Climate of the City of Mexico. 
DQCTOE 
GOIMITSTGO ORYA^ANOS, 
MEMBER OF TlIE 
Supreme Board of Health of Mexico. CLIMATE OF THE CITY OF MEXICO. 
BY DOCTOR DOMINGO ORVANANOS, 
MEMBER OF THE 
SUPREME BOARD Of HEALTH OF MEXICO. 
The influence which the atmosphere, the waters and 
the locality, exercise on individuals when congregated in 
societies and living on determined points of the earth, is 
very positive, and for this reason very important to the 
sanitary physician of each place. It is also of importance 
to him to have a thorough understanding of the climate 
in which his patients reside, this being a very important 
consideration in the resolution of the many problems which 
affect public health. 
In this paper I propose to make a brief study of the 
principal meteorological data of the city of Mexico, in 
everything referring to its situation, elevation, humidity, 
temperature, winds, ozone and light. 
SITUATION. Mexico (place of Mexitli, God of War) 
is situated in 19° 26’ 26” Nerth Latitude and 99* 6’ 45” 
Longitude West from Greenwich. The greatest width of 
the city from east to west is five kilometers and 53 meters; 
and the greatest width from north to south is 4 kilometers 
800 meters: the superficial area is 14 square kilometers; 
without including those grounds on which they are at 
present forming colonies. The semi-tropical situation of the city causes the rays 
of the sun to fall vertically upon it twice a year, whilst 
in all the rest of the year they are very little far from 
perpendicular. The longest day lasts 13 hours and 10 
minutes and the shortest, 10 hours 50 miuutes. 
ELEVATION. The City is situated at an elevation of 
2268 metres over the sea level. The mean barometric 
pressure is 586.40 m. m. the maximum in 15 years has 
been 594.19 m. m. (1879) and the minimum within the 
same period has been 579.80 m. m, 
The extraordinary elevation at which this city is situated 
causes a diminution of oxigen, which, instead of being in 
the proportion of 0.28331 litres of atmosphere with a 
temperature of 15“ as at the sea level, is only in the pro- 
portion of 0.20685. This means to say, that at each respi- 
ration we introduce almost one fourth part less of oxigen 
into our lungs than the inhabitants of the lower elevations, 
and this necessarily causes certain physiological and patho- 
logical changes in our condition. 
As might very well be suposed, ail the inhabitants of 
Mexico ought to suffer a certain amount of anaemia and 
for many years this has been the opinion of many intelli- 
gent observers, but nevertheless, the experiments practised 
during the last few years in this City, by Drs Cordero, 
Gaviho and especially Lope de Vega, appear to prove the 
following facts: 1st, The number of red corpuscles which 
each individual has are generally rather more numerous 
than on the coast; 2nd. The diminution of oxigen in 
the air is compensated by the increase in the number of 
respirations, which are from 22 to 24 por minute. These 
same conclusions are deduced from the experiments which 
have been lately made by Drs. Muntz and Viault, each one 
independently of the other, and at altitudes of 2877 me- 
tres above the sea level. These experiments were carried out 
on the “Pic de Midi” and on the summits of the Andes. 
Dr. Gaviho has also made three experiments to discover the amount of carbonic acid which was exhaled, and ob- 
tained a mean of 4. 25 per hundred of air, arriving there- 
fore to the following conclusion; In Mexico the produc- 
tion of carbonic acid manifests itself almost in the same 
proportions of volume as in those places which have a pre- 
ssure of 7(3 centimeters, and as this is an indication of the 
organic combustions, we may conclude that we consume 
approximately the same amount of oxigen, for which 
purpose we increase our respirations by one fourth part. 
Dr. Coindet came to the conclusion after various expe- 
riments made in the valley of Mexico at the time of the 
French Invasion, that the mean volume of carbonic acid 
exhaled was 3.96% among newly arrived Frenchmen, 
4.35% by Mexicans and 4.51°/0 by indians. 
But with respect to the results obtained both by Dr. 
Gavino and by Dr. Coindet, we cannot but reflect on the 
remarks previously made by Lombard, that even when 
the quantity of carbonic acid exhaled, might represent the 
same volume in Mexico as at the sea-level, taking into 
account the rarity of the atmosphere at an altitude of 
2268 meters, we And a remarkable deficit on the central 
table land as compared with the lowlands, 
The results obtained by Drs. Lope de Vega, Muntz and 
Viault are not founded solely on a limited number of ex- 
periments, and for that reason we await the continuation 
of these labors so as to be able to form an exact judge- 
ment on the point. 
ATMOSPIIEBIC HUMIDITY. Absolute Humidity. 
The city being situated at the considerable elevation 
that we have above mentioned, the general rule follows 
respecting the amount of water vapor held in the atmos- 
phere which applies to elevated situations, that is to say, 
that it ought to be scarce considering the latitude. So it 
turns out to be, as the medium quantity contained in a cu- 
bic meter per annum is 8.16 grammes or measured in mili- 
meters of mercury 8,24 m. m. This amount is small com- 6 
pared with the towns at a low'er elevation within the re- 
public, where the tension of the water vapor is 12 to 14 
m. m. As hot air absorbs a greater quantity of water 
than cold air, the progress of the tension of the vapor 
follows an analogous process to that of the temperature, 
that is to say, that it is is weaker in winter and more 
elevated in summer. Thus it is that wre see in Mexico a 
general increase from February to July, and a correspon- 
ding decrease from the latter month to the following Fe- 
bruary. Respecting the daily rates of tension, the obser- 
vations follow the same course, which is parallel to that 
of the temperature, notwithstanding that at mid-day 
and in the first hours of the afternoon the lines diverge to 
a slight dergee, caused by the extra elevation of the at- 
mospheric temperature which carries off a great quantity 
of waiter vapor that cannot be quickly replaced by evapo- 
ration from the earth. 
EVAPORATION. The evaporation in the city is extra- 
ordinary, the mean by day being 6. 6 m. m. in the sun and 
2. 5 m. m. in the shade. 
RELATIVE HUMIDITY. The absolute amount of 
vapor contained in the atmospere in not of so much impor- 
tance to us as the relative humidity. The atmospere may 
contain a great amount of vapor and appear to us dry, and 
on the contrary it may hold very litle vapor and appear to 
us full of humidity, as our senses can only appreciate re 
lative humidity, and more especially wdien this approa- 
ches the degree of saturation. 'The progress of relative hu- 
midity is in inverse ratio to the absolute humidity and can 
be easily understood when we reflect that with the increase 
of heat in the atnosphere, the latter requires a greater 
amount of vapor to become saturated. The mean annual re- 
lative humidity in Mexico amounts to 61 in place of-75 or 
80 that wTe find at lower elevations. The result is that we 
have a very dry atmosphere, a circumstance remakable 
wTien we think of the five lakes that are close to the city. In spite of this fact the atmosphere in this city is as 
dry as that of Puebla, San Luis Potosi and other ci- 
tios of the central table land. Some years ago, when we 
discussed in the National Academy of Medicine, the possi- 
ble evils that might result to the towns of the valley from 
the drainage of the lakes, I had the honor to read to that 
honorable corporation a paper in which I made an ex- 
tensive study of everything relating that question, arri- 
ving to the following conclusion: “ From all this we find 
that even if the lakes were a great deal larger than they are 
they would hardly increase the hygrometry of the city; first • 
ly because it is not situated in the course of the prevail- 
ing winds from. N. W.; and secondly because the winds 
which pass over these lakes are excessively dry and hot 
and therefore require enormous quantities of vapor before 
they arrive to the point of saturation. 
Fron what is above said, it will be easily understood 
that no prejudice will arise to the hydromet'ric conditions 
of the city from the drainage of the lakes. 
DEW. As the nights are generally very clear, especial- 
ly in spring, and the air is calm, the dew falls in abun- 
dance on vegetation of all kinds. 
FOGS. Fogs are extremely rare in this city, although 
they do sometimes occur in the early spring mornings. 
CLOUDS. During the winter and the greater part of 
spring the sky over the Mexico valley is perfectly clear; 
we have a few cloudy days in December and January, 
but they are by no means common. What is commonlv 
observed is a species of cloud cap sorrounding the valley 
and resting on the crests of the mountains which limit 
the view. The current of air which rises from the ground 
through its own heat, becomes rarified as it elevates itselfr- 
and for the same reason gives off a great deal of its 
heat. Itbalso meets with the almost frozen tops of the 
mountains which oblige it to precipitate its vapor in the 
form of clouds. These are most commonly seen in the afternoons, but when night draws on and cools the soil 
of the valley, the ascending current ceases, the clouds 
disappear and the sky remains perfectly clear for the rest 
of the night. 
RAINS. The rains are not very abundant in the 
valley of Mexico, although as a general rule they are not 
rare, the mean annual rainfall reaching 614.4 m. m. and 
the number of rainy days averaging 139. 
The rainy season can be said to last from June to Octo- 
ber, as in the other months of the year we rarely have any 
rain. The amount of water that falls in each precipitation 
varies a great deal, being sometimes very excessive, and 
has occassionaly reached 63.3 m. m. (1888.) 
The relative abundance of the rainfall dees not prevent 
our atmosphere from being very dry, and it is curious to 
observe, how after a heavy storm the rays of our tropical 
sun shine afresh, the sky clears and the air is as dry as 
before. 
SNOW. It may he said that we do not know what a 
snow storm is, as we only see one about once in every forty 
years, so that they are hardly worth taking into considera- 
tion; nevertheless we have the pleasure of observing the 
perpetual snows all the year round on the summits of Po- 
pocatapetl and Ixtac.ihnatl. 
TEMPERATURE. We have seen that the atmosphere 
of the valley is very dry, as happens more or less in all 
elevated localities. As the water vapor absorbs the caloric, 
this being so scarce in our atmosphere renders it colder 
than that on the slopes of our mountains or on our coast. 
On the other hand, the ratification of our atmosphere by 
the decrease of pressure in its ascent to the central table 
land is another powerful cause for the diminution of its 
heat. That is one reason why our dry atmosphere is 
always fresher and more agreable than that of other cities 
in the Republic that are situated in the same latitude as 
Mexico but at a lower elevation. The elevation of our valley is the principal element of its climate, because as 
we say in another part “The dimished pressure of our 
atmosphere which is the cause of the ratification of the 
atmosphere, of its coolness, and to a great extent, of its 
dryness, is the principal meteorological cause which gives 
rise to all our tliermometrical phenomena/’ The mean 
annual temperature in Mexico in the shade is 15°5 centi- 
grade (59"9 F.) The knowledge of these figures cannot 
give us even an approximate idea of our climate, seeing 
that it is well known that there are plenty of places which, 
with the same annual mean temperature, show a very 
great difference in the progress of that temperature during 
the different seasons, during the different months and 
generally in the modifications which that same tempera- 
ture undergoes. 
The mean temperature in Mexico during the winter is 
12." 9, that of spring 17.0 8, that of summer 16.° 6, and 
that of autumm 13.° 5. The difference between the mean 
temperature of the hottest month which is April, and Ja- 
nuary which is the coldest is 7.° 5. Thus we find that the 
climate in Mexico can be classed as both temperate and 
equable, seeing that the annual variation of temperature 
is exceedingly slight. But if instead of taking into account 
the general variations of temperature we look at the ex- 
treme degrees of heat or cold we find that our climate 
is also variable. Thus we find that the maximum in the 
shade for 16 years has been 31.° 6 (1878) and the maxi- 
mum in the sun has been 49. (1878) the minimum in the 
shade has been l.° 7 below zero (1887) and the minimum 
in the sun 7.° 2 below zero (1887.) The difference between 
the absolute maximum and minimum in the shade reaches 
up to 25. degrees in the course of a month, and the differen- 
ce between these same maximum and minimum in the sun 
has reachead as high as 53.° 9. 
The daily progress of the temperature is as follows: The 
temperature between five and six in the morning is the low- est in the day, and from that point it rises gradually until 
two or three in the afternoon when it reaches its maximum, 
and thence decreases in as gradual a manner until five or 
six in the morning. 
With respect to the daily oscilation and other phenome- 
na incidental to the inconstancy of our climate, we have on 
another occasion expressed ourselves as follows on the sub- 
ject. (1). “The daily oscilation has been 21° 2 in the shade 
and 50° 7 in the sun. All the changes of temperature 
are divided by the mean temperature between the annual 
mean from 10° below zero at 80° latitude and the annual 
mean of 32°; consequently, between these two extreme 
temperatures there is a scale of 50°; so that we go through 
that scale in one single day, as it has been seen, and we 
can properly say that in one and the same day we expe- 
rience all the changes, and it seems as if we were conve- 
yed from the Polar frozen regions to the burning zones 
of Africa. 
This difference of temperature constitutes one of the 
characteristic features of our climate. In Mexico with her 
dry atmosphere, the sun’s rays produce an extraordinary 
heating power, so as to give our soil the appearence of 
being scorched. When ths sun sinks behind the tops of the 
mountains, the radiation towards the heavenly regions in 
the Valley of Mexico begins, the air which on account of 
its dryness is enabled to prevent the heat from reaching 
the soil, is also enabled to prevent its departing from it, 
and this takes place with extraordinary frequency. We 
thus have in Mexico, on one side, the burning soil, and on 
the other, not only in the regions of great altitude but in 
the moderate altitude of the valley, perpetual eold. In our 
region the eternal snow begins at an altitude of little more 
than four kilometers, as can be seen on Popocatepetl and 
(1,) The Federal District in the Republic of Mexico as a suitable place for 
persons predisposed to tuburculous affections and for relief of pulmonary con- 
sumption. Public Health Papers and Reports, Vol. XVI, Ixtacihuatl. The Valley of Mexico, as already said, is si- 
tuated at 2600 meters above the sea level, and only about 
two kilometers distant from the eternal snow; that is to 
say, as distant as a town would be that were situated at 
50° or 60° degress latitude.” 
FROSTS are very frequent in the valley of Mexico, and 
as they always take place with a clear and transparent, at- 
mosphere the ’fays of the sun beat on our bodies to an 
excessive degree, wdiilst in any other place, perhaps only a 
yard distant but in the shade, we feel a remarkable degree 
of cold. 
I have myself experienced a cold of 15° C. below 
zero in New York, and have felt it less than in the 
City of Mexico with a temperature of one or two degrees 
above zero, and this is because the irradiation of our bo- 
dies is very rapid and extraordinarily intense during a 
heavy frost, Fortunately these are not very frequent and 
when they happen, the cold is only felt for one or two 
hours in the morning and evening.' In all other respects 
the City of Mexico is a place well adapted for winter re- 
sidence, seeing that here we have not the excesive and con- 
tinued cold waves of the north or its snows; but it is even 
a better summer residence on account of the absence of 
the excessive heat found in the more northern countries. 
In Mexico we require some covering over our beds during 
the whole year, on the days of greatest heat we have some 
delightfully fresh mornings, and even on the coldest days 
we do not require any fires in our houses. It will there- 
fore be easily understood that our city is a pleasant, re- 
sidence in winter, and although it appears a paradox to 
advise the inhabitants of the United States and Canada to 
take a journey of several thousand kilometers to the south 
during the month of July; the fact is that they will find 
in these tropical latitudes, elevations of two or three thou- 
sand meters with a fresh aud agreeable temperature. 
WFNT)S. The prevailing winds in Mexico are from the N. W. The breeze m the City is generally slight, its 
mean velocity per second not going beyond O.8.M., and 
the maximum velocity that has been observed up to date 
is 21 m. m. (1886.) The strongest wind is that which 
comes from the north east. With regard to the humidity 
of each one of these winds that blow over the city we 
can give them these respective positions: 1st.; N. W.; 2nd, 
W.; 3rd, S. W. and N.; 4th, S.; 5th, S. E.; 6th, E.; and 
7th, JST. E. Their respective temperatures beginning with 
the coolest, can be rated as follows:-lst, N. W.; 2nd, W.; 
3rd, N.; 4th, is. E.; 5th S. and hi. E.; 6th, E, and 7tli S. E. 
OZONE. The annual mean quantity of ozone in the air 
of the city is 4. 2; but as the methods that have been hi- 
therto discovered for determining the amount of ozone in 
the atmosphere are defective, I will not enter further into 
this subject. 
LIGHT. The geographical situation of the city and 
its great altitude are the cause of the extraordinary inten- 
sity of light in its atmosphere. The ratification of the at- 
mosphere and the water vapors as we have above shown 
being very scarce, the solar rays exercise on the ground 
their full luminous, calorific and chemical powders unim- 
peded by any disturbing influence. 
The interesting observations carried out by Downs and 
Blunt (1) on the effect of light on bacteria and other orga- 
nisms which are perfectly developed as well as on the 
protoplasm, has brought to our knowledge some most in- 
teresting facts with regard to this question. By exposing 
to light the Pasteur solution, urine and infusion of hay, 
and afterwards depriving them of light, we find that this 
agent is not favorable to the developement of bacteria 
and of microscopic spores whicr are produced dnring pu- 
trefaction. It has also been noted that the direct rays of 
the sun act in a more powerful manner than when they 
(1.) Proceedings of the Royal Society Vol, 26, 1878. are difused, and that the germs already developed can be 
destroyed by the sole influence of solar light. 
Doubtless it is to this benevolent influence that we owe 
our almost total exemption from tuberculosis and the 
fact that when found amongst us it takes such a very 
mild form, also the fact that the greater part of the infec- 
tuous diseases are either rare amongst us or entirely 
unknown. 
It is to this beneficent influence also that we have the 
pleasure of seeing you again in our own country, and we 
do not doubt that in your own hearts yon will always 
preserve a sincere affection for, and desire to see, the beau- 
tiful light as well as enjoy the delicious climate of the pi- 
turesque valley of Mexico, 
Mexico, Od. 3rd, 1892, 
I). Orvaxaxos. ..... memoria es propiedad de la Asociacwn 
[Am pa Icana de Salub ridad JPublica y sin per- 
mi so desu Comisiondepublicaciones no podrd 
reimp Hmirse ni copiarse ni eqptractarse.