# Weather Question

#### mightymike

##### New Member
I’m having a hard time with this concept. Air expands and becomes less dense when it is heated, it’s molecules spread apart. As air cools, the molecules become packed more closely together, making it denser and heavier than warm air. Ok, then why is air denser at the ground where it is warmer and less dense the higher you go when it is cooler? I must be missing something but I can’t figure out what it is. I study weather theory, it says one thing, and airplane performance (density alt) says another. Thanks for any help on this.

All of what you say is true, however the fact that the air aloft is less dense is more of an overwhelming factor. Colder air is more dense, but is negligible compared to pressure.

Thanks for the info. So is it that the pressure of the atmosphere is much greater near the ground and much less aloft and atmoshereic pressure has nothing to do with heat, but rather the weight of the atmosphere acting on the air? I may be getting this here.

Try and read up on Charles Law and Boyles law. Pressure and temperature react with each other.

Well, let me try not to butcher this too much...

air is very compressable, so this is why the pressure of the atmosphere at the surface is much higher then aloft. There IS a correlation between pressure and heat, though there are other variables involved that effect that, but generally speaking, heat from compression is why it's 60 degrees at sea level and -50 degrees at 40,000 feet.

Paul

Pressure. As you increase your altitude, pressure decreases. Think about it like this: if you boil a pot of water on the stove, the water will eventually evaporate (at sea leve l pressure and temp that is), but if you place sealed lid over the water, the water will boil at a much higher temperature. You see, air is just like the water. At lower altitudes, there is more air pressure presant, so it is like having a lid on the boiling water, and makes the air a bit harder to change its' state. But when you go up to higher altitudes, the pressure is less, so air can change state much more easily without the restrictions. Here is one more visual experiment that you can try. You can make water boil at room temperature. Put water into a glass jar, and rig up a fitting so that a vacuum attatchment can be used. Turn on the vacuum and watch what happens. Most people will tell you that the vacuum is pulling all of the water out of the jar, but when you look in the bag, it will be completley dry.

A variant of the famous ideal gas law (PV=nRT) is the relationship that pressure is proportional to density times temperature. That is, more or less, P = D * T.

You're right, as temperature goes up, density drops (volume increases), AT A FIXED PRESSURE.

As we go down in the atmosphere, the air does indeed get hotter (T goes up), which would make you think that density should go down (D goes down), which is the same thing as volume increasing.

BUT, as commented by another person, pressure goes up as we go down in the atmosphere as well. If P goes up, then D * T has to go up by a proportionate amount. Analysis of actual atmospheric conditions shows that for every 100 "units" of increase in P, D goes approximately up by 80 units, and T goes approximately up by 20 units. That is, Density goes up (volume goes down) by about four times as much as Temperature rises, per unit increase in Pressure. It's a good thing that density takes so much of the "brunt" of the increase in pressure... otherwise it would be a lot hotter at the Earth's surface, like well over boiling!

This too all confused me until I started classifying in my mind that volume and density are essentially the same thing (since volume is inverse to density, and vice versa), and that pressure is conceptually different than density. They're related by temperature, but that's the only relation. Just remember, P is proportional to D times T. Just a few more conceptual examples: If D goes up and T goes down by the same amount, P remains unchanged. If P goes up then either D or T has to increase. If P goes up and, for example, D decreases, than T has to increase by even more to "equal out" the increase in P.

Hope this helps! The figures for the "standard atmosphere" (pressure, density, and temperature for a given altitude) that you can use to play around with to help you understand it better conceptually (at least it helped me out anyway,) can be found at http://www.usatoday.com/weather/wstdatmo.htm . I'm happy to try to answer any further questions, or to elaborate further on this question, if anyone's interested. I'm trying to learn weather very well myself as I'm thinking about being a dispatcher someday in the future, so the better I can teach it, the better I understand it thoroughly myself!

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