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.

0:42Half 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.

1:24You do not need to be a meteorologist. Start with a few cause-and-effect relationships, then learn the report groups. Some groups are optional and others change with the weather, so recognize the pattern rather than expecting every report to contain the same fields.

1:45Host: Density altitude describes air density as an equivalent altitude in the standard atmosphere. As density altitude rises, the propellers can produce less thrust at the same speed. The aircraft may need more power for the same task. Use its performance data to judge the size of that effect.

2:07High temperature, high elevation, and high humidity can combine to raise density altitude. Lower pressure also raises pressure altitude and can add to the effect. Compare hot with cold or moist with dry at the same pressure when you reason about density. Water vapor is lighter than the dry-air molecules it displaces, so moist air is less dense than dry air at the same temperature and pressure.

2:38Hot and high conditions tend to raise density altitude and reduce performance. Humidity adds to that effect. Cold, dense air may help aerodynamic performance, but cold can also reduce a battery's usable energy. There is no single weather setting that guarantees the best flight. Use the aircraft's limits and performance data, then account for the battery, load, and wind.

3:07And 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.

3:32Host: Stable air resists vertical motion. Moist stable air often produces layered clouds, steady precipitation, or haze and fog near the surface. That can mean a smooth ride with poor visibility. These are typical clues. Dry stable air can be clear, and stable air flowing over mountains can produce waves and severe turbulence.

3:57Unstable air favors vertical motion. It commonly produces cumulus clouds, showers, and turbulence. Visibility may be good between showers, but precipitation, blowing dust, smoke, or other obstructions can still limit it. Unstable air alone does not prove that a thunderstorm exists; moisture and lift matter too.

4:21Use cloud shape, precipitation, turbulence, and visibility together. Layered clouds with steady rain and limited vertical mixing point toward stable air. Growing cumulus, showers, and strong vertical motion point toward unstable air. These are clues for interpreting a scenario, not guarantees that the flight is safe.

4:47One 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.

5:36Host: A METAR reports weather observed at a station, usually an airport, at the stated time. It is normally issued on a schedule. A S P E C I reports certain changes between routine observations. Check the timestamp: an old METAR is not current weather just because it is an observation. Conditions at your launch site can differ from the station.

6:01Here 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.

6:35A wind group written two four zero one five G two five K T means wind from two hundred forty degrees true at fifteen knots, gusting to twenty-five. Direction comes first, then speed, then any gust. But not every group begins with a numeric direction. V R B means variable direction, and zero zero zero zero zero K T means calm. Decode the group you have rather than forcing every report into one example.

7:08One 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.

7:27F E W means few clouds, S C T scattered, B K N broken, and O V C overcast. Cloud heights are in hundreds of feet above the reporting station's ground level. The ceiling is the lowest broken or overcast layer, or reported vertical visibility into an obscuration. Few and scattered layers do not establish a ceiling. S K C means clear sky, while an automated C L R report means no clouds detected below twelve thousand feet. The altimeter group A two niner niner two means twenty-nine point nine two inches of mercury.

8:09Host: A T A F is a Terminal Aerodrome Forecast for the airport and roughly five statute miles around it. It shares wind, visibility, weather, and cloud codes with a METAR, but it is not the same template. It has an issue time, a valid period, and forecast change groups. F M gives a new set of prevailing conditions. T E M P O gives temporary changes within an interval. P R O B thirty gives a thirty-percent chance of the stated event. A U S T A F does not normally include the METAR temperature, dewpoint, or altimeter groups.

8:51So 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.

9:18Temperature-dewpoint spread is the difference between those two values. A small spread means the air is near saturation and fog or low cloud may form if cooling continues. A wide spread reduces that particular fog risk. It does not prove good visibility: dust, smoke, rain, or other obstructions can still be present.

9:43For example, eighteen over seventeen means a one-degree Celsius spread. That is a cue to watch for saturation and fog, not proof that fog is already present. Thirty over five gives a twenty-five-degree spread, suggesting drier air. You still read the visibility and weather groups. The temperature pair alone cannot tell you whether the sky is clear or a drone flight meets the visibility rule.

10:12Notice 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.

10:46Host: Review two kinds of weather knowledge: effects on the aircraft and report decoding. Higher density altitude can reduce thrust and climb margin. Stable and unstable air have typical clues, with exceptions. A METAR is a time-stamped observation; a T A F is a forecast with changing periods. Coded wind uses true north. Cloud heights use the station's ground level, and the ceiling can include vertical visibility. A narrow temperature-dewpoint spread suggests fog risk but does not prove current fog. Check the whole report and conditions at your own site.

11:29Weather 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.