Sat. night so obviously I was just thinking about Barometry...

Red_Marauder

And I was wondering how isobar readings are arrived at.

How does a meteorologist apportion barometric readings in a geographical area as broad as the Atlantic ocean. In other words, who decides where the isobars currently lie or where they are going to spread to, and widen, and how do they specifically do this?

And to what point in the sky (or is it at sea level) do pressure readings relate, since this reading will obviously vary?

I tried looking this up but it was either to basic to mention or I didn't come across it.

darkman2

Red_Marauder wrote: »

And I was wondering how isobar readings are arrived at.

How does a meteorologist apportion barometric readings in a geographical area as broad as the Atlantic ocean. In other words, who decides where the isobars currently lie or where they are going to spread to, and widen, and how do they specifically do this?

And to what point in the sky (or is it at sea level) do pressure readings relate, since this reading will obviously vary?

I tried looking this up but it was either to basic to mention or I didn't come across it.

Pressure readings relate to all levels. They are measured in hectopascals. The charts used on the weather bulletins are at 1000hpa level - which is ground level. The hpa level reduces with altitude. 850hpa is about 1,500m for example.


As for drawing the lines. There are buoys scattered all over the Atlantic which provide a constant stream of data. This is real time data fed into computer models constantly. Aircraft are also constantly in use for measurements. Obviously all of the ocean cannot be covered. It is simply a case for a meteorologist to join the dots. Synoptic charts are never 100% percent accurate. There is an element of guesswork involved and a margin of error.

The models then fill in the dots.




Humans do still have an input.

Su Campu

darkman2 wrote: »

Pressure readings relate to all levels. They are measured in hectopascals. The charts used on the weather bulletins are at 1000hpa level - which is ground level.

This isn't correct, 1000hPa (or mbar) is not ground level, I think you're confusing things.

The charts we see in bulletins have isobars, which are lines that join points of equal sea level pressure. So elevation is held constant, and the pressure is the variable. This is the opposite to the upper level charts, in which pressure is held constant, and elevation is the variable. So the lines in the 850hPa chart are not isobars but are isohypses, lines connecting points where the geopotential height of 850hPa pressure level is the same.

Over oceans, sea level pressure is measured by buoys and ships. These reports are all fed into the computer, and in areas where there are no reports, previous computer model forecasts runs are used to "fill in the blanks". This data assimilation give the "analysis" chart, the starting point for forecast runs.

Over land, where the elevation is higher than sea level, the actual pressure measured by a barometer at a weather station is converted to sea-level pressure using an algorithm that takes into account the station's elevation and temperature over the last 12 hours. Usually the pressure drops by 1hPa every 27ft rise in elevation, but that can vary with different airmasses, with less than 27ft for cold airmasses and vice versa.

Here's the NWS surface analysis plot, showing the actual ship and buoy reports used in generating it. (The number at each plot is the the pressure, in tens of hPa. eg. 292 = 1029.2hPa, and the "arrow" is the wind barb, showing direction and speed)

Su Campu

You said the bulletins show the 1000hPa level - they don't. And you said 1000hPa is ground level - it isn't. Now maybe I've completely misread things, so if I have then sorry.