The variables.nl file contains the definitions of all variables that we can produce
forecasts for. This includes temperature, humidity, pressure, etc:
T units=^oC min=-50 max=45 mean=0 std=5 description=Surface_temperature standardName=air_temperature
P units=kPa min=50 mean=100 max=120 std=2 description=Pressure_surface standardName=air_pressure
The most important part of this file is the tag names (T and P) as these are
used later on to specify which variables to produce forecasts for. The min, mean, max, and std
(standard deviation) are used by the quality control system, which throws out bad
observations/forecasts.