Calculating cloud heights with radar?

[PFM]

Dusting off the cob webs on radar usage, does anyone remember a formula for calculating cloud tops using tilt and distance?

Thanks.

[twistedenginestarter]

Isn't this yer ole one-in-sixty? Thus the cloud tops would be:

tilt X nautical miles X 100

This of course has to be added/subtracted to/from your current altitude.

[Captain Airclues]

PFM

Just get a glass of water and look along the surface of the water. If you can see the cloud you'll hit it, if you can't you won't.

Airclues

PS. The clever people will tell you about the water meniscus and the curvature of the earth, but it's worked for me for 35 years.

PPS. You do get some strange looks from the F/O though

[5150]

Radio Nav theory recommends the following equation for the ATPL writtens;

(Tilt Angle - 0.5 beamwidth) x range (nm) x 6080 = Answer in feet.

Answer, like above, needs to be applied to current altitude.

[Manflex55]

As twisted said, the 1 in 60 rule works fine. To determine the cloud ht, the wx beam is raised by turning the tilt knob until the cloud just disappears from the screen. Let's say our beam has a width of 5°:

- if the cloud disappears as tilt setting reads, for example, "4½° UP" (which means a beam centreline 4½° above the horizontal), then the beam's LOWER line (& hence the cloud top) is 2° above your current alt.

- should U B, for example, 45nm away, using the 1 in 60 rule U get a cloud top of 9,000' above your current alt.... so plenty of time to climb & pass above it. Don't forget the recommended 2,000' margin if your cloud is a CB !!

MF

[fireflybob]

Funny but I thought wx radar showed significant amounts of "precipitation" as opposed to cloud?

This seems to be the latest favourite interview question!

[Checkboard]

That's true, the above formulae give the radar height of the cloud - the top of detectable precipitation. The actual cloud will extend a third or so higher, depending on the cloud.

If I feel keen tomorrow, I will include an article on radar I have filed somewhere.

In the meantime, have a read of these two articles from the TechLog Archive:
Terminal Area Weather Radar Technique, and
X band radar tips?

[dv8]

No pilot manual of any A/C I have flown have dared to tell me the beam width of the AWR.

Hush Hush, need to know and all that.

[411A]

In NO aircraft that I have flown in the last 35+ years has the radar tilt indication been especially accurate.
As with the glass of water routine, many years ago flew with Captain who carried a small bubble level to sight along to estimate cloud tops. Seemed to work for him anyway.

[LightenupFrancis]

I agree with what Manflex55 said regarding the tilt and 60 to 1 rule. I take it one step further by remembering that the earth is round and its horizon constantly falling. Hence, for every 60nm away a return is, you subtract 1 deg of tilt from the equation. 60 x 360 = 21600nm (pretty close to the earth's circumference). So a storm I paint with precip tops of 30,000ft at 30nm with a hypothetical tilt of 2 1/2 deg up bottom of the beam, would/should show up with 1 1/2 deg up tilt at 90nm. If it shows up at 90nm with the same tilt, it would be 6000ft higher, or 36,000ft. I used to fly a lot of 10hour flights, what else was I supposed to do with my mind?

[twistedenginestarter]

kelbill

Registered January but this is your first post. Glad to see you lost your viginity on Tech Log where the elite people hang out.

Well, welcome. And let's hope we don't have to wait so long for Post No 2.

[ITCZ]

411A, you are right about the tilt calibration -- rarely will the "indication" equal the actual tilt.

Easiest way to "calibrate" it yourself for each flight is to use the 100'/1nm rule of thumb. Tilt down till ground return = height (not altitude AMSL) in thousands. Eg F350, closest ground return @ 35 miles. Bottom of beam is 10 degrees down. You do the maths.

Generally speaking, according to company 'coneheads' a 10 inch diameter wx radar dishes produce a beam 8 degrees wide, and the beam width reduces by one degree for each increase of one inch diameter of the dish. Generally speaking. Writer makes no warrantee as to the accuracy of above statement. Break the habit of a lifetime and talk to a radio tech yourself before you sledge me. Even better -- ask the radtech to find out the beamwidth for your installation from the bucketload of manuals he/she has.

Armed with those two slightly dodgy bits of info, you can then do some more 1/100 with your tilt in flight and come up with handy guesstimates. Not accurate to the millimeter, but we don't give our ETA to the decimal of a second either.

[ 05 November 2001: Message edited by: ITCZ ]

[Checkboard]

Antenna type vs dish size & beam width

Size   X-band    C-Band

10"       10°          NA
12"        8°          NA
18"        5°          NA
24"        4°          6.5°
28"       3.5°          6°
30"        3°           5 °

(From "X band radar tips?", posted above.)

[ 06 November 2001: Message edited by: Checkboard ]

[Agaricus bisporus]

PFM, if you'll provide an e-mail address I can mail you a 4 page article on advanced radar techniques which covers this and many other systems for scientific use of radar. Originally published in an American aviation mag some years ago, very useful.

[4dogs]

Agaricus,

Would you mind mailing me a copy as well, please?

[Tee]

The formula I was given for the Radio Nav ATPL writtens is:-

(tilt - half beam width) x range (nm) x 101.3

101.3 is a constant

[Agaricus bisporus]

Sorry chaps, if I'd read the above posts properly I'd have seen the article in Checkerboard's link. It's the Trammell terminal area radar article, very useful, and as long as you know the diameter of your antenna it is independant of tilt calibration, ie it is self calibrating.Thoroughly reccommended.