BELL RINGER Get a handout from the bookshelf. Label page 9 “Isobars and Air Pressure”. Then answer the following question: What are two tools we use to take weather measurements? Explain what they’re used for.

Air Pressure What type of gas primarily makes up our atmosphere?

Nitrogen makes up 78% of our atmosphere Oxygen only makes up 21% of it!

Isobars Isobars are lines connecting equal air pressure to each other Isobars when connected make loops that do NOT cross other loops Differ by 4 milibars *High Pressure = Good weather (ex. Sunny skies) *Low Pressure = Bad weather (ex. Thunderstorms)

Example of Isobars

Can you feel air pressure? When you take off or land in an aircraft, your ears may hurt or feel uncomfortable. This is because your eardrums can feel changes in air pressure as the aircraft moves quickly up and down. But what is air pressure?

What is Air Pressure? It is caused by the weight of all the air in the atmosphere pressing down on Earth. It is also known as atmospheric pressure. Air pressure changes with the height and also when air warms up or cools down. Changes in air pressure cause changes in the weather.

Differences in air pressure Low Pressure When air rises, it leaves behind an area of lower pressure, because the upward- moving air is not pressing down so hard on the surface.

High Pressure Areas of high pressure are formed where air is sinking back down, and so pushing down harder. >> see the diagram

Since there are many areas of high and low pressure above the Earth’s surface due to uneven surface heating. Air moves from high pressure to low pressure forming winds. As a result, the greater the difference between the high pressure and low pressure areas is, the higher the wind speed is.

Highs and Lows Pressure is different all over the world. Lows are areas of low pressure with the lowest pressure at the center. Lows usually bring wet, cloudy weather.

Highs are areas of high pressure with the highest pressure at the center. Highs bring sunnier, and dry weather. The way these move from day to day causes the changes in the weather.

Measuring Air Pressure Air Pressure is measured in millibars (mb) on a barometer. The simplest kind of barometer is a mercury barometer and pressure is measured in mm (in) of mercury. An aneroid barometer can measure move conveniently. A barograph is a special kind of barometer, which records continuous changes in air pressure.

Photos I A aneroid barometer

Photos II A barograph

BAROMTER