Low and high pressure help

[Driver 170]

Hi, i'm studying the oxford aviation PPL books. I'm not wanting to fly or sit any exams, i have my own 737 800 pmdg setup for FSX also the FCOM, FCTM. I would like to start from the basics, so i got myself books on amazon from oxford aviation.

I can't seem to figure out pressure systems and warm/cold fronts! I'll try and explain below...

A warm front is in conjuction with low pressure as the air is less dense and rising?

Cold front is a high pressure system its more dense and air is sinking.


Now this is where i get confused, why on a hot/warm day there is high pressure as the isobars expand and low pressure its cold and dense and the isobars are closer together?

Can someone please clean this understanding up for me.

Thanks, vernon.

[darkroomsource]

Hot air rises.
Cold air descends as a result of the hot air rising (something has to come in to fill the void)

Secondly. As a gas heats up, it expands, so the same mass takes up more space. Therefore a volume of hot air "weighs" less than the same volume of cold air. Since the air is in somewhat of a column, the "weight" of the air pressing on the air at the bottom of the column is more in cold air than it is in warm air.

[Heston]

To be honest you are confused in your opening post earlier than you think. Low pressure areas (depressions) give rise to both hot and cold fronts. High pressure areas (usually) don't have fronts.
Fronts are just divisions between warm and cold air. Whether they are called warm or cold fronts depends on their movement and whether an observer on the ground experiences a rise or fall in temperature as they pass.
Bear in mind that a lot of the words we use to name things in the weather are based on poorer understanding than we now have - names given to things by Victorian gentlemen enthusiasts who had no satellite pics or computer modelling. The person (this guy Jacob Bjerknes - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Jacob_Bjerknes) ) who first called the division between airmasses a "front" did so because he thought it reminded him of the armies facing each other across the "front" of the trenches in France in the First World War.


Its too complex to explain properly without diagrams, so best not to try. If you want to explore meteorology further, and its a fascinating subject because even the professionals don't understand as much as they'd like to, I think you should also read this
Meteorology and Flight: A Pilot's Guide to Weather: Amazon.co.uk: Tom Bradbury: Books

[RatherBeFlying]

Heston has got it right.

I would add that low centres tend to follow the jet stream; also that lows manufacture most of the weather that makes difficulties for pilots.

Following animated satellite IR imagery and matching it to reports and forecasts will help you learn how weather works in real time.

Radar shows you precip - very useful for any outdoor pursuit. The computer in my old tennis club had a tab for radar ;)

[Driver 170]

Thanks guys for taking the time to write back!

Heston i just ordered that book from amazon.

It can all be confusing where it says in books in warm air the pressure is high and the isobars expand, but i've also read high pressure is when the air is descending from above so its cold air sinking!

[Brooklands]

It can all be confusing where it says in books in warm air the pressure is high and the isobars expand, but i've also read high pressure is when the air
is descending from above so its cold air sinking!

Driver 170, yes, but as the air descends it warms up as explained here (http://www.ecn.ac.uk/what-we-do/education/tutorials-weather-climate/tutorial-welcome/anticyclones-and-depressions/anticyclones).

Have a look at Simon Keeling's weather web (http://www.weatherweb.net/wxwebtv2.php) site - there's lots of interesting stuff there

Brooklands

[victor tango]

Driver 170
After youve read the excellent book suggestions from the other chaps may I suggest you look at synoptic charts from the Met Office. Over a 3-4 day period they map the areas of Hi and low pressures and associated forming of frontal systems?

[Driver 170]

Hi guys slowly understanding it now. I just got tom bradburys book today from amazon.

I picture high pressure like mountains and low pressure like valleys. Gives me a 3D picture of the systems :)

[Whopity]

Another good book, aimed at Microlight Pilots but written by a former Met Observer who flew during the war"Pilots Weather" written in an understandable manner.

[Driver 170]

Can anyone explain why the contour lines are straight in a warm front and curves at a cold front around a polar front depression?

So does low pressure systems always form at polar front depressions?

[Whopity]

If you look carefully, they are not straight, but curved slightly in the opposite direction. They are part of a different air mass that is being overtaken by the cold air and forced up. Low pressure is more defined at the centre than high pressure giving rise to different curvature.

Frontal depressions are only one cause of low pressure; others include Polar air depressions, Tropical revolving storms and Oragraphic lows.

[Driver 170]

Ahh right i see, thanks for that its all coming together slowly. I was looking at the ATPL videos on youtube regarding this subject and in upper winds video, they say low pressure systems are associated with cold air and high with warm air? Is this only in the upper atmosphere?