18 NAT OPE 
| NOVEMBER 1, 1906 
Mean Maximum Temperature. 
Berber, Jacobabad, Massawa, 
Lat. 15° N. Lat. 28° N, Lat. 15° N. 
January 000 86°7 73°60 " 84'2 
February. go'o Mag, 00 $5°3 
March 96 6 gI'l 87:1 
April 1060 103°I 90°5 
May ... I10°5 1116 94°5 
June... 112"1 {12°7 995 
July ... 108°5 107°3 tl TOTO) 
August as 110°3 1038 LOLS 
September ... 108°5 1035 97°7 
October 1040 936 95:0 
November 960 86°8 89°6 
December 89°6 76°7 86 9 
The data show that at the hottest period, from May 
to September, the high-temperature conditions are as 
intense in the Soudan hot area as in Upper Sind, and 
are more prolonged and persistent. This hot area plays 
a very important part in the meteorology of the Nile basin. 
It is throughout the whole year much hotter than Lower 
Egypt. The difference between the mean day temperature 
at Berber and Alexandria increases from 8° in January to 
16° in April and May. It thence diminishes under the in- 
fluence cf the monsoon rainfall in the Soudan region to 
8° in August, and increases to a second maximum (12°) in 
November. It is undoubtedly due to the presence of this 
permanent hot area in the central Nile basin that 
northerly winds prevail almost continuously in the northern 
half of the basin (i.e. north of Berber). The Massawa 
data also indicate that the hottest portion of the Red Sea is 
from 10° to 16° cooler during the day hours from March 
to October than the land area to the west. As the width 
of the Red Sea in lat. 15° to 20° N. is about 300 miles, it 
is evident that the presence of this relatively cool area will 
modify considerably the air movement and pressure dis- 
tribution in the adjacent land areas, more especially the 
Soudan comparatively low-lying area. 
Much less is known of the pressure distribution than of 
temperature in the Nile basin. Barometric observations are 
being taken at a considerable number of stations. When 
the elevations of the observing stations have been accurately 
determined by the Survey Department, it will then be 
possible to give, for the first time, a satisfactory statement 
of the changes of the distribution of pressure during the 
year. It is to be hoped that this information will be 
available in Captain Lyons’s monograph on the meteor- 
ology of the Nile basin, which we believe he has under 
preparation. Comparison of the temperature conditions of 
northern India and of the Nile basin suggest the probable 
pressure scheme. Pressure in January and the following 
three or four months is probably lowest in the interior 
regions of Africa to the south of the equator. An inde- 
pendent local low pressure begins to form in the Soudan 
hot area in March, and intensifies to some extent in April 
and May. This low-pressure area limits the advance of the 
monsoon winds in that region in the same manner that the 
low-pressure area in Baluchistan and Sind, and the Hima- 
layan mountain barrier, limit the northward extension of 
the south-west monsoon winds in India. During the period 
from June to September, an extensive low-pressure area 
extends from the Soudan across south-west Asia to Upper 
India, but it is probable that the Soudan depression, due 
to the local thermal conditions, maintains an independent 
existence from the Upper India depression, and is separated 
by a belt of somewhat higher pressure across the Red Sea. 
This is not confirmed as yet by observation. Captain 
Lyons, however, indicates in the chart of the mean dis- 
tribution of pressure in northern and central Africa in July 
his conviction that a local belt of low pressure stretches 
across central Africa between lat. 12° N. and 18° N. This 
either fills up in October and November or is transferred 
southwards. 
The air movement in the Nile basin is on the whole com- 
paratively simple. It is almost continuously from north 
to the north of lat. 17° or 18° N. (Berber), and is hence a 
drift up the valley due to permanent temperature and 
pressure differences between the east Mediterranean and 
Upper Nile valley. Also in the extreme south of the 
NO. 1931, VOL. 75] 
basin (in the basin of the Victoria and Albert Lakes) it is, 
so far as is indicated by the available data, almost equally 
persistent, but from the opposite direction, that is, from 
south and south-east. That region is hence, during nearly 
the whole year, within the sphere of the south-east trades. 
The movement is apparently for a short period in the early 
months of the year light, variable, and irregular, but 
chiefly from north. 
The air movement in the intermediate region between 
the equator and lat. 16° N. to 18° N. is typically monsoon. 
During one period of the year dry land winds (from the 
north) prevail, followed during the remainder of the year 
by humid oceanic winds (from south or west). The in- 
fluence of the Soudan hot area begins to be shown in 
March, and winds alternate between northerly and 
southerly directions in April and May. Thus at Khartoum 
the percentage of steadiness decreases from about 90 per 
cent. in January to 4o per cent. in May. In the beginning 
of June a change similar to that occurring in India in 
that month is initiated. Steady winds, the continuation of 
the south-east trade winds, which have previously given 
heavy rain to the equatorial lake area, prevail during 
the next three months. The direction of the air move- 
ment rapidly changes in proceeding northwards from 
south to west, determined by the position of the Soudan 
low-pressure area and action due to the earth's rotation. 
The current hence advances directly to the Abyssinian 
mountain or plateau area, the axis of which runs due 
north and south, its forced ascent over which gives rise 
to the heavy precipitation over the greater part of the 
plateau. No rain falls at this time in the Red Sea coast 
districts on the lee side of the plateau. The plateau hence 
plays (but much more completely) the same part for the 
Abyssinian branch of the south-west monsoon current that 
the West Ghats play with respect to the Bombay branch. 
This movement holds steadily until September, when the 
monsoon current contracts southwards, and light, northerly 
winds extend slowly to the neighbourhood of the equator. 
There is hence a clearly marked monsoon alternation of 
winds and of season (dry and wet) in the intermediate 
area between lat. 5° N. and Jat. 18° N. 
The distribution of the rainfall in the Nile basin is 
very clearly exhibited in a series of monthly charts in 
Captain Lyons’s monograph. A chart showing the amount 
and distribution of the average annual rainfall would have 
been a useful and valuable addition to the series. Charts 
of annual or seasonal rainfall are, as a rule, even more 
valuable for comparison than charts of monthly rainfall. 
The air movement has shown that the Nile basin may 
be divided meteorologically into three areas, viz. the area 
of dominant northerly winds (north of lat. 17° N.), the area 
of alternating monsoon winds between lat. 17° N. and 
the equator, and the area of dominant south-easterly 
winds south of the equator. The rainfall differs greatly 
in its characteristic features in these three areas. In the 
northerly region it occurs during the winter months, as in 
Syria, the Euphrates valley, and the Iran plateau, and is 
small and very variable in amount. The average annual 
fall at Alexandria and Suakim is about 5 inches, at Port 
Said 2 inches, and at Suez 3 inch. In the intermediate 
monsoon region practically no rain falls from November 
to April. Thunderstorms occur in May, chiefly in the 
southern districts, and frequent heavy rain from June to 
September or October, according to position. The rain- 
fall is heaviest on the western and central portions of the 
plateau. In the Himalayas the rainfall is, as a rule, 
heaviest at an elevation of about 4000 feet. The 
Abyssinian data are too scanty to show whether there is 
any line of maximum rainfall lower than the level of the 
interior plateau or higher mountain ranges, where the 
highest elevations exceed 15,000 feet. 
The precipitation in the equatorial lake region has a 
double maximum and minimum in its annual variation, 
related, as Captain Lyons points out, to the apparent 
movement of the sun. The rainfall is small in amount 
during the period of heavy rainfall in the monsoon region 
from June to September. It is heavy from October to 
December, and again in March and April, and is light 
to moderate during the intervening months of January and 
February, and moderate in May. 
