Dots and Punch

Dots. They are a good thing for us windsurfers and kiters (we're too lazy to have to add the "boarders" every time we type it out). On our wind graphs of current or forecast conditions, dots represent wind gusts. On the forecast graph on the front page, dots usually don't appear until gusts are forecast to be around 15 mph or more - enough for many of us to plane.

Dots are less frequent in the summer time since, for a number of reasons, summer is less windy. Recently, we went a few weeks without seeing dots. And, when we did, they were more indicators of thunderstorms rather than sailable wind. Each day, we'd scan the forecast graphs for dots to no avail. Even looking forward through the week showed not much more than a bunch of sixes and sevens. But, no dots.

bad dots
Bad dots, bad. These dots didn't predict sailable wind but rather the likely chance of thunderstorms.

It's all about the dots

Yes, it is; yes, it is. (On Roswell Rd. in Sandy Springs.)

In the last week, the dots reappeared and even gave a few good sailing sessions over the July 4 holiday. Welcome back, dots. We missed you.
.

Punch

There have been various characters over time named Punch. In England, they've long had Mr. Punch, who for 450 years has been knocking the missus, Judy, around (to be fair, she gets in her licks, too).

Punch and Judy
"Punch and Judy is a traditional, popular, and usually very violent puppet show featuring Mr Punch and his wife, Judy."
- Wikipedia
(Mr. Punch is on the left.)

The Hawaiian version of Punch is no less violent, either.



In our little world of windsurfing, punch describes how air density affects the feel of our sails. For those of you who remember the Ideal Gas Law from high school chemistry (PV=NRT) and a bit of math, we find that air density varies by temperature. Air is less dense at higher temperatures. Air density is also less at altitude which affects us a bit on Lake Lanier at almost 1100 ft above sea level; and, humidity, which we have in abundance in the summer time.

Air density is important because it determines how much "power" there is in the air, or to think of it another way, how many molecules of air are interacting (pushing, pulling, being deflected by) with your sail. More molecules means more force. Pilots, too, are especially concerned with air density because it affects how fast (and far) they need to go to take off and stay in the air.

This all sounds "science-y" but has a point. This time of year, it takes more wind to get going or for the same wind speed, you need to use a bigger sail. The effects can be significant. More than a few of us have seen whitecaps here or at the beach in the summer, rigged what we usually rig, go out on the water and then wonder why, as the gusts roll through, nothing seems to happen. Conversely, it's also why on winter days with temperatures in the mid-40s that we seem so delightfully powered even in modest gusts.

There are various calculators on available on the interwebs for calculating air density. Running some numbers through one, we find that air is roughly 12% less dense on a hot and humid summer day (93 degrees) than a typical winter day (45 degrees). Your 7.5 has shrunk to a 6.7. Conversely, if you think your 7.5 is your big enough sail in the winter, you'd need an 8.5 to deliver the same power in the same wind speed conditions in the summer.

To add insult to injury, we lose about 4% air density at our altitude of almost 1100 feet above sea level at Lake Lanier or Allatoona vs. someone at the beach or Cape Hatteras all other conditions being equal. Your 7.5 at the beach shrinks to a 7.2 up here.

This isn't all bad. Our lower air density in the summer means you can ride your bike faster on these hot windless days. Something to think about while we wait for dots.

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Randy
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Joined: 05/05/2002 - 10:38
Posts: 4161
Re: Dots and Punch

If you bother to wait till the end of the wind talker's report in the warmer weather months you will hear him say something like Density Altitude 3000 feet. This is like golf - a low density altitude is better. In fact, usually he doesn't bother even reporting it when its a good number. But that's the altitude equivalent of air at 59 degrees above sea level. So when its 3000 here it is the same density here as the air 3000 feet above sea level. So that helps to quantify how badly we are going to get screwed by whatever little wind there is.

The More You Know.......

What happens in a black hole stays in a black hole.

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webguy
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Joined: 12/31/2000 - 22:01
Posts: 11183
Re: Dots and Punch

Randy, good point. I used a couple of density altitude calculators online to verify my numbers when I was writing.

Here's one that's used by pilots and engine tuners (auto engines, too, are affected by these same factors so to accurately calculate HP, air density needs to be known):
http://wahiduddin.net/calc/calc_da.htm

btw, Gilmer is at 1276 feet altitude (Lake Lanier full pool is 1071). Altimeter setting is also read as barometer or sea level pressure - usually a number around 29-30 inHg or about 1000 mb. In the summer, the density altitude can be 3000+ feet while in the winter it even be below sea level.

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