Understanding the basics of the atmosphere helps anticipate conditions.
By Karsten Shein
Comm-Inst Climate Scientist
However, every year, pilots neglect to obtain weather briefings, and encounter adverse weather conditions beyond their own and/or their aircraft’s capabilities.
While weather is a substantial part of flight training, and many instructors try to instill in their students a healthy respect for thunderstorms, turbulence, or icing, we tend to rely on forecasts and weather briefings to help us avoid the worst conditions.
But simply taking a forecast as gospel without understanding the way the atmosphere functions is often why pilots find themselves in weather trouble.
Basic meteorological knowledge can go a long way toward receiving more informative briefings and staying a step ahead of the weather.
Weather is mostly about 2 things – energy and water. However, to truly understand the atmosphere, one must see the atmosphere not as a layer of “air,” but rather as a layer of molecules which are held in place by gravity but are always trying to escape it.
The force applied by these molecules is what gives us lift and thrust, but it’s also what produces drag. Weather happens because the molecules that make up the atmosphere are energized, de-energized, and displaced. The atmosphere is always trying to maintain a balance, and it does so by moving these molecules around.
The molecular atmosphere is mostly (78%) nitrogen and 21% oxygen. Carbon dioxide, ozone, argon, and a few other gases, including water vapor, make up the last 1% or so, although, in some cases, their properties give them an outsized influence on the overall atmosphere.
Each of these molecules has a tiny bit of mass. Mass is critical to all this, since it is a primary variable in the equation of force (Force equals mass times acceleration.) With their tiny mass, each air molecule is influenced by the gravity of Earth keeping it from dissipating into space.
Mass also means that each molecule itself exerts its own gravitational force. Earth’s gravity is the reason why these gas molecules are most concentrated near the surface, and decrease exponentially in quantity with increasing altitude.
But the reason why gravity doesn’t compact all of the molecules into a super-dense atmosphere a few meters thick is something called hydrostatic equilibrium, which is related to the density of molecules present in the atmosphere at any given altitude. In fact, just about every motion in the atmosphere is related to air density.
Hydrostatic equilibrium means the forces acting on a molecule are in balance. Vertically, those 2 primary forces are gravity, which acts downward, and a pressure gradient force acting upward. The pressure gradient force (PGF) is the force exerted by the molecules themselves at a given density.
Fewer molecules in a volume of air (density) means lower mass and thus a lower force (barometric pressure). So, although density difference drives the dynamics of the atmosphere, it is easier to measure and think of those differences in terms of pressure.
The concept of a PGF is not new to most pilots. It is why the wind blows from high pressures to low on the weather map. More molecules in a place will always want to push outward to places where there are fewer. The same is true vertically, and, of course, the ultimate lack of air density acting on our atmospheric system is outer space.
The vertical PGF acting to draw molecules upward, and the force of gravity drawing them downward, support a fixed density of molecules at any given distance from the Earth’s surface. At that altitude, barring any outside influences, the density of the air allows a balance of the 2 forces.
Both forces decrease exponentially with increasing altitude until density can no longer support flight. This happens at the Kármán line, around 62 miles (100 km). Without any external inputs, the ideal atmosphere would always be absolutely stable, lacking any horizontal or vertical motion, and no weather.
But this is not the case, due to solar radiation. As with most gases, the atmosphere is controlled by the ideal gas law that relates temperature, pressure, and volume. If volume is kept constant and temperature is increased, pressure increases. If pressure is kept constant and temperature increases, volume must increase.
This is because temperature is a measure of sensible energy that, when absorbed by the air molecules, causes them to accelerate, exerting greater force. Either the volume in which they are acting must increase to maintain a constant pressure, or the pressure in the volume will increase.
This give and take is essential to creating weather, because the input of more solar energy in one place than another creates a pressure imbalance that establishes a PGF, which allows the air to flow as wind.
Most flight and most of what we consider weather occurs in the lowest layer of the atmosphere – the troposphere, which extends to around 66,000 ft (20,000 m) over the tropics and 23,000 ft (7000 m) over the poles, and contains around 75% of the atmosphere’s molecules.
Some aircraft flying at higher latitudes may also cruise in the stratosphere, which extends up from the top of the troposphere to ~180,000 ft (55,000 m). These layers are defined by environmental temperature. The troposphere is heated from the Earth’s surface, and so temperature tends to decrease with height.
The stratosphere is heated by the interception of solar radiation by the ozone layer in its upper reaches, meaning it is heated from above, and temperature normally increases with increasing altitude. The temperature inversion at the boundary between the 2 layers ensures that most of what we consider weather is limited to the troposphere.
Globally, around 55% of the solar radiation that reaches the Earth makes it to the surface, being either absorbed or reflected by the land and water. Earth’s rotation around the sun, the spin on its axis, and even variations in the type of soil or distribution of water at scales from a few square meters to an entire hemisphere will affect the distribution and handling of the incoming energy.
For example, due to its low capacity to absorb energy before it increases in temperature, dry land will quickly reradiate absorbed solar energy as sensible heat (measured by increased temperature) that heats the overlying air through conduction.
Similarly, at night, without solar input, dry land will not retain its energy, so it absorbs it from the overlying air, cooling the air. Water, on the other hand, can absorb more energy before increasing its temperature.
Water is also an element that, at normally-occurring air temperatures, can use the absorbed energy to change state, melting or evaporating instead of heating. When the air over a location is heated by a warm surface, its molecules increase movement, pushing apart.
Fewer molecules over the heated surface mean that the air is less dense. This does 2 things. First, it decreases the downward gravitational force relative to the upward PGF, making the air buoyant. Second, it creates an inward pressure gradient, and air flows toward this less dense area.
As it converges it must go somewhere, and the only place available to it is up. As the heated air rises through its denser (colder) surroundings, it travels further from its heat source and rapidly loses heat to its surroundings, gradually becoming denser until it eventually balances with the density of the air around it.
Water is the other key factor in the weather equation. Without water, our ‘weather’ would consist solely of wind and occasional turbulence in clear skies. Water vapor contributes to the density effect because it is lighter than atmospheric nitrogen or oxygen, meaning that, as more water is added to the air, air density decreases.
Water plays much more important roles in creating weather, as it is capable of using absorbed energy to evaporate (or melt if it was ice). When it becomes a gas, water can be entrained by the atmosphere, along with the energy it has stored, and carried with the air flow to new places or heights.
But water can’t hold on to that energy forever. In fact, most water molecules only hold on to their gaseous energy for a few milliseconds before releasing it and condensing back to a liquid state, but in sufficiently warm and unsaturated air, there is a surplus of heat energy that gives those molecules a new infusion of energy to reevaporate.
Several things can happen when there is ample water vapor in the air. First, at any density, the air has a finite amount of energy to impart to keeping entrained water vapor in its gaseous state. When the rates of evaporation and condensation balance each other, the air is said to be saturated.
If saturated air becomes denser, usually via cooling, the air molecules will draw energy from the condensing water vapor and not return it. Instead, that heat slows the cooling of the air. This is important in 2 ways. As the surface air cools overnight, it may saturate, producing fog and and moderating nighttime temperatures.
This is why night-time temperatures tend to grow so much colder in arid regions than humid places. In air rising convectively, saturation slows the cooling rate by the air drawing from the energy stored in the water vapor, which in turn allows the rising air to remain warmer than its environment for a bit longer, rising even further.
Critically, if there are abundant aerosols in the air, they may be of a type that attract and scavenge condensed water into cloud droplets. Without aerosols, such as sea salt, dust, and even some pollutants, clouds and fog would have a very difficult time forming.
With those aerosols, the liquid water molecules attach themselves, and when enough have done so, they create cloud droplets. Caught in updrafts, some of those cloud droplets collide and merge with other droplets.
Most frequently, they do so well above the freezing level of the atmosphere, and grow as ice crystals that are able to preferentially attract and absorb nearby water molecules, either in the form of liquid droplets or directly from water vapor. Eventually, these crystals gain enough mass to overcome the updraft and fall back toward the Earth as precipitation.
Importantly, above the freezing level, to temperatures of around -40° C (-40° F), but primarily in temperatures of 0° C to -15° C (32° F to 5° F), most cloud and precipitation droplets will remain liquid. These supercooled liquid droplets will freeze spontaneously to any subfreezing surface they strike.
Fortunately, while water may remain liquid at temperatures well below freezing, ice will melt quickly as soon as temperatures rise above 0° C. Pilots should expect the possibility of airframe icing any time they operate in subfreezing air where visible moisture (clouds or precipitation) is present.
A final primary concept we should address is the spin of the Earth and how it affects the weather. At any place on the planet, the speed and direction of the wind at the surface or aloft is a direct function of the pressure (density) change over the area.
However, a look at any surface or upper air weather map will show that wind never blows directly from high pressure to low. The reason for this is that there is another, apparent, force acting on the moving air. That force is due to the spin of the Earth, and is known as the Coriolis effect.
The strength of the Coriolis effect is determined by the latitude, and is relative to the rotation of the Earth. The effect is zero at the equator and becomes more pronounced at higher latitudes, and always acts 90 degrees to the right to the wind direction in the northern hemisphere (to the left in the southern).
This effect causes the wind to appear (relative to the rotation of Earth) to veer to the right in the northern hemisphere and to the left in the southern. Aloft, the wind veers until the PGF and Coriolis forces are in direct opposition. When this happens, the wind is moving perpendicular to the PGF around the highs and lows, and is known as the geostrophic wind.
At the surface, however, friction acts to slow the wind, reducing the Coriolis effect relative to the PGF, which is dependent only on the pressure difference. The result is that the wind turns back toward the low pressure. This is why flow at the surface appears to spiral cyclonically into low pressure centers.
The angle of that deflection is dependent on the magnitude of the friction, which in turn is affected by the roughness of the surface. Understandably, the dynamics of the atmosphere are far more complex than described here, and the various weather phenomena that occur within its fold are affected by many things beyond preferential heating, water availability, and a rotating planet.
But these are the core fundamentals that explain weather on scales large and small, and understanding them provides a greater ability to read important information into standard weather forecasts. As always, if you experience weather conditions that other pilots should know about, be sure to send a Pirep.
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.