See Heat Pump and Defrost Cycle. A winter phenomenon produced by a heat pump, for which conditions have to be just right. If you actually catch a heat pump giving a steam show and have a video camera handy, it makes for one hell of a YouTube video.
Many heat pumps have interval defrost and will have a defrost cycle every 30, 60, or 90 minutes whether they need to or not, even if they are NOT frozen or iced over. The ones with more modern demand defrost will oly defrost on an as-needed basis, basically when there is ice/frost buildup on the coils. The best conditions for a steam show are when it is below 40 degrees, and a heat pump will freeze up much faster when it is wet, rainy, or snowy. However, a heat pump is capable of producing a good steam show on a nice, sunny day with no precipitation. The best steam shows typically occur below 37 degrees regardless of weather conditions.
How much steam a heat pump produces depends on the outside temperature, how much ice/frost/snow has built up on the outdoor coils, and the heat pump itself. To someone not familiar with a heat pump, a steam show can be misleading, and calls to the fire department in the winter by people claiming their heat pump is on fire are not out of the ordinary. Some stubborn customers also wind up spending hundreds of dollars on unnecessary service calls only to find out their heat pump is doing exactly what it is supposed to do.
The heat pump at the elderly couple's new condo produced such a big steam show that they thought the unit was on fire, shut it off, and called the HVAC company, only to find out that their heat pump was doing exactly what it was supposed to.
An HVAC tech once told me the story of a customer who observed a steam show and thought their heat pump was on fire, and paid over a hundred dollars for a guy to come out only to tell them it had been defrosting.