When flying around a thunderstorm, it is advised that you fly in the upwind portion of the thunderstorm. First off, how do you figure out which side is the upwind and downwind of a thunderstorm? Second, why fly in the upwind?
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4 Answers
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The "anvil" of a Thunderstorm is always on the downwind side. Airline pilots routinely deviate around the upwind side in hopes of a smoother ride.
photo source: CRAZY CLOUDS: The Thunderstorm Anvil
answered May 25, 2017 at 0:40
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$\begingroup$ Ahh, thank you. However, isn't the upwind side parallel to the downwind? Wouldn't you still be penetrating the anvil? If the left side of the thunderstorm is the upwind, wouldn't that side actually be a crosswind since the wind is blowing across? $\endgroup$– nyorkr23May 25, 2017 at 0:43
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3$\begingroup$ @nyorkr23 I think you're confusing the term a bit. In this context, upwind is like saying upstream, it means the direction the wind is coming from. It's also relative - to the storm, upwind is to the left (in this picture) and downwind is to the right. So the aircraft could be upwind to this storm but at the same time be downwind to another one to the left of the picture. The aircraft will be experiencing a crosswind sure, but it is still on the upwind side of the storm cell. In the circuit, the upwind leg is parallel to the downwind one. $\endgroup$– BenMay 25, 2017 at 3:31
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1$\begingroup$ Oh I see. Its in relation to where the storm is moving, correct? $\endgroup$– nyorkr23May 25, 2017 at 3:34
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15$\begingroup$ @nyorkr23: Just in case it isn't clear: "upwind" and "downwind" don't mean the same as "updraft" and "downdraft". The word "upwind" doesn't mean a wind that's blowing vertically upwards - it means "in the direction from which the wind is blowing", and "downwind" means "in the direction towards which the wind is blowing". For example: if you are downwind of a cheese factory, you will smell it, but if you are upwind of it, you won't. In the diagram in this answer, "upwind of the storm" is on the left of the diagram, because the wind is blowing left-to-right in that diagram. $\endgroup$– psmearsMay 25, 2017 at 10:28
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2$\begingroup$ @nyorkr23: From a more meteorological (less aviation) background... it's not so much primed on where the storm is moving towards (in fact, many storms are stationary... and while storms commonly do move similar to the wind at "upper-levels" (i.e. cruising altitudes) [since those winds are usually strongest], it's not guaranteed, as movement is a factor of wind at all levels, plus storm growth direction) $\endgroup$ May 26, 2017 at 4:55
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You don't figure out which side of the storm (mature stage) has updrafts or downdrafts. You simply don't fly near, in, or under a thunderstorm. It's too dangerous. The FAA advises that you stay at least 20NM away from intense radar echos associated with thunderstorms in order to avoid clear air turbulence and hailstones. These factors may be present on all sides of a mature thunderstorm.
answered May 24, 2017 at 23:49
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3$\begingroup$ Yeah I get that. But if you were already near one and you can't avoid it because you screwed up, that's the scenario I'm talking about. The reason I ask is because a DPE asked this to a couple people I know and they tried to explain it, but I was confused. $\endgroup$– nyorkr23May 25, 2017 at 0:19
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1$\begingroup$ That's all going to be dependent upon what the lifting factor creating the storm is. For airmass thunderstorms forming over flat terrain on warm summer days, this may be very difficult to discern; for frontal storms, usually the air fwd of the front is rising and the air aft is falling. At the point you are talking about, you probably will not have the option to pick where you want to penetrate the storm at; if such time and luxury exist, you could probably maneuver to avoid or make a forced landing to get away from it. $\endgroup$ May 25, 2017 at 0:44
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2$\begingroup$ How do you deal with the "line of storms spanning from Canada to Mexico" case then? Better to be on the ground wishing you were in the air, eh? $\endgroup$ May 25, 2017 at 0:59
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3$\begingroup$ Don't take off in the first place. A line of storms that size is an easily predictable thing and will be present on a proper pre-flight weather briefing. This isn't a spontaneous thing; you would know about this long before you departed, therefore you have a lot of options available to you. $\endgroup$ May 25, 2017 at 1:45
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5$\begingroup$ RIght but if you're flying a B777 on a 12 hour flight across the equator you're going to come across a thunderstorm more often than not. You don't stay on the ground in that scenario, you simply give it a wide berth, upwind if possible. In that case it's easy to determine which was is upwind because of onboard wind displays. $\endgroup$– BenMay 25, 2017 at 3:05
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One reason that you stay away from the downwind side is hinted at by the funny quote by @psmears:
If you are downwind of a cheese factory, you will smell it, but if you are upwind of it, you won't.
Here is my take on it:
If you are downwind of a thunderstorm, you will get hit with hail, but if you are upwind of it, you won't.
Of course, it is possible there is no hail on the downwind side, but still it is a risk that is not usually necessary to take, and it is almost always possible to deviate as needed.
Below is an image taken from Professor Frank Ludlam's (Imperial College) 1961 article in Weather. The storm is moving right to left and affects are out to 20 km. You can see the large area of hail downwind.
I remember coming in to NAS Beeville in an A4 finishing up an instrument training hop. We were in a slow descent at 25k feet heading east at 250 kts. It was around sunset, with the sun behind and encroaching darkness in front of us. There were numerous large thunderstorm cells around us, some as high as 60-70k feet. As I looked up towards their tops I could see the lightning arcing from one side to the other of a cell in that weird, mesmerizing twilight. It was like a fireworks show, and quite impressive. There wasn't anybody out there that evening and we asked to deviate around weather. We got approval and did these lazy turns in our descent to the approach. One of those most pleasant fligths I visit often in my memory.
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Simply put, flying downwind of thunderstorms increases your risk of hail.
The downwind portion of the thunderstorm is characterized by an anvil cloud. Typically the downwind side has a much higher probability of hail.
It is not uncommon for anvil clouds to extend a hundred miles down wind from larger cells. Hail may be can be obscured by precipitation. Hail can damage an aircraft even though other flight conditions such as turbulence are benign.
