Where damaging straight-line wind comes from, the size that separates a microburst from a macroburst, and which of the two your own air mass hands you. Which is the wet kind.

In this lesson you will

  • Distinguish a microburst from a macroburst by Fujita's size definition.
  • Identify wet and dry microburst environments from the sub-cloud moisture profile.
  • Distinguish straight-line divergent damage from tornadic convergent damage.
  • State the SPC derecho criteria.

Air coming down

A downburst is a downdraft that hits the ground and spreads out radially. That is all it is, and it produces winds that match a respectable tornado with none of the warning signs people are watching for. Fujita split them by size:

Microburst
Affects an area 4 km, about 2.5 miles, or less across. Small, intense, and over in minutes.
Macroburst
Larger than 4 km. Same mechanism, wider footprint, usually less extreme at any one point.

The split matters to a spotter mostly as a reminder of scale: a microburst can flatten a neighborhood and leave the next one untouched, so "it was fine where I was" is not evidence about the next street.

Wet and dry

Dry microbursts fall through deep dry air, cool by evaporation, and arrive with dust and little rain. They belong to the High Plains. Wet microbursts, buried in heavy rain, dominate the humid Southeast, and Atkins and Wakimoto's study of them over the southeastern United States is the anchor for this region: in a pure wet-microburst environment the cloud base sits near 850 mb.

Why this is the lesson that matters here
In a warm, deeply moist air mass you will meet far more wet microbursts than tornadoes, most of them from ordinary summer pulse storms with no watch in effect and nothing dramatic to look at. The rain is the camouflage. Lesson 8 goes further.

What outflow looks like on the way

The leading edge of outflow is a gust front, and the cloud that marks it is a shelf cloud, with the ragged turbulence on its underside sometimes called a whale's mouth. Module 3 lesson 6 is the full treatment; the point here is the same one that module made: a shelf cloud is outflow, so it is a wind warning to you, not a rotation report.

A rain foot, where the bottom of a rain shaft splays outward sideways, and a dust foot doing the same thing over dry ground, both say a downdraft is reaching the surface. Those are the cues you get, and you get them late.

Derechos

When a fast-moving line of storms produces damaging wind along a very long swath, it earns its own name. SPC sets five conditions, and all of them have to hold.

  1. A wind damage swath at least 250 miles, about 400 km, long.
  2. Wind gusts of at least 58 mph (93 km/h) along its length.
  3. Several well-separated gusts of 75 mph (121 km/h) or greater.
  4. It originates from an MCS moving faster than the mean wind, so it is primarily driven by its own cold pool.
  5. The system shows the radar signatures that go with that: bow echoes and rear-inflow jets.
What the 2025 revision actually changed
Not the numbers. The 250 miles, the 58 mph and the well-separated 75 mph gusts are the same as the 2016 revision. What was added is the last two conditions: the system has to be cold-pool driven and has to look like it on radar. The point was to make the definition physical rather than purely a damage tally, so that a long swath of wind produced some other way does not qualify just for being long.

The cool-season serial derecho, a line-echo wave pattern rather than one long bow, is characteristic of the southern Mississippi valley and the humid Southeast, which makes it the version most likely to arrive here.

Check yourself

A damaging wind event flattens an area about a mile and a half across, in heavy rain, from a single summer storm with no watch in effect. What is it?

  1. A wet microburst
  2. A macroburst, since the damage covers more than a few hundred meters
  3. A derecho, since the winds were damaging
  4. A weak tornado, since the damage was concentrated
Hint

Two of the options have size definitions attached, and one of those sizes is 4 km.

Answer

A wet microburst

Under 4 km across makes it a microburst, and heavy rain with it makes it the wet variety, which is the dominant damaging-wind mode on this coast. A macroburst is larger than 4 km, and a derecho needs a swath of hundreds of miles.

Sources

  1. Fujita 1985, SMRP Research Paper 210: The Downburst, Microburst and Macroburst
  2. Atkins & Wakimoto 1991, Wea. Forecasting 6, 470–482: wet microburst activity over the southeastern United States
  3. Fujita & Wakimoto 1981, Mon. Wea. Rev. 109, 1438–1456
  4. SPC, About Derechos, definition of a derecho (updated April 2026)
  5. Corfidi et al. 2016, Bull. Amer. Meteor. Soc. 97, 935–949, and Squitieri et al. 2025, Bull. Amer. Meteor. Soc. 106(1): the two revisions behind the current definition