Want to keep track of the weather you of course need the temperature which can be measured be using e.g. the TMP36. This is not enough we also need to know the barometric pressure to be able to do some weatherforecast. 
This pressure can be measured with the BMP085 (datasheet); mounted on a breakout board (that can be found at Sparkfun) will be a good idea if you want to be able to connect it to your Arduino. An extra feature of the BMP085 is that it can also measure the temperature.
At Sparkfun you can find some useful links and Bildr has a nice example of how to connect the breakout board and get useful information from it.
It don’t have the BMP085 myself so I cannot tell you my experiences. I’ll write about it some time when I bought and installed one myself.