Enhanced energy can create a volatile weather cocktail.
By Karsten Shein
Comm-Inst Climate Scientist
Every time of year has distinct weather that can make flying a challenge. In the middle-latitude summer, the added heating of the atmosphere by a sun that is more directly overhead helps to reduce air density and intensify convective activity.
The result is that, for aviation, summer is a time associated with greater and more energetic thunderstorm activity, more frequent low-level thermal turbulence, and a general decrease in aircraft performance.
When we talk about summer as a season, we are generally speaking of those months in the middle latitudes (between about 30 and 60 degrees) where there is a marked increase in temperature from the winter months when the noon sun favors the other hemisphere.
This distinction is less pronounced in the tropics and subtropics, where temperatures are more or less consistent through the year, and seasons are marked more by wind and rainfall patterns.
Lower-latitude monsoons are tied to the annual progression of the sun. The Intertropical Convergence Zone (ITCZ) migrates northward between April and October, and southward from November through March. Having the sun more overhead in the summer months provides the added heat and evaporating water necessary to power and shift the ITCZ.
The heating also strengthens and expands the semi-permanent subtropical high-pressure cells that, in connection with the inflow of the ITCZ, produce the easterly tradewinds that blow throughout the tropics. However, in expanding, the cells may envelop some places, resulting in stagnant airflow – the so-called doldrums that are characterized by clear skies, high temperatures, and calm winds – often for weeks at a time.
The poleward migration of the ITCZ and the strengthening subtropical highs establish routine seasonal patterns of rainfall as the tropical front is directed over land areas such as southeast Asia and the Indian subcontinent. Such regular rainfall can facilitate abundant agriculture, but, simultaneously, produces low ceilings, embedded thunderstorms, and often flooding rainfall that can significantly disrupt aviation operations in these regions.
There is also less of a seasonal difference in temperature at polar latitudes between the cold and warm months, but it’s more substantial than the temperature changes in the tropics – and becoming more so. In recent years, late summer (August–September) at arctic airports has seen temperatures sustained at a level which has enabled the thawing of the permafrost that underlies airport runways.
The softening underlayment has caused runways to warp and, in some places, collapse, making them dangerous and potentially unusable without substantial repair and reinforcement.
So, pilots operating in high latitudes during summer should expect the potential for smooth runways to develop bumps and undulations. It is always wise, especially at the many uncontrolled or remote airstrips in the far north, to overfly the strip before landing. If it doesn’t appear safe, the landing should be aborted.
It is in the middle latitudes, however, where summer becomes a potent mixture of heat and moisture to produce a cocktail of adverse and dangerous flying weather. Thunderstorms have been, and likely always will be, among the most dangerous and disruptive of weather events for aviation, and the atmospheric heating enhanced by the sun being more directly overhead makes them even more dangerous.
Even the most ordinary thunderstorm is dangerous. They form when the surface air is heated and humidified to a point at which it can break through a capping temperature inversion and rise into the free atmosphere to form a convective circulation cell.
Most storms will grow and die as simple airmass storms, exhausting their supply of water vapor into drier air aloft and dying within an hour as a lack of windshear aloft causes the denser, dry air to collapse back on the updraft. However, in summer months, 2 things happen that affect thunderstorm frequency and intensity.
The first is that the increased solar radiation reaching the surface during the day heats the surface and the overlying air. Hotter air can hold more water vapor, and the surplus of energy, where water is available, means a lot of the energy will go into evaporating the water, allowing it to join the atmosphere as a gas containing a great deal of stored (latent) heat.
While the sensible (kinetic) heat facilitates evaporation and the temperature difference that allows the hot, humid surface air to rise, it is the latent heat that sustains the lifting when the cooling air causes the water vapor to condense, releasing the latent heat as sensible heat that helps to keep the temperature of the rising air above the temperature of the air through which it is rising.
As long as the rising air remains warmer than its environment, it will keep rising. So, naturally, more heating of a wet surface and the air above it means more latent energy available in the rising air to sustain and even strengthen a thunderstorm. The second important consideration is that the surface acts as the heat source for the lower atmosphere.
More heat in the daytime means greater warming of the overlying air. At night, however, the ground is cut off from its heat source, so it cools, drawing in heat from the overlying air. In the summer, when nights are more frequently clear (allowing more rapid heat loss), this often means that, by morning, a temperature inversion (warmer air over colder) has set up.
It is these inversions that prevent the surface air from rising as it is heated after sunrise. Capping low-level inversions, or caps, are critical to thunderstorm development. In their absence, or if they are too weak, the surface air will need only a little heating before it rises, limiting the energy available and causing the rising air to cool and stabilize quickly with the surrounding atmosphere.
At best, this rising air may produce fair weather cumuli and a little bit of light chop on approach or departure. Conversely, if the cap is too strong, perhaps 4° C or more – meaning the air above the surface layer is at least 4 degrees warmer than the air beneath it – it is highly unlikely that the surface air will accumulate enough heat during the day to break through the cap, and, again, storms are unlikely.
The sweet spot is a cap between about 2–3° C. This is just strong enough to support sufficient energy accumulation in the surface layer that the air is able to punch through at some point during the day, and ensuring that the air can remain warmer than its environment as it develops into a potential thunderstorm.
Caps that are conducive to thunderstorm formation can be found on atmospheric sounding profiles at around 2000–5000 ft AGL. Inversions above or below that zone mean that the surface layer is either too thin to build up heat, or too deep to prevent the heat from diffusing throughout the layer.
Similarly, the air in the surface layer must be both warm and humid. In general, thunderstorms are unlikely if the dew point is less than 13° C (55° F). This is a significant reason why thunderstorms are more likely during summer months. Cooler months in most parts of the middle latitudes will generally not attain such a dew point.
Of course, energetic air busting through a cap is not enough to generate a thunderstorm. The air above the cap must remain colder than the ascending air as it cools. It should also be relatively dry in the mid levels of the atmosphere to maintain convective instability aloft.
For a thunderstorm to become severe, winds aloft must provide both speed and directional shear to tilt the cell and promote mesocyclonic rotation within the cell.
Surface heating warms the troposphere
The seasonal increase in solar heating of the hemisphere warms the entire troposphere. While the upper limits remain quite cold, the overall warming of the layer means the air molecules spread out as they gain energy.
The easiest way to spread out is, of course, to expand into areas where there are fewer molecules. For the atmosphere, this means upward. In the winter, a cold troposphere in the middle latitudes tops out at around 30,000 ft.
But, during the hot summer, the warmed air pushes the upper limit of the troposphere to around 50,000 ft. In the tropics, it can be upward of 60,000 ft MSL. In cases where the troposphere is unstable throughout, a convective cell may rise unimpeded to the top of the troposphere, where it will inevitably encounter the strong temperature inversion that marks the base of the stratosphere.
This interaction is normally identified by a flat anvil on the thunderstorm. At such heights, even the most capable business jets may not be able to overfly a summer storm, nor should they if they could.
The same conditions that produce the chance of thunderstorms also tend to reduce visibility. The capping temperature inversions in the lower atmosphere that keep the surface layer air from rising and mixing with the free atmosphere also trap aerosols and water vapor in the surface layer. The pollutants will often rise to concentrate near the base of the inversion.
When seen from the cockpit, that boundary is often marked by a brownish line. In areas where water is in greater supply than polluting aerosols, the increasing concentration of water vapor in the warming surface layer air creates a milky white haze.
The hazy appearance is due to light in all visible wavelengths being scattered in every direction by the microscopic droplets. An extreme manifestation of this scattering is a billowing cumulus cloud, but at lesser concentrations of water, the air maintains some transparency. Hazy conditions reduce visibility, as well as diminishing both depth perception and the ability to distinguish colors.
Among the more impactful, yet underestimated, aspects of summer heat is the increase in density altitude. The heat that expands the summer troposphere, allowing thunderstorms to grow to impressive heights, does so by imparting the air molecules with energy, forcing them to spread apart.
The decreased density at a given altitude due to heating is equivalent to the density that would normally be found at a higher altitude. Naturally, with fewer molecules passing around lifting surfaces and through engine inlets, lift and thrust are both decreased relative to what would be attained in colder air at that same altitude.
Increased density altitude and reduced performance are normally most pronounced in the heated surface layer during takeoff or landing. In most cases, it’s a matter of needing additional runway to get in the air or onto the ground safely. However, on very warm and humid days, when operating close to the aircraft’s weight limits, at higher-altitude airports, or at those with shorter runways, the tradeoff may be to reduce aircraft weight in order to have enough runway for departure.
Even then, the density may simply become too low for effective operation. Some heat waves have become so intense that even the main runways at major airports may not be long enough at any takeoff weight for an aircraft to reach rotation speed and deflect enough air with its control surfaces to leave the ground.
In June 2017, PHX (Phoenix AZ) made headlines when daytime temperatures of nearly 50° C (122° F) grounded many aircraft, especially those with smaller lifting surfaces, control surfaces, and powerplants, such as business and commuter aircraft.
During hot weather, following aircraft operating handbook guidance is critical, and pilots operating aircraft near weight limits may want to avoid midday arrivals or departures. Fortunately, summer also brings a great deal of good flying weather.
Crews can do preflight walk-arounds and passengers can deplane without needing to don a parka. There’s generally no need for an expensive coat of deicing fluid. Also, where winter weather patterns may cover half a continent with low overcast and howling winds for several days, summer squall lines usually move faster, cover less geography, and are interspersed with days of clear air.
Compared to other times of year, summer weather can be a bit trickier to forecast. While large-scale weather systems such as fronts and lows can be tracked and forecast with confidence, smaller-scale conditions such as temperature inversions and airmass storms – even those that become supercells – remain difficult to predict with precision.
Rather, forecasters will issue convective sigmets for large areas in which conditions are likely, and those alerts are updated frequently as conditions evolve. If a forecaster does suggest that thunderstorm activity is likely along your route of flight, that information should be taken seriously because it is based on the existing dynamics being very favorable, even if the forecast can’t give you precise information about exactly where the storms may form.
If possible, rerouting around the area of greatest danger is always the safest course of action. In all cases, where convective activity is forecast, it is critical that pilots obtain updated weather information throughout the flight.
Karsten Shein is cofounder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg University and a climatologist with NOAA. Shein holds a commercial license with instrument rating.