It is possible for hot air balloons to go up in the rain but it's more difficult to get the hot air balloon into the air during rain.
Also when it rains the rain is gonna run off the top of the balloon and get into the basket and on the passengers so most hot air balloon places will not take off in a hot air balloon during the rain.
Because it leads to a bad experience and could potentially be bad and cause a crash.
So if it's raining it's unlikely anyone who runs a hot air balloon business will go up in the rain with the hot air balloon.