See Weather Conditions in Ubuntu’s Top Panel with this Extension
The GNOME Weather desktop app (available in the Ubuntu repos) is a great tool as not only can you open the app to see weather information for any location you want, but it integrates a 4-day weather forest in the notification shade (calendar applet? Never sure what to call it).
But if you want to be able to glance at the top panel of your Ubuntu desktop to get the current weather conditions and temperature, without the hassle of opening a separate weather app or opening a pop-up menu, check out the Weather or Not GNOME extension.
Weather or Not leverages the same backend to display the current weather in the panel. Clicking on its panel icon will open the GNOME Weather app so that you can get dive deeper into more detail.