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## Section5.2Pressure gradient force

The pressure gradient is a change in pressure over a distance; the pressure gradient force over some distance $$n$$ is thus

\begin{equation*} PGF = \frac{\Delta P}{\Delta n} \end{equation*}

The force always points from areas of higher pressure to areas of lower pressure. The steeper the pressure gradient, the stronger the PGF and the stronger the winds.

Mapping variations in pressure can thus be used to indicate the PGF. There are two main ways to show this:

• surface pressure map: shows the atmospheric pressure adjusted to sea level (MSLP). Isobar contours can be drawn to indicate pressure values.

• isobaric chart: shows the altitude of a given pressure surface. For example, how high is the "500-mbar" level above the surface.

We can again consider how pressure reflects the weight of the air above: areas of high pressure have higher isobaric altitudes than areas of low pressure; this will influence air movement and subsequent weather.

If PGF always points from high pressure to low pressure, and pressure decreases with altitude, why doesn't the atmosphere just fly away?Answer.
The Earth's gravity prevents this from happening; the balance between gravity and the pressure gradient force is otherwise known as the hydrostatic balance (see Section 2.4) which governs how quickly the pressure changes with altitude.