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When you look at a surface pressure chart e.g.:

Weather map

(from http://www.metoffice.gov.uk/public/weather/surface-pressure/#?tab=surfacePressureColour&fcTime=1437480000)

you can see low pressure zones over the North Atlantic ocean. How are these zones tracked? I cannot imagine there are many monitoring stations in the Atlantic ocean.

Possible ways:

  1. Ships - but I doubt there are enough to get readings over a wide enough area.

  2. Comparison of pressure readings from coastal monitoring stations. Are there enough points for accurate interpolation of the readings?

  3. Aircraft - comparison of GPS altitude to barometric pressure readings could give the surface pressure in a given area. That said, most aircraft travelling over the Atlantic are on specified tracks which may not cover a given area.

  4. Monitoring stations on buoys at well spaced locations in the ocean. Potentially impractical and a hazard to shipping.

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It is a combination of points 1,2 and 4.

Ships

Ships can provide observations, but they are generally confined to shipping lanes rather than distributed all over the oceans. NOAA operates a volunteer observing program and you can find recent ship obs here. The ships can report location, wind direction, wind speed, pressure, pressure tendency, air temperature, water temperature, dew point, wave information and a few other fields. Not all ships report all of the data, but pressure is generally reported by all ships participating.

Buoys

NOAA operates a global buoy network though these are generally located in coastal regions. There are regions of regularly spaced buoys in the ENSO regions. There are also some buoys in the Indian Ocean (not pictured).

enter image description here

Drifters

The buoy network leaves vast swathes of ocean unmonitored and to get these obs NOAA uses a network of drifters that, as the name implies, drift along the ocean surface.

enter image description here

The blue points in the map above are drifters than can report sea level pressure.

Summary

Weather models that assimilate these observations will use sophisticated algorithms to "fill in the gaps" and produce a continuous field of global sea level pressure.

Aircraft can report pressure, but knowing pressure at altitude won't yield sea level pressure without knowing (or making assumptions) about the state of the atmosphere between the airplane and the surface. Upper air observations are good though, and quite important to forecasting and generally much more sparse than ocean surface observations are.

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    $\begingroup$ It is also worth mentioning that this data assimilation will blend the forecast from a previous model run with more recent observation data. So the initial state of the atmosphere, including surface pressure, will be a blend of simulation and observation data. $\endgroup$ – hertzsprung Jul 28 '15 at 12:15

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