Thunderstorms Part 1: Defining what a thunderstorm is, the types, and the hazards

What is a Thunderstorm?

It’s a cumulonimbus cloud that contains lightning.  But lightning isn’t the only concern: wind shear, downbursts, hail, and heavy precipitation can often exist within a thunderstorm.

You probably recall that a thunderstorm forms when there is:

  1. Sufficient water vapor,
  2. An unstable lapse rate, and
  3. An upward lifting action.

What Is an Unstable Lapse Rate?

A lapse rate is the rate of temperature change with altitude. The faster the temperature decreases, the “steeper” the lapse rate and the more “unstable” the atmosphere becomes.

Life Cycle of Thunderstorms

The 3 stages of a thunderstorm’s life cycle are cumulus, mature, and dissipating.

All thunderstorms develop as cumulus clouds (or the cumulus stage), which are formed by updrafts. Updrafts start the process and are due to:

  1. Surface heating
  2. Converging winds
  3. Sloping terrain
  4. Frontal surface
  5. Local winds from lakes, valleys, sea, etc.

An initial updraft causes air containing water vapor to rise. Eventually the water vapor condenses, forming a cloud. The condensation releases latent heat, which partially offsets cooling in the saturated updraft and increases cloud buoyancy. This drives the updraft still faster, drawing more water vapor into the cloud. This action can become self-sustaining.

As the water vapor continues to increase in the cloud, the water droplets grow to raindrop size and the cloud grows larger and darker.

Eventually the water droplets grow heavy enough to fall through the cloud, causing a frictional drag in the air that generates downdrafts. When the cumulus cloud has coexisting updrafts and downdrafts, the cloud is in the mature stage. Confirmation of the mature stage occurs when precipitation hits the ground.

In the mature stage, downdrafts remain cooler than the surrounding air. The consequence is that the downward speed accelerates and can exceed 2,500 feet per minute. On the other hand, updrafts can reach maximum speeds of 6,000 feet per minute. These updrafts and downdrafts are in close proximity, and they create violent wind shear. This wind shear creates friction that results in lightning.

At some point, the clouds’ updrafts decrease or cease. In this instance, the cloud is in the dissipating stage, where there are more downdrafts than updrafts. Eventually no more rain hits the surface, and the downdrafts abate.

How Big Can Thunderstorms Get?

Horizontally: thunderstorms are often between 5 and 30 miles in size.

Vertically: in general, thunderstorm tops are between 25,000 and 45,000 feet, but they occasionally can extend to as high as 65,000 feet.

Prominent Hazards of Thunderstorms

1) Turbulence is hazardous in any thunderstorm regardless of its size and strength. A pilot should never contemplate purposely entering a “small” thunderstorm because the risks go beyond simply damaging the structural integrity of an aircraft.

a) It would be almost impossible to hold a constant altitude in a thunderstorm and maneuvering in an attempt do so greatly increases stresses on the aircraft.

b) If a pilot were to find themselves in the proximity of a thunderstorm, the recommended course of action is to reduce speed to below the maneuvering speed and prioritize attitude control, allowing the aircraft to “ride the waves.” Always disconnect the autopilot and monitor input and resist the urge to over-control the aircraft.

2) Hail competes with turbulence as the greatest hazard to aircraft. Hail forms when the water vapor reaches freezing point. The weight of the hail causes it to fall, where it either melts and turns to rain or remains solid and hits the ground as hail.

a) Pilots should assume hail is in ANY thunderstorm.

b) The anvil of a large thunderstorm can have hail falling underneath it.

3) Lightning can puncture the skin of an aircraft and damage communication and navigation equipment.

a) Fuel vapors in a gas tank can light and cause an explosion. Some tip tanks are more susceptible than other types.

b) Lightning can disrupt radio signals.

c) Lightning can also blind a pilot, momentarily limiting their ability to fly, especially in low-light or night flight.

d) Lightning may not affect every electronic device in the aircraft, but typically the stress of the strike can cause the equipment to fail in the weeks and months afterward. It’s not uncommon for an aircraft to ultimately require replacement of most of the electronics within a year or so of a lightning strike.

4) Downbursts can create hazardous conditions because they can cause low-level wind shear. When the wind velocity changes, it can lead to a loss of lift, which can be deadly in all flight phases, especially low altitude operations.

a) Microbursts are small-scale, intense downdrafts which, on reaching the surface, spread outward in all directions from the downdraft center. This causes the presence of both vertical and horizontal wind shears that can be extremely hazardous to all types of categories of aircraft, especially at low altitudes.

• These downdrafts can be as strong as 6,000 feet per minute, and winds at the surface can be as strong as 45 knots. This equates to a potential 90-knot wind shear when a headwind changes to a tailwind.

• Microbursts can last as long as 15 minutes, but the maximum intensity usually lasts between 2 and 4 minutes.

• Microbursts are a severe hazard for aircraft within 1,000 feet of the ground. Recovery potential is severely limited if encountered at low altitudes.

5) Icing is another hazard facing pilots. Updrafts bring large amounts of water vapor to high altitudes. There is a propensity for supercooled water to freeze and form clear icing immediately when it hits aircraft.

• The NASA Glenn Research Center found that exposure to clear icing for 2 minutes could lead to significantly higher stall speeds due to the doubling of drag, reduction of maximum lift by 25%-30%, and reduction of the critical angle of attack by 8 degrees.

Types of Thunderstorms

Thunderstorms can occur year-round, but in the summer, much of the country can experience every type of thunderstorm. As a result, thunderstorms are more common in the summer than in other seasons.

  • Single Cell thunderstorms are rare because they typically build and form into multicell storms. These cells can be safely navigated, though additional care should be given when flying at night or if the cell is embedded in other clouds.
  • Multicell thunderstorms are typical thunderstorms that contain clusters of cells at various stages of their life cycles. As the first cell matures, it’s carried downwind, and a new cell forms upwind to take its place. The lifetime of a multicell thunderstorm is several hours or more. This type of thunderstorms is tougher to circumnavigate.
  • Squall Lines are multicell thunderstorms that form a line and can extend for hundreds of miles. New cells continually re-form at the leading edge of the system with rain and sometimes hail following behind. Squall lines are the most difficult to navigate because they are too high to fly over (even for commercial jets), too dangerous to fly through or under, and often require too great of a distance to fly around, given their size, speed, and direction of movement.
  • Supercell thunderstorms are dangerous convective storms that consist primarily of a single, quasi-steady, rotating updraft that persists for an extended period of time. Updraft speeds may reach 9,000 feet per minute, equal to 100 knots. It’s possible for multicell and squall lines to contain cells that are supercell in nature.
  • Air mass thunderstorms often result from surface heating, and they are most frequent in the middle to late afternoon during the summer months. When they reach the mature stage, rain falls through or immediately beside the updraft. The falling precipitation induces frictional drag and retards the updraft, reversing it to a downdraft. Thus, these storms become self-destructive and usually have a lifecycle of 20 to 90 minutes.
  • Steady state thunderstorms are associated with weather systems such as fronts, converging winds, and troughs aloft that induce an upward motion. These storms intensify with afternoon heat, but unlike air mass thunderstorms, precipitation falls outside the updraft and does not retard the updraft of air. Thus, these thunderstorms can persist for several hours.


Why Should You Care about the Type of Thunderstorm?

Knowing the type of thunderstorm is helpful because it allows pilots to understand what is fueling their growth. If it’s an air mass thunderstorm as a result of a hot summer day, then at night there is a higher probability that thunderstorms may be dissipating and not rebuilding.  (This is not always the case, as a hot surface can create a lifting action.)  On the other hand, if there is a cold front creating steady state thunderstorms, then pilots need to be aware of where the cold front is going because new thunderstorms will be rebuilt on that frontal line.

Isn’t it Easy Enough to Stay Away from Thunderstorms?

Irrespective of our technological advancements and ability to forecast near-term weather, thunderstorms can still provide a challenge in flight. Thunderstorms can quickly build, then dissipate and regenerate. Therefore, their size is ever changing, and pilots needs to remain far enough away from them to not to get caught when they are building or get too close and make an ill-informed assumption that they are dissipating.

Check back next week for Part 2, where we discuss technology that can help pilots contend with thunderstorms.