Hi.

 

Don't think of low or high pressure as anything other than where air is moving into or out of. As the air warms, it expands and its pressure increases. On the other hand, a large mass of air squashing another part of the atmosphere will also increase its pressure.

 

Air will naturally flow out of those places. Wherever it starts from, unless it's flowing directly west or directly east, the Coriolis effect will cause it to turn away from its original path. The effect is caused by the fact that different points on the planet's surface have different distances to travel in 24 hours as the earth rotates - a point on the equator has a much bigger circle to travel around than does somewhere near to either pole. That means stuff nearer to the equator is moving faster than stuff nearer to the poles.

 

The equator is warmer than the poles. Warm air at the equator expands and rises and cold air at the poles contracts and sinks so there's a sort of conveyor system that sends warm air up from the equatorial surface, across to the poles, where it sinks and is sucked back to the equator by the space caused by the original equatorial air's departure.

The conveyor system is called a Hadley Cell.

 

Now, think about the shape of the planet. It has a big East-West circle around the equator and a weenie little on around either pole. Anything that is turning with the planet (palm trees, houses, equatorial air, foxes...) goes once around its own east-west circle in 24 hours. Things at the equator have a much greater DISTANCE to travel in 24 hours than things closer to the equator. That means they're going much faster.

 

The planet is rotating from west to east (that's why the sun rises over there) so, going back to the Hadley Cell... all that air doing grillions of miles per hour west to east when rising from the equator will continue to do grillions of miles per hour when it heads for the poles. Ground closer to the poles isn't going so fast so the air in the Hadley Cell overtakes it.

 

As the air moves towards the poles it loses energy, some goes into absorbing moisture from the sea, some goes into the surrounding atmosphere (that cell does have to shoulder air out of its way) but the important thing is that it does slow down as it travels. It has to keep moving towards the poles because the whole Hadley Cell is driving it around so the only component of its speed it can lose is in west-east direction.

 

That means the general trend in a Cell is for air to curve westward relative to the general flow. In the northern hemisphere Hadley cells curve anti-clockwise and give rise to low-pressure systems (into which the air is flowing) that turn anti-clockwise. In the southern hemisphere, the Cells flow northwest to southeast and curve westwards so air flowing into a lower pressure part of the atmosphere turns clockwise.

 

Cells themselves don't usually stretch all the way from the equator to the pole - there's a string of four or five each covering different latitudes.

 

The picture below isn't the best illustration but you can at least see the diagonal flow, and the blobby cloud around the equator indicates some big thermals where air is rising ready to start its journey towards the poles.

 

Cheers,

D

 

Thanks but i think i'll skip this chapter i kinda get the adjust of it

Understanding that the wind will turn in a certain direction is what will be most helpful to you.

 

What is really important is to be able to visualize what the winds are doing when you open up a prog chart. Take a moment to study the fronts, the isobars and the pressure differential. Envision where the winds are going and how strong they will be.

 

Then look at a wind and temp chart and study the wind directions and speeds.

 

With time you will likely get a good feel for what to expect In terms of winds, visibility, types of clouds, thunderstorm risks.

 

Now as for the coriolis effect...

 

Air moves from high pressure to low pressure and it will take a direct path to get there. Picture a single particle of air moving from high pressure to low pressure as the tip of your pencil. The earth is the page underneath the top of your pencil.

 

If the earth stopped rotating you can draw a straight vertical line on the page between the two points. Now try to rotate the page at the same time while you move that pencil along that vertical path. You will trace a curved line instead. This is the path the air traces over the ground as it moves from high pressure to low pressure.

 

Hopefully this helps you understandwussy is happening. The air isn't actually "turning". It is the ground moving underneath the air mass that gives the appearance of the air turning. In reality the air and ground do meet and interact with each other, but this should be good enough to get started if you really want to know.

Thats brilliant i understand the PGF and the coriolis force act opposite of each other and the geostrophic wind is 90 degrees to the CF slowly getting my head around it.

 

BUT still can't picture why the wind turns opposite way in a low pressure and a high pressure?

 

Also why are isobars straight in a high pressure (WARM AIR) and in a low pressure COLD AIR always round?

Think i mean't cold and warm fronts

Thats brilliant i understand the PGF and the coriolis force act opposite of each other and the geostrophic wind is 90 degrees to the CF slowly getting my head around it.

 

BUT still can't picture why the wind turns opposite way in a low pressure and a high pressure?

 

Also why are isobars straight in a high pressure (WARM AIR) and in a low pressure COLD AIR always round?

Think i mean't cold and warm fronts

Again, in the northern hemisphere, if each parcel of air moving towards a low is turned by coriolis to the right, then the air around the low will circulate anti-clockwise. Draw this out on a piece of paper, a big L and four arrows pointed towards it from north, south, east, and west. The arrow of air moving west towards the low gets turned 90 degrees right towards north, while the parcel moving south into the low turns right towards west, the parcel moving east towards the low turns right towards the south and a piece moving north towards the low turns right towards the east. Connect the turned arrows and you have a counterclockwise flow.

 

The opposite happens with a high in the northern hemisphere. Each parcel of air moving outwards in each direction gets turned to the right. This forms a clockwise flow around the high.

Light bulb just came on lol

 

What about the isobars being straight in a warm front and in a cold front there round?

This link is a good source to answer all of your interesting questions ;-)

 

http://www.aos.wisc.edu/~aalopez/aos101/wk11.html

 

And once you're there, start getting back and see also:

 

http://www.aos.wisc.edu/~aalopez/aos101/wed1205.html

...the air around the low will circulate anti-clockwise.

Yes, cyclones go anti-clockwise, while anticyclones go clockwise 

It makes sense really since positive angular rotation is anti-clockwise. 

Hi.

 

You might all enjoy this (though sadly the author also mentions the B word...):

 

http://physicsfocus.org/colin-white-physics-myths/

 

Kevin, the Coriolis effect causes atmospheric flow from equator to pole to deviate eastwards, not clockwise or anti-clockwise. It makes no difference whether the flow is from high pressure to low, or from low to high. The westward twist is caused by a loss of kinetic energy.

 

There's a lot of stuff in Google about Coriolis fallacies, especially concerning water draining out of a bath. Some of it is right, some wrong but a lot of it is quite funny. Try http://en.wikipedia.org/wiki/Coriolis_effect#Draining_in_bathtubs_and_toilets and the rest of that Wikipedia page.

 

Cheers,

D

Not flows move from equator to the pole. There are three zones of flows north of the equator and also south of it.

 

In the northern hemisphere, the flow from 30n degrees to the equator is southward. This is because the hot air at the equator is lower pressure than the air 30n of it. From 30n degrees to 60n degrees it is northward, and once again southward from 90n to 60n. The flow then becomes predominantly westerly or easterly at the boundaries of these zones because of the coriolis effect.

 

At the slightly smaller scale of high and low pressure systems in these large air masses, the exact same behavior occurs.

I agree with you mostly - I was referring to direction. See above:

 

Cells themselves don't usually stretch all the way from the equator to the pole - there's a string of four or five each covering different latitudes.

 

Did you notice the mistake in the Wikipedia entry? there's one in my first post too but it's too late to edit that...

 

D

Hi.

 

You might all enjoy this (though sadly the author also mentions the B word...):

 

http://physicsfocus.org/colin-white-physics-myths/

 

Kevin, the Coriolis effect causes atmospheric flow from equator to pole to deviate eastwards, not clockwise or anti-clockwise. It makes no difference whether the flow is from high pressure to low, or from low to high. The westward twist is caused by a loss of kinetic energy.

 

Cheers,

D

Coriolis causes any flow to twist to it's right, not just eastward. Air can also flow from the poles towards the equator, and the earth's rotation will affect that as well. The coriolis will twist that flow towards it's right, which would be westward (northern hemispherewise). The effect of air flowing into a low or away from a high being twisted rightwards causes the apparent clockwise or counterclockwise flows.

Getting a bit confused with pressure systems and warm/cold fronts!

 

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

 

Cold front is a high pressure system? More dense and air is sinking and cooling?

 

Can someone clean this up for me. Thanks!

Don't mix the two up. I'm looking at a current prog chart for the US and we have a number of lows over the center of the country right behind and in front of cold fronts with a couple of occluded and stationary fronts off to the north.

 

Fronts are air masses with mostly uniform properties of temperature and humidity.

 

BTW, the current prog is showing good examples of fronts rotating around those lows.

Yes but i'm reading, high pressure is where its warm and the isobars expand and in the cold isobars are closer together?

 

Then i read high pressure is when the air is descending and getting warmer as it sinks to the earth