I am having trouble finding weather data for San Luis Potosi, Mexico. 
Its exact location is 22N09 100W59 which is about 4 hours North of
Mexico City. If anyone could lead me in the right direction of finding
historical international weather data in the categories of annual
precipitation and evaporation rates I would greatly appreciate it. 
I've wasted a lot of time searching and have still come up with
nothing.

Hi Lindseyann,

The International Research Institute (IDI) maintains an international
climate data library, where you can access this type of information
for any location. The main webpage for the Institute is here: 
http://ingrid.ldeo.columbia.edu/ and if you scroll to the bottom of
the page, there is a link to a very good tutorial on using the system.

San Luis Potosi’s World Meterological Organization (WMO) station call
sign is MMSP, and its identifier is 765390.  Here is its main daily
data page:  http://ingrid.ldeo.columbia.edu/SOURCES/.NOAA/.NCDC/.DAILY/.STATION.cuf/IWMO+765390+VALUE/
Also, here’s a link to a page of daily precipitation data for 1994-99:
http://ingrid.ldeo.columbia.edu/SOURCES/.NOAA/.NCDC/.DAILY/.STATION.cuf/IWMO+765390+VALUE/.prcp/
There doesn’t appear to be any data specifically on evaporation, but
you can extrapolate evaporation rates from other information provided.

And then, here is a link to Mexico’s national meterological site,
which has historical weather data for the country:
http://216.239.39.120/translate_c?hl=en&ie=UTF-8&oe=UTF-8&langpair=es%7Cen&u=http://smn.cna.gob.mx/productos/map-lluv/hmproduc.html&prev=/language_tools
 I’ve had to translate the page as my Spanish is somewhat limited, but
if yours is better, you might prefer to use the original language
site, here: http://smn.cna.gob.mx/SMN.html with this link to climatic
data: http://smn.cna.gob.mx/productos/map-lluv/hmproduc.html

If you would like to order historical weather data on CD from the
National Climatic Data Center (NCDC), there is information here on
what’s available: http://www.lib.ucdavis.edu/govdoc/CDROM/climate.html

As for the NCDC, you might also find pertinent information on their
site, though be aware they do charge a small fee for downloading some
reports.  Their main site is here:
http://lwf.ncdc.noaa.gov/oa/ncdc.html and specific climatic data is
available here: http://lwf.ncdc.noaa.gov/oa/climate/climatedata.html 
You will have to do a little digging here, but the amount and types of
data are voluminous and cover the globe.

Also, the Washington Post maintains some limited historical weather
data for a number of locations, including yours, here:
http://www.washingtonpost.com/wp-srv/weather/longterm/historical/data/san_luispotosi_mexico.htm

And if you’re interested in hard copy, there is a list of publications
at the U. of West Florida, here:
http://www.lib.uwf.edu/eli/Science/meteorology.shtml

The search terms I used were:
historical climatic data mexico evaporation OR precipitation -new
and the Google results are here:
://www.google.com/search?num=50&hl=en&lr=lang_en&ie=UTF-8&oe=UTF-8&as_qdr=all&q=historical+climatic+data+mexico+evaporation+OR+precipitation+%22+%22+-new&btnG=Google+Search&lr=lang_en

I hope you’ll find this information useful.  Please let me know if
anything’s not clear.  Best of luck in finding the data you need.

Regards,
Byrd

Thank you very much for your prompt and informative answer.  This was
info required for work - i'm an environmental consultant intern, and i
don't have any extra time to waste.  These weather data websites will
be a continual help for me, and i appreciate all of the work that you
put into your response.  I will be recommending and using GoogleAnswer
in the future.