Notes on Marsden Point Weather Time Series 

The time series of atmospheric pressure and winds come from NOAA's high resolution global atmospheric model called GFS.
The data were extracted for the node nearest Marsden Point at 175 deg E -36 deg N (about 48 km ESE of Marsden Point).

Atmospheric Pressure

The top plot shows atmospheric pressure. Sea level generally follows the inverse of this, so when pressure goes up, sea level falls and vice versa. Each hPa of change in pressure results in 1 cm of sea level change in the opposite direction (it's called "inverted barometer" and there's more about it here).

The small oscillations in pressure are atmospheric tides. Unlike ocean tides which are driven primarily by the Moon, atmospheric tides are mostly solar, though there is a tiny lunar effect. Atmospheric tides are driven by the absorption of water vapour in the troposphere. Their amplitude decreases with distance from the equator.

Winds

The lower two plots show the wind speed and direction. The winds used here are those 10 m above sea level. They are the large-scale winds derived from the shape of the isobars (taking account of pressure gradient, Coriolis force and centrifugal force), not local winds such as a sea breeze which is driven by the difference in temperature between the land and the sea.

Confidence Intervals

The dashed blue lines in the pressure and wind speed plots are confidence intervals based on a year of data (from July 2004 to July 2005). They can be interpreted as follows:

• for 97.5% of the time, pressure (or wind speed) will be above the top line;
• for 2.5% of the time, pressure will be below the bottom line;
• for 95% of the time, pressure will be between the top and bottom lines;
• for 50% of the time, pressure will be above the 50% line;and
• for 50% of the time, pressure will be below the 50% line.

If the pressure goes above the top line, sea level will generally be more than 17 cm lower than average.