The Power of Air-Pressure
Air pressure changes with weather patterns. For instance, air pressure around storms is relatively higher than the air pressure within storms. And, differences in air pressure causes wind to blow from areas of high pressure to areas of low pressure. Using a barometer, you can observe changes in air pressure to get forecasting clues. To better understand what initiates a change in weather patterns, it is helpful to grasp certain concepts such as air density, pressure, and the affects of high and low pressure.

Air Density
Air is made up of molecules that move at high speeds and collide into each other and surrounding objects. Temperature and the speed of molecules have a "positive" relationship: when temperature rises, molecules move at a faster speed. When moving at faster speeds, the molecules push harder against their surroundings. This expands the area of air, giving that volume of air less density. Just the opposite happens with lower temperatures of air. With colder temperatures, molecules move at slower speeds. That puts less force on the surroundings and the volume of air remains with higher density. Air density is dependent on temperature, water vapor, and pressure.

Pressure
Pressure also has an effect on density. When more pressure is applied to a volume of air, that volume of air will have higher density. For example, air density is lower at higher altitudes, where the air pressure is low. High and low pressures at the earth's surface level affect weather conditions. These readings can be recognized as "H" and "L" in weather forecasting.

High-pressure symbols indicate that the specified area of air has higher air pressure relative to its surrounding air. Low-pressure symbols indicate low air pressure relative to its surrounding air. Meteorologists do not have exact numbers to signify the difference between high and low pressures, so measurements are always based on relative differences. The National Weather Service reports air pressure in two ways: surface air is measured in inches of mercury; higher elevations are measured in millibars, known as hectopascals (hPa). Hectopascals are a direct measure of pressure, such as pounds per square inch, and are what scientists generally refer to.

Clouds
When air is descending due to high pressure, it warms up and inhibits the formation of clouds. This is why high-pressure air is generally correlated with good weather. Air that descends in high-pressure areas will rise again when it reaches an area with low surface pressure. As air rises, it cools and water vapor in the air condenses to water droplets, resulting in clouds. These clouds may then lead to rain or snow, which give reason to why low pressure is correlated with bad weather.

Winds
The difference between high and low air pressure produces wind. A force is sent from high to low air pressure, which becomes wind. The larger the gap between high and low pressure, the more indication there will be for stronger winds. The distance between the areas of high and low pressure determines how fast the wind is accelerating.