The land-sea distribution within the geo-graphic tropics is illustrated in fig. 9-1-1. Note that most of this 47-degree stretch of latitude is oceanic, and that many islands and large portions of some continents are also found here.

Topography and location play the significant roles in weather differences experienced in this belt, but migratory weather disturbances also contribute to daily and seasonal changes in the weather. We will

The tropics are the source region for equatorial and tropical air. Both of these air masses are warm, and the interaction between these air masses is minimal. Since locating frontal boundaries between these air masses is almost impossible, I will not differentiate between them in this unit. Any reference to tropical air will pertain to both tropical and equatorial air unless specifically stated otherwise.

The midlatitudes are the primary battle-grounds for tropical and polar air, but polar air masses do invade the tropics. As polar air pushes through the tropics, temperature contrasts between the polar and tropical air rapidly disappear above the surface layer. At the surface, the passage of polar fronts causes changes in the temperature, as well as other weather elements. The changes are not great, and frontal contrast continues to diminish as the polar air modifies. The trailing edge of strong polar fronts are often reclassified as SHEAR LINES, because low-level cyclonic wind shear becomes the significant feature separating the polar and tropical air. Shear lines are usually accompanied by lines of convective activity. At times, the cyclonic wind shear and convective activity are your only clues to the continued progress of polar air into the tropics. Tracking these fronts and shear lines is made somewhat easier by satellite imagery, because lines of convective activity occur along the boundary, and the convective buildups are readily apparent in the pictures. Although not a frequent occurrence, a strong polar outbreak does occasionally push from the hemisphere experienc-ing winter, across the equator, into the hemisphere experiencing summer.