Notes
As in all things meteorological, this rule of thumb is not 100% accurate. However it is a good way to think about what might be happening in a particular area.
As in all things meteorological, this rule of thumb is not 100% accurate. However it is a good way to think about what might be happening in a particular area.
By “large area” we are talking at least hundreds of square km but more likely thousands!
This pattern gets modified by El Nino/La Nina events, cyclones etc.
The maps are in order top left, top right, bottom left, bottom right.
There is 24 hours between each of the maps.
Again, this can be modified by such events as east coast lows.
The maps are in order top left, top right, bottom left, bottom right.
There is 24 hours between each of the maps.
Convection occurs over the warm waters – and this rising, moist air creates clouds.
The wind is towards the west, blowing moist air over towards Australia's north and east coast, which brings rain south of the summer monsoon.
It is over these warmer waters that tropical cyclones form.
Notes:
With the warmer water further west, the convection/precipitation cycle is closer to the eastern seaboard, bringing increased rainfall.
The area in which tropical cyclones form is also closer to Australia's east coast – concentrating the tropical cyclone formation into Australian waters (rather than spread out across the western Pacific).
Notes:
The convection/precipitation cycles moves away from Australia – as does the area in which tropical cyclones form.
Wind in the tropics is generally away from Australia, so reduced rainfall results.
The IOD is still reasonably controversial as an independent meteorological phenomena.
Arguments have been mounted that it is part of the ENSO (El Niño Southern Oscillation) system.
However, evidence appears to support that the IOD is independent and a major factor in droughts through southern Australia.
This year's -ve value is however not relieving the drought (so far) in SW Australia.
In Nov 1973, the SOI reached 31.6 compared to a current value of 25 (Oct 2010).
The diagrams are for the northern hemisphere, but the southern hemisphere is essentially a mirror image.
As can be seen the tropopause altitude is not fixed, but is lowest at the poles and highest at the equator.
The tropopause is a layer that is some kilometres thick and it is in this effectively constant temperature layer that the jet streams exist.
Notes:
The second link may be a bit easier to follow whilst you are learning how to read the BoM charts.
It also provides an animation over the last three days
Notes:
The jets mark the boundaries between warm and cooler air masses
A meander marks a surface cold front
Notes:
The atmosphere may be up to 5% water vapour at low level in the tropics The Dry Adiabatic Lapse Rate (DALR - 3C/1000ft) and Saturated Adiabatic Lapse Rate (SALR - 1.5C/1000ft) are not really constant (as can be seen from the SkewT/LogP charts).
However, for the bottom 10,000ft of the atmosphere, they are reasonable approximations.
Notes:
At 30C, the latent heat of vaporisation/condensation is about 2.4 x 106 Joules/kg.
A small puddle could easily contain 1 kg (1 litre) of water – so it is easy to see how much energy gets bound up in water vapour (and released in a thunderstorm).
The Hiroshima nuclear bomb released 84 million million Joules. It would only take a thunderstorm of about 7 square km (about 2.65km x 2.65km) dropping 5mm of rain (a small storm) to equal the energy released by the Hiroshima bomb!
Notes:
NOAA is the National Oceanographic and Atmospheric Administration of the USA
Notes:
Although the DALR is curved, a straight line is a reasonable approximation in the lower atmosphere.
Notes:
I use the 12 hour javascript sounding cycle option which allows an examination of the whole day (every 3 hours).
The 7am and 10am soundings are particularly important in terms of determining how the day will start (see later).
HCRIT allows for the fact that your glider is descending all the time relative to the air around it. So once lift drops this low you are no longer able to climb.
On the Downs, we have regularly soared in conditions where the B/S ratio was below 5 (as low as 2 on occasions).
It is worth looking at the Cu potential plot. This will tell you where cloud is likely to be thin (or even non existent).
RASP gets its starting data from NOAA (the GFS data set we use in the soundings etc).
Experience on the Downs has shown that NOAA fairly consistently underestimates surface temperature by 2-4C and this results in RASP generally being on the conservative side in terms of convective activity.
“something” to start vertical air movement can be a number of things – convection, convergence (frontal, maritime etc), geography (hills/mountains) etc
Notes:
However technical our forecasting becomes, a reality check is ALWAYS worthwhile. My forecasting activity always commences by looking out at the sky and seeing what's happening right now, even when down on the coast 150km east of the Downs.
CAPE around 500 J/kg suggests thundery showers
CAPE around 800 J/kg suggests thunderstorms
Values above 800 J/kg show increasingly severe storms (convection well above 500hPa)
However, the CAPE value alone is not enough – something has to get convection started!
I use SeeYou to plan tasks in association with Google Earth. Google Earth gives me an idea of the terrain and its potential for out landings.
If this sounds like a lot of work, you are probably right for tasks up to about 500km as these are (relatively) simple and can occur within a day.
Longer tasks require more of a day (if not all of it) and knowing that such a task might be 'on' is vital to the planning process.
Knowing the 'first cloud' temperature will help greatly in timing your launch. For this day, an initial cloud base of 5,000ft suggests you can take off before 23C
Be prepared to get it wrong sometimes – my shortest cross country is 21km and I phoned for a retrieve before anyone else took off!
