17-1    Air mass:

1. Refers to huge body of air that has similar temperature and moisture. Small scale nonuniformity exists.

2. Source region: when the air mass acquire its characteristics.

3. Classification based on source region:

   • Polar (P)

   • Tropical (T)

4. Classification based on surface feature:

   • Continental (c)

   • Maritime (m)

5. Combination of 3 and 4: CP, CT, mP, mT.

6. Where they are? Fig. 17.3.

7. Weather associated with 

   • cP: not with heavy precipitation, cold and dry.

   • mT: with precipitation, warm, moisture, unstable.

   • cT: southwest, hot and dry.

17-2    Fronts:

1. Fronts: boundaries separate different air masses.

   • relatively narrow band.

   • mixing occurs along fronts.

2. Warm front: warm air occupies where cold air occupied before. Thus, warm invades.

   • Use w to represent. w on the cold air side.

   • slope is gentle, rising.

   • represented by cirrus cloud ==> cirrostratus ==> altostratus ==> nimbostratus. Fig. 17.5.

   • rate of advancing: slow.

   • light to moderate precipitation.

   • temperature rises.

   • moisture rises.

3. Cold front: cold air advances to where warm air occupied before. Cold air invades. Fig. 17.7.

   • Use 4to represent. 4on the warm side.

   • Slope steep, subsiding.

   • rate of advancing fast.

   • temperature decreases.

   • moisture decreases.

4. Stationary front: no movement of either air masses. Use both 4and w to represent. w on the cold side and 4on the warm side.

5. Occluded front: when cold front meets warm front: Fig. 17.8. Use both 4and w to represent. w and 4on the same side.

17-3    Middle-Latitude cyclone:

1. Middle-latitude cyclones. 

   • larger centers with low pressure.

   • counterclockwise circulation.

   • moving west to east.

   • cold front and warm front from center. Fig 17.10.

2. Life cycle:

   • Easterly cold air and westerly warm air meet. Their relative movement is counterclockwise. Fig. 17.9A.

   • Front not strait but wave ==> warm air move north and cold air south. Warm front and cold front forms. Fig. 17.9C.

   • Cold front moves faster. As cold front meets warm front ==> occlusion forms. == cyclone mature.

   • warm air aloft, cold air lower elvel. Both counterclockwise flow ==> cyclone dissipates.

17-4    Weather associated with a cyclone:

Use Fig 17.10 as an example. Along section A-E:

1. High cloud.

2. Low cloud, precipitation, T rises. why? warm front.

3. Tropical air, warm, wind direction: SW.

4. Cold front, wind NW, dark cloud, P increases, T decreases.

5. High P, clear sky.

Around L in Fig. 17.10 wind is inward. Unless other outlet exists, the inward move won't persist. Therefore, aloft provide a way for constant air filling into low pressure center.

17-5    Thunderstorm:

1. Occurrence:

   • associated with cumulonimbus clouds.

   • occurred in tropical region often, warm air.

   • often form along or ahead of cold front.

2. Stages:

   • supply of moisture. When heated up, air rise ==> cumulus stage.

   • when size of rain drops too large to experience uplift, precipitation begins ==> mature stage.

   • downdraft dominates: air cool ==> dissipating stage.

17-6    Tornadoes:

• Often associated with severe thunderstorms.

• Often occur in late spring when air temperature contrast bigger.

• Air pressure lower in the center.

• Classified on Fujita (F-scale) based on degree of damage. Most are F0 -F1 low damage tornadoes.

• Often occur in great plain where cP meets mT.

17-7    Hurricanes:

• Sometimes called typhoons or cyclones.

• have to have minimum speed: 119 km/h rotary circulation.

• pressure low in center, thus inward wind direction.

• near center wind upward to form cumulonimbus tower ==> eye wall.

• at center, no precipitation, light wind ==> eye.

• initial: large volume of warm air normal on ocean. Typically late summer when water temperature is high.

• release of latent heat cause upward movement.

• scale set 1-5 based on wind speed and central pressure.

• evolution from tropical depression to tropical storm to hurricane.

• damage due to wind, surge, and flood.

Homework:

• Read chapter summary on p.487.

• Use your own word to explain the key terms on page 488.

• Answer the review questions on page 488.