If you want to talk about temperature with an American, there are a few things you need to know. Start by watching this video:

Celsius vs. Fahrenheit

Not all Americans know about Celsius, and even if they know about it, they don't have a good understanding of what 30 degrees or 10 degrees Celsius feels like.

The exact mathematical formula is Fahrenheit = 1.8 x Celsius + 32, and Celsius = (Fahrenheit - 32) x (.555).

But that's hard to do for some people, so here's a few important temperatures to know:
0 degrees Celsius = 32 degrees Fahrenheit
10 degrees Celsius = 50 degrees Fahrenheit
20 degrees Celsius = 68 degrees Fahrenheit
30 degrees Celsius = 86 degrees Fahrenheit
40 degrees Celsius = 104 degrees Fahrenheit

Word Order

Here are three ways to say the temperature:

Number + "degrees" + measurement: It's 50 degrees Fahrenheit today.
Number + "degrees": It's 10 degrees in Berlin today (the other person must know or guess which measurement you are using)
Number: It's 30 outside (the other person knows you are talking about temperature from the context).