Frost

The liquid oxygen used at the hospital where I work is turned into a gas before it goes into the building and is piped into each patient room. The picture shows the liquid line as it enters the evaporator section of this distribution system.
There are two evaporators and the liquid is piped into them alternately so that neither one will be so covered with frost as to become useless. this picture was taken on a cold, dry, December day so you can imagine what it would look like on a humid summer day.