Equivalent potential temperature, or theta-e, is the temperature an air parcel would have if it were lifted until all its water vapor condensed out, releasing its latent heat, and then brought back down to a pressure of 1,000 millibars. It folds an air mass's heat and moisture into a single number.
What raises it
Theta-e goes up when air gets warmer, more humid, or both. That is why a warm, moist air mass stands out as high theta-e on a map even when its temperature alone looks ordinary.
Values are usually given in kelvin (K). The useful information is in the pattern and the differences, not in one absolute number.
Why forecasters track it
Theta-e stays nearly constant as a parcel rises or sinks, whether the air is dry or saturated. The American Meteorological Society (AMS) Glossary describes it as conserved in a reversible moist adiabatic process, so it works as a tracer for following an air mass.
A theta-e ridge, or axis, is a band of relatively high values. NOAA's JetStream notes that severe weather and excessive rainfall often occur near or just upstream from one, and an NWS technical attachment says theta-e ridges are favored for storms only if other ingredients are present.
The NWS office in Louisville notes that mesoscale convective systems (MCSs) tend to move toward the highest low-level theta-e air.
How it changes with height
When theta-e decreases with height, the layer is potentially unstable. The AMS Glossary explains that if such a layer is lifted until fully saturated, it becomes unstable.
A large drop from the surface to a minimum aloft means dry air over moist air. The Louisville office lists a surface-to-minimum-aloft difference of 20 degrees or more in its wet microburst guidance, and notes that dry air above moist air can enhance downburst potential if storms develop.
What it does not tell you
High theta-e is not a storm forecast. It marks warm, moist air, but storms still need instability through the column and a lifting mechanism, and a strong cap can prevent widespread storms.
Guides like the 20-degree difference are local rules of thumb, not switches, and they work best read on a full sounding.