Frost along a transect By Linda De Wet

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

CONDITIONS FOR FROST FORMATION RADIATION FROST Calm, cloudless skies No (or very light) wind Inversion ADVECTION FROST Cloudy skies Strong winds No inversion

FROST FORMATION Frost forms through deposition White or hoar frost Inversion Black frost No inversion Above-ground formation first

NIGHT (inversion) DAY Height (m) Temperature (oC) Temperature variation with height above earth surface, during day and night.

FROST DAMAGE Frost damage influenced by: Differences in climate during the season Cultivar types Morphology or growth stage Yield size Rate of Ta decrease Minimum Ta (Tmn) Time duration of Tmn

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 radiation frost conditions (Rosenberg, 1983)

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 probabilities

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.)

Reference by Loik and Redar (2003) Cold is an NB factor for productivity of plants > half of the earth’s surface = average Tmn of < 0 oC for a part of each year Ta near to soil surface = greatest variation, eg. Ta (1cm) = X oC (1m) – 7 oC Forest management – Forests react quicker wrt climate variations

Reference by Loik and Redar (2003) Frost risk calculated from energy balance and heat transfer processes at soil surface Experiment carried out by Loik en Redar: Aim: Seedlings at different heights exhibit ecotype differentiation with freezing tolerance & ability to undergo cold acclimation Higher gradient – different heights Seedlings of Great Basin Desert crop 3 x 2 different locations AWS data + Hobos (Ta) at 1 h intervals (autum 1997 to spring 1998) Loggers were placed underground

Reference by Loik and Redar (2003)

Reference by Loik and Redar (2003) Results:

Reference by Loik and Redar (2003) Note the differences in monthly Tmx and Tmn air temperatures for Bishop (1270 m) and Aspendell (2575 m) (38 oC – 6 oC = 24 oC vs 25 oC– 10 oC = 15 oC) Diff in Ta of lower-lying area is higher = not the norm (higher-lying = more sheltered?) Not shown: The lowest Ta (for all areas) was found at the highest altitude in Nov (winter) with 13 oC, and at the lowest altitude in December (9.2 oC) advection frost and radiation frost conditions, respectively.

Reference by Loik and Redar (2003) Ta at soil surface remained at 0 oC (snowy conditions). Even at the lowest altitude, the Ta was lower with less snow. Seedlings at higher altitudes were protected by crop covered surfaces (vice versa at flat surfaces-see Snyder). Crop covering and seedling establishments were greater at 2575 m than at 1775 m en 2175 m elevations.

Reference by Loik and Redar (2003) Freezing tolerance and cold acclimation depends on both meso- and microclimate Ta, crop covering, snow depth and snow melt patterns. Further research along gradients from high-elevation desert to montane (crop-covered) ecosystems are required.

Reference by Loik and Redar (2003) Conclusions for reference: Seedlings are more sensitive than adult plants Crop-cover influences surface Ta Snow can lead to increase in surface Ta Depending on frost conditions, Ta in valleys can be lower under radiation frost, calm conditions. Or Ta on mountain slopes could be lower (advection frost, windy conditions)

Generally Cold air drainage is less in open, flat areas and so frost risk is higher. Large differences in altitudes increase turbulence at the soil surface. Exposed surfaces (open areas)  higher radiation loss and larger energy losses. But in forests, a mixing of surface air with air above ↓ stratification of air inside forest areas  low frost risk. Because Ta increases with height at night, the Ta is higher at higher altitude.

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