Baja California displays a strong contrast of climatic conditions – both along and across the peninsula. There are three main factors that determine a location’s climate in Baja California.
Its latitude. The position in the subtropical belt and the relative influence of winter and summer storms. Winter rains associated with mid-latitude cyclones (e.g. cold fronts) decrease as one proceeds southward. Summer rains associated with tropical storms or afternoon thunderstorms increase in frequency as one proceeds south.
Its proximity to the Pacific Ocean. This determines whether daytime high temperatures are moderate of high, and whether the location will experience low stratus clouds and frequent fog. The importance of low clouds and their diurnal variation can be seen in this video (you must download and open in Quicktime for a full resolution view).
In the lee of the higher mountains of northern Baja California there is a strong rain shadow. During cool-season storms, moist southwesterly winds ascend and precipitate on the upwind/Pacific side of these mountains and descending air on the downwind/desert side results in cloud-free or at least much reduced rainfall. The “Lower Colorado Desert” appearing on most maps of the subdivisions of the Sonoran Desert is for the most part an especially dry desert associated with the lack of much winter rain and even less summer rain. The winter dryness (relative to the Pacific slopes of California and northern Baja California) is due to this rain shadow effect.
Since rainfall from winter storms diminishes southward in Baja California and the mountain height also diminishes on average the rain shadow effect is less apparent in southern Baja California.
Altitude and location relative to the major mountain masses. Although most of the easily accessible parts of the Baja California peninsula are of relatively low altitude (less than about 2000 ft), there are some areas, especially in northern Baja California, where one can drive to 5000 – 9000 ft. Here altitude does have a strong impact on temperature. However, it isn’t always what one might expect. During the summer months as one climbs from the Pacific coastal region towards the mountains one passes through a strong temperature inversion and the daytime temperatures at 5000-6000 ft elevation in the mountains are usually warmer than at the coast. It is certainly much drier. So, in Southern California and northern Baja California, to cool down in summer you head to the coast, not to the mountains.
Mean cloud cover
Cloud cover, especially low stratus, has a large impact on the underlying vegetation in Baja California. Clouds have multiple effects. In the daytime they reflect sunlight and reduce the surface temperature, When low clouds impinge on the underlying terrain (“fog”) the cloud droplets wet the surface. Areas where this is common favor the presence of lichens, an epiphytic bromeliad (Tillandsia recurvata) and a variety of plants that can best survive in such relatively cool and moist environments.
There are no really reliable maps of cloud cover over the peninsula, but satellite imagery can be used to show daytime cloudiness. In this section we will show some of these.
The Summer “monsoon”
The dramatic difference between meteorological conditions on the Pacific and Gulf sides of the Baja California peninsula is seen most clearly on summer days. An example can be seen here (download the loop for best resolution animation).