This is what I know, low or no wind and pretty cold temps..........but can someone explain the phenomenon of frost a little bit better. How can you have frost and no freeze? Is there any way to predict when it will happen? What is the relationship to the dew point?.......I feel really dumb that I don't know this so I thought I would ask you folks here on PT.

Conditions for frost to form are:

Temps can be as high as the low 40S

Clear skies

High humidity with dewpoint at or below freezing

Light or no wind

Under windy conditions, surfaces such as car tops, grass, roofs, etc. are pretty much the same as the air temp but under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees. If these surfaces' temp drops to the dewpoint, condensation will occur on them, and if the dewpoint is at or below 32, it will be in the form of frost.

Under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees.

This is something that I don't understand. One of the principles of the second law of thermodynamics is that heat can not flow from a cold body to a warmer one, so how would this happen?

Under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees.

This is something that I don't understand. One of the principles of the second law of thermodynamics is that heat can not flow from a cold body to a warmer one, so how would this happen?

I am no PhD Thermodynamicist, but -----

Heat transfer on something like a windshield would occur as conductive transfer (glass touching the car transferring heat), convective transfer (heat transfer due to the movement of air, which can naturally occur due to the differences in density of the air, or can also occur if the wind is blowing) and radiative transfer (transfer related to the difference in radiation enetering the glass from that leaving the glass).

In the case of the skyward facing windshield, if the loss due to radiative effects to the sky are greater than the gains from the conductive and convective means, then the surface temp will drop. The drop will stop I believe when these heat transfer modes equalize (i.e. the temperature of the glass drops to the point that the heat transfer in from the conductive and convective means ends up being equal to that loss due to skyward radiation).

Conditions for frost to form are:

Temps can be as high as the low 40S

Clear skies

High humidity with dewpoint at or below freezing

Light or no wind

Under windy conditions, surfaces such as car tops, grass, roofs, etc. are pretty much the same as the air temp but under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees. If these surfaces' temp drops to the dewpoint, condensation will occur on them, and if the dewpoint is at or below 32, it will be in the form of frost.

I tested this a number of years ago and noted that, with very still air at ~41F and perfectly clear skies, the surface of a car windshield was measured (with a thermocouple) to be at 32F. I then observed (since dewpoints were high) slush beginning to form with time, although the air temps remaining at ~40-41F.

Under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees.

This is something that I don't understand. One of the principles of the second law of thermodynamics is that heat can not flow from a cold body to a warmer one, so how would this happen?

Heat flows from a warm body (your car windshield) to a cold body (outer space) via blackbody radiation. No violation of thermodynamics here. Air is a great insulator for thermal conductive processes but the drier it is, the more transparent it becomes to blackbody radiation.

During a radiative freeze, it is not the air that cools objects and the soil, but it is objects and the soil that cools the air as the objects and soil cool from radiation losses to outer space.

Under clear, calm conditions, these surfaces radiate more of their heat away and can drop significantly below the air temp, by as much as 10 degrees.

This is something that I don't understand. One of the principles of the second law of thermodynamics is that heat can not flow from a cold body to a warmer one, so how would this happen?

Heat flows from a warm body (your car windshield) to a cold body (outer space) via blackbody radiation. No violation of thermodynamics here. Air is a great insulator for thermal conductive processes but the drier it is, the more transparent it becomes to blackbody radiation.

During a radiative freeze, it is not the air that cools objects and the soil, but it is objects and the soil that cools the air as the objects and soil cool from radiation losses to outer space.

That clears things up, thanks!

Awesome explanations guys. I hate frost! I bet we lose more palms here from frost than from cold.

if the dewpoint is at or below 32, it will be in the form of frost.

This last bit may not be correct. My dewpoint last night was 37 degrees and frost formed. .......Should this be if the dewpoint is below the temperature of the air frost will form?..............My temps got down to 34 F. last night and dew point was 37 F....as I mentioned.

I like this topic! I understand better now how cooling works in our gardens! Can our fingers get cooler than the air temperature? I know it's a little off topic. But I could swear my fingers do. I have never checked with a thermometer though.

Just curious!!!

Randy

if the dewpoint is at or below 32, it will be in the form of frost.

This last bit may not be correct. My dewpoint last night was 37 degrees and frost formed. .......Should this be if the dewpoint is below the temperature of the air frost will form?..............My temps got down to 34 F. last night and dew point was 37 F....as I mentioned.

You are right, the original statement wasn't correct, frost can form on surfaces if the dewpoint is above 32F because surfaces cool below the air temperature.

However, your own temp measurements sound off. Your temp isn't likely to have been below the dewpoint, if your dewpoint was 37F and it was below 37F then you would have been completely fogged in, i.e. the air would have been saturated with moisture droplets that would precipitate out of the air. You can read about it here: http://lukemweather.blogspot.com/2011/05/why-is-it-so-difficult-to-cool-below.html.

I like this topic! I understand better now how cooling works in our gardens! Can our fingers get cooler than the air temperature? I know it's a little off topic. But I could swear my fingers do. I have never checked with a thermometer though.

Just curious!!!

Randy

Your body produces heat, so I seriously doubt your fingers would get colder than the air, it just feels that way.

In cold weather, your body reduces blood flow to your extremities to preserve heat to your core. So, your fingers are colder than say your armpits. You can warm them up by putting them in your waistband.

In cold weather, your body reduces blood flow to your extremities to preserve heat to your core. So, your fingers are colder than say your armpits. You can warm them up by putting them in your waistband.

But not any lower...

Back to frost ....... We had 2 mornings this week at 37 and 38 F. Very heavy frost! Heavier than the thanksgiving freeze IMO. We had zero wind and that's what did It I believe. Anyway, I lucked out in that I had used my thermal foam jackets on a few plants. The plants were undamaged. I noticed however that the frost that formed on the outside of the jackets was extremely thick almost like solid ice. I wonder if somehow the radiant barrier facing inward on the jackets allowed the outer surface to supercool even more.

Monday morning when I left for Texas, when had a really hard frost. I think I even say a thin sheet of ice over a few puddles. Now enough of this, it is time for spring.

BTW, when you have a day free, let me know and I will attempt top setup a Louisiana PRA. There are possibilities.

I stand corrected. The dew point doesn't have to be 32 degrees or lower, just the temp of the object. But the temp of the object has to be at or below the dew point. The dew point cannot be higher than the air temperature. When dew point= air temp, the humidity is 100%. The farther apart they are, the lower the humidity. Right now in Denver, the temp is 51* and the dew point is 11. The RH is 19%.

hand me some duc-tape....I think my head is about to explode....lol

Makes sense... you have to have dew to have frost. Frost is frozen dew. Surfaces cool below air temp if there is no wind. Ex. low temp is 35, dew point 36.... so dew forms.... there is no wind, so the surface of your king palm fronds cools to 32 due to radiation losses..... the dew on the fronds turns to ice..... you have new free mulch!

Makes sense... you have to have dew to have frost. Frost is frozen dew. Surfaces cool below air temp if there is no wind. Ex. low temp is 35, dew point 36.... so dew forms.... there is no wind, so the surface of your king palm fronds cools to 32 due to radiation losses..... the dew on the fronds turns to ice..... you have new free mulch!

That's almost how it works, except air temp will always be at or above the dewpoint. Keep in mind that if the dew is above 32F, then radiational losses are much lower, and the odds of the surfaces on king palms to drop below freezing are minimal. Live plant surfaces take considerably more cold to freeze than a car windshield. That's because the leaf is full of water.

So in the scenario above, dewpoint of 36F means when the air is around 39F, surfaces will cool faster and water condenses. As the air drops to the fewpoint, it saturates. So if the dewpoint is 36F and the air temp drops to 36F, then you have fog, and the fog prevents any further radiational losses.

Same is true with freezing fog. If the dewpoint is 31F and the air is 31F, you have freezing fog. That will not harm a king palm.

If the dewpoint is 26F, and the air temp is 30F, now you have a much worse situation. The king palm will slowly drop to the dewpoint, and when it reaches the dewpoint of 26F, frost forms on the leaves, which is below the freeze threshold of king palms. The fronds freeze solid and you have damage.

Sweet!. Your explanation. not frost. Why do lawns get frost way before leaves of trees?

Sweet!. Your explanation. not frost. Why do lawns get frost way before leaves of trees?

Two reasons:

1) It's on the ground where it's always several degrees colder than anywhere else because cold air sinks to the ground,

2) the grass actually insulates the soil, so the heat from the soil is trapped beneath the lawn and can't radiate upwards. So it gets even colder right above the lawn. A lawn is most definitely going to be the coldest place in the garden no matter what.

Put a thermometer flat on the lawn during a frost or freeze. When it might be 28F out there, the top of the lawn may already be 20F. It could be 35F 6 feet above the ground and the lawn might be totally iced over at 27F.

This is why all weather stations are generally put 6 feet off the ground.