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HeliTorque :: View topic - An idiot's guide to the semi-diurnal variation of pressure..
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HeliTorque Forum Index » Ground School

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An idiot's guide to the semi-diurnal variation of pressure..
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niho
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PostPosted: Mon Jul 20, 2009 6:18 pm    Post subject: An idiot's guide to the semi-diurnal variation of pressure.. Reply with quote

Have been having a read of meteo and the first problem I have come across is this semi-diurnal variation of pressure. What I don't get is why the maximum pressure occurs at about 10am and 10pm. The book says it is due to the daily heating and cooling effects of the sun but I don't understand what the heating has to do with the pressure at the earth's surface...

I've had a quick search of the internet but only turned up academic essays, if someone could give me a quick idiot's guide to this I would be very grateful!

Thanks,
Nick
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veeany
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PostPosted: Mon Jul 20, 2009 6:32 pm    Post subject: Reply with quote

In really simple terms

Air pressure at the surface is effectively the weight of the column of air above that point (divided by the area of the point but that doesn't matter in this instance as the size fo the point wont change)

Hot air rises, this reduces the downforce the air presses on the ground with at the point in question, so reducing its weight.

I could pick all kinds of holes in this, but I am trying to keep it simple.
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niho
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PostPosted: Mon Jul 20, 2009 6:39 pm    Post subject: Reply with quote

Ok, thats what I originally thought, but then I had read abit earlier about advection - surely that causes the air to stay roughly the same pressure as cool air is constantly being brought in to replace the hot air which has risen?
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niho
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PostPosted: Mon Jul 20, 2009 6:41 pm    Post subject: Reply with quote

Also, if pressure is linked to the temperature of the earth, wouldn't the pressure be diurnal like the temperature instead of semi-diurnal?

Thanks for your help!!
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PostPosted: Mon Jul 20, 2009 8:19 pm    Post subject: Reply with quote

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Also, if pressure is linked to the temperature of the earth, wouldn't the pressure be diurnal like the temperature instead of semi-diurnal?


Thinking about it yes it would, or so I reckon.

A quick look at some of my Met books, and a couple of the scientific papers on the net, they seem to imply that the atmosphere is heated from the top (contrary to popular teachings and the water vapour and ozone seem to be the most sensitive bits), and then the surface re radiation heats the atmosphere from the bottom, this appears to be what causes the twice daily cycle.

The other interesting (or not) bit is that at our (uk) latitudes the entire thing looks like it only makes about 1mb difference over the entire cycle.
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PostPosted: Tue Jul 21, 2009 6:28 pm    Post subject: Reply with quote

Ok, so I'm assuming its safe to just learn that its semi-diurnal and that the most pressure is at 10am and 10pm rather than fully understanding it?

Also (slightly OT) is the helicopter air law different to the FW one? There seems to be alot of information in the book that only seems to affect FW pilots... For example, would I need to know the section on runway characteristics (TORA's, TODA's, clearways and stopways) or stuff like runway threshold markings? Surely they're of little use to a heli pilot?
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PostPosted: Wed Jul 22, 2009 12:42 am    Post subject: Reply with quote

you'll be surprised how much you use, especially in regards to runway markings; getting it wrong could result in an accident so worth persevering.

At coventry the FATO is across the main runway and more often than not you'll be advised to cross behind or ahead of a departing or landing plank. understanding take off distances and roll on distances will allow you to make a more informed decision when timing your movements to be as safe as possible.
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