low pressure 
noun [mass noun]
A condition of the atmosphere in which the pressure is below average (e.g. in a depression): areas of low pressure almost always bring rain [as modifier]: low-pressure systems across the Alps...
- Light-ice years are associated with more low-pressure activity in the East Siberian Sea.
- At the same time the cold air moving southwards behind the low-pressure centre is losing height.
- The first thing is, remember, a hurricane is just an area of extremely low pressure, and there's a lot of circulation around that center of low pressure.