Aim: What is Pressure and what does it make? Do Now: Define Pressure. Give an example of pressure change from an experience you had.

I. Air Pressure - the pressure exerted by the weight of air (molecules)

II. Causes of Air Pressure -Differences in temperature cause differences in air pressure. -temperature changes the density of the air. -Water vapor changes the density. Warm Cold

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

-air pressure is measured by using a barometer. -As pressure increases the liquid in the barometer rises. -As pressure decreases the liquid lowers in the barometer. III. Measuring Air Pressure

** A decreasing barometer (air is rising and pressure is low) means the weather will be cloudy and wet. (water vapor is less dense) ** An increasing barometer (air is sinking and pressure is high) means the weather will be dry and sunny.

IV. Pressure gradient: - describes the difference in air pressure seen on a map. Drawn using isobars. - This difference creates wind. Air moves from high pressure areas to low pressure areas (H L).

- The movement of air due to differences in air pressure - Closely spaced isobars create a strong pressure gradient & high winds (steep gradient) - Widely spaced isobars create a weak pressure gradient & light winds (gentle gradient) V. Winds

Barometric Pressure Reading for April 10 th 2013 Barometric Pressure Reading for April 10 th 2013 (Wednesday) 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? High pressure = happy weather (rising barometer) Low pressure = lousy weather (falling barometer) 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.

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