Trowal
A trowal (Trough of Warm Air Aloft) is a trough-like extension of warm air aloft — a warm, moist air mass that wraps around the circulation of a maturing low-pressure system at upper levels. The trowal is one of the primary sources of freezing precipitation in Canadian aviation.
How the Trowal Forms
As a mid-latitude cyclone matures, the warm sector (warm, moist air between the warm and cold fronts) is wrapped around the low. At upper levels, this warm air forms a tongue that extends poleward and curves around the system. Where this warm air aloft overrides cold surface air, classic freezing rain conditions exist: rain falling from the warm layer, passing through a shallow sub-freezing layer, and arriving at the surface as freezing rain.
Aviation Hazards of the Trowal
- Widespread freezing rain associated with the trowal region — potentially covering thousands of square kilometres
- Embedded icing within cloud at all levels in the trowal area
- Poor forecast accuracy — the exact trowal position and intensity can shift
- Long duration — trowal icing events can persist for 12–24 hours or more
On the TC Written Exam
Trowal is tested at the CPL level in Canadian meteorology. Know: it is a warm air aloft feature wrapping around a mature low; it produces widespread freezing rain; it is associated with the most severe and widespread icing in Canadian winter operations.
Related Terms
See also: Freezing Rain, Warm Front, Cold Front, Icing.