Weather: METARs, TAFs, and density altitude

0:00Host: This is the Drone Authority study podcast, episode three, Weather. Weather is a smaller slice of the exam than the heavyweights we have covered, five percent of scored items under the current PSI blueprint, but do not blow it off, because it is also some of the easiest points on the whole test once you understand the format. The reason weather feels hard at first is that it tests two completely different kinds of things, and people try to study them as if they were one. So let me split them apart right now, because seeing the two halves clearly is most of the work.

0:35Half one is physics. Do you understand how weather actually affects a small, lightweight aircraft, your drone, in the air? This is conceptual. It is about lift, air density, and stability. Half two is decoding. Can you read a coded METAR or a coded TAF, those strings of letters and numbers that look like a secret cipher, without a translator? This half is pure pattern recognition. Once you know the format, it is almost mechanical. So we are going to do physics first, then decoding, and by the end both halves will feel scoreable.

1:15Here is the encouraging part. Neither half requires you to be a meteorologist. The physics is just a few cause-and-effect relationships, and the decoding is a fixed template that never changes. So let us knock them out one at a time.

1:50Host: We start with the single most tested physics concept on the entire weather section, and that is density altitude. Density altitude sounds technical, but the idea is simple: it is a measure of how thin the effective air is. And thin air is bad for your drone, because your propellers need air to bite into to generate lift and thrust. When the air is thin, your props are grabbing less, so you get less lift and less efficiency. The drone has to work harder to do the same job.

2:30So what makes the air thin? Three things, and you must know all three. High temperature, high elevation, and high humidity. All three of those raise density altitude, and high density altitude means thin effective air, which means worse performance. Hot air is less dense than cool air. Air at high elevation is less dense than air at sea level. And humid air, surprisingly to a lot of people, is actually less dense than dry air, because water vapor is lighter than the nitrogen and oxygen it displaces. So heat, altitude, and humidity all push in the same direction: thinner air, worse performance.

3:20Here is the memory hook that makes this stick. Hot and high equals weak. If you remember nothing else about density altitude, remember hot and high equals weak. So your drone performs at its worst on a hot, humid afternoon at a high-elevation site, and it performs best on a cold day at sea level. When the exam describes a scenario with heat and elevation and asks what happens to performance, the answer is always some version of reduced lift, reduced climb, reduced propeller efficiency. There is never a scenario where thin air helps you.

4:10And keep this concept in your back pocket, because it comes back in episode four, Loading and Performance. Density altitude and a heavy payload stack on top of each other. Thin air plus extra weight is a double hit. The exam loves to combine them, so the work you do understanding density altitude here pays off twice.

4:40Host: The second physics concept is air stability, and the simplest way to hold it is as a contrast between stable air and unstable air. Stable air resists vertical motion. It does not want to rise or churn, so it just sits there. That gives you smooth, calm flying conditions, which sounds great, but it comes with a downside: stable air often traps moisture and haze near the surface, so you get poor visibility, and it tends to produce flat, layered clouds called stratus.

5:25Unstable air is the opposite. It rises and churns and mixes. That mixing gives you turbulence, the bumpy ride, but the same mixing also clears out the haze, so you get good visibility. And unstable air produces puffy, vertical clouds, the cotton-ball cumulus clouds you see building up on a summer afternoon.

6:00So here is how to answer a stability question fast. If the scenario describes bumpy, turbulent air but great visibility and puffy clouds, that is unstable air. If it describes smooth, calm air but hazy, poor visibility and flat layered clouds, that is stable air. Smooth and hazy is stable. Bumpy and clear is unstable. Notice the trade-off built into nature: you generally get either a smooth ride or good visibility, but stability rarely gives you both. Lock in smooth and hazy equals stable, bumpy and clear equals unstable, and you have got it.

6:25One more physics piece that affects you directly, surface wind. Near the ground, wind is slowed and twisted by friction with the terrain and by obstacles like buildings and tree lines. So the wind you feel at takeoff can be gustier and shiftier than the smoother flow a little higher up. That matters for a drone because you fly in exactly that friction layer. Expect turbulence and sudden gusts near structures, on the downwind side of buildings, and over uneven terrain. When the exam describes mechanical turbulence near obstacles, that is friction at work, and the practical lesson is to give yourself extra margin when flying low near anything solid.

7:00Host: Now we switch to the second half, decoding, and we start with the METAR. A METAR is an hourly observation of the current weather at a specific reporting station, usually an airport. The key word is current and observed. A METAR tells you what is happening right now, not what is forecast. And the beautiful thing about a METAR is that it is always read left to right in the same fixed order, every single time. So if you memorize the order of the fields, you can decode any METAR you are handed.

7:45Here is the order. First, the station identifier, the airport code. Second, the date and time. Third, the wind. Fourth, the visibility. Fifth, the present weather, things like rain or fog. Sixth, the sky cover, the clouds. Seventh, the temperature and dewpoint. And eighth, the altimeter setting, the air pressure. Station, time, wind, visibility, weather, sky, temperature and dewpoint, altimeter. That fixed order is your decoder ring.

8:30Let me decode the field people fear the most, the wind, because it looks the most cryptic. Suppose you see two four zero, one five, gust two five, knots. Written out it is the digits two four zero one five G two five K T. That decodes as: wind coming from two hundred forty degrees, at fifteen knots, gusting to twenty-five knots. The first three digits are always the direction the wind is coming from, in degrees true. The next two digits are the steady speed in knots. And if there is a G, that stands for gust, followed by the peak gust speed. So the pattern is always direction first, then speed, then gusts. Direction, speed, gust.

9:35One subtlety worth a point: the wind direction in a METAR is given in degrees true, referenced to true north, whereas the tower might give you wind in degrees magnetic when you are talking on the radio. For the written exam, just remember that the coded METAR uses true.

10:10Next, sky cover, which uses four codes you simply have to memorize. FEW means a few clouds. SCT means scattered. BKN means broken. And OVC means overcast. There is also SKC or CLR for a clear sky. The two that matter most are broken and overcast, because broken and overcast clouds define what is called a ceiling, the lowest layer that covers most of the sky. If you see a wall of letters, find the cloud codes and you will know how much sky is covered. And the altimeter setting shows up as the letter A followed by four digits. A two niner niner two, written A two nine nine two, means twenty-nine point nine two inches of mercury, which happens to be the standard sea level pressure, a nice number to recognize.

11:00Host: Now the second coded product, the TAF, which stands for Terminal Aerodrome Forecast. Here is the wonderful news: a TAF uses the exact same coded format as a METAR. Same wind grouping, same sky cover codes, same style throughout. So once you can read a METAR, you can read a TAF, you are just reading the same kind of code. The only real difference is what the code represents. A METAR is an observation of current conditions at the station. A TAF is a forecast for roughly the five-statute-mile area around an airport, and it is valid for a stretch of time, typically twenty-four to thirty hours.

11:55So the mental model could not be simpler. METAR is what is happening now. TAF is what is expected. If a question asks which product gives you the current observed conditions at a station, the answer is METAR. If it asks which gives you the forecast, the answer is TAF. Same code, different time frame: one is now, one is the future.

12:35And here is the one trick that shows up constantly across both products, the single highest-yield weather concept after density altitude: the temperature and dewpoint spread. The spread is just the gap between the temperature and the dewpoint. When the temperature and the dewpoint are close together, the air is near saturation, which means high relative humidity and a real chance of fog or low visibility forming. When they are far apart, the air is dry and visibility is good.

13:20Let me make that concrete. If a METAR reads eighteen over seventeen, meaning eighteen degrees Celsius temperature and seventeen degrees Celsius dewpoint, that is a one-degree spread. One degree is tiny, the air is almost saturated, so that is essentially a fog warning. Expect reduced visibility. But if it reads thirty over five, a twenty-five degree spread, that is dry air, clear skies, good visibility. So small spread means fog risk, wide spread means clear and dry. Temperatures close together, watch out for fog.

13:40Notice too that the temperature and dewpoint in a METAR are given in degrees Celsius, and they are separated by a slash, like one eight slash one seven. A small detail, but if a question gives you the numbers, read them as Celsius, and remember a negative value is written with an M in front, for minus, so M zero five over M one zero is minus five over minus ten. Knowing that little quirk keeps a cold-weather METAR from throwing you.

14:15Host: Let me recap the whole episode, because weather rewards a clean mental filing system. Two halves: physics and decoding. On physics, the headline is density altitude, where hot and high equals weak, because heat, elevation, and humidity all thin the air and cut your lift and prop efficiency. And stability, where smooth and hazy is stable air, bumpy and clear is unstable air, plus surface friction that makes low wind gusty near obstacles. On decoding, the METAR is the current observation, read left to right: station, time, wind, visibility, weather, sky, temperature-dewpoint, altimeter. Wind is direction, then speed, then gust. Sky codes are FEW, SCT, BKN, OVC. The TAF is the same code but a forecast for the area around an airport. And the killer detail across both is the temperature-dewpoint spread: close together means fog and low visibility.

15:15Weather is mostly format recognition, so the best thing you can do is drill a handful of real METARs until the fields are second nature and you can pick out the wind, the sky cover, and the spread at a glance. Do that, take the practice exam linked on this page, and these become some of your easiest points. And in real life, of course, always check the current conditions and the forecast before you fly. Verify everything at faa.gov slash u-a-s. This is educational, not legal advice. Next up, episode four, Loading and Performance.