Did I encounter "thermal spots" ?

Skyscraper

Skyscraper

Cleared for Takeoff
Joined
Mar 12, 2013
Messages
1,490
Location
Santa Rosa, Ca
Display Name
Display name:
Skyscraper
Hey all,

Went for a brief flight yesterday from KSTS to KDVO to practice a few touch n go's. It's a short trip so I only climbed to about 1,800 ft. Winds were calm, not much lateral gustiness at all. Out of nowhere I gained a few hundred feet in altitude and had to really fight the yoke. This happened again and again flying over large parking lots or other large black patches of what have you. At the first opportunity, I veered off to fly over green fields instead to see if it was more calm and as soon as I reached the fields it was very smooth flying with neutral controls all the way. My co-pilot suggested it may have been thermal spots. Does this sound like the case? or was it just random gusts being random? Or was it something I've not yet considered?

discuss...
 
from your description most likely thermals. was it in the afternoon?
 
Yup, could easily be thermals, especially in calm wind and in the afternoon. It's been severely sunny.

Those of us who train on the bayside learn about those ad nauseum. KPAO has a pond right under short final, and it sinks pretty good right there (and then blows you up when you pass it). On climbout, you can get rising air over the dry part of the bayside, and then sinking air when you turn across the bay.
 
Thermals, I remember that many years ago I could tell my PAX when we had passed over a major highway or the best, a KMart roof.
 
Sounds like it. Gnoss field though is known for having some really gnarly winds, from offshore breezes winding through the coastal mountains. I landed there once where the windsocks at both tends of the field were opposite of each other.
 
tmyers said:
Thermals, I remember that many years ago I could tell my PAX when we had passed over a major highway or the best, a KMart roof.
Click to expand...

I've had many soaring flights saved by wal-mart parking lots, including one competition flight last summer off the wal-mart in Hillsboro, TX.
 
You'll get more and more of those as it warms up.

If you fly over the haze layer, you'll usually be above it. That's going to get higher in the summer. It's not a safety of flight issue unless there are thunderstorms or moderate to severe turbulence in the area. But it will require you to develop your instrument scan to hold altitude.
 
Can sinking air be as strong as the lift from thermals? I remember only once encountering sinking air strong enough to have difficult climbing out of it.
 
yes. what goes up must come down. thermal sink can on strong days out power your loaded spam cans ability to climb. other sources of sink, like wind blowing down a ridge or the down side of a mountain wave, can very easily overpower your climb rate. if you find yourself in sink, do what glider pilots do and speed up to get out of it.
 
Thermals can be your friend once you know where to look for them. On the other side they can be a pain as well, but it is all part of the game. I used to cuss the fact that 99% of approaches to hospitals were over a fat parking lot. You home town of Santa Rosa has a peach on a summer day.
 
An oversimplified, non-helpful response - Yes! All weather is the result of uneven heating of the earth's surface - hence - thermal.
 
Skyscraper said:
Hey all,

Out of nowhere I gained a few hundred feet in altitude and had to really fight the yoke.
Click to expand...
You're not supposed to fight it, dude... slow to somewhere between Vs and Vbg, roll into a 45-degree bank, and enjoy the ride. :D
Joking, obviously... yeah, you flew through a thermal. As with sink, if you're not looking to use the thermal, speed up a bit and you'll get out of it sooner. But seeing as you may feel you want to descend anyway to maintain your desired altitude, you'll probably speed up anyway. But I wouldn't recommend speeding up beyond Va if you are getting hammered by a rough thermal.


Tips for avoiding thermals, assuming you have to fly fairly low during the peak thermal hours (midday- late afternoon):
- If you see a cumulus with very crisp details on top and a flat or even concave bottom, don't fly under it. Even a small one may be the "hat" atop a strong thermal. If you must fly under it, bear in mind that the base of the thermal may be upwind of the cloud a bit.
- Clouds or no clouds, take note of the terrain, looking to avoid the most likely thermal-generating stuff like parking lots, large buildings with black roofs, plowed fields, etc. Any man-made heat generator, like a smokestack, fire, cooling tower, or rooftop AC units can also produce strong, isolated lift. Again, bear in mind that if there is wind, it will be making the thermal lean so the top is downwind of the source.
- Know that sometimes thermals "pinch off", forming a bubble that keeps rising, with smoother air just below. Thermal sources sometimes cycle, rather than producing a steady stream of hotter air (particularly if clouds are passing by on a hot day). I mention this "bubble" thing because you may be flying low and go under what should be a thermal-indicating cloud, and think you are OK until wham! under the next cloud, you get your butt kicked.

- If you don't have any particular reason to fly low, usually if you are above the cloud bases (not necessarily the tops), you should get a smoother ride, unless there is shear.
- The flipside of being thermal-savvy is that as you avoid strong thermals, you will very likely encounter strong sink around them. There are also pretty reliable surface sources of sink, like deep water or tall, dense trees. And as you might suspect, where sink meets lift, there could be shear.

All of these tips can be very useful when you are in the pattern or entering/leaving the pattern, because you are low and slow, thus vulnerable to an upset due to turbulence. Thermals and sink can also mess up your nice precise altitude and heading control in the pattern.
 
Last edited:
rottydaddy said:
You're not supposed to fight it, dude... slow to somewhere between Vs and Vbg, roll into a 45-degree bank, and enjoy the ride. :D
Joking, obviously... yeah, you flew through a thermal. As with sink, if you're not looking to use the thermal, speed up a bit and you'll get out of it sooner. But seeing as you may feel you want to descend anyway to maintain your desired altitude, you'll probably speed up anyway. But I wouldn't recommend speeding up beyond Va if you are getting hammered by a rough thermal.


Tips for avoiding thermals, assuming you have to fly fairly low during the peak thermal hours (midday- late afternoon):
- If you see a cumulus with very crisp details on top and a flat or even concave bottom, don't fly under it. Even a small one may be the "hat" atop a strong thermal. If you must fly under it, bear in mind that the base of the thermal may be upwind of the cloud a bit.
- Clouds or no clouds, take note of the terrain, looking to avoid the most likely thermal-generating stuff like parking lots, large buildings with black roofs, plowed fields, etc. Any man-made heat generator, like a smokestack, fire, cooling tower, or rooftop AC units can also produce strong, isolated lift. Again, bear in mind that if there is wind, it will be making the thermal lean so the top is downwind of the source.
- Know that sometimes thermals "pinch off", forming a bubble that keeps rising, with smoother air just below. Thermal sources sometimes cycle, rather than producing a steady stream of hotter air (particularly if clouds are passing by on a hot day). I mention this "bubble" thing because you may be flying low and go under what should be a thermal-indicating cloud, and think you are OK until wham! under the next cloud, you get your butt kicked.

- If you don't have any particular reason to fly low, usually if you are above the cloud bases (not necessarily the tops), you should get a smoother ride, unless there is shear.
- The flipside of being thermal-savvy is that as you avoid strong thermals, you will very likely encounter strong sink around them. There are also pretty reliable surface sources of sink, like deep water or tall, dense trees. And as you might suspect, where sink meets lift, there could be shear.

All of these tips can be very useful when you are in the pattern or entering/leaving the pattern, because you are low and slow, thus vulnerable to an upset due to turbulence. Thermals and sink can also mess up your nice precise altitude and heading control in the pattern.
Click to expand...

Thanks for all the info and tips! Frankly this was a pretty new phenomenon to me as I have only been at it since last Fall, so each new season and the conditions it brings are a first for me.
 
Given where you operate, you're likely to get much more effect from winds at altitude flowing over the mountains. That's mechanical turbulence, and it can really suck. I had some of that try to turn me over on departure from KHAF once; I would have thought it was a wake turbulence encounter, but there was no one else around. The aircraft rolled sharply to the left at about 300 AGL, probably due to winds at altitude flowing over Montara Mountain. There was definitely significant windshear in the area; winds at the surface were from the southwest at around 5 knots, but were at least twice as strong in the opposite direction at pattern altitude. On final from 500 AGL, I watched the crab angle reverse direction....

But thermals happen locally when it gets warm. Generally NOT on "spare the air" days, though. The lower thousand feet or so (at least) is stable then.

We don't get really strong thermals much because mountains and winds tend to break them up. But weaker ones do happen. Thermals this time of year may be capped by fair weather cumulus if it's unusually warm, but that's not common in summer (too dry).

If there is an issue detected at STS, Tower will issue a "low level wind shear" advisory on ATIS, saying how much airspeed will be lost on final. It's very seldom bad enough for that. And it's not usually an issue at lower altitudes (lower than, say, 5000 feet) with thousands of feet of ground clearance. It may be an issue near your aircraft's performance limits or close to the ground, such as crossing over the high Sierra.
 
MAKG1 said:
Given where you operate, you're likely to get much more effect from winds at altitude flowing over the mountains. That's mechanical turbulence, and it can really suck. I had some of that try to turn me over on departure from KHAF once; I would have thought it was a wake turbulence encounter, but there was no one else around. The aircraft rolled sharply to the left at about 300 AGL, probably due to winds at altitude flowing over Montara Mountain. There was definitely significant windshear in the area; winds at the surface were from the southwest at around 5 knots, but were at least twice as strong in the opposite direction at pattern altitude. On final from 500 AGL, I watched the crab angle reverse direction....

But thermals happen locally when it gets warm. Generally NOT on "spare the air" days, though. The lower thousand feet or so (at least) is stable then.

We don't get really strong thermals much because mountains and winds tend to break them up. But weaker ones do happen. Thermals this time of year may be capped by fair weather cumulus if it's unusually warm, but that's not common in summer (too dry).

If there is an issue detected at STS, Tower will issue a "low level wind shear" advisory on ATIS, saying how much airspeed will be lost on final. It's very seldom bad enough for that. And it's not usually an issue at lower altitudes (lower than, say, 5000 feet) with thousands of feet of ground clearance. It may be an issue near your aircraft's performance limits or close to the ground, such as crossing over the high Sierra.
Click to expand...
Good points...it might have been terrain-induced.Thought of that, but I didn't look at the route or area wx... just figured it was most likely a thermal because he was fairly low, got lifted up pretty rapidly, and didn't mention being near high terrain. Anyway, I like to share what I've learned while actually hunting for thermals- it's useful knowledge for those just trying to avoid them or plow through them.
 
Yeah, there are both mountains and significant changes in albedo and heat capacity in the area. STS is in a valley, and prevailing winds at altitude tend to cross the mountains. Both mechanical and thermal turbulence are expected. But it's generally not that bad unless the winds are either howling or they are completely gone and it's hot as Hades.

DVO is on the bayside AND the lee side of the mountains (so are PAO and SQL, but they are a bit further from the mountains). Those make for some interesting interactions.
 
Thermals depend on surface heating AND adiabatic lapse rate. Look at the upper winds and temperatures forecast; if the temp drops more than 2°C per thousand feet, thermals are possible. If they're well over 2° per thou, it could get really rough. If the drop is less than 2°C per thousand, thermals are very unlikely.

Dan
 
Sac Arrow said:
Sounds like it. Gnoss field though is known for having some really gnarly winds, from offshore breezes winding through the coastal mountains. I landed there once where the windsocks at both tends of the field were opposite of each other.
Click to expand...

I did all my crosswind training at DVO. I had the same encounter with the windsocks at both ends of the fields opposite. :D
 
'Want a Phd in thermals? Just spend some time in the Desert Southwest. Sometimes, it's like riding a rodeo bull. +/- 500ft and continuous bumps. We always try to be on the ground by 10am or so, and that might be too late. 'Made the mistake of flying between Page and Phoenix one nice June afternoon... I've never been so close to just putting it down on a road, as I was beat and my hand was numb from the death-grip on the stick. Some say I should have gone higher but, as a rookie, I thought 9K was high enough.
 
You must log in or register to reply here.
Back
Top