Maleny Weather

Past Week’s Weather

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Monday August 24^th 2009 will go into the record books as the hottest August winter’s day on record when the mercury soared to reach a peak of 31ºC at 3.00pm. A ridge of high pressure along the north Queensland coast brought strong hot north-easterly winds, low humidity and an extreme bush fire danger to the region. During the period of the mini-heatwave overnight temperatures were 6 degrees above the norm.

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The week’s weather topic has been dominated by the approach of a mini heatwave. Temperatures soared to 28.8ºC, with a potential next week to go even higher and break all known August records on the Sunshine Coast hinterland. The synoptic pattern showing a slow moving large high off the mid-Queensland coast is responsible for bringing hot strong NW winds to the Ranges and is similar to the one in 2006 when on Friday August 25^th the temperature soared to 29.2ºC. Overnight temperatures are six degrees above the norm and relative humidity down as low as 35%.  Dew has been minimal and fire danger acute.

 Queensland is experiencing summer conditions in the finals days of winter, with record temperatures flagged and a high-fire-danger warning issued

Alice Springs has recorded its hottest August temperatures in almost three decades.

The mercury reached 35.2 degrees Celsius on Saturday, breaking the record of 34.7C set the previous day.

Mark Kersemakers, from the weather bureau, says the previous hottest August day was 34C in 1970.

He says the overnight temperatures have also been unseasonably high.

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Despite the fact we had three thunderstorms this week the amount of precipitation was minimal with only 6.2 mm recorded in rain gauges. Past records show that the month of August is a month of extremes.  It is likely to be very wet or very dry. In 2007 it was very wet with 533 mm recorded for the month. Last year on the other hand was very dry with only 3.2 mm. It looks as if the current month is going into the record books as another ‘dry’ month. So far this month we have seen very little wintry weather with temperatures well above average. 

OOUTLOOK ~ Unsettled – winds increasing.

El Niño slowly consolidating in the Pacific

Atmospheric indicators are increasingly showing patterns typical of a developing El Niño event. These indicators are driven by warm conditions in the tropical Pacific Ocean. If these warm conditions persist, as forecast by leading climate models, 2009 will be considered an El Niño year.

The Southern Oscillation Index (SOI) has fallen over the past two weeks, and is now near zero. Persistent negative values are a feature of El Niño events.

The last month has seen below average rainfall across much of eastern Australia with particularly dry conditions through Queensland. El Niño periods are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia.

Harry and Humidity:

Some people are still unclear on the concept of relative humidity. You seem to get it confused with another relative, Uncle Harry. Uncle Harry, who gets flustered easily and tends to perspire even when the air conditioner is set at 10ºC, does remind us of relative humidity, so we understand your confusion.

Just as Uncle Harry’s copious perspiration and cries of “Wow- it’s hot as Hades in here!” do not indicate the actual temperature in the room, relative humidity does not indicate the actual amount of water vapour in the air.

Instead, RH tells us how close the air is to being saturated. RH, which is what our weather station reports, is expressed as a percentage that indicates the amount of water vapour actually in the air compared to the amount of water vapour required for saturation at that particular pressure and temperature. Absolute humidity, on the other hand, is a measure of the actual water vapour density in a fixed volume of air – dew point is an indication of this.

Uncle Harry, on a warm, muggy day is often heard to sigh, as he sits in the shade of his oak tree and mops his brow with a soggy handkerchief, “I don’t mind the heat. I just can’t take this awful humidity.”

He is explaining the difference, in human terms, between relative and absolute humidity. When we are warm, we perspire. The moisture evaporates into the air and cools the skin. If the RH is low, this evaporation happens very quickly and we perceive a lower air temperature. But when the RH is high and the air is approaching saturation, evaporation slows or halts and we sense higher air temperatures. Uncle Harry is right. With his very effective perspiration system, he would be more comfortable sitting in the shade of a cactus on an even warmer day in the desert!

Relative humidity changes during the course of a day, but not because the amount of water vapours in the air changes much. It is the changing of air temperature that changes RH. As the air cools, RH increases. As it warms, RH decreases.

So when Uncle Harry waddles over to your thermostat and thumbs it far to the left, he effectively lowers the RH! When the air conditioner cools the air, it increases its RH up to the point of saturation. The water vapour then condenses into liquid and drains out of the system. The cooled and dried air is then pumped into the house. In the winter, your heating system also dehumidifies the air. The cold air outside has little water vapour. When this air is pulled into the house and heated, the vapour capacity of the air rises dramatically, and so the RH plummets. You end up with desert-dry conditions inside the house. Any perspiration evaporates very quickly, leading to the famous winter dry skin, scratchy throat, and dry mucus membranes.

So remember, while Uncle Harry is a humid relative, he’s not, we repeat, NOT, relative humidity.

 anon

• What falls as it rises and rises as it falls?
• The answer is, of course, the barometer because pressure decreases as altitude increases and so the barometer will fall as you climb a mountain or go up in a balloon!
• With a series of high pressure systems sweeping across the continent, as is usual during the winter months, one’s thoughts turn to the aneroid barometer found in many homes on the Range where the day often begins by the ritual tapping of the barometer to see which way the needle jumps!
• Although barometers should be adjusted for height above sea level by turning a small screw at the back, it does make sense to adjust for Mean Sea level instead so that it is easier to read weather maps on TV or newspapers.
• Always bear in mind it is more important to know what the barometer is doing rather than what the needle is showing.
• Early home barometers were very helpful by including a little blue and red disc to show whether the barometric pressure was rising or falling. This avoided the need for ritual ‘tapping’!
• If your barometer is a more recent model the face will show the units in hectopascals, the unit of pressure measurement accepted around the world. However, should the markings be in inches then multiplying by 33.85 will bring inches to hectopascals (hPa).
• The face may also show such words as Stormy, Rain, Change, Fair, Very Dry as a guide to current or future weather, but in fact has little practical application other than being decorative.
• Atmospheric pressure changes are irregular and knowledge of them is very important in the study of winds and weather patterns in forecasting weather.
• Barometers used to predict weather should be read in conjunction with outside air temperature and relative humidity. The following guide lines are used by some weather watchers.
• Barometer steady, humidity increase, temperature falls = Chance of rain
• Barometer steady, humidity decreases, temperature rises = Fine
• Barometer rises, humidity increases, temperature steady = Showers.
• Barometer quick rise/fall, humidity increases, temperature plunges = Thunderstorms.
• Barometer falling, high humidity, temperature rising = Heavy rain.
• Barometer rising, humidity falls, temperature rising = Wind change, fine.
• Barometer falling, humidity high, temperature plunging = Weather deteriorating rapidly.
• Finally, watch the Weather Bureau’s synoptic maps and compare your location’s barometric pressure with the isobars shown.

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For most of the week weather on the Range was static influenced by a large high covering the continent. Daytime temperatures were three degrees above normal with long hours of sunshine and light winds. A change came on Saturday with the approach of a cold front and a development of a small low in the north Tasman Sea. Winds backed to SW at 8.30 pm with gusts of 15 – 20 knots and a three degree fall in overnight temperatures.

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There was little change in our winter weather pattern over the past week with a slow moving high in the Tasman Sea extending a ridge up the east coast bringing stable conditions to the ranges

EXTENDED OUTLOOK

The weather across the Ranges is likely to remain fine for most of the week. However, there is a slight chance of an isolated shower or two from onshore SE moist winds toward the end of the week.