The new indicator-datetime (clock indicator) which was released in Ubuntu 10.10 Netbook Edition (and will also be default in 11.04 Desktop edition too) doesn't provide any weather info like the old Gnome Clock applet.
A while back, a weather indicator was released, however its PPA is empty for a long time and it seems its development has stopped but luckily, Atareao @ atareao.es has started a new project called "My Weather Indicator" designed to bring back the weather conditions to your panel.
Besides the info available in the appindicator (temperature, termic sensation, wind velocity, humidity and so on), 'My Weather Indicator' also displays NotifyOSD notifications on conditions change and you can get an extended weather forecast by clicking the "Forecast" option in its menu:
This being the first release, there are some bugs too, most of them probably because of the weather service it uses (The Weather Channel): sometimes it takes a long time for the Forecast dialog to open; also, I've tried various cities in my country and for some, the weather wasn't displayed correctly while for others, the city wasn't found at all. Further more, the application lacks an option to change the measuring unit (so for now you're stuck with °C and km/h).
Install "My Weather Indicator" in Ubuntu
To add the PPA and install My Weather Indicator in Ubuntu, use the following commands:
sudo add-apt-repository ppa:atareao/atareao sudo apt-get update sudo apt-get install my-weather-indicator
Once installed, run it by going to Accessories > My Weather Indicator.
Unfortunately My Weather Indicator doesn't have a Launchpad page so you can't easily submit the bugs may find. However, you can let the author know if something doesn't work @ atareao.es.