Weather: A study in variables

1. Weather: A study in variables Pressure

2. Variable Review: Atmosphere - Structure • Consists of four major layers (top to bottom) • Thermosphere • Mesosphere • Stratosphere • Troposphere - where we live & weather happens • Mnemonic: Too Many Stupid Tests • As you move upward from the surface, • Pressure decreases in each layer • Temperature “zigzags” ( as you go up)

3. Heat transfer in atmosphere • Only 3 options: • Conduction • Convection • Radiation • Heat flow is ALWAYS from high energy to low energy • Heat energy measured with temperature

4. Temperature affects air pressure Changing variables has effects!

5. Pressure Air pressure(book definition): the amount of force per unit area exerted by air. • Caused by gravity. • A measure of the force/weight of moving air molecules. • Measured with a barometer. • Milimeters of mercury (mmHg) • Milibars (mb) • One atmosphere at sea level = 14.7 psi

6. How does 14.7 psi feel? Two Volunteers? (must be prepared to summarize experience/observations)

7. Pressure • Gravity affects air pressure: • Keeps the atmosphere here! It causes air pressure! (Smaller planets/moons don’t have atmospheres due to LACK of gravity.) • Altitude affects air pressure: • Air pressure is affected by depth. The deeper we are in air, the more pressure there is. • Higher altitudes are like being in the “shallow end”.

8. Pressure • The deeper we are in a fluid, the more pressure there is! • There are more molecules on top of you!

9. Pressure: High/Low • High pressure = • there are more molecules bumping into a surface. • There are more molecules stacked on top of an area. • Low pressure = • There are less molecules bumping into a surface. • There are less molecules stacked on top of an area.

10. How does temperature and water affect air pressure? Temperature Hypothesis: If temperature increases, then air pressure will _______________. Water Hypothesis: If water increases, then air pressure will ____________________.

11. The effects of Water Vapor & Temperature • Temperature: • Higher temperature means less pressure. • Air is less dense and molecules are more spread out. • Lower temperature means more pressure. • Air is more dense and molecules are more compact. • Water vapor: • The more water vapor, the less pressure. • The less water vapor, the more pressure. • Water molecules weigh less(MW) than some gases.

12. Pressure and Temperature • Hot fluids rise, cold fluids sink. We know that! (helps explain convection) • Warm temperatures mean molecules spread out. • Molecules are moving more. (fast dance tempo) • Lowers the density. Less dense means it rises. • Rising molecules means less of them bumping or stacking on the surface of the earth. • Warmer temperatures mean lower pressure. • Colder temperature = molecules huddling together(moving less) = high pressure. (slow dance tempo)

13. Wait a minute. . . • If warm air means less pressure, then why do we get colder rainy days with a low pressure system? And. . . • Why does a high pressure system seem to go with clear warm days? • We will discuss these answers as we add more variables! 

14. Pressure and Wind • Nature abhors a vacuum! • Wind is a result of differences in air pressure. • Air will always move from areas of high pressure to areas of low pressure.(isobar map activity) • Isobars: lines connecting weather stations with equal pressure • Wind blows perpendicular to isobars. • The closer the isobars, the larger the pressure difference or pressure gradient. • The steeper the pressure gradient, the stronger/faster the winds in that area.

15. What causes differences in pressure? • Because the earth is heated unevenly, there will always be pressure differences. • So, as long as the sun will shine, the wind will blow. • Where on Earth, in general, is it the hottest? • Where on Earth, in general, is it the coldest?

16. Highest Pressure Highest Pressure Lowest Pressure Global Pressure Patterns

17. Global Wind Patterns