HQUATORIAL METEOROLOGY 
torial weather is the undercutting of stagnant equa- 
torial air by a surge of fresh tropical air, producing 
effects similar to those of a cold front in temperate 
latitudes. This situation has been treated in a most in- 
genious manner by Freeman [4], who draws an analogy 
between the undercutting of the equatorial air by a 
sudden surge of cold air and the supersonic gas flows 
resulting from the action of a piston. In his discussion 
Freeman assumes that the air is at rest under an inver- 
sion of constant height near the equator and that a 
cold-air mass, acting as a piston, moves into the area, 
undercutting the stagnant air. He then computes the 
characteristic or Mach lines for the resulting flow and 
shows that the theory admits of three types of dis- 
turbance oriented normal to the current, that is, a 
rarefaction wave, a compression wave, and a “jump.” 
The compression wave, of course, arises from a surge 
of cold air, while the rarefaction wave may be caused 
by the falling off in the trade, although this latter case 
is relatively unimportant. Freeman considers that the 
‘“ump”’ may be the explanation of the “easterly wave” 
because, as he points out, when a “jump” moves into 
an atmosphere at rest, the energy required to main- 
tain the “jump” is derived from the atmosphere at 
rest and no other source of energy is required to main- 
tain the flow. The moving fluid will show rapid vertical 
motions at the “jump” which would lead to the typical 
weather phenomena associated with the easterly wave. 
The theory also provides a model for a sloping surface 
of discontinuity in these low latitudes which, so far, 
has been produced by no other theory. In the example 
which Freeman gives for a case in the New Guinea 
area, the agreement between the calculated and the 
observed values is most remarkable in view of the 
difficulties inherent in the problem. 
One of the major problems that remain to be tackled 
is how to allow for the effect of the diurnal and semi- 
diurnal variations of pressure which dominate the baro- 
grams of the equatorial region. The daily change in 
pressure is very much greater than the secular change 
and if one treats it loosely as the result of pressure 
waves that travel from east to west around the equa- 
tor, the effect is to produce variable gradients of pres- 
sure which attain values of the same order of magnitude 
as the geostrophic and space-acceleration terms in the 
equations. The problem is a difficult one and not much 
progress appears to have been made up to now in 
solving it. 
On the whole it is fair to say that although some 
progress has been made, we are still very far from a 
satisfactory solution to the dynamical problems of the 
equatorial region, but the problems are well worth at- 
tention and success in solving them might have a pro- 
found effect on meteorological theory as a whole. 
FORECASTING 
Since most of the meteorologists in the equatorial 
region owe their presence there to the necessity of pro- 
viding weather forecasts, forecasts have to be made 
even when the methods of preparing the forecasts are 
only incompletely understood. Generally speaking, the 
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forecaster labours under great difficulties, not only the 
economic ones of poor equipment and inadequate com- 
munications but also those which result from the fact 
that he is usually a stranger to the area and lacks the 
long familiarity with the weather changes which help 
to guide forecasters in temperate climates. He has no 
textbook methods which he can study and he has to 
obtain his knowledge of local conditions from woefully 
mcomplete climatological statistics which hide rather 
than emphasize the variations in weather which he 
must account for. 
For a forecaster trained in the temperate zone the 
greatest difficulty is caused by the breakdown of the 
geostrophic relationship between wind and pressure 
which makes the drawing of isobars within the region, 
especially in view of the small number of stations, 
appear to be a matter for imagination rather than 
reason. The result is that the isobaric pattern has to 
be drawn independently of the wind distribution and 
it has become usual to construct additional charts of 
lines of flow for the various levels independently of the 
pressure. By superimposing the streamline and isobaric 
charts it is sometimes possible to determine areas of 
acceleration or retardation of the streams and even- 
tually to formulate some empirical rules for develop- 
ment. 
One of the difficulties which have exercised the minds 
of many people is how to handle the diurnal variation 
of pressure and weather on the synoptic chart. Now 
that universal time has been adopted for synoptic ob- 
servations the effect of the diurnal variation must be 
very carefully watched on any chart which has a large 
east-west extent so that afternoon convective effects 
will not be confused with frontal effects. There is a 
body of opimion that the pressure readings should be. 
“corrected” for the relatively large diurnal variationg 
of pressure in order to obtain “representative’’ pres: 
sures for each station, but the day-to-day irregularities, 
especially in the diurnal wave, make the correction of 
the pressures a rather doubtful project. It is indeed. 
questionable whether such a ‘representative’ pressure 
chart would have any dynamical significance and. 
whether more might not be lost than gained in the 
process. It is, however, beyond question that the com- 
parison between successive three-hourly synoptic charts 
is exceedingly difficult because of the diurnal variation 
of pressure. The tendency has been to compare each 
chart with the one prepared 24 hr earlier and thus 
obtain an overlapping series of 24-hr changes. There 
appears to have been no prolonged investigation into 
the application of corrections for diurnal variations, 
owing no doubt to the pressure of routine work. How- 
ever, such an investigation would be of great value 
whatever the result. 
Some workers prefer to ignore the isobaric chart al- 
most completely and rely on the streamline charts for 
their analyses. It is usual to draw the streamlines for 
different levels in order to get a three-dimensional pic- 
ture of the atmospheric structure. Usually the charts 
are not drawn conventionally, with the distance be- 
tween the lines being inversely proportioned to the 
