The weather dropped below zero.

When discussing the weather, it is generally not said how much it will fall.

If you have to describe it in percentage, you should use thermodynamic temperature (Kelvin).

25°C = 298k

17 C=290K

Decrease =(298-290)/298=2.7%

It is meaningless to directly discuss how many percent the temperature has dropped-because the temperature can be negative (otherwise, if the temperature drops from zero to below zero, how many percent will it drop? )