Where you are changes the hazard
Two numbers, the freezing level and the boundary-layer moisture, decide which of these two hazards your region actually hands you. They are not the same everywhere, and the training photographs are from somewhere else.
Two dials, set differently everywhere
Everything in the previous seven lessons is physics and applies wherever you are standing. What is not the same everywhere is which hazard that physics delivers to you, and it comes down to two quantities.
- Freezing level height
- How far a stone has to fall through air above freezing before it reaches the ground. Lesson 1 noted that between 40 and 70% of what forms aloft can melt on the way down. A high The height at which the air reaches 0 °C. A hailstone has to survive the fall from there to the ground, so a high freezing level means more melting and smaller hail at the surface. pushes that fraction up and takes the hail away.
- Boundary-layer moisture
- Whether a descending Air descending through and out of a storm, driven by the weight of the precipitation and by rain evaporating into drier air below. It is what produces the gust front and the damaging winds. arrives wet or dry. A deep dry sub-cloud layer lets evaporation accelerate it, producing a A downburst falling through deep dry air, cooling by evaporation and arriving with dust and little rain. A High Plains phenomenon rather than a Gulf Coast one. under a storm that may not be raining at the ground at all. A moist one produces a A downburst arriving inside heavy rain, which is the dominant damaging-wind mode in the humid Southeast. The rain hides it, so the warning you get is short., buried in rain.
What that produces, region by region
Boundaries are soft and every region gets every hazard occasionally. The column that matters is your own, and the point of reading the others is knowing which training material was written for somebody else.
The coastal case, with numbers
The Gulf coast row is worth expanding because it is the one where the figures are published and the contrast is sharpest. Two comparisons make the scale concrete better than an average would. On the Texas Gulf coast, one coastal county recorded 18 severe hail days since 2000 against 80 for the county containing San Antonio, a few hours inland. And when NWS Jacksonville logged two-inch hail in August 2020, it was only the third August two-inch report in Florida since 1955.
The wind you are most likely to actually witness
This is the practical half of the lesson. Ask which damaging-wind event is commonest where you live, rather than which is most dramatic, because the commonest one is what you will be standing under.
Tropical systems
Landfalling tropical cyclones bring their own reporting problem, and they bring it a long way inland: A band of low cloud running along the low-level wind and moving toward a storm, marking the air being drawn in. Bands curved as though turning cyclonically may mean a mesocyclone. produce brief convective gusts and spin-up tornadoes on top of a background of sustained tropical wind. Module 4 lesson 9 walks the rotation side of that; the wind side is subtler.
- Separate the gust from the background. A 55 mph Wind speed averaged over a period, typically two minutes at an automated station. A gust is a brief peak, and reporting one as the other changes the meaning entirely. with 70 mph gusts is a different report from a 70 mph convective gust in a 30 mph background, and the second is the one a band produced.
- Timing matters more than usual, because bands arrive in a sequence somebody is tracking.
- The measured or estimated flag matters more than usual too, because anemometers in a landfalling cyclone are often the only instruments still reporting, and often not exposed the way lesson 5 requires.
A tropical rainband is not just heavy rain, and treating it that way is how people are surprised by a gust or a brief tornado hours from the centre and hundreds of miles from the coast.
An August afternoon in a warm, deeply moist coastal air mass. A single storm with no watch in effect produces damaging wind over a small area. Which hazard was this most likely, and why?
Two dials. One of the wrong answers is the hazard a high freezing level takes away, and one confuses a dial setting with a definition.Wet microbursts from summer pulse storms dominate damaging wind in warm moist air masses, and they arrive without a watch, in heavy rain, from a storm that looks unremarkable. Large hail is the hazard a high freezing level suppresses. And wet against dry is set by the sub-cloud moisture, not by whether the storm was a single cell: the same cell over the High Plains would give you the dry version.