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The weight of air resting on a given area of the Earth's surface is known as air pressure. Air pressure (or atmospheric pressure) is always greatest at sea level, where the air is at its most dense. Therefore at the top of a mountain the air is less dense and therefore the pressure is lower.

The air is composed of billions of tiny particles that are constantly moving in all directions, bouncing off whatever they encounter. These collisions constitute what is known as air pressure. The more collisions occurring within a certain area then the greater the air pressure will be.

We are completely unaware of this, but the air is constantly exerting pressure on us, on average this is 14 pounds per square inch. (1 kg per sq cm ) . Air molecules are naturally drawn towards the earth by gravity, and as a consequence the density of the air is greater near the surface of the earth. Therefore the number of molecules in a given area, the air pressure, decreases with altitude. These molecules are in constant motion and this prevents them from settling at ground level.

At sea level, standard air pressure is 1013, but typically the pressure varies between 980 and 1040 millibars (mb). As with any aspect of the atmosphere there are extremes and the highest and lowest recorded pressures are as follows:

The highest recorded atmospheric pressure, 1085.7 mb, occurred at Tonsontsengel, Mongolia, 19 December 2001.

The lowest sea level air pressure ever recorded was 870 mb in the eye of Typhoon Tip over the Pacific Ocean on October 12th 1979

Air pressure is measured using a barometer. Although the changes are usually too slow to observe directly, air pressure is almost always changing.

Weather maps showing the pressure at the surface are drawn using millibars. Air pressure can tell us about what kind of weather to expect as well. Winds blow in an attempt to combat the differences in air pressure. Wind is the movement of air over the surface of the Earth, from areas of high pressure to low pressure. A large change in pressure over a relatively small distance, a large pressure gradient, can result in far stronger winds. When the isobars are tightly packed, locations within that large pressure gradient can expect windy conditions. As air rises and creates an area of low pressure, water vapour in the atmosphere will condense and form clouds. However sinking air, in an area of high pressure, means that no condensation will take place. This is why low pressure is associated with cloudy skies and unsettled conditions, and high pressure is associated with clearer skies and drier conditions.

Winds near the Earth's surface rotate anti clockwise toward the centre of areas of low pressure and clockwise outward from the centre of areas of high pressure in the Northern Hemisphere, with an opposite flow (clockwise around areas of low pressure and counter clockwise around areas of high pressure) occurring in the Southern Hemisphere. The main reason for this pattern is the Coriolis force, which results from the Earth's rotation on its axis and deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

So keep tapping your barometers and feel the weight of the air pressing in around you!

After a childhood interest in the Weather Mark Boardman went on to study Climatology at University and has continued his studies for the subsequent 20 years. He is considered an expert in his field.

All About The Weather

Unlike how it might sound, a popcorn science fair project can actually be quite interesting. One can learn a number of facts from the data that a popcorn science fair project can supply regarding the kinds of food that we consume in our day-to-day lives. The following is a very simple popcorn science fair project, which explores the reason as to why and how popcorn pops the way it does.

The materials required are quite simple and easily available at any store keeping laboratory supplies. The basic materials required are as follows:

* Unpopped popcorn kernels

* A clean and dry beaker

* A stand for mounting the beaker

* A Bunsen burner to heat the beaker

* A closed glass box with one small outlet for performing the experiment.

* A source of light for illuminating the background

The apparatus for the popcorn science fair project may be set up quite simply. The underlying principle is that the popcorn pops when the water vapor inside the kernels heats up and bursts the seed coat. The inner parts can then expand.

First, place around 50 kernels of popcorn inside the beaker and mount it at an angle to the vertical. Let the background near its mouth be illuminated by a light source. Place the burner underneath the beaker, keeping it at a distance of at least ten centimeters from the base of the beaker. Place the entire set-up inside the glass case.

The experiment for the popcorn science fair project can be initiated by lighting the Bunsen burner at a very low rate, letting the flame impinge lightly upon the base of the beaker. Illuminating the light source will show a steady stream of steam coming out of the beaker. Eventually, the popcorns will start popping and the stream of steam will become more prominent.

If the steam is allowed to impinge on a cold surface, water droplets will be formed immediately. All the components used in the popcorn science fair project (except the kernels) would be perfectly dry. This demonstrates that the steam originates from the water present inside the kernels.

This popcorn science fair project might sound simple, but precautions must be taken:

* The beaker should not be allowed to overheat, otherwise it might crack.

* The box should have at least one outlet for the air expanding inside it due to the heat.

* Adult supervision is recommended in the handling of burners and light bulbs.

Jordan Matthews is a High School Math and Science teacher who has worked as a judge and a coordinator of many science fairs. Check his Science Fair Project ideas website for some more ideas and information.