight level so unless my plane has anti ice systems I would not consider it.
If you want more info search Radiative cooling in wikipedia.
(sorry for the bad English I am not a native English speaking person)
Watched several videos about dewpoint (lectures, physics and etc.) and the your explanation (through aviation optics) was the one that really made me understand the concept! thank you.
Glad it was helpful!
thank you never under stood tiil now
Hey it's John from fly8ma.com!
He does a great job!
This was so simple and easy to understand. I love it.
At 1 degree difference, you need to stay away, Far Away... End of story
great video it should have more views thanks
Excellent explanation.👍
View all the free courses here fly8ma.com/courses/weatherxplore-lessons/
You get frost on a telescope and the reason you radiate so much is because it's radiating into space which is 3 degrees above absolute zero.
The beat explanation on UA-cam
Thank you for kind reply and feedback.
Please America, got metric and Celsius.
Thank you
Thank you!
Why would you use farenheit when talking about aviation? Pleease go metric.