Opinion

OPINION: Cold? It’s Colder Than They Say

Dr Sushil K Sharma BA MA MEng PhD is an Associate Professor of Meteorology at the Fiji National University. The views expressed are his alone and not that of his
30 Jun 2015 13:56
OPINION: Cold? It’s Colder Than They Say
jackets, pullovers are out on temperatures decrease says writer.

Dr Sushil K Sharma BA MA MEng PhD is an Associate Professor of Meteorology at the Fiji National University. The views expressed are his alone and not that of his employers.

 

Fijians definitely have been experiencing very cold conditions over the last month to date. In fact we have been experiencing thermal comfort conditions as much as 5-6 degrees Celsius lower than that measured by our National Meteorological Centre observing network, or temperature forecasts by them—especially so, during periods of recent 25-30 knot southerly winds gusting to 45 knots over the entire region, over and south of Fiji.

The entire Southern Hemisphere  during the last month to date, as is also the case during this time of the year,  around latitude 30-40 degree South has had as many as 7 to 8 major anticyclones (areas of high pressure) with closed anti-cyclonic wind circulation.

These high pressure cells have been very intense indeed with central pressures of around 1022 hPa to 1041 hPa. The 1041 hPa pressure is almost unprecedented and definitely one of the few highest ones noted historically.

Further over the South Pole and covering the entire continent of Antarctica between 80 degrees south and 90 degrees south, is covered with an intense high pressure system, due to the extreme cold with high density subsiding air, from the upper atmosphere.

Between this high pressure region over the South Pole, and the belt of high pressure systems circling the entire Southern Hemisphere in a west to east direction, in a latitudinal direction of about 35 degrees south, is also a “grave yard of lows”.

 

Graveyard of lows

This “grave yard of lows” are intense low pressure systems, as many as 12 -15 of them with central low pressure systems, between the range of 950 hPa and 985 hPa. These lows are also moving eastwards zonally around the entire Hemisphere, in a west to east direction at around latitudes 60-70 degrees South.

Thus in essence the area around the entire Southern Hemisphere between latitudes 15 degrees south to 40 degrees south is covered with subsiding air from the upper atmosphere, leading to enhanced, persistent, and stable high pressure systems.

This leads to marked downward motion of air which hardly allows for much thermal convection. In contrast the air is very stable. On the surface, these mobile eastward moving high pressure system lead to almost a constant supply of cool and dry air mass, moving onto the tropical regions from the far south.

It has been noted that in the last two months, these mobile, but very intense high pressure systems, which have often been more enhanced over the colder regions of the state of Victoria and South Australia have been upwards of 1040 hPa in strength, and have often extended a ridge of very high pressure towards the immediate southern parts of Fiji.

The grave yard of lows traversing just south of Tasmania and South Island of New Zealand, have kept the pressure gradient very intense, and thus maintained  very strong to near gale force winds from the southerly quadrant over vast areas south of Fiji, and even extending over us.

 

Enhanced cold air

In the last 2-3 weeks we have had enhanced cold air moving onto us directly from the South at the surface, but also from the upper atmosphere, transporting cold air onto us from the high latitudes, via the Hadley circulation—which transports air from the colder higher latitudes to the mid-latitudes over and south of us.

These processes are in fact helping with the transport of air masses from one global region to the other, to ensure that we have a global equilibrium of temperature. If this was not the case, our polar regions would freeze over and the tropical regions would heat up, much more than possible for human habitation.

The Hadley circulation including the synoptic systems of high pressure and low pressure systems, cold fronts, depressions and pressure gradient and wind flow—are there helping with the transfer of mass, momentum, energy and thus a natural part of the earth’s weather and climate system.

It should be noted that due to the intense high pressure systems and the ridges of high being aligned in the last 3 weeks in a manner conducive to very strong southerly winds over Fiji, Fijians have quite rightly observed that they felt that it was much colder than usual.

This is true as, the persistent transport of cold air from the south together with very strong winds, have definitely reduced the temperatures over us.

Fijians definitely has been experiencing very cold conditions over the last month to date. In fact we have been experiencing thermal comfort conditions as much as 5-6 degrees Celsius lower than that measured by thermometers.

It is true that the Fiji Meteorological Service cannot tell you how cold it is, and neither can many of the global National Meteorological Services, who are relying on the thermometer —maximum, minimum and dry bulb measurements to do so.

Thermometers still accurately measure the air temperature in a ventilated enclosure away from sunlight, wind and rain. But humans do not perceive weather like thermometers. We endure humidity, wind chill, direct sunlight and a host of other factors.

 

Apparent temperature

People have recognised in Fiji the last few weeks that they felt much colder than what the observed or forecast meteorological temperature had indicated.  This “felt” temperature is officially known as “apparent” or “perceived” temperature.

Apparent temperature —the felt temperature—can be calculated by using measures of the thermometer temperature, humidity and wind chill, down to obscurities such as walking speed, “skin resistance to heat transfer” and “clothing resistance to moisture transfer”. Some apparent temperature models are basic, such as wind-chill and wet-bulb temperature, whilst others are very complex and involve more than a dozen factors.

It should be noted that recently the observed overnight minimum temperatures in Fiji were about 16 degrees Celsius. It should be noted that the human “felt temperature” during only a 24 km per hour wind a relative humidity of 45 per cent would give the nation a “felt temperature” of 10 degrees Celsius—over 6 degrees lower than the observed thermometer temperature.

Similarly even during the day time, when we were feeling very cold indeed during very strong winds, a 20 degree Celsius day time thermometer measured temperature will have a “felt temperature”  of 14 degrees in a 39 km per  hour wind, with relative humidity of 75 per cent.

Thus we note that in both cases the “felt temperature” is about 6 degrees below that measured by the thermometer.  Thus Fijians were correct when they said in huge numbers that it was very cold indeed, much more so than that historically experienced every dry season.

 

Main factor

The main factor for the cold has been the persistent strong to gale force winds, as the apparent temperature is a direct function of the wind speed.

It can be clearly said that the “felt temperature” also referred to as the apparent temperature were about 8 to 12 degrees Celsius lower than the measured thermometer temperatures,  in the last few weeks.

This is much lower than the observed and forecast 16 degrees by the meteorological service. Fijians were correct in saying that it was much colder than forecast, as they were referring to the perception that was felt by their bodies.

Feedback: jyotip@fijisun.com.fj

 


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