Hi Andy, found the following on
Wikipedia after a quick search.
"Total air temperature is a term used generally in aviation. In other applications it is called stagnation temperature. Total air temperature is measured by a specially designed temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest kinetic energy is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore total air temperature is higher than the static (or ambient) air temperature.
Total air temperature is an essential input to an air data computer in order to enable computation of static air temperature and hence true airspeed."
The probe they are referring to is generally known as a Rosemount probe.
Hope this helps
, regards TS