Thunderstorms and Microbursts

11.thunderstorms-and-microbursts. Thunderstorms and Microbursts

A thunderstorm is a local storm produced by a cumulonimbus cloud and accompanied by lightning, thunder, and often gusty surface winds, heavy precipitation, hail, and severe turbulence. Three ingredients are required for a thunderstorm to form:

• Sufficient water vapor (high moisture content in the air)
• An unstable lapse rate (warm air at the surface, cool air aloft)
• A lifting force (frontal lift, convergence, terrain, or surface heating)

Life Cycle of a Thunderstorm

Every thunderstorm progresses through three distinct stages:

1. Cumulus (towering cumulus) stage — Strong updrafts, sometimes exceeding 3,000 feet per minute, dominate the cell. The cloud builds vertically as warm, moist air is lifted and condenses. Precipitation has not yet reached the ground. This stage typically lasts about 15-20 minutes.

2. Mature stage — Begins when precipitation starts falling out of the base of the cloud. The falling rain drags cool air downward, creating a downdraft that coexists with the continuing updraft. The presence of both updrafts and downdrafts side-by-side produces the most violent turbulence, lightning, hail, heavy rain, and possibly tornadoes. Cloud tops frequently reach 40,000 feet or higher and may form an anvil of cirrus blown downwind by upper-level winds. The mature stage is the most dangerous and typically lasts 15-30 minutes.

3. Dissipating stage — Downdrafts dominate the entire cell, cutting off the supply of warm, moist inflow. Precipitation tapers, the cell rains itself out, and the cloud erodes into stratiform remnants.

Types of Thunderstorms

• Single-cell (air mass) — Isolated, short-lived storms triggered by daytime heating; usually mild but can still be hazardous.
• Multicell — Clusters of cells in various stages; the gust front of a dying cell can trigger new cells.
• Squall line — A non-frontal, narrow band of active thunderstorms, often ahead of a fast-moving cold front. Squall lines often produce the most severe weather: tornadoes, large hail, damaging winds.
• Supercell — A single, persistent rotating thunderstorm (mesocyclone) that can last for hours and produce the most violent weather, including strong tornadoes and giant hail.

Hazards to Aviation

Thunderstorms threaten aircraft with virtually every weather hazard known:

• Turbulence that can exceed airframe limits — extreme turbulence has broken aircraft in flight.
• Hail that can shatter windshields and damage leading edges; hail can be thrown miles from the parent cell, including out of the clear air beneath the anvil.
• Icing — supercooled water droplets above the freezing level cause rapid airframe icing.
• Lightning — can puncture skin, damage avionics, and temporarily blind the pilot.
• Tornadoes — capable of destroying any aircraft.
• Low ceilings, heavy precipitation, and reduced visibility.
• Pressure variations that cause altimeter errors of 100 feet or more.
• Wind shear and microbursts near the surface.

The FAA recommends pilots avoid any thunderstorm by at least 20 nautical miles, especially severe or intense cells with anvils, because hail and turbulence routinely extend that far from the visible cloud.

Microbursts

A microburst is an intense, localized downdraft that spreads outward in all directions when it strikes the surface. Microbursts are produced by convective activity but can occur with rain showers that are not full thunderstorms; virga (rain that evaporates before reaching the ground) is a classic visual cue, particularly in the dry West.

Key characteristics:

• Horizontal diameter typically less than 2.5 statute miles.
• Downdraft speeds can reach 6,000 fpm.
• Horizontal outflow winds may exceed 45 knots, producing a 90-knot shear from headwind to tailwind across the cell.
• Lifespan rarely exceeds 15 minutes — peak intensity typically lasts only 2-4 minutes.

The Microburst Encounter

An aircraft flying through a microburst experiences a predictable, deceptive sequence:

1. A performance-increasing headwind as it enters — airspeed and altitude increase, tempting the pilot to reduce power.
2. A strong downdraft in the core — sink rate increases dramatically.
3. A performance-decreasing tailwind as it exits — airspeed bleeds off, lift decreases, and the aircraft may be unable to climb out.

This sequence is most lethal on approach or departure when the aircraft is low, slow, and configured. The recommended escape if encountered is maximum thrust, pitch for the published wind-shear escape attitude, and accept stick-shaker if necessary. The best defense is avoidance: do not take off or land when a microburst is suspected; delay 15-20 minutes until the cell decays.