You will be able to recognize tropical systems and snow squalls, and know what makes each one
different from the thunderstorms you have learned so far.
Tropical systems and winter convection get their own lesson because they behave unlike anything else,
and because each one is a good test of whether the earlier concepts actually generalize or whether you
just memorized some pictures.
Hurricane / Tropical System
A pinwheel wrapped around a center.
A mature tropical cyclone has three useful radar features:
Eye. A relatively echo-free center.
Eyewall. The ring of intense precipitation around it, usually containing the
strongest winds.
Spiral rainbands. Curved bands wrapping around the circulation.
Laura gives you the textbook view: eye in the middle, eyewall around it, rainbands spiraling outward. You will not always see a perfect eye. Follow the curve. Even when the center is off the radar image, the rainbands still bend around it.
The Green and Red test, scaled up
A tropical cyclone on velocity is one enormous rotational couplet. Apply the test
from Green and Red and it works exactly as advertised: the inbound and outbound winds sit on the
opposite sides of the center, so it is rotating rather than converging. Same test, scaled up several
hundred miles.
The peak winds sit at the eyewall, not the eye and not the rainbands.
It is the same test you already learned: the two sides straddle a radial through the center, so you are looking at rotation rather than convergence.
One side is stronger
In the Northern Hemisphere, the storm's forward motion adds to its rotating winds on the right side
and subtracts from them on the left.
The storm does not need to get stronger for its wind field to become uneven.
Try it
Watch the wind field
Rotation stays the same. Forward motion changes the wind around the storm.
Expect artifacts
Tropical wind speeds routinely exceed what the radar can measure cleanly, so aliasing is common
throughout the velocity field. When Radar Lies covers how to recognize it. In a hurricane, assume some of what
you are seeing is folded.
Snow Squall
A narrow band moving fast, in winter, where nothing else is.
A sudden burst of heavy snow that can drop visibility quickly. It may only last a short time at one
spot even while the band keeps moving.
Snow squalls are shallow, so the radar beam can pass over them at distance. A band that is obvious
near the radar may become weak or disappear farther away.
When Radar Lies explained why.
Why snow is hard to read
Snow has no dependable dBZ range. Large flakes, wet snow, and the melting layer can all make
reflectivity stronger than you might expect, so color alone cannot tell you what is falling.
Some apps add a precipitation type layer that combines radar with temperature and other data:
One storm, four precipitation types. Reflectivity is underneath; the precipitation-type layer adds the blue, purple, pink, and rain colors.
Blue = snow
Purple = sleet
Pink = freezing rain
Green to red = rain
This is a layer, not another radar product. The radar still measures reflectivity
underneath; the added colors are an estimate of precipitation type.
Useful clue, not proof. The classification can be wrong, especially near the
rain/snow line.
Layers, not another product
Everything we've shown you on Live radar so far has been a product: a different
way of looking at the same radar scan. Reflectivity, velocity, CC, a different tilt, all still the
radar, just read differently.
A layer is different information altogether, placed over or under the radar
picture from a source that has nothing to do with the radar at all. Depending on the app, that might
include lightning, satellite, warnings, or ground temperature. The live radar in this course includes
ground temperature as its one example.
Lake effect snow
Cold air crossing relatively warm lake water can produce narrow bands of snow downwind.
Follow the band backward. It begins over the lake and stretches downwind.
Lake-effect bands often line up with the low-level wind. Follow a band backward and you can often
trace it toward the lake that is feeding it.
A small change in wind direction can swing the band somewhere completely different.
Try it
Read the pattern
Both rules from this lesson, tested one real image at a time.
Check yourself
On velocity, a tropical cyclone appears as one enormous couplet. Applying the same test from the velocity lesson, what does it show?
Rotation, and the test works unchanged at a scale of hundreds of miles. The inbound and outbound maxima straddle a radial through the center, which is the side by side arrangement.
In the Northern Hemisphere, which side of a moving hurricane has the strongest winds?
The right side. Forward motion and rotation point the same direction there and add together, while on the left they oppose and partly cancel. Embedded tornadoes cluster in the right front quadrant for the same reason.
Lake effect snow bands are oriented which way relative to the low level wind?
Roughly parallel. The bands are convective rolls in the boundary layer stretched along the flow, which is why a real shift in wind direction can move the heavy snow to a different set of towns — though shoreline shape, convergence, and wind shear can complicate the exact alignment from case to case.