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That's perfectly understandable. Except, it's just that I always thought dewpoint forms when the temperature MEETS the dewpoint temperature. Whereas, in this case, you'd see dew on the ground at a RH of 70%. Remember, there is no trace of rain. I see this 'phenomena' happening after a clear day.
That's what I thought, too. Are you sure the relative humidity wasn't 100% or very close to it at some point overnight. Just because the temperature and dewpoint are far apart late in the evening doesn't mean they will be at sunrise.
The other difference is as said, the ground cools faster than where air temperature is recorded.
^^^ A scientist at the local weather station, told me that the 1mm rain day level, was used to prevent dew measuring as a rain day -makes one wonder how rain day stats from 0.1 mm countries,can be taken seriously.
A light dew here this morning, even though it's 15C and 80% RH -must have been cooler during the night.
I don't know if they use it to count as a rain day, but the dew totals are certainly added to the monthly totals here. I just added up the dew from December at Luqa & it adds up to 3.0mm...
That's what I thought, too. Are you sure the relative humidity wasn't 100% or very close to it at some point overnight. Just because the temperature and dewpoint are far apart late in the evening doesn't mean they will be at sunrise.
The other difference is as said, the ground cools faster than where air temperature is recorded.
And this is of course what causes a "ground frost" in cooler climates where frost is visible on surfaces despite the air temperature not dropping to 0C/32F...
And this is of course what causes a "ground frost" in cooler climates where frost is visible on surfaces despite the air temperature not dropping to 0C/32F...
Isn't that the same in any climate that gets frost?
My climate gets 3x as many ground frosts as air frosts.
Isn't that the same in any climate that gets frost?
My climate gets 3x as many ground frosts as air frosts.
Yes obviously, but it was aimed at cooler, rather than cold, climates that may only get ground frosts & no air frosts. Some people don't understand how there can be a visible frost on grass, cars, roofs etc.. when the temperature didn't drop below 0C...
Colder climates would still have many nights with ground frost, but not air frost. I would have thought?
Nowhere as many as having both. You'd need to have nightly lows frequently in the 32-36°F range to get a frost but no freeze. Not all lows in the 32-36°F range creates frost on the ground — if its cloudy or windy, or the dewpoint is just above freezing, no frost. For my location, I get about 150 nights 32°F or below, 180 36°F or below.
That's what I thought, too. Are you sure the relative humidity wasn't 100% or very close to it at some point overnight. Just because the temperature and dewpoint are far apart late in the evening doesn't mean they will be at sunrise.
The other difference is as said, the ground cools faster than where air temperature is recorded.
Yes, very sure. Because I have always checked the current weather just after noticing the dew on the grass. The relative humidity has always been around 75-80%.
And it happened last night (after a hot 30C day). It was 12am. The temperature was 19C. The relative humidity was 75%. The grass was very wet. I'm guessing the ground temperature was much cooler (as what many people are saying here)?
And it happened last night (after a hot 30C day). It was 12am. The temperature was 19C. The relative humidity was 75%. The grass was very wet. I'm guessing the ground temperature was much cooler (as what many people are saying here)?
I've recorded grass frost in every month of the year here in Saskatoon, in most years. This past year, there was solid ice on my car windshield on the 22nd of August; a thermometer placed on the windshield indicated -1 C (30 F). Metal surfaces cool off rapidly just like grass does. The official low that morning was +2 degrees C (36 degrees F).
Air frosts on the other hand are rare in June and August, and virtually unheard of in July.
The most crazy difference that I observed in the last year was a -14 C (7 degrees F) ground temperature sometime in October when the official air temperature was only -6 C (21 F).
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