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LINUX GAZETTE
...making Linux just a little more fun!
Python Weather Station
By Phil Hughes

This program is a simple interface that allows you to build a web page from the Metar data output from weather stations around the world. I wouldn't call it exciting but it does work.

Rather than try to describe what you will see, go here and take a look. You should see at least two and possibly as many as five weather reports from around Costa Rica. That is, the program has a list of five weather stations to check and displays the information from all that report.

System Structure

The heart of this system is the Pymetar package available here. This is a Python program which fetches Metar data described here. Pymetar is a command-line tool but it does all the dirtywork.

My goal is to make this information available on a web page. I didn't want to turn this into a huge programming project but I want the implementation to make sense. The most basic approach would have been to run the program as a CGI script for each request. However, this was potentially very inefficient because it would require the program to grab all the data each time a CGI request came in. More important, it would mean the user would have to wait for all these requests to complete.

I decided the best compromise was to set up a cron job to fetch the data and build a weather page. Then, each page request would just be displaying a static page. As the weather data does not change all that often, this actually offers pretty much current information.

Implementation

First, here is the code:


#!/usr/bin/env python

import sys
import time
sys.path.insert(0, "/home/fyl/pymetar-0.5")
import pymetar

def stations(args):
    for arg in map(lambda x: x.strip(), args):
        try:
            weather = pymetar.MetarReport(arg)
        except IOError, msg:
            # uncomment the following and remove pass line to see the errors
            # sys.stderr.write("Problem accessing the weather server: %s\n" % msg)
            pass
        else:
            if weather.valid:
		print "<h3>"
                print weather.getStationName()
                print " ( Lat: %s, Long: %s, Alt: %s m)" % \
		  weather.getStationPosition()
		print "</h3>"
		print "<table border=\"2\">"
                print "<tr><td>Updated</td><td> %s</td></tr>" % \
		  weather.getTime()
                if weather.getWindDirection() is not None:
		    print "<tr><td>Wind direction</td><td> %s</td></tr>" % \
		      weather.getWindDirection()
                if weather.getWindSpeed() is not None:
                    print "<tr><td>Wind speed</td><td> %6.1f m/s</td></tr>" % \
		      weather.getWindSpeed()
                if weather.getTemperatureCelsius() is not None:
                    print "<tr><td>Temperature</td><td> %.1fC (%.1fF)</td></tr>" % \
		      (weather.getTemperatureCelsius(), \
		      weather.getTemperatureFahrenheit())
                if weather.getDewPointCelsius() is not None:
                    print "<tr><td>Dew point</td><td> %.1fC (%.1fF)</td></tr>" % \
		      (weather.getDewPointCelsius(), \
		      weather.getDewPointFahrenheit())
                if weather.getHumidity() is not None: 
                    print "<tr><td>Humidity</td><td> %.0f%%</td></tr>" % \
		      weather.getHumidity()
                if weather.getVisibilityKilometers() is not None:
                    print "<tr><td>Visibility</td><td> %.1f Km</td></tr>" % \
		      weather.getVisibilityKilometers()
                if weather.getPressure() is not None:
                    print "<tr><td>Pressure</td><td> %.0f hPa</td></tr>" % \
		      weather.getPressure()
                if weather.getWeather() is not None: 
			print "<tr><td>Weather</td><td> %s</td></tr>" % \
			  weather.getWeather()
                if weather.getSkyConditions() is not None: 
			print "<tr><td>Sky conditions</td><td> %s</td></tr>" % \
		          weather.getSkyConditions()
		print "</table>"
            else:
                print "Either %s is not a valid station ID, " % arg
		print "the NOAA server is down or parsing is severely broken."


print "<html>"
print "<head>"
print "<title>Costa Rica weather from PlazaCR.com</title>"
print "</head>"
print "<body>"
print "<h1>Costa Rica weather from PlazaCR.com</h1>"
print "<p>Latest reports as of %s CST" % time.ctime()
gm = time.gmtime()
print "(%d.%02d.%02d %02d%02d UTC)" % (gm[0], gm[1], gm[2], gm[3], gm[4])
print '<p><a href="images/costa_rica.gif" target="_blank">Costa Rica map</a>'

stations(["MROC", "MRLM", "MRCH", "MRLB", "MRPV"])

print "</body>"
print "</html>"

I chose to just import the pymetar.py code in the wrapper than generated the HTML page. To do this, I added the Pymetar directy to the path being searched by Python.

Next I define stations, a function that queries the weather stations using the Pymetar code and then formats the output into HTML. It looks pretty ugly because it is just some long print statements building HTML strings with some if statements tossed in to see if we actually got the data. The important point is that you pass it a list of the station names and you get the body of the web page back.

Finally, the last maybe 15 lines of code just build the HTML boilerplace and call stations to produce the guts.

Testing and Installation

Because of the design, testing is very easy. There are no web-based dependencies in the design so you can just run the program from the command line.

In my case, I called the program wcr, so just typing ./wcr will run the program and display the HTML on standard output. If all goes well, run the program again, redirecting the output to a file. For example,

./wcr > /tmp/weather.html

You can now point a web browser at the file and see if it renders the page the way you want. If not, now is the time to make changes in wcr and continue testing.

Once you are happy with the output, upload the code to your web server and set up a cron job to run it. Normally, crontab -e will allow you to edit your crontab entry.

I elected to run the program twice an hour, at 5 and 35 minutes past. The crontab entry must execute the program and write the output file to a location the web server can get to. I used:

5,35 * * * * /home/fyl/pymetar-0.5/bin/wcr > /var/www/htdocs/weather.html

The four asterisks tell cron that the 5 and 35 minute times apply to every hour of every day. The next field is the name of the program to run. Finally the redirect operator (>) is followed by location where the HTML file is to be stored.

Assuming you set all the permissions right--that is, the program can write to the file and the web server can read the file, you are all done. Just point to this file and you have a weather page.

Conclusion

For the perfectionist, you probably need a fancier soluution. Why? Well, there will be a point in time when the contents of the HTML file will not be valid. When cron fires of the job the contents of the output file are truncated. Then the program runs and builds a new file.

Because of the way the program works this time is not just a short execution time of some Python code as the program queries the various weather stations and has to wait for a response. With the five stations I poll, I see elapsed times between one and ten seconds. If having bad data on the site for a maximum of 10 seconds every 30 minutes is acceptable to you, all is well. If not, write the output to a temporary file and then move it to the real file when all is done. Still not perfect but really close.

Now, for us mortals, we have a quick and dirty weather page. Have fun.

 

Phil Hughes is the publisher of Linux Journal, and thereby Linux Gazette. He dreams of permanently tele-commuting from his home on the Pacific coast of the Olympic Peninsula. As an employer, he is "Vicious, Evil, Mean, & Nasty, but kind of mellow" as a boss should be.


Copyright © 2003, Phil Hughes. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 94 of Linux Gazette, September 2003

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