A number of rules have been compiled regarding the movement of low-pressure systems in relation to the mean isotherms or thickness lines. These rules are outlined as follows:

. Unoccluded lows tend to move along the edge of the cold air mass associated with the frontal system that precedes the low; that is, it tends to move along the path of the concentrated thickness lines. When using this method, you should remember that the thickness lines will change position during the forecast period. If there is no concentration of thickness lines, this method cannot be used.

. When the thickness gradient (thermal wind) and the mean windflow are equal, the low moves in a direction midway between the two. This rule is more reliable when both the thermal wind and the mean windflow are strong.

. When the menu windflow gradient is stronger than the thickness gradient, the low will move more in the direction of the mean windflow.

. When the thickness gradient is stronger than the mean windflow gradient, the low will move more in the direction of the thickness lines.

. With warm lows, the mean isotherms show the highest temperature directly over the surface low, which is about halfway between the 700-hPa trough and ridge line. This indicates the mean isotherm and 700-hPa isoheights are 90 degrees out of phase. Since warm lows move with the mean speed of the warm air above them, they will be rapidly moving systems.

. If the highest mean temperatures occur under the 700-hPa ridge (isotherms and contours in phase), the ridge itself is warm while the low is cold; therefore, the low will move slowly.

. Lows move with a speed of approximately 50 percent of the thermal wind for the 1,000- to 500-hPa stratum, and approximately 75 percent of the thermal wind of the 1,000- to 700-hPa stratum.

Some of the rules for moving lows in relation to the jetstream position were mentioned previously. One basic rule however, states that "highs and lows situated under or very near the jetstream behave regularly and follow the steering indications." Minimum deviations occur when the upper flow does not change with time.

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