Physics 137 - Introduction to the Atmosphere and Weather                                                                        Homework Key - Assignment #12
Revised 11/02/2009

12-QT2. Explain how an anticyclone during autumn can bring record-breaking low temperatures and cP air to the southeastern states, and only several days later very high temperatures and mT air.

As an anticyclone moves into a region that region is usually on the eastern side of the anticyclone where winds are northerly and cold and often deliver cP air. Later, because of the common eastward shift of long wave patterns in the upper-air flow, the axis and finally the western side of the anticyclone move over the same region. When that happens that region is then exposed to the southerly, warm air flow along the western boundary of the anticyclone which may carry mT air into the region. If the anticyclone is especially large and strong, both the cold, northerly flow on the anticyclone's eastern edge and the warm, southerly flow on the anticyclone's western edge are especially strong and record-breaking temperatures can be set at both extremes by the same system.

12-QT5. When a very cold air mass covers half of the United States, a very warm air mass often covers the other half. Explain how this happens.

This happens when the upper-air flow pattern is meridional with a wavelength comparable to or somewhat longer than the distance across the United States with an air-mass transition about halfway across the country. Under these circumstances a warm mT air mass can dominate the weather on one side of the country while a cold cP or cA air mass dominates the other side.

12-QT9. Why does the same cold front typically produce more rain over Kentucky than over western Kansas?

Most of the precipitation produced by a cold front comes not directly from the cold air mass, but from the displaced warmer air which is lifted by the advancing wedge of cold air. Since the average absolute humidity is generally higher in Kentucky than in Kansas, more precipitation condenses and falls from the wetter displaced warm air in Kentucky than in Kansas.

12-QT10. You are in upstate New York and observe the wind shifting from East to South accompanied by a sudden rise in both the air temperature and dewpoint temperature. What type of front has passed?

The wind change, the temperature rise and the dew point rise are all consistent with the passage of a warm front. No other type of frontal passage is consistent with all of these changes.

12-PE1. Make a sketch of North America and show the upper-air wind-flow pattern that would produce: (a) very cold cA air moving into the far western states in winter, (b) cold cP air over the Central Plains in winter, (c) warm mT air over the Midwest in winter, (d) warm, moist mT air over southern California and Arizona during the summer.

See the sketch at the right.

12-PE2. You are presently taking a weather observation. The sky is full of wispy cirrus clouds estimated to be about 6km (20,000 ft) overhead. If a warm front is approaching from the south, about how far away is it (assuming a slope of 1:200)? If it is moving toward you at an average warm-front speed of about 10 knots, how long will it take before it passes your area?

If the assumed slope is correct then the distance of approaching front can be obtained from the direct proportion: distance/(6 km) = 200/1. Hence the distance is 1200 km. Let Dt represent the elapsed time until the front arrives.
Then Dt = 1200 km / (10 knots ×1.85 km/s/knot  = 65 hours.