What Is Frost ?

[Gardeneer]

Last night AccuWeather forecast 38F, Weather Channel 39F and my own weather station indicated 35F and we got frost.
Now how to explain that ?
What is FROST ?

[weaselbean]

Frost is that white stuff that makes tomato plants look like boiled turnip greens.
Mine spent the night in the house. They go in the ground Monday.

[Gardeneer]

Originally Posted by weaselbean

Frost is that white stuff that makes tomato plants look like boiled turnip greens.
Mine spent the night in the house. They go in the ground Monday.

Thanks to all for commenting.
Nothing happened to my over 50 tomatoes , 20 peppers, cukes, watermelon, corn an potatoes.
Only some eggplants got some bite.
As I mentioned the temperature did not go under 35F

[rhines81]

I Googled the cause of frost (source: weather.gov)

The following list are some meteorological conditions that can lead to frost conditions:

• Clear skies lead to radiational cooling, allowing the greatest amount of heat to exit into the atmosphere.
• Calm to light winds prevent stirring of the atmosphere, which allows a thin layer of super-cooled temperatures to develop at the surface. These super-cooled temperatures can be up to 10 degrees cooler than 4-5 feet above the surface, where observations are typically taken. For example, if conditions are favorable, air temperatures could be 36 F, but the air in contact with the surface could be 30 degrees or colder.
• Cool temperatures, with some moisture, that promote ice crystal development. If the super-cooled, freezing temperatures can cool to the dew point (the temperature at which, when cooled to at constant pressure, condensation occurs; moisture will have to come out of the atmosphere as fog, frost, etc) frost could develop on exposed surfaces. A local study done on frost formation relating temperature to dew point has these guidelines for frost: temperatures from 38 to 42 F can lead to patchy frost, 33 to 37 areas of frost, and 32 and below widespread frost/freeze. Note that the study did not factor in other considerations to frost, such as sky cover and wind speeds.
  
• Local topography has a large role in determining if and where frost develops. Cold air will settle in the valleys since it is heavier than warm air, therefore frost conditions are more prone in these regions. Valleys also shelter the area from stronger winds, enhancing the potential for frost.

Other local effects, such as soil moisture/temperature and stage of vegetation "greenness" are factors that can affect the possibility of frost forming.

[Cole_Robbie]

Originally Posted by rhines81

Clear skies lead to radiational cooling,

Radiational cooling is what causes a single-layer plastic greenhouse or high tunnel to drop below the outside temperature on cold, clear nights. So it is possible to cover your plants with plastic to protect them from a frost, yet have them freeze under the plastic, even though the air temperature outside did not get below freezing.

[rhines81]

Originally Posted by Cole_Robbie

Radiational cooling is what causes a single-layer plastic greenhouse or high tunnel to drop below the outside temperature on cold, clear nights. So it is possible to cover your plants with plastic to protect them from a frost, yet have them freeze under the plastic, even though the air temperature outside did not get below freezing.

Rationale thought would think that the low enclosure would contain the heat? There are 3 modes of heat transfer: convection, conduction and radiation. Radiation of heat is the oddest of the bunch. Insulation and materials of construction control the other two modes.

[Cole_Robbie]

Good point. If the plastic is low enough, it will get heat from the ground. The cooling below outside effect is from Cornell's research on high tunnels, but I don't know if it is the same with a low tunnel or row cover.

[oakley]

Sometimes early Spring and late Fall my neighbor gets hit with frost even without
an announced warning. Tenders like basil get hit first in the Fall. Mine have held on weeks past his and he is right across the street though different terrain. Our gardens
are about a football field apart. I'm uphill and he is nestled in a dip.

My soil is just a tad warmer. The morning dew and often fog holds longer just that small
dip downhill. My garden dries out quicker by almost an hour.

[Worth1]

Hoar frost is the coolest frost I have ever seen in my life.
Hoar coming from the old English word for old age and white hair as in hoary old witch.

Worth

[KarenO]

http://www.courierpostonline.com/sto...oint/70707594/

It's a combination of factors
KarenO

[slugworth]

A 5 letter word that really should be 4 letters.
I once had a neglected garden where the weeds were taller than the plants.
The weeds actually protected the plants from the frost.

[Gardeneer]

Originally Posted by slugworth

A 5 letter word that really should be 4 letters.
I once had a neglected garden where the weeds were taller than the plants.
The weeds actually protected the plants from the frost.

[Hoosier]

If you have a light frost where the temperature does not go below freezing for very long you can wash the frost off the plants to prevent damage. The damage comes when the sun comes up and the ice crystals act like hundreds of tiny magnifying glasses. If you can wash it off before the sun gets very high or if they are shaded for long enough that it melts before the sun hits them.

[KarenO]

The link explains the difference between a freeze and a frost.
KarenO

[Gardeneer]

Right. Radiation heat loss to the space is what happens in the deserts .
So that is what frost at 35F happened last night. It might even happen with tonights's low of 40 with clear starry skies.