Humid landings

[Push 0]

In snowy Ontario we are in the middle of spring thaw. I've done two jumps this weekend, and I've noticed that the Manta 288 (or so) that I'm under descends a lot faster than usual. This is particularly pronounced at flare time. The flare is about the same as always, but the downward speed is greater. As a result, what was once a stand up landing changed a little. My leg still hurts a bit. In fact, my instructor, who has an 84 Velocity and is very good with it, almost tumbled on his first landing of the day.

So my question is, can the performance of a parachute change in such conditions? My guess is that the high humidity from the thawing is absorbed by the nylon as it flies. This increases the weight of the parachute, and hence increases the rate of descent. Could this actually happen?

-- Toggle Whippin' Yahoo
Skydiving is easy. All you have to do is relax while plummetting at 120 mph from 10,000' with nothing but some nylon and webbing to save you.

[nicknitro71 0]

It's not the weight of the chute. It has to do with air density and water vapor. Taken to the extreem think about flying your canapy in the water!

Memento Audere Semper

903

[quade 4]

Water, as a gas, is less dense than dry air.

However, temperature plays a -much- more significant role than humidity.

See THIS.

If you were used to jumping in chilly temps during the winter months and this last weekend was much warmer, then that alone should pretty much explain everything.

quade -
The World's Most Boring Skydiver

[billvon 2,990]

>It's not the weight of the chute. It has to do with air density and
> water vapor.

Well, that's right, but -

>Taken to the extreem think about flying your canapy in
> the water!

It's actually the opposite. On a humid day the air is _less_ dense. A hot, humid day is the worst in terms of landings (or best, if your objective is to go as fast as possible.)

[Push 0]

Thanks a lot quade. That was very helpful. It's remarkable how much difference this effect really makes. Definitely something to watch out for.

-- Toggle Whippin' Yahoo
Skydiving is easy. All you have to do is relax while plummetting at 120 mph from 10,000' with nothing but some nylon and webbing to save you.

[CanuckInUSA 0]

I'm only kidding, but why don't you come out and visit us here in Colorado for a jump or two on one of our hot July summer days when the density altitude is pushing 10000 feet. Ok I'm sure that was uncalled for. But we've got some interesting challenges here that's for sure. But I like it that way.

Try not to worry about the things you have no control over

[PhillyKev 0]

Why does your chart show a higher equivalent altitude at lower temperatures. Isn't the opposite true? As the temperature increases performances is as if you were at a higher altitude?

[bmcd308 0]

The chart shows altitudes and temperatures that are equivalent to 59 degrees F and 29.92 inches of mercury at sea level.

So let's say you are on the beach (at sea level), and you see that it is 59F and your barometer reads 29.92 inches of mercury. You take careful notice of how your canopy flies when it lands on the beach, ignoring the scanitly clad women there (you're more of a scientist than a skydiver, in this example).

Now we build a 1,000 foot tall tower on the beach, and you want to land on the roof. In order for you to notice that your canopy flew exactly the same as you landed on the tower as it did when you landed on the beach, the temp on the tower would need to be 55F (4 degrees colder than it was on the beach).

Lower, colder air is more dense.

BMcD...

EDITED to add: You're actually correct in your understanding of how air density works, you're just reading the chart wrong.

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www.jumpelvis.com

[PhillyKev 0]

Ahhh...thought is was showing what the equivalent air mass at that temperature would be as related to a change in altitude with temperature staying the same. Gotcha.

[quade 4]

I'm not completely sure I understand your question, but I'll give it a go.

What the chart portion of the page is showing is the ISA standards (more or less, I did some rounding to make things a bit easier).

What it means is that the ISA standard sea level pressure and temperature is 29.92 inches of mecury at 59 degrees F.

Under those conditions (the ISA standard conditions) at 1000 feet above sea level you would expect to have a temperature of 55 degrees F -- so on and so on.

If you were at a DZ 1000 feet above sea level and the temperature was 70 degrees, then according to the rule of thumb I had laid out, the density of the air would be the same as if you were at a DZ located 2000 feet above sea level at the ISA standards.

The variation in temperature at your drop zone is by far the most significant factor with respects to air density and how your canopy will react on a given day or even from the cool early morning hours to mid-day heat.

At Perris, for instance, it would not be unheard of to have a temperature swing of 30 to 45 degrees. That's the equivalent of raising the drop zone an additional 2000 to 3000 feet.

quade -
The World's Most Boring Skydiver

[nicknitro71 0]

Quote

It's actually the opposite. On a humid day the air is _less_ dense. A hot, humid day is the worst in terms of landings (or best, if your objective is to go as fast as possible.)

Did not I say the same? On a hot humid day there is LESS atmospheric pressure so less air per square feet (i.e. molecules are further apart) than in a dry day when the pressure is high. Less air means less lift.

Memento Audere Semper

903

[quade 4]

Quote

On a hot humid day there is LESS atmospheric pressure so less air per square feet (i.e. molecules are further apart) than in a dry day when the pressure is high. Less air means less lift.

This is pretty confusing and I'm hoping that it's just the sentance structure.

Just to make certain, let's talk about a few different things.

Hot and humid doesn't always go together. I'll admit that in some parts of the country it may seem like it, like Florida for example, but in California and Arizona, just the opposite may be true.

Higher temps and lower barometric pressure at a single location doesn't always go together either. In fact, that's almost always not true. Think about the pressures in a hurricane for instance. Really old and basic weather forecasting was using nothing more than a set of eyes and a barometer. The general rule was that the lower the pressure the worse the weather.

So, anyway, let's not confuse the terms atmospheric pressure with density-altitude. Atmospheric pressure being a measurement of pressure without regard to the temperature. Density-altitude being a term to describe the density of the air compared to a certain standard pressure and temperature (mostly).

quade -
The World's Most Boring Skydiver

[nicknitro71 0]

I'm with you on that one. If we are just talking about a sealed pot filled with air then the following is always true:

As you increase the temperature, the volume stays the same and the pressure increases.

Density and Pressure are not the same thing though.

When we are dealing with the atmosphere, dense air corresponds to a rapid change of pressure with height. The opposite is true for less dense air.

Also, not only elevation and temperature infuence air density. Think of a hot day at the north pole and a hot day at the equator both with equal pressure and equal temperature. Does the air have the same density? Proplably not due to different layers of upper air with different densities. Remember that air pressure is the sum of all the layers above the point of mesurament. So you can have the same temp with the same pressure but different densities.

To finish when it comes to canopy flying all you are ultimately interested is air density, i.e. the denser the more lift you have.

Memento Audere Semper

903

[PhillyKev 0]

Yeah...I was just reading the chart wrong. I thought it was trying to show that as temperature decreases the result is as if you had increased altitude, which would mean the air gets less dense as temperature decreases. Which is obviously not true but at first glance I thought that's what it was trying to convey.

[bmcd308 0]

>>Under those conditions (the ISA standard conditions) at 1000 feet above sea level you would expect to have a temperature of 55 degrees F -- so on and so on.
<<

Well, delete my post then, before anyone else reads it!

BMcD...

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www.jumpelvis.com

[quade 4]

Unfortunately, only billvon and Sangiro can do that in this forum.

quade -
The World's Most Boring Skydiver

[bmcd308 0]

Immortalized as an idiot.

----------------------------------
www.jumpelvis.com

[JohnPolk 0]

hey quade..

i've never really understood this density altitude thing... when an awos report says that the density altitude is 3000ft, what does that mean exactly? i know how as density altitude increases it reduces the efficiency of an airplane (or parachute) but how do they compute the Density altitude?

[quade 4]

Quote

when an awos report says that the density altitude is 3000ft, what does that mean exactly?

It means that whatever elevation the reporting station is located at had the same air density that you would expect to find at 3000 feet under ISA conditions at the time the recording was made. In other words, your airplane (or canopy) will perform as if it were at 3000 feet at 48 degrees F (the ISA standard temp for that altitude) with an altimeter setting of 29.92 inches of mercury.
For the precise calculation you could look HERE, but that's very unwieldy and not very handy in an airplane or at the dz.
The rule of thumb graph and formula HERE, is about as simple as it gets and is so close to accurate that it'll get you through Private, Instrument and Commencial written & oral exams.
For highly critical applications, there are hand-held computers where you input all the data and it spits out a number and there are automatic electronic density-altitude calculators and readouts that you can install on airplanes.

quade -
The World's Most Boring Skydiver

[riddler 0]

Quote

Immortalized as an idiot.

Welcome to dropzone.com! We're all idiots. If you don't like someone, feel free to browse through their past posts looking for idiotic things they say. BTW - yours is not a stupid post at all - other will learn from it. There are plenty of drunken posts that are much worse. But you can only edit/delete your own for up to 6 hours after you post, so your drunken words and typos are also immortalized after you wake up with that nasty hangover.