FROSTFROST PREVENTION PREVENTION

ByByLinda De WetLinda De Wet

WHAT IS FROST?WHAT IS FROST?Frost is defined in the Frost is defined in the Glossary of Glossary of Meteorology Meteorology as the condition that as the condition that exists when the temperature (Ta) exists when the temperature (Ta) of earth’s surface and earth-bound of earth’s surface and earth-bound objects fall below freezing, i.e. 0 objects fall below freezing, i.e. 0 C C (Rosenberg, 1974).(Rosenberg, 1974).

CONDITIONS FOR FROST CONDITIONS FOR FROST FORMATIONFORMATION

RADIATION FROSTRADIATION FROST– Calm, cloudless skiesCalm, cloudless skies– No (or very light) windNo (or very light) wind– InversionInversion

ADVECTION FROSTADVECTION FROST– Cloudy skiesCloudy skies– Strong windsStrong winds– No inversionNo inversion

FROST FORMATIONFROST FORMATION

Frost forms through sublimationFrost forms through sublimation

White or hoar frostWhite or hoar frost– InversionInversion

Black frostBlack frost– No inversionNo inversion

Above-ground Above-ground

formation firstformation first

NIGHT(inversion)

DAYHe

igh

t (m

)

Temperature (oC)

Temperature variation with height above earth surface, during day and night.Temperature variation with height above earth surface, during day and night.

Note

On earth’s surface at different altitudes:

•Windy day or night – Ta is lower at a higher altitude Advection frost

•Calm day or night – Ta is lower at a lower altitude (valleys) Radiation frost

Course of air and soil temperature and wind speed during Course of air and soil temperature and wind speed during radiation frost conditions (Rosenberg, 1983)radiation frost conditions (Rosenberg, 1983)

-10

-5

0

5

10

15

20

25

30

0:00

1:00

2:00

3:00

4:00

5:00

6:00

7:00

8:00

9:00

10:0

0

11:0

0

12:0

0

13:0

0

14:0

0

15:0

0

16:0

0

17:0

0

18:0

0

19:0

0

20:0

0

21:0

0

22:0

0

23:0

0

Time (30min intervals)Half-hourly temperatures over a 24 h period measured along a transect at four

locations at different altitudes for two frost days (12-07-2003 and 20-07-2003) in Bloemfontein.

Tem

pera

ture

(oC

)Saint Church Sue Police Saint Church Sue Police

Hill

House

Church

School

SAWS

School

Hill

House

Church

SAWS

1340

1360

1380

1400

1420

1440

1460

1480

-10 -8 -6 -4 -2 0 2 4 6 8

Tmn(oC)

Alti

tude

(m)

Results for an experiment measuring minimum temperature profile for 4 different locations at different altitudes as compared to the measured Tmn

(SAWS) for 22/8/03 and 10/05/03 for a suburb in Bloemfontein.

FROST DAMAGEFROST DAMAGEFrost damage influenced by:Frost damage influenced by:

Differences in climate during the seasonDifferences in climate during the season

Cultivar typesCultivar types

Morphology or growth stageMorphology or growth stage

Yield sizeYield size

Rate of Ta decreaseRate of Ta decrease

Minimum Ta (Tmn)Minimum Ta (Tmn)

Time duration of TmnTime duration of Tmn

Frost probabilitiesFrost probabilities

HOW TO PREVENT FROSTHOW TO PREVENT FROST

Consider energy balance of soil surface Consider energy balance of soil surface at nightat night

Aim: to maintain energy status of soil-Aim: to maintain energy status of soil-plant-air volume to be > critical Ta in 3 plant-air volume to be > critical Ta in 3 ways:ways:

Retard Retard energy loss from systemenergy loss from system

RedistributeRedistribute existing energy in system existing energy in system

AddAdd new source of energy to system new source of energy to system

1. Site selection1. Site selection

Cold air drainage to valleysCold air drainage to valleysLocal weather records Local weather records Soil types- Dark soilsSoil types- Dark soils

- Light- Light- Gravel - Gravel

Windbreaks – frost pocketsWindbreaks – frost pocketsFences/shadenetsFences/shadenetsHeavy vegetation Heavy vegetation Water bodies Water bodies Wind direction.Wind direction.

2. Radiation Interception2. Radiation Interception

Artificial clouds Artificial clouds back-radiation back-radiation

-Spraying water into the air-Spraying water into the air

-Burning oil or tyres for smoke-Burning oil or tyres for smoke

Environmental authoritiesEnvironmental authorities

Smaller burners Smaller burners labour intensivelabour intensive

3. Thermal insulation3. Thermal insulation

Clear day Clear day solar radiation warming soil solar radiation warming soil

radiation frostradiation frost

Small plants covered with black plastic or Small plants covered with black plastic or

brown paperbrown paper

Trees – wrap trunks withTrees – wrap trunks with

fiber-glass, cardboard,fiber-glass, cardboard,

polystyrenepolystyrene

Cover cold-tender plants e.g. basil, Cover cold-tender plants e.g. basil,

peppers, tomatoes, geraniums and peppers, tomatoes, geraniums and

impatients with blankets, sheets, impatients with blankets, sheets,

newspapers, etc. (excluding plastic).newspapers, etc. (excluding plastic).

Protection at -6.7 Protection at -6.7 ooC using frost-cloth.C using frost-cloth.

Drape plants completely – no openingsDrape plants completely – no openings

3. Thermal insulation3. Thermal insulation

Above reduces rate of Ta change and Above reduces rate of Ta change and

decrease in time period where decrease in time period where

T < 0 T < 0 CC

Disadvantage = disease / pest problemsDisadvantage = disease / pest problems

3. Thermal insulation3. Thermal insulation

4. Air mixing4. Air mixing

FansFansWind machinesWind machinesHelicoptersHelicopters

HeatersHeaters

5. Direct air- and plant-heating5. Direct air- and plant-heating

6. Soil manipulation6. Soil manipulationIrrigation and compaction or rolling soilIrrigation and compaction or rolling soil

of 0.4 of 0.4 C in soil TaC in soil Ta

Rolling soil + irrigation Rolling soil + irrigation > 0.6 > 0.6 C in soil C in soil Ta (1.1 Ta (1.1 C to 1.4 C to 1.4 C)C)

Soil covers hold soil heat in, e.g. plastic Soil covers hold soil heat in, e.g. plastic but should be but should be

opened early opened early

morning to release morning to release

heat to the plants.heat to the plants.

Factors influencing soil surface Factors influencing soil surface Ta at nightTa at night

Soil surface TaSoil surface Ta

HighHigh LowLow

Good exposure to sunGood exposure to sun Open to sky at nightOpen to sky at night

Cloudy nightCloudy night No cloudsNo clouds

Windy weatherWindy weather No windNo wind

Compact soilCompact soil Loose soilLoose soil

Moist soilMoist soil Dry soilDry soil

Bare soilBare soil Cover crop or mulchCover crop or mulch

7. Application of water7. Application of water

Water temperature > 0 Water temperature > 0 CCWater can give 0.374 MJkgWater can give 0.374 MJkg-1-1 + heat of + heat of melting (by 0 melting (by 0 C) = 0.334 MJkgC) = 0.334 MJkg-1-1

Sprinkler or flood irrigationSprinkler or flood irrigation

Non-sprinkled and sprinkled apple treesNon-sprinkled and sprinkled apple trees

Average bud temperature for non-sprinkled and sprinkled apple trees, as compared with air and wet-bulb temperature in May at Logan, Utah

(Rosenberg, 1983).

In the same time period (15:30):In the same time period (15:30):Ta = 25 Ta = 25 ooC and Tw = 15 C and Tw = 15 ooCCNon-sprinkled apple treesNon-sprinkled apple trees– Ta = 45 Ta = 45 ooCC

sprinkled apple trees sprinkled apple trees – Ta = 15 Ta = 15 ooC C

Difference of 30 Difference of 30 ooC for sprinkled and non-C for sprinkled and non-sprinkled!sprinkled!

a)a) Sprinkler irrigationSprinkler irrigation-At onset of freezing T, water freezes on plants and heat of -At onset of freezing T, water freezes on plants and heat of

melting is releasedmelting is released

-As freezing process proceeds-As freezing process proceeds – – ice stays at 0 ice stays at 0 C, but there C, but there must also be enough heat for must also be enough heat for back radiation, evaporation and back radiation, evaporation and convection, sprinklers mustconvection, sprinklers mustcontinue after sunrise till all icecontinue after sunrise till all ice has meltedhas melted

b)b) Flood irrigation:Flood irrigation:-gives heat to soil-gives heat to soil-Can increase Ta by up to-Can increase Ta by up to 3 3 C.C.

8. Chilling to prolong dormancy8. Chilling to prolong dormancy

Use sprinkler irrigation during the day in Use sprinkler irrigation during the day in winter and early spring to cool air and to winter and early spring to cool air and to delay bud development.delay bud development.

Must be for minimum time period under Must be for minimum time period under specific Ta.specific Ta.

Can delay development byCan delay development by

8 to 17 days for apples 8 to 17 days for apples

and peaches.and peaches.

9. Other methods9. Other methodsIn Vineyards:In Vineyards:

– InsuranceInsurance – Frost AlarmFrost Alarm – Fog machinesFog machines – Vine guardsVine guards – Physical Physical

coverings coverings – Electric currentsElectric currents

- - ViniCable.ViniCable.

The average number of frost-free days per year is projected to rise across much of the globe by the 2080s, with the largest increases (red and orange) across the western fringes of North America and Europe. (Illustration courtesy Gerald Meehl and Claudia Tebaldi, NCAR.)

CONCLUSIONSCONCLUSIONS

Frost is a great threat to most cropsFrost is a great threat to most crops

Effects of frost Effects of frost – Differences during seasonDifferences during season– CultivarsCultivars– Morphology stageMorphology stage– Yield sizeYield size– Rate of temperature decreaseRate of temperature decrease– Minimum temperature Minimum temperature – Time duration of minimum temperatureTime duration of minimum temperature

High value fruit and vegetablesHigh value fruit and vegetables

Little data available on yield decreases Little data available on yield decreases relative to minimum temperatures relative to minimum temperatures scope scope for research in this area is relatively widefor research in this area is relatively wide

There is no cure for crops or plants There is no cure for crops or plants damaged by frost, so the only other damaged by frost, so the only other alternative left is to practice the prevention alternative left is to practice the prevention methods described. Prevention, after all, is methods described. Prevention, after all, is better than curebetter than cure

CONCLUSIONSCONCLUSIONS