[lgl]

It's clearly in Celsius, which is the metric system's temperature unit and it is clearly stated on the website's about page:

By default the page loads and displays distances in miles, temperatures in Fahrenheit, ie English units (also known as Imperial or USCS system units). If you wish to have the page load and display in kilometers and temperatures in Celsius, ie metric system units use the urls below to select your preferred units.

[jacquesm]

GP means they should have used a Capital 'C'.

[danbruc]

It is a [small] capital C but a capital C is the symbol for coulomb, the correct symbol for degree Celsius °C.