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Lesson 14: Aviation Weather

Lucky Weather

Probably not two things you’ll hear associated much in aviation.  Generally, when it comes to weather, Murphy’s Law is in full effect and what can go wrong will go wrong.

Check out the video above for a nice intro to weather and clouds, and then kick back and peruse through the TOPICS below to start learning about how weather forms, and the key traits we’re looking for to ensure good flying weather.

That looks like it’ll clear up by the time we takeoff…-Said no pilot ever…


Weather happens because of only two things:

  • The uneven heating of Earth’s surface from solar radiation
  • Heated air becomes less dense (lower pressure) creating circulation vertically and horizontally

That is what weather really is.  All the extra stuff is a byproduct of those two key factors.

A few things to talk about first:

  • Coriolis Effect
  • Airflow patterns
  • Air density
  • Lapse Rate “adiabatic principle”
  • Temperature Inversions
  • Stability

Coriolis Effect

This is what we call the effect the earth’s rotation has on fluids in the northern and southern hemispheres.  Ya know how when you flush the toilet in Australia the water spins the other way?  Well, that’s Coriolis effect (mostly).  Due to the Earth’s rotation, the Oceans and high-pressure air systems in the northern hemisphere spin clockwise (opposite down under in the southern hemisphere).  However, in the northern hemisphere, when you have a low-pressure system of air, it will spin counter-clockwise. (see diagram below)

coriolis effect weather course flying surface wind weather online course

The Earth’s rotation causes the air to spin in these directions.  However, when you get close to the ground, the friction from the Earth’s surface interrupts this movement and the air flow follows a more logical pattern where the high-pressure air flows out towards low-pressure areas.  (if you are wondering what an “isobar” is on that map, we’ll cover that when we talk about charts in another TOPIC).

Air Density

There are a number of things that affect the density of the air we are going to be flying through.  The key ingredients for us is the pressure, temperature, and humidity.  As you heat a gas (the air) it expands, making it less dense.  As you squeeze a gas, it obviously becomes denser as you pack more molecules into a tighter space.  Lastly, when you introduce lots of moisture into the air, the water molecules push the air molecules farther apart, making it less dense (humidity isn’t taken into account when calculating density altitude, we just want you to know your airplane will not perform as well when the humidity level is high).

As pilots, we refer to the total air density as DENSITY ALTITUDE.  It makes sense that we would have a standard to compare by, and we do.  Our “standard day” weather conditions are 15 degrees Celsius (59F) and the altimeter setting or “pressure” being 29.92″ of mercury (inches of mercury) at sea level.  Any warmer than that at sea level and our air is going to be less dense (assuming pressure stays the same), thus with less dense air, the DENSITY ALTITUDE would be higher (Density Altitude, abbreviated DA is the “feels like altitude”. As the airplane climbs in altitude, we get less performance from it, when taking off from an airport by the beach (at sea level) on a hot summer day, the DA is going to be higher than sea level as there is less air to “feed the wing”, you might as well at this point be taking off from an airport at a much higher elevation since the airplane will perform as if it was already operating at a higher elevation due to the “thinner” less dense air.

Now if the pressure increases, that squeezes the air making it denser, thus lowering the DA or “feels like altitude”.  The airplane will perform better with higher altimeter settings (i.e 30.25″ instead of 29.92″).  With an H (high pressure) in the area, you can get better performance from your airplane.  The air weighs less when it is less dense as shown below.

air density altitude hot and humid air

I.E. Taking off from Miami in August with an altimeter setting of 29.92″, and outside air temperature (OAT) of 35 degrees Celcius would give you a DA of about 2,800 feet (the airplane will perform taking off as if it was already flying at 2,800′ and not climb as well compared to having a cooler temperature and more dense air).


Lapse Rate

Before discussing Lapse Rate, it’s first important to note exactly how the air on Earth is heated or cooled.  For our purposes, the air is heated directly by the ground as solar radiation hits the ground and warms it.  Thus the air near the ground is always warmer than the air that is higher up during the day (especially when the sun is out and very strong).

lapse rate weather courseNow, the air gradually cools the higher we get away from the ground.  The rate at which the air cools is what we refer to as the LAPSE RATE.  For example:

The air at the surface is 90 degrees F, and the air at 1,000′ is 86F and 2,000′ 82F.  The LAPSE RATE is 4 degrees per thousand feet.  We will always refer to the lapse rate as the rate of change in temperature per thousand feet (sometimes in Fahrenheit and sometimes Celsius).

Why is the Lapse Rate significant to us?

The LAPSE RATE is truly a measurement of stability in the atmosphere.  When the atmosphere is UNSTABLE (high lapse rate), you are likely to have clear air, turbulence, and the possibility of storms forming.  This is because, with an UNSTABLE atmosphere, the air has an easier time rising forming updrafts.  These updrafts are what “lift” haze and smog away from the ground, increasing visibility.  The same updrafts are also responsible for creating the bumps we feel as turbulence, and they also lift warm moist air from the ground up to cooler altitudes, where that moisture will condense to eventually form clouds and rain (and even thunderstorms if it is lifted high enough).

The problem with a large lapse rate (unstable atmosphere) is it really doesn’t take much lifting force at all to get air moving up.  If you have a “parcel” of air on the ground that is heated to 90F, and it is surrounded by air that is 85F (maybe the difference between the air over the grass at the airport and the air over the dark taxiway), then it is going to start to rise.  As the warm parcel of air rises it will carry moisture with it from the surface (provided that there was some moisture at the surface).  As that parcel of air climbs higher in the sky, it will quickly find the air around it is cooler than it and continue to rise.  Although that warm 90F parcel of air from the ground will cool at a steady rate as it rises due to the decreasing pressure around it (we call this the ADIABATIC LAPSE RATE, more on this later), it will probably still be warmer than the air around it, and continue to rise and rise, carrying up with it moisture, turbulence (updrafts), and lifting the haze out of the way so we have great visibility.

Now does it just keep rising and rising to outer space?  No, that is where the ADIABATIC LAPSE RATE comes into play.  ADIABATIC means conservation of heat.  What this really means to us is you can’t destroy the heat, it has to go somewhere.  Each parcel of air will have a specific ADIABATIC LAPSE RATE of its own, that ultimately means how much its own temperature will change per thousand feet.  The dry adiabatic lapse rate of any parcel of air is always 5.4 degrees F (3.0 degrees C) for every 1,000′.  The moist adiabatic lapse rate varies greatly based on just how much water (humidity/moisture) is in the parcel of air (it is roughly 1.5 degrees C or 2.7 degrees F per thousand feet).  What this means to us is dry air will cool more quickly than moist air when rising (hence why the air is more turbulent in the summer, moist air rises easier than the dry air in the winter).

Ya, but what about it going into outer space?

Okay, so we gave an example earlier of the lapse rate of the atmosphere being a nice even 4 degrees F per thousand feet (90F at the surface, 86F at 1,000′ and 82F at 2,000′).  Let’s say we have a dry parcel of air that was hanging out over top of some darker colored rocks in the desert, and it became heated to 92F and started to rise.  It, of course, would cool as it rose at the dry adiabatic rate of 5.4F per 1,000′ as depicted below.  Given this, it would cool at such a rate that it would be around exactly the same temperature as itself just prior to 2,000′ above the surface and stop rising.  In this case, we would say the atmosphere is “STABLE”, because the air from the ground does not want to rise very high.  We could also expect the bumps and turbulence to stop above 2,000′ because there should not be updrafts rising past 2,000′ in this case.

lapse rate weather course adiabatic rate lapse rate weather course adiabaticlapse rate weather course adiabatic rate

Now if we had been using a moist parcel of air, having an adiabatic lapse rate of maybe 3 degrees F instead of 5.4, we can see how it would have just kept on rising until either the ATMOSPHERIC LAPSE RATE varied or it encountered a TEMPERATURE INVERSION.  The atmospheric lapse rate, in reality, does vary and has a large impact on how air does or does not rise in the atmosphere.  Meteorologists routinely send up instruments attached to small balloons several times per day to measure the temperature at different altitudes in the sky.  The data received from those instruments is the basis for all weather forecasting worldwide.

What if we turn it all upside down?

Now, you must be talking about a TEMPERATURE INVERSION!

A temperature inversion is simply when the temperature does not continue to decrease as you increase in altitude (as you would often expect it to).  Take a look at the illustration below:

temperature inversion aloft

This change in temperature in the atmosphere can act as a sort of “ceiling”, as warm air from the ground may start to rise upwards as it normally does, only to stall out when it reaches the temperature inversion since it is no longer warmer than the air around it.  This “stable atmospheric” condition results in less bumps and turbulence aloft, poor visibility as there are not the strong updrafts needed to take away the haze and pollution that occur down low, and often results in drizzly precipitation (but not heavy rain or thunderstorms, you need an unstable atmosphere for those to occur).

So a pretty interesting thing here is flying with a temperature inversion can make for a nice smooth ride, unfortunately, poor visibility is also associated with it.  The good news is there are two main types of “inversions”, surface inversions, and inversions aloft.  A surface inversion is just when a temperature inversion exists close to the surface, compared to an inversion aloft that may exist at several thousand feet in altitude (as depicted above).

Surface inversions are very common on a daily basis right around sunset.  As the sun begins to set, the ground starts to cool and also cools the air that is near the ground, while the air above the ground that was being heated all day stays a little warmer.  This creates a low-level temperature inversion or “surface inversion”.  This is most common on calm clear evenings (a great time to go for a flight!).

An inversion aloft is simply that, a temperature inversion that exists at some altitude above the ground (maybe a few thousand feet agl-60,000′).  There is one reason why weather stops always by around 60,000′ at the absolute highest and doesn’t keep going higher and higher to outer space.

The reason the weather “stops” at around 60,000′ is this is the top of the Troposphere, and after that, you enter the Tropopause (the boundary between the troposphere and the stratosphere).  In the tropopause, there is a naturally occurring temperature inversion that exists worldwide and stops warm air from rising further as the temperature does not continue to decrease above the top of the troposphere.

Can a temperature inversion aloft stop an aircraft from climbing?

Well yes.  Maybe not your Cessna, but as hot air balloons climb with the warm (low density air) in the balloon and ascend into cooler air they continue their climb upward.  If they encounter a temperature inversion as they climb higher, that will certainly put a damper on their high flying dreams…if the air they are climbing into is warmer than the air in the balloon, then they will not be able to climb up into it and instead sink back into the cooler denser air below that can support the “buoyancy” of the balloon.  (bonus question: What category aircraft is a hot air balloon? Answer in the forum link below!)


So flying around a temperature inversion sounds great since it should guarantee some smooth air aloft, right? Well remember a few paragraphs ago how we said a temperature inversion is like a “ceiling”?  It really is a boundary layer and can act as a buffer between two different wind layers.  It is very common with low level temperature inversions that exist right around sunset to have a nice light breeze at the surface of 2-3 knots and a stronger wind (even coming from a different direction perhaps) at altitude.  There’s many evenings I’ve been out flying where the wind is 20 knots at 300′ agl and 5 knots at the surface.  This makes things interesting, especially if you are flying along and about to land with a 20 knot headwind at 300′, and all of a sudden that wind drops to just 5knots.  If you were flying only 10 knots above your stall speed, you are going to have a very exciting “low speed event” aka, stall.

Although temperature inversions can help us to have stable conditions to fly in, you must always be alert to the possibility of windshear when operating when a temperature inversion exists.

Lastly, before moving on here, let’s define WINDSHEAR.   Wind shear is a change in wind speed or direction, either vertical or horizontal, over a short period of time or distance.

You can expect windshear when: