| A | B |
|---|
| wave cyclones | midlatitude or extratropical cyclones |
| cyclonic shear | created from opposing air streams sliding by each other causing the air to spin. |
| polar front jet stream | a zone of faster moving air in the upper troposphere |
| midlatitude cyclone | large low pressure systems with diameters often exceeding 1000 kilometers (600 miles) that generally travel from west to east. |
| polar front theory | wave cyclones develop in conjunction with the polar front. |
| Norwegian cyclone model | wave cyclones develop in conjunction with the polar front. |
| Fronts | boundary surfaces that separate air masses of different densities, one usually warmer and more moist than the other. |
| overrunning | As one air mass moves into another, the warmer, less dense air mass is forced aloft |
| warm front | occurs when the surface (ground) position of a front moves so that warm air occupies territory formerly covered by cooler air, |
| cold front | where cold continental polar air actively advances into a region occupied by warm air |
| stationary front | occurs when the air flow on both sides of a front is neither toward the cold air mass nor toward the warm air mass |
| occluded front | develops when an active cold front overtakes a warm front and wedges the warm front upward |
| dryline | a boundary between dry, dense air and less dense humid air often associated with severe thunderstorms during the spring and summer |
| cold-type occluded front, | the air behind the cold front is colder than the cool air it is overtaking |
| warm-type occluded front | the air behind the advancing cold front is warmer than the cold air it overtakes. |
| speed divergence | speed variations within the jet stream cause air to converge in areas where the velocity slows, and to diverge where air is accelerating |
| directional divergence | the horizontal spreading of an air stream |
| vorticity | the amount of rotation exhibited by a mass of moving air) also contribute to divergence (or convergence) aloft |
| blocking highs | stagnant anticyclones that block the eastward migration of cyclones, keeping one section of the nation dry for a week or more while another region experiences one cyclonic storm after another. |
| cyclogenesis | cyclone formation |
