GENERAL DRAWING PROCEDURES
Where do I start? You can begin your analysis anywhere, but it may prove easier if you start in an area of numberous reports. Most analysts begin over a continental area and proceed to later cover the adjacent ocean. It may also be easier if the region you select is experiencing moderate wind speeds (15 to 20 knots). The reason for this is isobars follow the general wind flow, and where
the wind speeds are stronger, the isobars are more
the wind speeds are stronger, the isobars are moreapt to conform to the wind direction.
After you’ve chosen an area in which to start, your next step is to select an isobar to draw. Again, the choice is yours. Many analysts like to begin with a specific isobar, the 1016 often being preferred. They feel it outlines more pressure systems than any other isobar when completed. Others may choose a value that is well defined by the reports in the area. In figure 7-2-2, the analyst chose the 1012 isobar. Note that of the seven re-ports in this area, only one has the exact pressure of 1012.0 mb. Finding stations that report the exact pressure value of the isobar you’re draw-ing isn’t common; therefore, you must learn to interpolate between pressures.
Interpolation is a fitting and placement pro-cess. When the exact value of the isobar you’ve chosen to draw isn’t reported, you must inter-polate its location between stations. For ease of understanding, let’s look at our example again. Moving downwind from point A, you first come to two stations that are reporting sea level pressures of 1011.2 mb and 1013.2 mb. The 1012 isobar definitely fits between these two stations, but where? As the analyst, you must decide on its placement.
Figure 7-2-2.—Situation requiring interpolation between
Figure 7-2-2.—Situation requiring interpolation betweenpressures.
No matter what interpolation method you use, you’ll work with the pressure difference between stations. In our example, the pressure difference between the station reporting 1011.2 mb and the station reporting 1013.2 mb is 2 mb. Half of this difference is 1 mb. By adding this half value to the lower pressure or subtracting it from the higher pressure, you can determine the pressure occurring halfway between the stations. This is called the midpoint pressure. The midpoint pressure be-tween the stations in our example is 1012.2 mb.
The placement of the 1012 isobar just became easier. Keeping in mind Buys Ballot’s law, which states: With the wind at your back (in the Northern Hemisphere), lower pressure will always be to your left, left of the midpoint.
NOTE: For ease in computing, round off the pressure values to the nearest whole mb. There will be times when more than one isobar will fit between two stations. The procedure out-lined still applies. For example, if the next two stations downwind were to show pressures of 1006.8 mb and 1015.3 mb, the 1012 isobar would fit between them, as would the 1008 isobar.
Through the interpolation process, the midpoint pressure is determined to be 1011.0 mb. The 1012 isobar would be drawn to the right of the mid-point, and the 1008 isobar would be drawn to the right of the station reporting 1006.8 mb. This drawing procedure is repeated with every isobar until the isobar extends to the edge of the chart, returns to the origination point (forms a closed loop), or enters an area of no reports (and there’s no justification for continuing).
When there is a large pressure difference be-tween two stations on the chart and several isobars must be drawn between them, it is a good idea to determine the number of isobars you’ll be draw-ing. You can then space a series of dots between the stations to use as a guide. This procedure applies more so over ocean areas than continents because of the sparsity of reports.
I know it is difficult visualizing doing this analysis quickly, but with practice, it becomes routine. Contact your training petty officer for the latest practical training publication (PTP) contain-ing practice exercises. You may also be able to obtain ozalid copies of plotted charts on which to practice.
In drawing isobars, we have discussed the im-portance of following the wind. The relationship of wind and pressure is such that, in general, winds blow across isobars from higher to lower pressures. Refer to figure 7-2-2 again. The wind does not parallel the 1012 isobar. It blows across it at an angle toward lower pressure. The angle the wind makes with isobars is an effect of friction be-tween air and the surface over which it is passing.
Over a smooth surface, this angle is small. Over a rough surface the angle is large. Normally, the rougher the terrain, the greater the angle. The angles are 10° to 20° over oceans, 15° to 30° over smooth terrain, and 45° or more over rough ter-rain. Another factor controlling the angle the wind makes with isobars is wind speed; the greater the wind speed, the greater the angle.