As a pilot, you know more about the weather than most of our earthbound friends. You can amaze and dazzle laypersons with your meteorological acumen and have likely become the resident weather authority in your household. Yes, dear, you should take your umbrella to work today.
However, within our flying community, we know we consult a finite number of weather maps and data screens regularly, either online or within an EFB, before launching skyward. None require an advanced degree to understand them, provided you learn the basics of weather science.
Sure, our non-pilot friends and family regard us for our atmospheric prognostication as if we know as much as a meteorologist. However, realistically, there’s always another chart or data set around which we don’t/can’t wrap our brains. You know, the ones that really do require PhD-level analysis.
Today, let's look at a fantastical reporting tool on the border of intellectual accessibility that we’ve all heard of and perhaps even learned a little about. However, any deep dives in pursuit of mastering it might have led to realizing the commitment required to tame this beast.
It promises crystal-ball-like visions of climatological foresight, yet many pilots’ eyes glaze over after just a few minutes trying to interpret its messages. I am, of course, referring to the enticing yet unapproachable Skew-T Log (P) diagram.
Our goal today is to learn just a few things about it to glean useful information after a few minutes of looking at it. What follows is not a comprehensive education on Skew-T Log (P) diagrams but rather enough information to allow you to get weather data that can help confirm or contradict conclusions you’ve drawn from studying your usual WX products. It will provide more granular information and give you a better visualization of what’s going on up there.
Let’s start by simplifying and demystifying the Skew-T Log (P) diagram, usually referred to just as “Skew-T.”
Skew-T means the isothermic lines (temps) on the chart aren’t straight up and down or side-to-side. They’re skewed at a 45° angle.
Log (P) simply means that the atmospheric pressure isobar lines are represented logarithmically. That’s fine because atmospheric pressure increases/decreases with altitude logarithmically in real life, so the isobars on the chart closely match reality.
Here’s an example of a Skew-T on a pretty dull weather day (i.e., good for flying).
The right side of the chart sometimes shows a pressure altitude scale and/or wind barbs at various altitudes.
Those are the metrics we’re going to use. You’ll notice green index lines and some dashed as well, but we’re not going to dive into anything so deep in this article. Here are the “at a glance” tools and tips you were promised.
If you see directionally divergent wind barbs in a cluster at a particular altitude, especially if wind speeds also vary, that’s a bumpy ride up there.
A small (or non-existent) gap between dewpoint and temperature often indicates the presence of warm stratiform clouds. Altitudes at which the air and dewpoint temps diverge are where the probability of cloudiness decreases.
Of course, there’s a catch. Inside a large temp/dewpoint spread, cold stratiform clouds or convective cumuliform clouds could still shoot upwards, unconcerned with the ambient air around them until they rain out. Keep that in mind if the air is unstable below divergent temps/dewpoint…and it’s thunderstorm season.
Area of uncertainty
As with any fluid, there is uncertainty in our atmosphere. Just because the air temperature and dewpoint diverge doesn’t mean there’s a clean top or ceiling for a cloud deck.
Where air and dewpoint temps begin to diverge from having been close together at different altitudes, you should think about probabilities of the presence of clouds, not a stark presence or absence. This graphic provides a good example of “eyeballing it” regarding cloud layer altitudes.
Stratocumulus clouds and temperature inversions
On a Skew-T chart, a sharp right turn of the temperature plot as altitude increases signals temperature inversion, which caps clouds in unstable air below. This is an example of what a temp inversion looks like. Note that if it is above 0°C at a higher altitude, precipitating, and below freezing at lower altitudes, you can expect to see FZRA on the METAR/TAF.
Supercooled liquid water
It can exist from 0°C to below -40°C. Most airframe icing in saturated conditions happens around 0°C to -15°C. Pay close attention to where the temperature plots cross 0°C.
So, there you have it. Again, this is very far from being a comprehensive explanation of Skew-T analysis. However, now you know how to get a little more weather information, at a glance, that can broaden your view of conditions, thereby increasing your situational awareness.