18th Air Support Operations Group
U.S. Air Force
Cairns Army Airfield
Fort Rucker, Alabama
For many adults, summer is the time we look forward to taking family vacations. For kids, the season provides a much-needed break from school work. For teenagers, it’s a time to bask in the sun at the beach. For aviators, though, summer is a time when many flight hazards exist. Understanding and preparing for these hazards are the keys to staying safe.
As we transition into summer, gone are the days of cold fronts and nor’easters. Most of summer’s weather is tied to thunderstorms. Whether you’re talking about the sea breeze, air mass thunderstorms, severe weather or a tropical system, all thunderstorms have certain inherent dangers: severe turbulence, severe icing, low-level wind shear (LLWS), heavy rain, hail and lightning. It even says so right there in Block 22 of Department of Defense (DD) Form 175-1! All thunderstorms have the potential to turn severe with little or no notice. And finally, any thunderstorm can produce a microburst. There are a few other summertime hazards besides thunderstorms, but I’ll address those later.
It is important to understand the basic dynamics of a thunderstorm. To put it simply:
Moisture + Instability + Lift = Thunderstorm
When these three ingredients come together, it is the perfect recipe for a thunderstorm. As a harmless cumulus cloud grows into a dangerous thunderstorm, it goes through three stages: developing, mature and dissipating.
During the developing stage, the cumulus cloud grows vertically and an updraft develops. Little, if any, precipitation falls, but there is some occasional lightning. Expect some light turbulence in and around these convective clouds.
In the mature stage, the updraft continues to feed the storm and a downdraft is formed. Precipitation falls, sometimes heavily, often reducing flight visibility to < ½ statute mile. Frequent lightning, hail, damaging winds and tornadoes are possible.
As the downdraft reaches the surface, it cuts off the updraft and the storm reaches the dissipating state. Although the precipitation tapers off, lightning is still present and isolated instrument flight rules ceilings are possible. It is during the dissipating stage that you have to watch out for microbursts.
Types of thunderstorms
There are three types of thunderstorms: single-cell, multicell and supercell. Let’s take a closer look at each type.
A single-cell storm (also known as a pulse storm or air mass thunderstorm) is generally short-lived, lasting only 30-60 minutes, and usually does not produce severe weather. It contains a single updraft. Strong winds and marginally severe hail are possible, but tornadoes are rare.
Multicell storms are clusters of single-cell storms. New storms tend to form on the rear edge of the cluster, mature cells are in the center, and dissipating storms are along the front edge. New cells can continue to develop every five to 15 minutes. Flash flooding, large hail and strong winds are the main hazards, but short-lived tornadoes are also possible.
A supercell contains a single, rotating updraft and is usually isolated from the main thunderstorm outbreak — a renegade, if you will. Large hail, severe winds and tornadoes are possible. The lifespan of a supercell can be several hours.
As Air Force meteorologists, we have many tools at our disposal to detect and predict thunderstorms. As you probably guessed, we use the operational weather squadron’s hazard charts. These forecast turbulence, icing and thunderstorms in three-hour increments out to 120 hours. We also look at surface and upper-air analysis as well as the Skew-T Log P diagram. The Skew-T gives us a vertical profile of the atmosphere at a particular location so we can identify the availability of moisture, instability and lift. (Remember those three ingredients?)
Every day is different, as is every thunderstorm. I’ve seen days when I’d have bet the farm on getting severe thunderstorms and received absolutely nothing. I’ve also seen days when I had to amend the mission execution forecast before it was even valid because thunderstorms fired up hours earlier than expected. That’s what makes thunderstorm season so interesting and exciting!
Satellite imagery is a great tool for identifying clouds, areas of turbulence and thunderstorms. I’m particularly fond of visible imagery, but it is only available during the daytime hours. With a 1 kilometer resolution image, you can watch the cumulus clouds build, the thunderstorm bubble up, the anvil top blow off and the outflow boundaries form. It’s quite a sight. At night, we use infrared imagery. There are various enhancements that help identify thunderstorms.
Of all the tools at our disposal, radar is the most important in monitoring thunderstorm development and progression. Using radar imagery, we can track the location and movement of precipitation, identify thunderstorms versus showers, pinpoint any areas of hail or rotation, and interpret whether the storms are building or weakening.
So how does the radar work? Basic radar theory is that the radar shoots a beam at the lowest elevation angle. The returns are bounced back to the radar, where the data is processed into the imagery we all know and love. Once the radar has completed a 360 degree sweep of the lowest elevation angle, it moves on to the next highest elevation angle and so on and so forth. Within minutes, we have a full picture of all slices of the atmosphere. Each individual slice is plotted as a base reflectivity product, and the composite of all slices is plotted on a composite reflectivity product.
Radar imagery can be deceiving. Don’t fall into the trap of seeing red and thinking it means a thunderstorm. That is not necessarily the case. While red on the radar could mean a thunderstorm, it could also mean large raindrops or a great number of raindrops. The interpretation of red also depends on which product you are viewing. Most forecasters I know use the composite reflectivity as a briefing tool because it gives a nice overview. However, this product is a compilation of all layers in the column, so the presence of red might make you think the situation is worse than it actually is. A scan of the lowest two to three layers of base reflectivity is a better way to check out a storm. Or better yet, ask your local weather experts at the Air Force weather team!
Some other things to watch out for: Beware of the bow! When you see a line of thunderstorms begin to bow outward, that’s an area you want to avoid. A bow echo is usually indicative of damaging straight-line winds.
Outflow boundaries are no picnic either. An outflow boundary marks the dissipating stage of a thunderstorm. That sounds like good news, but it’s not. Across an outflow boundary you’ll notice a wind shift, gusty winds, and cooler, drier air. What does that sound like? A very small-scale cold front, perhaps? Yes, and it is along these boundaries new thunderstorms are likely to form. When outflow boundaries converge with each other or with other storm cells, things can go downhill quickly. Since most commercial websites and apps (including ForeFlight) filter the outflow boundaries and air traffic control radar doesn’t detect them, it’s challenging to identify one unless you know what to look for.
So what does one look for? First, use a data source that does not filter outflow boundaries. The National Oceanic and Atmospheric Administration (NOAA)/National Weather Service (NWS) and College of DuPage are excellent sources. Be sure you’re looking at base reflectivity, the lowest elevation/slice/tilt. Finally, look for the classic arc-shaped return that is moving away from the parent storm. The screenshot below captured four outflow boundaries (yellow arrows) near Cairns Army Airfield in summer 2019.
I mentioned earlier that every thunderstorm has the potential to produce a microburst. A microburst is a sudden, violent downdraft of wind in a thunderstorm that is less than 2 ½ miles in scale. The winds rush down and out, radially, in all directions. The damage from a microburst can be even worse than that of a tornado, and microbursts present a grave danger to aviation, particularly to rotary-wing pilots.
Unfortunately, predicting microbursts is a challenge. We can’t tell you which cell will produce a microburst and when that microburst will occur. The science and technology just isn’t there yet, at least not on the military side. We can examine radar data for signs a storm might collapse by looking at the echo tops trends and vertically integrated liquid trends; but by the time we receive the data, it’s already several minutes old. Unfortunately, we usually only find out about a microburst after the fact — when one of our weather sensors reports a gust of 65 knots. That’s not a good feeling!
Air mass thunderstorms are prevalent during the summer months. In and of themselves, they are usually manageable and easy to pick around. However, certain interactions can quickly wreak havoc on aviation. We constantly monitor the radar for outflow boundaries, merging cells, sea breeze and other interactions that tend to ramp up convective activity. Any of these interactions increases the risk of warning-level winds. Thunderstorm hazards exist up to 20 miles outside of the core of a thunderstorm. The best way to avoid thunderstorm hazards is to steer clear of thunderstorms.
We’ve been talking a lot about thunderstorms, but they are not the only game in town during the summer months. Let’s not ignore other summer aviation hazards like turbulence, LLWS and fog. Hurricane season also spans the summer months.
Turbulence: The two main types of turbulence are thermal and mechanical. Thermal turbulence occurs when the surface heats up and warm air rises. Thermal turbulence is usually light and confined to the lowest levels of the atmosphere. As you probably guessed, the peak times for thermals to occur is from late morning until late afternoon, since that is when heating is at its max. Mechanical turbulence is caused by horizontal and/or vertical wind shear and can be the result of pressure gradient, orographic effects or frontal zones. Mechanical turbulence is generally found in a thin layer with a width of 10-40 miles and a length much greater than that. Watch out for rotor, lenticular and cap clouds. These clouds are associated with mountain wave turbulence and should always be avoided.
In general, the effects of turbulence for rotary-wing aircraft are amplified with increased airspeed, decreased weight of aircraft, decreased lift velocity and increased arc of the rotor blade. The turbulence intensity depends on aircraft type. The table below shows the turbulence intensities relationship across Category I, II, III and IV aircraft.
LLWS: This is drastic changes in speed and/or direction in the lowest levels of the atmosphere. Outside of thunderstorms, LLWS most often occurs near/along frontal zones and in mountainous areas.
Fog: It is common after heavy rain to have periods of dense ground or tree fog. This happens frequently during the summer months. The good news is this is usually localized and short-lived. Radiation fog is also common during the summer months. More good news: Radiation fog burns off when the inversion breaks, usually by mid-morning. Terrain can also impact fog formation. Upslope winds will cause fog to form; it will persist until the winds change direction. Upslope fog can last for days.
Hurricanes: Hurricane season runs from June 1 through Nov. 30. Hurricanes form over the warm ocean waters and can even impact areas well inland. Impacts vary by location. Coastal locations will see the strongest winds along with storm surge. Winds weaken as a tropical cyclone moves inland, but these locations can still see hurricane-force gusts, heavy rain, flooding and tornadoes in the right-front quadrant as the rain bands rotate around the center of circulation.
It’s a little early for the NOAA or Colorado State University 2020 hurricane season predictions, so I’ll end with a wrap-up of the 2019 season. The 2019 Atlantic season featured above-normal activity with two subtropical storms; two tropical depressions; 10 tropical storms; and six hurricanes, three of which were major hurricanes. The average season consists of 12 named storms, six hurricanes, and three major hurricanes. What will 2020 bring? Only time will tell.
A final word
Many aviation hazards exist during the summer months. We are all impacted by weather in our daily lives, but probably no one more so than aviators. Mother Nature can quickly turn a routine mission into a life-threatening situation. Ensuring you have the most current weather brief, understanding these hazards and how to mitigate them is critical to keeping your aircrew safe. Weather happens … are you prepared?