| What is the temperature? If at
sunset the temperature is close to freezing then there is a better chance
for the formation of frost. Remember the temperature reported by your
local weather station is measured 1.5 meters above the ground, much higher
than your plants. So if the forecast is for a temperature of 4°C (39°F),
the temperature of your plants may be close to freezing. |
| How fast is the wind blowing? If
the wind is blowing strong then there is vertical mixing that inhibits the
formation of a radiation inversion and the cooling of the surface, and
thus frost. |
| What is the general weather
pattern? Be aware of the meteorological condition causing the wind! Is
a cold mass of air heading your way? If so, you had better
protect your plants or harvest. |
| What sort of cloud cover is
there? Clouds are good emitters of terrestrial energy so they reduce the
energy losses at the ground during the night. If it is cloudy, and will
stay cloudy, then the likelihood of frost is reduced. If it is to be a
clear night then frost is more likely to develop. |
| What is the condition of the
ground? If the soil is warm and moist, then the plants may be O.K.
Conduction will transfer heat upwards from below the surface and inhibit
the development of the temperature inversion. Also know your backyard. Is
your garden located in a local low spot which will allow cold air to drain
into it? |
| How long will the night be?
Energy losses exceed energy gains during night. The longer the night, the
more time the ground has to cool and so the colder it will get. |
| What is the dew point? This is a
very important question to ask yourself. A rule of thumb--if the dew point
is above 7°C (45°F) at sunset then you are probably OK. Below 4°C (40°F)
you better cover your plants. |
| Are you in doubt? If you're not
sure if frost will form, you might as well spend a few minutes and cover
the plants to enjoy them a little longer. |