Zone 8 makes up most of the valley floor in California’s Sacramento and San Joaquin Valleys. Only a shade of difference exists between Zone 8 and Zone 9, but it’s an important difference—crucial in some cases. Zone 9 is a thermal belt,meaning that cold air can flow from it to lower ground—and that lower ground is found here in Zone 8. Citrus furnish the most meaningful illustration. Lemons, oranges, and grapefruit, which flourish in Zone 9, cannot be grown commercially in Zone 8 because the winter nights are frequently cold enough to injure the fruit or the trees; the trees would need regular heating to deliver decent crops. The same winter cold can damage many garden plants. That cold often shows itself in winter, when cold air rolls off the Sierra Nevada and pools on the valley floor, condensing into thick tule fog. Zone 8 differs from Zone 14, which it joins near the latitudes of north Sacramento and Modesto, in that Zone 14 occasionally gets some marine influence. Low temperatures in Zone 8 over a 20-year period ranged from 29 to 13°F (–2 to –11°C). Certain features that Zones 8 and 9 share are described under Zone 9.