Here in the American west, we have a tremendous degree of climate variability, and those variables are some of the principle factors to take into account while you prepare.
Does this seem too basic? Well, consider that, every year, hundreds of people require rescue due to heat related injuries because they started an inland hike in the middle of the afternoon with no water. With more knowledge about climate, those people might have been able to prepare accordingly (or would have avoided altogether) hiking under those conditions.
Weather vs. Climate
One common mistake we commit is to equate climate and weather. To use an analogy, think about personality. It’s a collection of traits, tendencies, and temperaments that characterize a person and which remain consistent over time. That collection of traits reacts with external and internal influences to produce moods, which vary from day-to-day and sometimes moment-to-moment. Climate, therefore, is much like a region’s “personality,” which produces its “moods,” which we refer to as weather.
There exists a common misconception that California has a monochromatic climate where it’s always sunny and there are no seasons – an erroneous idea since California experiences seasons like anywhere else. Winters are cooler, with periodic storms originating in the eastern and northern Pacific basin. Springs are variable between cool and warm weather, with a mixture of stable weather patterns broken up by occasional periods of storminess. Summers are warm with almost no measurable precipitation. And autumns are often warm and dry alternating with periods of cool and wet weather.
California’s geography ensures that these general patterns get expressed in very different ways. The ocean’s impact is profound, as it has a moderating influence on climate. Thanks to the California Current, our coastal waters are generally quite cool, and the presence of a major body of cool water keeps coastal air temperatures very close to water temperatures most of the time. The further away from the ocean you travel, the more the ocean’s influence wanes, which means that temperatures frequently increase the further inland you travel. While it’s 70 degrees in Santa Monica, it may be 120 in Palm Springs.
California’s mountain ranges exert dynamic impacts on air masses through a variety of physical principles. For every 1,000 feet of elevation gained, air temperatures decrease by 3 to 5 degrees Fahrenheit; this process also works in reverse, as we’ll discover with Santa Ana winds. If the sea level temperature is 80 degrees, the temperature at 10,000′ may be anywhere from 50 degrees to 30 degrees. This cooling trend influences maximum vapor capacity, which refers to air’s ability to hold water vapor. Warmer air can hold more water vapor, and colder air holds less. When air masses rise and cool, they lose their capacity to store water vapor, causing the vapor to condense and possibly fall as rain.
Mountains also form a physical barrier for incoming storms that travel from west to east. Storms stall out on higher elevation, dropping most of their precipitation thanks to lower maximum vapor capacities. Vastly diminished storms might make it past the mountains, but they have little rain left, producing dry climates on the eastern sides of mountain ranges. This is clearly evident if you visit the Sierra Nevada on the west and east sides. At 6,000′, you might find sequoia trees on the west side. On the east side at the same elevation, you’ll find sagebrush and pinyons. This phenomenon, called “rain shadowing,” explains why we have deserts.
Finally, because temperatures drop as you gain elevation, much of the precipitation that falls in the mountains comes down as snow, especially above 5,000 feet.
Quirks in the Climate
During the summer, a quasi-stationary high pressure cell over the Four Corner states shunts periodic incursions of tropical moisture originating from the Gulf of California westward across Arizona into California. As hot air rises from the Earth’s surface, it cools, causing condensation. That condensation then builds up as the atmosphere becomes more and more unstable thanks to the mixture of hot air rising and cold air falling. The instability can produce thunderstorms, which are most common over the deserts and mountains. Thunderstorms produce lightning, a common hazard in high altitude travel.
During the fall, high pressure systems over Nevada can push warm, dry desert air east and southeast. That air rises up and over the mountains, and when it drops down the west side of the mountains, it heats up in a process known as katabatic (Greek for “flowing downhill) heating. This process works in the inverse of how air masses cool as you gain elevation, and by the time the winds reach coastal cities, they are hot, dry, fast, and ready to whip wildfires into a frenzy. In SoCal, we call them Santa Ana winds. In NorCal, they’re often called Diablo winds.
Now that you’ve absorbed all of that information, you can begin applying it to hiking. We know that temperatures stay fairly consistent on the coast, but that temperatures increase the further inland you travel. We know that elevation gain also leads to colder temperatures, and it also produces increased precipitation that often falls as snow. We also know that mountains wring out a lot of moisture, so much so that they create deserts through rain shadowing.
So, let’s say it’s July, and you want to take a hiking trip. With the knowledge you picked up here, you’ll know that anywhere near the coast is likely to be comfortable year round. Also, the mountains will be much warmer than they were in January. However, anywhere more than 10-15 miles inland will be uncomfortably hot unless it’s over 7-8k feet of elevation, with the deserts being particularly hot. Conversely, if it’s January, you know that the deserts may be comfortable, while the mountains will be blanketed with snow.
With that in mind, here are the best seasons for hiking for each respective region of California:
The Coast: The coast from north to south is generally comfortable for hiking year round, although Santa Ana winds can produce triple-digit temperatures along the coast from September to November.
Inland Valleys and Foothills (500′ to 5,000′): This region is generally at its most comfortable from late October to late May, with a lot of variation during early fall and late spring. Winter time can mean heavy rainfall, but most days will be cool and clear.
Mountains (4,000′ to 14,500′): This region experiences the greatest variability given the profound influences of elevation on temperatures and rainfall. However, you can count on mid-spring to early fall for the best conditions for hiking below 7,000′. Above that elevation, the optimal conditions window is much smaller, with early summer to early spring being optimal.
Deserts: (-282′ to 7,000′): This regions gets extremely hot during the summer and is only really safe for hiking from late October to early May. At the lower elevations, that window is even shorter, lasting from mid-November to late March.
Recommended Reading: This is the first but certainly not the last time that I’ll make this recommendation, but Allan Schoenherr’s Natural History of California is a fantastic book that breaks down climate, geology, geography, flora, and fauna across all of California’s regions.