Why are there areas of both low pressure and high pressure in the United States?
147828
High and low pressure regions result from uneven heating of the surface and the movement of air masses.\n\nAreas of low pressure occur where air is being displaced upwards either due to convection or the movement of a front (usually a cold front). This usually results in unstable air, clouds, and rain. Wind circulates counter-clockwise around low-pressure centers in the northern hemisphere. \n\nAreas of high pressure occur where air is sinking - if you are in a high pressure zone where the air column is stable you experience clear skies.\n\nAlso there are regions around the United states that experience persistent highs or persistent lows - places where these pressure conditions are almost always present.
17085
High Pressure: "A mass of air that presses down strongly on the surface of the Earth because it is being cooled and is therefore more dense."  High pressure systems are characterized by fair weather, cooler temperatures (generally speaking -- though there are exceptions, see Santa Ana conditions in Southern California), and lower humidity.\n\nLow Pressure: "A mass of air that presses down only weakly on the surface [or rises from the surface] of the Earth because it is being warmed and [is] . . . less dense."  Low pressure systems are characterized by higher humidity, warmer temperatures, and more precipitation.
132266
Ignore harsha.  A 1 atm decrease in pressure would mean zero pressure at 100 meters above sea level, which is absurd.\n\nPressure decreases as you rise because the total mass of the atmosphere weighing down on you decreases.  High pressure means you need more energy to make water boil, so at sea level the boiling point will be higher than it will be on a mountain top.
429160
At a higher elevation the pressure on the water is less therefore it will boil at a lower temperature than it would at sea level.\n\nA liquids state of matter depends on pressure and temperature to make something from liquid to gas you use high temperature and low pressure.
335783
There are a lot of great sites that give basic weather predicting information.\n\nDifferences in air pressure are what causes the fronts--high and low pressure. High pressure areas generally have dry, good weather and areas of low pressure have precipitation.
216133
Well most (but not all) U.S. hurricanes are caused by heated water near Africa evaporating and rising quickly to form a localized storm cell. These occur in regular tropical waves, which is why hurricanes arrive at intervals. The initial storm cell is pushed west towards the U.S.A. by easterly winds and as it moves the storm can pick up more energy from warm water and combine with other unorganized storms, growing in intensity. It becomes a hurricane when it develops multiple cells and its windspeeds are over 74 miles an hour. It then can be pushed around by large air masses and other pressure zones and winds, usually it gets directed northward as it approaches the U.S. because of high pressure zones like the one around Bermuda (the Bermuda high). Depending on the exact location of these pressure zones and their intensities, the hurricane can be directed up early and miss the U.S., or up into Florida, or in the case of Katrina, into the Gulf of Mexico and then up between Mississippi and Louisiana. Once the hurricane comes onshore, it loses energy quickly as water starts to rain out and reverse the low pressure zones at the center and between the cells of the hurricane. It eventually dies but not before causing lots of wind and water (flood) damage.
144302
Hot air is lighter than cold air.\nWhen the ground is heated up by the sun, the air above it heats up too.\nThe warmer air near the ground gets pressed down on by the cold air above it.  When this happens, the air is essentially 'squeezed' out as the colder air moves into the area.\nAny time there is a difference between temperature of different layers of air, there is often movement between those layers to create a blance between them.  They cannot just switch places because one blocks the other, so they tend to move out of each other's way.\nIf you have enough of a difference in temperatures, you can end up with differences in pressure over an entire area.  When these pressure zones collide, they can result in heavy winds or even large storms.
595431
For the most part, yes. Low pressure systems in the Northern Hemisphere spin counterclockwise and Highs spin clockwise. The reverse is true in the Southern Hemisphere.
612443
coz it is part of USA
217992
Uneven heating of the Earth. This makes some places get hotter than others, which according to gas laws makes the pressure of places higher than others. Air moves from places of high pressure to places of low pressure to equilibrate.
543373
We have air around us. The atmosphere has high preasure and low preasure. When the high preasure area turns into low preasure, the air moves from high preasure area towards the low preasure. This movement of air from one area to another causes the wind. \n\nIt has nothing to do with clouds. The faster the low preasure area is formed, depends on whether you have clouds, or thunderstorms. Low preasure areas causes rain. This is why normally you have winds before the rain.
183470
Because wather instability happens whenever there is contrasting temperatures. In March, you have a cold air running up against hot air, so this causes there to be areas of low pressure.\n\nLow pressure = wind.
281316
1.  Because the rivers would flood due to no levees having been built to hold back water.  Being built on stilts allowed kept damage from flooding to a minimum, if at all.   2.  For airflow, as no air-conditioning existed and this was a way to create circulation for cooling in the hotter months.  Most houses built in the south also wer designed for maximum airflow and movement through the home.  For an ideal example, look at Thomas Jefferson's Monticello, designed and constructed to allow air to move easily through it for more comfortable summers.
831034
Because Texas is so geographically spread out, at any given time in the winter, you could have summerlike conditions in one part of the state and Arctic like conditions in another.  It really does depend on where in the state are you referring to.
1085318
There are a couple reasons.  First off, Texas is the second-largest state in the US, next to Alaska.  So, there is more area in the state for tornadoes to occur.\n\nSecondly, Texas is right in the heart of Tornado Alley.  This is a place in the continental US where the airflow is just right for supercell thunderstorms (tornado-causing storms) to form, with warm, moist air rolling off of the Gulf of Mexico, dry air coming from the Mojave Desert, and cool air flowing off of the Rockies.  The combination of these air masses provide a lot of atmospheric instability, prompting thunderstorms.\n\nDue to the winds around Texas/Oklahoma, the storms are more likely to be supercells, and more likely to form tornadoes.\n\nTexas might have the most number of tornadoes in a given year, but Oklahoma actually has the most tornadoes per unit area in the US because of the wind patterns being even better for tornado formation there.
11130
Wow, we are sitting at the bottom of a sort of "ocean of air".\n\nHere is one simple source for the newspaper ruler air pressure demonstration.\n\nJust search yahoo for "air pressure rule newspaper" for others.
605725
If you think of pressure as being the weight of a column of air at sea level, lets just call that 760 for example, then the column would be shorter and not as heavy if you were on the top of a mountain and therefore would have a lower pressure, say 500 and out in space it would be almost 0.  In the reverse if the column were going below sea level, it would be even heavier and therefore the pressure would be greater than 760. The deeper you go the heavier or greater would be the pressure.