Winds and Air M asses

1. Winds and Air Masses Unit 4 – Atmospheric Processes

2. Winds… • Earth’s atmospheric circulation is an important transfer mechanism for both energy and mass • The imbalance between equatorial energy surpluses and polar energy deficits is partly resolved • Wind – the horizontal motion of air across Earth’s surface • Differences in air pressure (density) between one location and another produce wind • Names for the direction from which they originate

3. Driving Forces within the Atmosphere • Gravitational force • Pressure gradient force • Coriolis force • Frictional force

4. Earth’s Gravitational Force • Earth’s gravitational force on the atmosphere is practically uniform, equally compressing the atmosphere near the ground worldwide • Density decreases as altitude increases

5. Pressure Gradient Force • High- and low-pressure areas exist in the atmosphere because of uneven heating of the Earth’s surface • A pressure gradient is the difference in atmospheric pressure between areas of higher pressure and lower pressure • A steep gradient causes faster air movement from a high-pressure area to a low-pressure area. • High and low pressure areas are caused by differences in surface heating and cooling. • Land versus water. • Different albedos and energy absorbing abilities of land, vegetation and human built surfaces

7. Isobars • Isobars are lines of equal atmospheric pressure drawn on a map. • Widely spaced isobars display a gradual pressure gradient. • Gradual pressure gradients imply light winds. • Closely spaced isobars display a steep pressure gradient. • Steep pressure gradients imply strong winds.

8. Coriolis Force • Deflects from a straight path any object that flies or flows across Earth’s surface • Wind, airplane, ocean currents • Deflection to the right in the Northern Hemisphere; to the left in Southern Hemisphere • The strength of the Coriolis force strengthens with an increase in latitude.

10. Friction Force • Drags on the wind as it moves across surfaces; up to 500m • Varies with surface texture, wind speed, time or day and year, and atmospheric conditions • Since surface friction decreases wind speed, it reduces the effect of the Coriolis force • Anticyclone • Winds in the Northern Hemisphere spiral out from a high-pressure area clockwise • Cyclone • Spiral into a low-pressure area counterclockwise • In the Southern Hemisphere these circulation patterns are reversed

12. When the “forces” combine… • When the gravitational force, pressure gradient force, Coriolis force and frictional force combine we see the development of ‘predictable’ global wind patterns.

13. Air Masses • Air masses are bodies of air that develop over large areas of the Earth’s surface • Two conditions needed: • Air must stay over the source region for a long period of time so that it can take on the characteristics of that region • Air must be stable • Usually form where air is descending back to Earth • These high-pressure cells line up roughly over the polar circles and the Tropics

14. Your task • Look at the defining characteristics of the 6 major air masses. • This will be done by completing the “Characteristics of Air Masses” chart • You will then look at cloud formation along fronts • Complete the “cloud development in fronts” diagram and associated questions on handout • Additional questions: • Explain a mid-latitude storm. • Why are storms more common in the mid-latitudes than elsewhere? • Explain the differences between continental and maritime air masses. • Explain the role of convections currents and the rotation of the Earth in the development of a storm. • Explain how studying jet streams could help long-range weather forecasting. • All info is found in Chapter 16, pages 262-268