#32 - Oct 2019 - Garden Stuff - © Sandy Lang - slang@xtra.co.nz
FROST September/October: Mid spring. Warmer. Pruning done. Buds breaking. Frost unlikely…
Frost: When air temperature falls below 0°C you get a ‘frost’...
•A ‘slight frost’ (−1 to −3°C) or a ‘moderate frost’ (−3 to −6°C). In Wellington we don’t usually get ‘severe frosts' (-6 to -9°C) or ‘very severe frosts’ (below -9°C).
•In Wellington we get about 10 (slight/moderate) frosts a year: Jun (2), Jul (3), Aug (3), Sep (1) plus a 50% chance of a frost in May and Oct.
•In Eastbourne we get fewer frosts and milder frosts (usually only 'slight' frosts) because we’re close by to the sea and it's especially windy…
Death: If plant cells freeze, they die... •Some plants are ‘frost hardy’. The cells contain antifreeze chemicals and can withstand a frost without freezing. •Most plants develop some ‘frost resistance’ as temperatures gradually fall in autumn. But they lose it again as temperatures gradually rise in spring. •So they can sometimes be caught out by an unexpected late-spring frost (e.g., after an early warm period) or by an unexpected early-autumn frost (e.g., after an especially warm autumn period).
Sea: The ‘thermal inertia’ of water is high, so sea temperatures change little through the year. The maximum is in February (about 17°C) and the minimum in August (about 12°C). Even in deep winter, Eastbourne air rarely gets to freezing (0°C), with the 'warm' sea so close by.
There are two sorts of frost: (a) Advective and (b) Radiative.
(a) Advective frosts - There’s no protection from a sub-zero Antarctic blast. Such low-temperature air flows (below 0°C) can destroy whole orchards. Happily, advective frosts are rare in NZ.
(b) Radiative frosts - At night, with no cloud and no wind, a leaf radiates heat to the sky. But the ‘night sky temperature’ is about −10°C, so there’s almost no ‘back radiation’. •So, leaf temperature falls to about 3C° below the surrounding air temperature (e.g. if air temperature is 7°C then leaf temperature falls to about 4°C). •This 'temperature depression' is enough for water to condense on the leaf (we call it dew).
But, if air temperature is less than about +3°C, then leaf temperature dips below 0°C and the dew freezes (we call it frost). Then, if the plant is not frost resistant, it freezes and dies.•Almost all Wellington's frosts are radiative.
Inversion: Under the same conditions (night, no cloud, no wind) the air at ground level (e.g., your lawn) soon cools down to leaf temperature (i.e. about 3C° below the temperature of the main body of air in your garden). This allows the radiating grass leaves to cool down even more.
•Cold air is heavy, so the cold air forms a ‘puddle’ on the ground.
•If you stand there with a thermometer, you will find the air at your toes is 3C° colder than the air at your nose.
•Colder air below, warmer air above is called an ‘inversion’ because, more usually, air gets colder as you go upwards (up a hill or up in a plane). •Even a light breeze will mix the cold air on the ground with the warmer air just above and prevent a radiative frost.
•Horticulturists use wind machines to mix up the air and prevent/mitigate inversions and associated radiative frosts. You see many of these in Marlborough to protect teh winegrapes. ___________________________________
FROST September/October: Mid spring. Warmer. Pruning done. Buds breaking. Frost unlikely…
Frost: When air temperature falls below 0°C you get a ‘frost’...
•A ‘slight frost’ (−1 to −3°C) or a ‘moderate frost’ (−3 to −6°C). In Wellington we don’t usually get ‘severe frosts' (-6 to -9°C) or ‘very severe frosts’ (below -9°C).
•In Wellington we get about 10 (slight/moderate) frosts a year: Jun (2), Jul (3), Aug (3), Sep (1) plus a 50% chance of a frost in May and Oct.
•In Eastbourne we get fewer frosts and milder frosts (usually only 'slight' frosts) because we’re close by to the sea and it's especially windy…
Death: If plant cells freeze, they die... •Some plants are ‘frost hardy’. The cells contain antifreeze chemicals and can withstand a frost without freezing. •Most plants develop some ‘frost resistance’ as temperatures gradually fall in autumn. But they lose it again as temperatures gradually rise in spring. •So they can sometimes be caught out by an unexpected late-spring frost (e.g., after an early warm period) or by an unexpected early-autumn frost (e.g., after an especially warm autumn period).
Sea: The ‘thermal inertia’ of water is high, so sea temperatures change little through the year. The maximum is in February (about 17°C) and the minimum in August (about 12°C). Even in deep winter, Eastbourne air rarely gets to freezing (0°C), with the 'warm' sea so close by.
There are two sorts of frost: (a) Advective and (b) Radiative.
(a) Advective frosts - There’s no protection from a sub-zero Antarctic blast. Such low-temperature air flows (below 0°C) can destroy whole orchards. Happily, advective frosts are rare in NZ.
(b) Radiative frosts - At night, with no cloud and no wind, a leaf radiates heat to the sky. But the ‘night sky temperature’ is about −10°C, so there’s almost no ‘back radiation’. •So, leaf temperature falls to about 3C° below the surrounding air temperature (e.g. if air temperature is 7°C then leaf temperature falls to about 4°C). •This 'temperature depression' is enough for water to condense on the leaf (we call it dew).
But, if air temperature is less than about +3°C, then leaf temperature dips below 0°C and the dew freezes (we call it frost). Then, if the plant is not frost resistant, it freezes and dies.•Almost all Wellington's frosts are radiative.
Inversion: Under the same conditions (night, no cloud, no wind) the air at ground level (e.g., your lawn) soon cools down to leaf temperature (i.e. about 3C° below the temperature of the main body of air in your garden). This allows the radiating grass leaves to cool down even more.
•Cold air is heavy, so the cold air forms a ‘puddle’ on the ground.
•If you stand there with a thermometer, you will find the air at your toes is 3C° colder than the air at your nose.
•Colder air below, warmer air above is called an ‘inversion’ because, more usually, air gets colder as you go upwards (up a hill or up in a plane). •Even a light breeze will mix the cold air on the ground with the warmer air just above and prevent a radiative frost.
•Horticulturists use wind machines to mix up the air and prevent/mitigate inversions and associated radiative frosts. You see many of these in Marlborough to protect teh winegrapes. ___________________________________