WEATHER GURU | Unpack those winter woollies, cold conditions about to start

Garth Sampson
Garth Sampson

A few weeks ago, many were asking if winter was late, with some thinking it would  never come after maximum temperatures hit the 30s mark with the berg wind conditions we were receiving.

Little did most know that winter was waiting in the wings and creeping closer like a thief in the night.

On Sunday morning, while enjoying a lie-in, the minimum temperature dipped to well under 10°C (7.4°C to be exact) for the first time this year.

The dew is forming more often than not, and we know that next on the list is that beautiful white carpet of frost, which we often see on suburban lawns in winter.

Many ask why dew and especially frost is more prevalent in winter.

Dew and frost need clear skies for maximum outgoing radiation, little to no wind and sufficient soil moisture.

Hence, the dew point temperature would be close to the surface temperature.

Dew thus forms when the temperature and dew point temperatures are equal.

For frost to occur, the temperature must be under freezing point, 0°C, causing white crystals on vegetation and other objects.

Black frost is a farmer’s nightmare and forms when the temperature is well below freezing with low humidity.

In other words, instead of ice-crystals forming on the plant, the sap actually freezes.

We are less than a month away from the winter solstice (longest night and shortest day) which many believe is the middle of winter, but it is in fact the start of winter.

There are many reasons for this and it is what is called “seasonal lag”.

The winter solstice (shortest day) denotes the lowest angle of the sun, and thus maximum heat loss, which continues for some time thereafter due to thermal inertia.

Longer days are a slow process and eventually lead to warmer temperatures.

The initial heating is, however, outweighed by the release of heat stored in the oceans, one of our main drivers of weather patterns.

The statistics tell us that the most number of days with minimum temperatures below 5°C in the Bay, are in July with an average of 5.6 days, followed by June with 4.1 days.

On the odd occasion, the minimum has dropped to -0.5°C in June right through till early in September, when -0.7°C was measured in 2008.

By the time of reading, the conditions should be windy, cloudy with showers, giving us the impression that winter is really starting to set in.

The real cold conditions will only be realised later in the season, when we have a few days of cold and wet conditions with snow on the high-lying grounds and mountains.

When conditions clear, the cloudless nights following are when the temperatures drop to all-year lows and blankets of frost turn our Bay into our own little winterland “of sorts”.


Combined dam levels

Algoa System: Down from 71.66% to 71.23%

This week in history:

2007:  East London and the Bay experience very cold conditions with two fatalities in East London and four  in the Bay.

Weather Safety Tips:

In winter, whenever undertaking long trips out of town, always check if passes are open or closed due to icing or snow.

Always have extra clothes and water in your car in case you get stuck.

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