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At this point, your analysis conforms to the needs and normal practices of most Naval Ocean-ography Command activities. However, there are additional analyses that are often needed or desirable to depict on the surface chart.

Learning Objective: Identify the proce-dures for depicting and analyzing move-ment, air masses, precipitation and obstructions to vision, and isallobars. Movement Analysis

Past history provides you with past positions of fronts and pressure systems. Your analysis pro-vides the current positions. The direction and speed of a system or front are based on its former and current positions. The direction is simply the line of movement from the old to the new posi-tion. The speed must be computed.

1. Measure the distance the center or front moved since the last analysis. Measure this same distance along a longitudinal line adjacent to and at the same latitude as the front or center. A pair of dividers works well for this purpose.

2. Convert the degrees latitude into nautical miles; 1° of latitude is equal to 60 nautical miles.

3. Divide the distance by the time frame in-volved in the movement. For example, a low-pressure system that moved a distance equivalent to 3° of latitude (180 nmi) in a 6-hour period would have a speed of 30 knots. (180 ÷ 6 = 30). A faster method for the above is to divide the degrees latitude by one of the following factors based on the number of hours involved: 6-hour movement known; divide by .1 12-hour movement known; divide by .2 18-hour movement known; divide by .3 24-hour movement known; divide by .4 36-hour movement known; divide by .6 For example, if a high-pressure center moved 2.8° latitude in 12 hours, its speed of movement is determined by dividing 2.8 by .2. This gives a speed of 14 knots.

Air-Mass Analysis

While an Aerographer’s Mate, familiar with a given weather situation, may have little need for air-mass classification, the distinction between air masses of different types is nevertheless impor-tant to weather analyses. It is a means of convey-ing a more complete picture to a user of the weather map, such as a pilot.

If air masses of different characteristics are isolated from one another, the transition zones between them can be determined as the first step in frontal analysis. The reverse procedure maybe used to advantage in examination of vertical temperature soundings, in which case the air masses over a station may be isolated from one another by locating the transition zones or fron-tal discontinuities on the soundings.

Through the use of hydrometeors, such as fog or drizzle and cloud types, a stable air mass can often be differentiated from an unstable air mass across a narrow zone. As air masses are classified according to characteristics common to their source region, many modifications have to be taken into account when they move. For example, when a warm air mass moves over a cold surface, if the flow is rapid, stratus may form, but if it is slow, fog will tend to result. In warm seasons, the warm air mass can be stable at night, producing fog, and unstable in the daytime. At sea, the diurnal temperature change is slight, with a tendency for greater stability in the daytime in lower layers.

Characteristics of the same general type of air mass vary to some extent with their source region. Arctic air, for example, varies somewhat, depending on whether it has developed in Siberia, over the polar cap, or in Canada.

Arctic air that invades mid-latitudes generally has a surface temperature of 0°F or below. No definite values of surface temperature or dew point can be stated for continental polar air masses, since these depend upon source region, length of time over the source, depth, trajectory, heating and cooling from below, and the like. Often there are marked horizontal gradients of temperature within these air masses. The amount of warming that takes place, both at the surface and aloft, when a cold air mass passes over a warmer surface is greater when there is subsidence (descending air motion) aloft.

The only air mass in mid-latitudes with approximate homogeneity at the surface, and to some extent aloft, is maritime tropical air. In the summer, mT air over the United States has a representative (maximum) surface tem-perature of near 90°F and a dew point of around 70°F. Corresponding values in the winter are roughly 75°F and 60°F. You have had some practice in locating and labeling air masses in unit 4. Though air-mass labeling is generally an optional entry, it can be a great help to the new analyst. Label the air masses according to the standard air-mass entries shown in table 7-4-1. Sometimes an alter-nate method is used to indicate the air masses. This method either shades the cold and warm air masses in light blue and red, respectively, or is indicated by a large arrow, shaded in lightly

Table 7-4-1.—Air-Mass Analysis Entries

and labeled appropriately, showing the direction of movement of the air mass (fig. 7-4-3).

NOTE: Indicate one air mass changing into another with an arrow joining the symbols for the two air masses. For example, the transitional state between maritime polar air and maritime tropical air would be shown mP mT. Show the mixing of two air masses by inserting a plus sign between the symbols for the air masses; for example, mP + mT. To indicate one air mass lying above another, place the letter symbols for the two air masses, one above the other, and separate them with a horizontal line; for example, . If an air mass has been modified, enter "mod" under the air-mass label.

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