Mid-semester Journal Check

1. Mid-semester Journal Check • Turn in journal for mid-semester check: • Last Thursday (Oct. 29): Last names starting with A-G • This Thursday (Nov. 5): Last names starting with N-Z • Next Thursday (Nov. 12): Last names starting with H-M • Feedback: • “✓” means OK (generally on track, or strong aspects partly compensate for weaker aspects) • “−” means not up to standard (quantity, quality, and/or focus of entries not addressing the assignment) • “+” means very strong (well exceeds minimum asked for in number and/or quality of entries)

2. Some Things We’ve Learned • On horizontal surfaces (such as at sea level), pressure varies from place to place. • Maps with isobars drawn on them help us visualize the spatial pressure pattern. • Pressure differences between places create a net force—the pressure-gradient (PG) force--on air, pushing toward lower pressure. The PG force pushes air into motion. • The strength of the PG force is greater where the pressure gradient (PG) is larger. • On a weather map, on a weather map, the spacing of isobars allows us to tell about the relative size of the PG and hence the PG force. • Once air is moving, the rotation of the earth affects that motion by apparently trying to deflect it. We “explain” this effect by inventing a Coriolis force. • The Coriolis force is larger when the wind is faster. • The Coriolis force pushes moving objects (including air) to its right in the N. Hem., left in the S. Hem., not at all at the equator. • The combination of PG force and Coriolis force tends to drive the wind close to geostrophic balance: the geostrophic wind. The wind is close to geostrophic aloft. • Geostrophic winds aloft tend to blow toward the east (or northeast or southeast) most of the time.

3. Some Things We’ve Learned (cont’d) • Near the earth’s surface, friction is a third important force (within the “friction layer”). • Friction opposes the wind, trying to slow it down. • Friction is larger over land than over water. (Land is “rougher” than water.) • The 3-way combination of PG force, Coriolis force, and friction drives winds across isobars at an angle. • As a result, surface winds tend to converge into low-pressure areas and diverge out of high-pressure areas. (We don’t see this aloft, though!) • As a result, air tends to move upward out of surface low-pressure areas and sink (subside) into surface high-pressurea areas. • Regardless of the combination of forces acting on air, winds tend to be faster where the PG (and hence PG force) is stronger.

4. Some Things We’ve Learned (cont’d) • Radiosondes are the main instrument for measuring the state of the atmosphere aloft. • Isobaric maps (upper air maps) show observations on a constant-pressure “surface”. A common isobaric map is the 500 mb map (about half way up in the troposphere). • The height of isobaric surfaces aloft vary from place to place. • Contours of constant height above sea level help us visualize the height pattern. • We can interpret height contours as if they were isobars. • Winds are faster where height contours are closer together.

5. Some New Things Learned • The areas aloft with the fastest winds tend to occur in the midlatitudes (30°-60° latitude). • Areas with fastest winds aloft form narrow belts around the midlatitudes. These are jet streams. • The fastest winds tend to occur where the PG is largest, which is where the isobaric surfaces slope the most steeply. • The height of isobaric surfaces aloft depends on the (average) temperature below the isobaric surface. • Colder air in the lower troposphere creates lower heights (lower pressure) aloft. • Warmer air in the lower troposphere creates higher heights (and higher pressure) aloft. • Globally, it’s colder at higher latitudes (farther from the equator, closer to the poles) • So heights (pressures) aloft are lower at high latitudes and higher at lower latitudes. • In the transition area between low and high latitudes (at midlatitudes), the temperature gradient is largest. • So the height gradient (pressure gradient) aloft is largest at midlatitudes. • So winds aloft are fastest at midlatitudes (and thus the jet streams).

6. Some New Things Learned (cont’d) • At midlatitudes, there are also east-west variations in temperature: “tongues” of colder air from higher latitudes “protruding” equatorward” alternate with tongues of warmer air protruding poleward. • So there are east-west variations in heights (pressure) aloft. • These features make height contours wavy. The wavy features we call troughs (lower heights/pressure) and ridges (higher heights/pressure). • The (geostrophic) winds follow these contours; jet stream has waves in it, too.