Estimating distances and sizes of storm features

[Jason Harris]

What are some good rules of thumb as well as other more precise formulas for
figuring out how far away and wide a wall cloud is, a tornado, the main updraft, etc. Can you estimate based on how much of the horizon is taken up, the angle
of your pespective--like the degree to which you find your chin tilting up? Anything and everything that can help with estimating distances and sizes would be very useful to know.

 

[Tony Laubach]

Hi Jason,

I typically use surrounding landmarks to judge the distance of something. I don't think I'm fast enough to sift through equations and angles in the field. One thing I have done is try to compare the road network to my location on a GPS map to give me a better idea. I've also tracked the distance I've driven. Say a tornado is near some trees I drove passed. I can quickly see how far I drove between those trees and my current position to get an idea of how far something is. Unless you're very well equipped or can do crazy things in your head quickly, you're always ball parking in the field. Its not til after the chase for me when I can go through and get more precise.

 

[Shane Adams]

Just a quick note....basic rule of thumb is however close you think the tornado is, it's usually much farther away....especially when you're trying to judge distance from close-range.

 

[Jason Harris]

Thanks Tony and Shane. Also, how about sound? How close is the tornado generally when you can actually hear it? And can you tell something about its strength by the sound? Or are there too many variables involved to make many judgments there? For instance, can a smaller weaker funnel have a more potent whistle to it than an actually stronger wider funnel?
Also, to go back to visual matters a bit more, at what distance will you no longer be able to see a storm's top? Or does lower level cloud cover vary too much to offer much of a rule there?

 

[Shane Adams]

How close is the tornado generally when you can actually hear it?

It depends on the tornado. Of course the stronger ones will be louder and thus more audible from a greater distance, but suffice to say if you're hearing the actual tornado, you need to be paying attention because you're close. I've only truly heard one tornado, and I was .7 mile from it at the time.

And can you tell something about its strength by the sound? Or are there too many variables involved to make many judgments there? For instance, can a smaller weaker funnel have a more potent whistle to it than an actually stronger wider funnel?

Obviously any wind that's strong enough to be heard is potent, and this transitions naturally to tornadoes. Even (relatively) weak tornadoes are intense wind-wise, but compared to the wind noise of a strong or violent tornado, they are basically "mute"....but this doesn't mean they aren't audible from close range. The notion that "the louder the roar the more intense the tornado" is a safe one, but it's not the only one. There is also the notion of "the louder the roar the closer the tornado". Knowing both is the key, and being constantly aware of your surroundings in the near-storm environment. I've only heard one tornado (which was small) so I can't compare the "roars" of small versus large tornadoes.

at what distance will you no longer be able to see a storm's top? Or does lower level cloud cover vary too much to offer much of a rule there?

In the clear sky, a few hundred miles at least. I've seen storms in southern Oklahoma from southern Kansas (DOH!). But as you mentioned, low clouds can ruin your view of a storm (not just the top but all of it in extreme cases). Fortunately, on typical Spring chase days, as the day wears on and the initial storm or storms begin to take over, the lingering cloud will eventually go away, leaving a largely-clear sky and your remaining dominating storm or storms. Once your LL clouds are gone, any storm within reach you will be able to see the top of.

 

[Jason Harris]

Good to have this information. And how about in terms of proximity: at what point (assuming relatively clear conditions) will you not be able to look up to see the storm top because you're ____ miles from the storm? And at what distance from the updraft would you generally expect to be encountering lower clouds (coming from various directions--I assume if the storm is coming from the SW heading NE and you're directly in that NE path you'd expect more outflow clouds and a far more difficult time sooner seeing much in general)

 

[Shane Adams]

at what point (assuming relatively clear conditions) will you not be able to look up to see the storm top because you're ____ miles from the storm?

If you mean the canopy (underside of the anvil): Depends on the storm and the anvil-level winds (around 30,000 feet). If the upper jet is strong, the storm's anvil will overspread the area outward and ahead of the storm much faster and reach much further...this means you can be a good distance away from the storm and still be in the shadow of the anvil (sorry Darin LOL). The slower the upper jet, the slower the anvil spread will be (and not as far out from the storm).

If you mean the top of the vault/updraft/tower: Barring cloud cover, this depends on how tall the storm is. There's really no applicable "field method" for determining this. You have to be fairly close to the storm to need to look "up" to see the top, so as the storm moves away, naturally your angle of view will gradually lessen. The taller the storm, the farther away you can be and still need to look "up"....the shorter the storm, the quicker your "view angle" will recede, as the storm moves away. As far as the distances required to meet/cross these "view angle" thresholds, I have no idea. I've never really considered it, as it's not really important to how the chase evolves.

Maybe I'm not understand your question?

And at what distance from the updraft would you generally expect to be encountering lower clouds (coming from various directions--I assume if the storm is coming from the SW heading NE and you're directly in that NE path you'd expect more outflow clouds and a far more difficult time sooner seeing much in general)

There is no rule here. Low level clouds are often a part of the pre-storm environment and hence linger about well into storm initiation. Their placement/density relative to a storm's updraft is as random as a gentle breeze and as unique as a snowflake.

 

[Mike Hollingshead]

Just a quick note....basic rule of thumb is however close you think the tornado is, it's usually much farther away....especially when you're trying to judge distance from close-range.

I would agree with this in half the cases I think. It seems if you are very close then you can fall victim to thinking it is closer than it really is. But, there seems to be a range in there(a hair further out but still rather close) where you swear it's further away, then all the sudden realize, it's not far away at all.

 

[David Wolfson]

I've become accustomed to estimating angles to the zenith, being aware of the cloud base height and estimating distances from those. For example, if a cloud feature is 30o above the horizon it's 1.7x the LCL ground projected distance away from you, if 45o then 1x the LCL away, and if 60o then only .6x the LCL away, etc.

With respect to distant clouds, due to the earth's curvature the formula for the distance in miles you can see something of height H in miles is approx. SQRT(8000H). So a fairly lusty eight-mile high cloud top disappears at about 250 miles away. Have a good idea from regional radar or whatever what the cloud top height may be and gauge accordingly by how much of the cloud you can see above the horizon.

[ed. to answer following post: SQRT is "square root". I've always just used the LCL as a synonym for cloud base, but the question made me think. Here's a "Haby hint" that gives the true answer: http://www.theweatherprediction.com/habyhints/160/. The LCL is pretty good for the type of storms we chase]

 

[Jason Harris]

Thanks all for the elaboration. Regarding these formulas David, can you further explain the abbreviations? Is the LCL basically the visible cloud base? (Lifting Condensation Layer?) What's SQRT? I'm pretty ignorant when it comes to math and physics.

 

[Jason Harris]

Thanks for the update David!

 

[Gene Moore]

What are some good rules of thumb as well as other more precise formulas for figuring out how far away and wide a wall cloud is, a tornado, the main updraft, etc. Can you estimate based on how much of the horizon is taken up, the angle of your perspective.

Perspective can be distorted by cloud base therefore I assume the tornado will be closer than it seems during a very low cloud base, and further away during a tall cloud base. For example, if you break out of the rain and see a tornado your cloud base may be the wall cloud base which is much lower to the ground. Tornadoes in foggy and hazy conditions, like along a warm front, are likely closer than they appear. This is a personal chase opinion from experience and is not written or blessed to my knowledge.