SHORT-RANGE WEATHER FORECASTING 
of stability is most important to the forecaster) the 
parcel method underestimates the resistance against 
lifting and overestimates the available energy. The 
slice method, first suggested by J. Bjerknes, does take 
into consideration changes in environment of the as- 
cending air, but the method is too complicated for prac- 
tical use. 
Constant-Pressure Charts. At district forecast cen- 
ters, constant-pressure charts are normally prepared 
for the 850-, 700-, 500-, and occasionally for the 200-mb 
surfaces. Fulks [22] has described procedures for the 
preparation of these charts which normally include 
entry of dry-bulb and dew-point temperatures, height 
of the pressure surface, and 12- or 24-hr height changes. 
Contours (isohypses) and isotherms are then drawn. At 
many centers, isolines of equal height changes are drawn 
and areas of certain moisture values, based on the dew 
points, are shaded. At weather centrals, other pressure 
surfaces may be useful, particularly the 1000-mb chart, 
as well as tropopause and density charts. 
The 850-, 700-, and 500-mb charts are among the 
finest tools available to the forecaster. A large number 
of techniques and rules, mostly subjective, have been 
developed or formulated during the past few years for 
use in forecasting. 
Constant-pressure charts, and the 850-mb chart m 
particular, can provide the forecaster with much of the 
information yielded by the isentropic chart. Since the 
isotherms are lines of potential temperature, they rep- 
resent the intersections of isentropic surfaces with con- 
stant-pressure surfaces. Means [33] has found the warm 
air advection in the lower layers of the atmosphere 
(2000-8000 ft and above m.s.l.) useful in forecasting 
thunderstorms. 
By far the greatest amount of work has been done 
on the 700-mb chart. During the past few years, how- 
ever, the 500-mb chart has been finding greater favor 
with forecasters, since the field of motion is more con- 
servative at this level, and for other reasons. The most 
important use of the 700- and 500-mb charts is the 
“steerme”’ of surface highs and lows or pressure rises 
and falls indicated by the field of motion at these levels. 
“Steering” appeared prominently in the publications of 
German meteorologists in the 1930’s particularly in the 
work of Baur [3], who found that the direction of mo- 
tion of regions of rise and fall of pressure was controlled 
by the steering at 5 km. The mean duration of a broad- 
weather situation (Grosswetterlage) was 5 to 514 days 
which, with the allowance of one transitional day, 
corresponds to the C.I.T. six-day type. Steering was 
broken down into four divisions: 
1. Westerly steering (high index)—about 18 per cent 
of the cases. 
2. Northwest and southwest steering (moderately 
low index and moderate troughs and ridges)—about 17 
per cent each. 
3. Northerly and southerly trough and ridge steering 
(very low index)—smaller percentages. 
4. Easterly steering (very low index)—observed very 
infrequently. Presumably a number of cases could not 
be classified. — 
751 
V. J. and M. B. Oliver [40] have summarized a large 
number of rules obtained from many sources for the 
use of the upper-air charts. Most of these rules have 
not been objectively tested but some of the more im- 
portant of them, applicable to the 700-mb chart, are: 
1. Surface cyclones move in the direction of the 700- 
and/or 500-mb flow. 
2. Surface cyclones move in the direction of the 700— 
500-mb mean isotherms, inclining slightly toward the 
colder air. 
3. If an upper isallobaric maximum (24 hr) is found 
in the direction in which the surface cyclone will move, 
the cyclone will move into the region, or just to the 
west of it, m 24 hr. 
4. Surface cyclones will move along a line from the 
center of the isallobaric minimum (24 hr) in the rear to 
the center of the isallobaric maximum in front. 
5. The smaller the angle between the upper isobars 
and the mean isotherms of the low troposphere, the 
closer the speed of the cyclone approaches the speed 
of the upper flow. But perhaps the most important and 
widely accepted rule in current use states: The surface 
low or surface katallobar moves with approximately 
half the speed of the 500-mb wind over it. 
Tasue I. AVERAGE DrvIATION oF THE DIRECTION OF 
MovreMENT or CYCLONES FROM THE ORIENTATION OF 
tHE Uprrr-LeEvEL ConTours AND ISOTHERMS 
Comparison A Comparison B 
Upper-level pattern 
Average | Number | Average | Number 
deviation | of cases | deviation | of cases 
Contours, 850-700 mb..... Bilis 212 24° 92 
Contours, 700-500 mb..... 28° 216 Zils 92 
Tsotherms, 850-700 mb....} 31° 215 Dom 92 
Isotherms, 700-500 mb....| 33° 218 BBS 92 
Contours, 200 mb......... 26° 159 ile 92 
The forecasting value of rule 1, if valid, is obvious. 
Austin [1] has tested this rule by comparing the direc- 
tion of movement of the cyclone on the surface with 
the orientation of contours and isotherms at various 
levels. This direction of movement was assumed to be 
Tasie Il. Frequency DisTRIBUTION OF THE DerVIATIONS IN 
DIRECTION 
—180° | —45° | —15° | +16° | +46° 
Upper-level pattern to to to to to 
—46° | —16° | +15° | +-45° | +180° 
Contours, 850-700 mb. ..... il 5 48 27 11 
Contours, 700-500 mb...... 2 7 60 12 11 
Isotherms, 850-700 mb..... 4 13 51 18 6 
Tsotherms, 700-500 mb...... 3 7 49 |. 22 11 
Contours, 200 mb.......... 3 14 50 7 8 
given by the direction from the position of the center 12 
hr before to the position 12 hr after the time for which 
the comparison was made. Austin’s conclusions are 
summarized in Table I. Comparison A includes all ob- 
servations, while Comparison B includes only those 
cases in which the speed of the cyclonic center exceeded 
20 mph. 
The frequency distribution (Table II) shows the 
spread of the deviations for the 92 cases in Comparison 
