# Geopotential height pressure relationship to flow

### Interpretion Guide

Extraction of Geopotential Height and Temperature Structure from Profiler and .. The divergence equation in pressure coordinates can be written (e.g., . ridge over the central plains is consistent with the anticyclonic flow over this region. Geopotential height approximates the actual height of a pressure surface above mean sea-level. Therefore, a geopotential height observation represents the. Calculate the geopotential height of the hPa pressure surface when the where p0 is the sea-level pressure and the relationship In (1 + x)⋍ x for x ≪ 1 has been used. .. Wave-relative air flow is indicated by heavy solid arrows and the.

We see that it increases from pole to equator see figure below. A schematic of isobars constant pressure on an x—y horizontal map at a constant height surface of m, as in the figure above. Brune If we now arbitrarily choose a constant-pressure surface of, say, mb, then the change in the height on an x—y horizontal plot on the pressure surface also shows an increase from pole to equator see figure below.

Brune Thus, low pressure on constant-height surfaces is related to low heights on constant-pressure surfaces.

### Geopotential Height: height of a given pressure

As a result of the hydrostatic approximation, for every height there is a unique pressure, so we can replace z with p as the vertical coordinate. We can then look at changes in variables as a function of x and y, but instead of doing this on a constant-height surface, we can do it on a constant-pressure surface.

We can now show how the equations of motion change when the vertical coordinate is switched from the height, z, to the pressure, p. Consider first the pressure gradient force PGF. The figure below provides a schematic of the math. A cross section of pressure change on a x—z surface.

Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather. The shaded contours indicate the vertical distance, or thickness, between the millibar surface and the millibar surface, measured in tens of meters. Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels.

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Thus, low values of thickness mean relatively cold air. The line is highlighted in black, since this line is often used as a rule of thumb to indicate the division between rain and snow for low terrain. When there is precipitation where the thickness is below dam, it is generally snow.

If the thickness is above dam, it is usually rain or sleet if the air next to the surface is below freezing. Negative values indicate ascending air, and positive values denote sinking air.

Ascending motion is associated with cloudiness and rain.

Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available. The remaining forecast panels indicate 12 or 24 hour accumulated precipitation, measured in millimeters. The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding the verification time in the lower lefthand corner of the map.

The green shading indicates the relative humidity percentage at the millibar level. High values indicate the availability of moisture. When areas of large upward vertical velocity are co-located with high moisture availability, heavy rainfall will likely occur. The streamlines indicate the wind flow. Advection of moisture by the wind can be inferred by noticing the direction and rate at which moist areas appear to be blown. Similarly, temperature advection can be inferred by noticing whether the wind is blowing cold air toward a warm region, or warm air toward a cold region.

Wind Streamlines and Isotachs and Divergence The streamlines indicate the direction of flow of the wind, which is generally from west to east throughout most of the subtropics, mid- and high-latitudes. Purple shading indicates the speed of the winds at the given pressure level, in meters per second. The mb level is near the core of the jet stream, so the tracks of the jet streams can be seen very clearly.

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The maps for the Tropics also have areas of blue and orange shading which indicate horizontal convergence or divergence of the flow at the given pressure level. Orange and red indicate strong divergence, and light and dark blue indicate strong convergence. Precipitable water is the total depth of liquid water that would result if all water vapor contained in a vertical column of air could be "wrung out", leaving the air completely dry.