Dew cover and Ice

[Arch Stanton]

At first light I have seen grass with a dew cover, but as it gets lighter towards sunrise the dew sometimes turns to frost - why the dip in temperature?

Is it as simple as the coldest part of the day in such calm anticyclonic conditions being the the minute before sunrise?

[Methuselah]

10 minutes ago, Arch Stanton said:

At first light I have seen grass with a dew cover, but as it gets lighter towards sunrise the dew sometimes turns to frost - why the dip in temperature?

Is it as simple as the coldest part of the day in such calm anticyclonic conditions being the the minute before sunrise?

I've often pondered that one myself, Arch, and I came to the conclusion that the coldest part of the day (under the conditions you mention) is an hour or so after sunrise -- until the sun reaches a certain angle, outward radiation will still exceed what's coming from the sun?

[Arch Stanton]

8 minutes ago, Ed Stone said:

I've often pondered that one myself, Arch, and I came to the conclusion that the coldest part of the day (under the conditions you mention) is an hour or so after sunrise -- until the sun reaches a certain angle, outward radiation will still exceed what's coming from the sun?

That makes sense Ed, thanks

[Nick L]

On 08/11/2022 at 10:25, Ed Stone said:

I've often pondered that one myself, Arch, and I came to the conclusion that the coldest part of the day (under the conditions you mention) is an hour or so after sunrise -- until the sun reaches a certain angle, outward radiation will still exceed what's coming from the sun?

This is exactly it. Doing roads forecasting means we have to monitor roads temperatures extensively through the winter months. Surface temperatures can keep dropping for up to an hour after the sun rises, particularly in areas of shade. Can catch local authorities out at times.

[ANYWEATHER]

Yes certainly true.  I've noticed this many times. The hour or so after sunrise in the Winter can be a harbinger for frost to develop and is often the coldest part of the day. Happens a lot when rain as cleared overnight say with a westerly airmass , but given a couple of hours of clear skies and zero wind ,a ground frost usually occurs with temperatures dropping to about 2/ or 3c..cold enough for grass frost and frost on the cars...

[mfhblue]

This might not exactly answer the question why frost forms on grass at sunrise, but it is an interesting (and possibly connected) phenomena.  I am sure many have noticed after a cold night how the temperature often falls a bit further at sunrise.  The last two mornings this has been really noticeable on my weather station.  It was already extremely cold at dawn but the temperature fell quickly by another 1-2c in the next half hour both mornings.  I did a bit of research earlier and found an explanation for this.

As I am sure we all know, on a still night the temperature is often colder at ground level than the temperature at 2m (the typical height at which it is measured), hence you can have a ground frost when the air temp is above 0c.  It seems that the first weak rays of sunshine in the morning warm the ground by a tiny amount, just enough to cause a little bit of convection and mixing of the air at the surface with the air just above.  Hence the very cold air at ground level is mixed with the air just above it causing the measured air temperature to fall at 2m level.

Maybe the phenomena are connected in some way and someone more qualified can explain.  I am thinking possibly something to do with some of the dew evaporating as part of this micro-scale convection process, and in turn causing a cooling of the remaining dew on the grass.