Pulse storms and downbursts
The storm that will not look severe, has no rotation to report, lasts half an hour, and puts a tree through somebody's roof. Most of our summer severe weather comes from this.
The A short-lived single cell in a high-instability, low-shear environment, which is most summer Gulf Coast convection. It goes up, produces a downburst and small hail, and kills itself inside about 40 minutes.
Take lesson 1's ingredients at their humid-summer settings: rich moisture, large Convective Available Potential EnergyHow much energy is available to an updraft, in joules per kilogram. Bigger numbers mean a stronger updraft is possible, and a number on its own says nothing about what kind of storm you get., sea-breeze lift. Then take lesson 3's fourth ingredient away. With almost no Change of wind with height, measured as the difference between the wind at the top and bottom of a layer. It is not needed to make a thunderstorm and it decides almost everything about what kind you get. the The column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. and 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. sit on top of each other and the cell has a short, violent, self-terminating life. That is a pulse storm, and it is what most of our June through September convection is.
The Field Guide's figures for it, used verbatim: a 30 to 45 minute life; brief small to moderate hail; A strong downdraft that hits the surface and blasts outward, producing damaging straight-line winds. It is the general term for what a microburst and a macroburst are size classes of. winds usually under 70 mph; a possible weak tornado; isolated damage.
"Usually under 70 mph" and "isolated" are why these storms are underrated. The damage is scattered rather than swathed, so it does not read as an event, and each individual cell is only dangerous for a few minutes. But those minutes land on somebody, and there are a hundred of these storms a summer.
Downburst, A downburst less than 4 km across, usually lasting 5 to 10 minutes. Small, brief and capable of winds over 100 mph, which is a bad combination for anything trying to detect one., A downburst more than 4 km across. Longer-lived and covering far more ground than a microburst, and generally less extreme at its peak.
- downburst
- A strong downdraft that reaches the surface and produces damaging outward winds. The general term; the two below are size classes of it.
- microburst
- A downburst less than 4 km across. Typically lasts 5 to 10 minutes, and winds can exceed 100 mph in the strongest.
- macroburst
- A downburst larger than 4 km across. Longer-lived, generally less extreme at its peak, and covers far more ground.
Fujita coined all three terms. The size cut at 4 km is his, and it is arbitrary in the way useful definitions usually are: it marks roughly where a downburst stops being small enough to slip between observing stations and start being resolvable.
The Southeast setup, in one number
Dry microbursts are a High Plains phenomenon: a deep, dry sub-cloud layer, Precipitation that evaporates before it reaches the ground, visible as streaks hanging under a cloud. Common in dry climates and a signature of the High Plains kind of downburst. rather than rain, and a dust foot when it lands. We do not get those. Ours are wet microbursts, and the environment that produces them was measured directly in northern Alabama during the MIST field programme in 1986.
The discriminator Atkins and Wakimoto found was the drop in equivalent potential temperature, equivalent potential temperatureOne number combining how warm and how moist a piece of air is. A large drop in it with height means dry air aloft over moist air below, which is the setup for a strong downdraft., between the surface and the mid-level minimum. Theta-e bundles temperature and moisture into one quantity, so a large drop with height means a moist boundary layer sitting under notably drier mid-levels, which is exactly the profile that lets falling rain evaporate hard.
Atkins & Wakimoto 1991, from the 1986 MIST programme in northern Alabama. The middle band is genuinely undetermined rather than marginal: the study separated two populations and did not fill the gap between them.
The mechanism is lesson 2's evaporative cooling with the volume turned up. Rain falls out of the storm into a dry layer, evaporates into it, chills that air below the temperature of its surroundings, and the resulting dense An imaginary blob of air, followed upward to see whether it stays warmer than its surroundings. Almost every number on this page comes from comparing one of these against the air around it. accelerates all the way to the ground and splashes outward.
What it looks like before it hits
- A A horizontal bulge near the ground at the bottom of a precipitation shaft, where the downdraft has hit the surface and spread outward. It is a visual sign of a wet microburst.. The bottom of the The visible column of rain falling between the cloud base and the ground. At a distance one can be mistaken for a wide tornado. splays outward near the ground instead of falling vertically. That splay is outflow already spreading, and it is the clearest single visual cue.
- A ring of dust, spray or debris expanding outward from a point under the storm. On water it is unmistakable and it is the reason mariners get almost no warning.
- A rapidly descending precipitation core. A shaft that visibly drops rather than drifts. What you are watching is the load letting go.
What to do about any of that is module 8, which covers positioning and safety and is the module that will keep you alive. The relevant preview: 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. gives you a very short amount of notice, and the safe distance is larger than the photograph makes it look.
Divergent, not convergent
A downburst and a tornado leave different signatures on the ground, and the difference is the single most useful thing a spotter can report about damage. A downburst pushes air outward from a point, so trees fall in a diverging, fanned or starburst pattern all pointing away from where the core landed. A tornado is a convergent, rotating flow, so debris and tree-fall vectors converge toward and cross the track, often lying in opposite directions on either side of it.
Damage survey technique proper is module 5. What belongs here is why the two patterns differ, which is that they are pictures of two different flows.
Where downburst science came from
Aircraft accidents. Fujita and Byers analysed the 1975 crash of an airliner on approach to JFK and identified the spearhead echo and the downburst behind it. The concept was resisted for years and then vindicated by dedicated field programmes, and the result is the on-board and terminal wind shear detection that every commercial flight now depends on. It is one of the clearest cases in meteorology of an observation nobody wanted to believe preventing a recurring disaster.
Locally
Sea-breeze-initiated pulse convection is the June to September pattern here, and the reports it generates are almost all wind and almost all brief. Two local cases worth knowing, both with measured gusts, which is what makes them citable:
- 2 August 2012, Pensacola. An early-afternoon storm produced a wet microburst around 12:30 to 12:45 pm, with a measured gust of 49 mph just west of Pensacola Regional Airport, downed power poles and 2 to 3 inches of rain.
- 27 March 2009, Baldwin County. A University of South Alabama surface station measured 86 mph near Robertsdale in a spring A segment of a line of storms that has surged forward into a bow shape on radar. The apex of the bow is where the strongest straight-line winds are. and downburst event. Same physics, different season, and a reminder that the 70 mph figure above is a typical value rather than a ceiling.
A summer sounding shows a moist boundary layer, a notably dry layer in the mid-levels, and a surface to mid-level theta-e drop of 24 K. What does that favour?
Compare the number against the MIST bands, then ask what a dry mid-level layer does to falling rain.24 K is above the 20 K threshold that characterised microburst days in the MIST sample. The mechanism is evaporative cooling: rain from a moist boundary layer falls into much drier air aloft, evaporates, chills that air, and the dense parcel accelerates down. It is wet rather than dry because the boundary layer here is moist, so the rain reaches the ground rather than evaporating away as virga.