Explain units for negative exponent

Question

From a textbook of recent publication;

"The average radiation on the Earth’s surface is $340 W m^{−2}$."

I get the 340W. I presume m is meters. How does a minus 2 affect a meter, rather a square meter?

Answer 1

It should be read as "per" and without the negative. So, in your case, "watts per square meter." You do the same thing with "miles per hour," which is $$ \frac{\text{miles}}{\text{hour}} = \text{miles} \cdot \text{hour}^{-1} $$