Aim: How does measuring air pressure tell us about the Weather? Do Now: Answer the following questions in your notebook. This a review from before the vacation. 1. What happens to the air molecules when heat is added? 2. How does adding heat to the air change the density? 3. How does adding heat to the air change the air pressure? 4. What would happen to the air molecules if heat were to be removed? 5. What substance can be added to air to make it less dense?

water vapor makes air less dense. I. Air Pressure - the pressure exerted by the weight of air (molecules) warm air = less dense cold air = more dense water vapor makes air less dense.

II. Temperature Changes Air Pressure Warm is less dense it exerts less pressure. Cold air is more dense and exerts more pressure. Warm Cold

A. Cold air - contracts (more dense) - sinks - creates more pressure B. Warm air - expands (less dense) - rises - creates less pressure

III. Measuring Air Pressure What instrument measures air pressure? As pressure increases the liquid rises in the barometer. As pressure decreases the liquid lowers in the barometer. What can be added to the air to make the air pressure decrease?

** A decreasing barometer (low pressure) means the weather will be cloudy and wet. ** An increasing barometer (high pressure) means the weather will be dry and sunny. WHY?

IV. Winds - Differences in air pressure creates wind. Air moves from high pressure areas to low pressure areas (H L).

Closely spaced isobars create high winds (steep gradient) V. Pressure gradient: - Drawn using isobars. (just like contour lines!) Closely spaced isobars create high winds (steep gradient) Widely spaced isobars create light winds (gentle gradient)

On which side of the country would the strongest winds exist? What do you look for to answer this question?

Closure: From the back table, take the worksheet on pressure and complete this working with your partner.

Barometric Pressure Reading for April 10th 2013 (Wednesday) High pressure = happy weather (rising barometer) Low pressure = lousy weather (falling barometer) Using the chart above, describe; 1) how the chart shows the weather was nice all during the day? 2) how the chart shows the weather was bad all during the night? 3) What changed in the air? 1) Pressure was rising from 7am to 1pm. 2) Pressure was falling from 5pm to 10pm. 3) More water vapor was entering the air, making the pressure decrease and causing lousy weather.