850hPa Heights is the distance from sea-level where you will find the
850hPa temperature. This temperature, as discussed before, is very roughly
1500 metres from the surface of the earth and is often used by forecasters
as it is much less affected by heating from the earth's surface
than the air at the actual surface. Its this temperature that can give a good guide to how cold it will be in the Blue Mountains during cold outbreaks, as the 850hPa temperature is usually only 200 to 400 metres higher into the atmosphere than our town at these times - Blackheath is around 1080 metres above sea level. So, if you know what the 850hPa temperature is and you know the height of it, all you have to do is subtract around 2 or 3 degrees from that temperature and bingo, you have a
forecast for the temperature in Blackheath! Read on for more details:
Go to this link:
Data and Links Here you will see the heading: "Climate, Weather
Analyses, Weather Forecasts". Its just down from the top of the page.
Under this title, click on any one of the three AVN Forecasts server
links, there are typically three of them named, 'server 1" etc. On the
next page, you will see a heading "Control file:" - select the most recent
time and date there. Next, go to "Options" and select "Include Variable Definitions
and Units". Under "Plot Type", select "Map". Click on "Next Page." Under
"Variable:" Scroll down about twelve listings until you get to "HGT" and
select it. Then, scroll a long way down until you get to "Level:". Select
"850" here. Next, go to "Time:" and select the time you want to know the
850hPa Height for. Remember that this is Universal Time, so the time in
Blackheath will be 10 hours (11 in summer) ahead of this time. So if you
want to know the 850hPa height for say, 10pm on September 12th, select 12Z
and the correct date. Now go to "Map Projection" just below this and
scroll down and select "Aust" for Australia. Then go to "Draw" underneath
this and select "Shaded and Lines". Finally, click on "Plot" and hey
presto, you can see the areas on the map that indicate the approximate
height of the 850hPa temperature for our region.
So, if you've established that the 850hPa temperature for September 12th was -2C from the information in my other article and you now have a more accurate height estimation of this temperature, how do you work out what the temp will be in Blackheath? If for example, you have established that the forecasted 850hPa temperature is -2C and you have also established that the 850hPa height is at an altitude of say 1480 metres above sea level and our town is at 1080 metres above sea level, just take off 0.6C (in dry conditions take off 1C) for each hundred metres below 1480 metres until you get to 1080 metres. Use the lapse rates given here as a guide only.
Stay tuned for more, I will add to this subject if its needed.