pressure charts

[pippymilly]

Hi there,

I would be so grateful if anyone could help me.  I have some surface pressure charts in which I have to measure the distance an area of low air pressure over the UK has travelling (pictures attached).  Can anyone advice me how you measure the distance?  the scales are not given so I cannot physically measure it.

 

Thank you

[mike Meehan]

The further north you go on a globe the lines of longitude become closer together so that eventually at the poles they meet but on a two dimensional chart they are parallel, meaning that at the northern latitudes the areas are depicted wider than what they actually are.

 

There is a device, a bit like a wide plastic see through ruler which can give you different readings of distance at different latitudes, depending on how you place this on the chart. I think you would need the device calibrated for the particular chart you are using.

[Interitus]

You could try an online latitude/longitude distance calculator like this one - http://www.nhc.noaa.gov/gccalc.shtml

Note it calculates the shortest distance along a great circle, which may not be the path the low pressure has taken.

[johnholmes]

roughly 1 degree lat is 60 nautical miles if my memory serves me correctly from years ago?