2.3: Precipitation

Precipitation Variation

While the three main climate zones provide a broad perspective of climate zones, regions within these climate zones may differ from each other based on other factors like precipitation. Regional variation in precipitation is a bit more complicated than that in temperature, which is largely controlled by latitude and elevation. Precipitation is influenced by numerous factors, each with a significant impact on the amount a location receives. The list below is not exhaustive, but includes the most important ones:

• Latitude: Latitude also influences weather patterns, but not in the same way it influences temperature. The Equator and 60°N &S typically experience low-pressure systems, which allow significant rainfall. Meanwhile, at 30°N & S and the Poles, high-pressure systems typically prevent significant rainfall. In fact, the world’s largest rainforests (Figure \(\PageIndex{1}\)) are located near the Equator, while the world’s driest deserts (Figure \(\PageIndex{2}\)) are located near 30°N & S.

Figure \(\PageIndex{1}\): A map of the world's rainforests. (Public Domain; Biome Rainforest via NASA) Alternative description of the image.

Figure \(\PageIndex{2}\): A map of the world's deserts. (Public Domain; Biome Desert via NASA) Alternative description of the image.

• Ocean Currents: Examine Figure \(\PageIndex{2}\) carefully, you will notice that the general rule that the world’s largest deserts straddle 30°N & S is generally true, but not always. The west coast of South America is home to the Atacama Desert, the world's driest place. The desert stretches well north of 30°S, almost to the equator. The reasoning for this has nothing to do with high or low pressure, but instead with the nearby ocean current. Figure \(\PageIndex{3}\) shows the location of the world’s major ocean currents. Warm ocean currents are shown in red and cold ocean currents are shown in blue. You'll notice that warm currents flow along the eastern coasts of continents, bringing warm tropical waters towards the poles. Cold water currents flow along the west coast of continents bring cold polar waters towards the tropics.

Figure \(\PageIndex{3}\): A map of global surface ocean currents. (Public Domain; Dr Michael Pidwirny via Wikimedia Commons) Alternative description of the image.

17. The Atacama Desert, on the West Coast of South America, is located next to a __________ ocean current. :
    1. warm
    2. cold
    3. neither warm nor cold

The average annual rainfall in Arica, Chile, is 0.76 to 1.8 mm (0.03 to 0.07 inches). On the other hand, the East Coast of South America, home to Rio de Janeiro, Brazil, lies next to the warm Brazil Current. The average annual rainfall in Rio De Janeiro, Brazil, is 1,100 to 1,300 mm (43 to 51 inches). Both cities have similar geographic features, such as coastal locations and similar latitudes and elevations, but very different rainfall patterns.

18. Compared to Arica, Chile, Rio de Janeiro, Brazil receives _________ rainfall.
    1. significantly less
    2. about the same
    3. significantly more
19. Considering what we learnt from Arica and Rio de Janeiro, cities near _________ ocean currents will experience ________ rainfall.
    1. cold; more
    2. cold; less

• Proximity to mountain ranges: If you live in Northern California, have you ever driven from Sacramento to Reno? If you have, you probably noticed how, during much of the drive, trees became taller and more lush, only to suddenly disappear into the desert by the time you arrived in Reno. The biggest reason for this, as you may have guessed, is the Sierra Nevada Mountain Range, which is between Sacramento and Reno. But why do mountains cause such a phenomenon? The reason is due to the Rain Shadow effect of the mountains (Figure \(\PageIndex{4}\)). As air traveling eastward encounters a mountain, it has to rise over it. When air rises to higher elevations, it cools, condenses, and rains out on the windward side of the mountain. Towns on the windward side of such mountains receive significant orographic rainfall. Once it crosses the peak, it travels down the leeward side of the mountain. When air descends to lower elevations, it heats and dries. Thus, towns on the leeward side of the mountains receive significantly less rainfall.

Figure \(\PageIndex{4}\): The Rain Shadow Effect. (CC BY 4.0; domdomegg via Wikimedia Commons) Alternative description of the image.

While there are many extreme examples of rain shadow zones around the world, let’s examine the San Francisco Bay Area. San Jose, CA, in the southern San Francisco Bay Area, is on the leeward side of the Santa Cruz Mountains, while the city of Santa Cruz, CA, is on the windward side, near the Pacific Ocean. Figure \(\PageIndex{5}\) shows average precipitation, and Figure \(\PageIndex{6}\) shows the topography of the San Francisco Bay Area.

Figure \(\PageIndex{5}\): Annual precipitation in inches over the San Francisco Bay Area. (CC BY 3.0; USGS via databasin). Alternative description of the image.

Figure \(\PageIndex{6}\): Topography of the San Francisco Bay Area. (CC BY 4.0; Topographical maps via USGS). Alternative description of the image.

20. According to Figure \(\PageIndex{5}\), the city of Santa Cruz, California, receives between 26 and 30 inches of rain annually. Meanwhile, San Jose, in the southern San Francisco Bay Area, about 30 miles north of Santa Cruz, receives ______ inches of rain per year.
    1. 11-15
    2. 16-20
    3. 21-25

21. According to Figure \(\PageIndex{5}\) and Figure \(\PageIndex{6}\), between Santa Cruz and San Jose is a mountain range that receives _____ rainfall annually than either Santa Cruz or San Jose.
    1. significantly more
    2. significantly less

22. This occurs because air coming from the Pacific Ocean travels eastward over the Santa Cruz Mountains, and it _______ over the mountains.
    1. cools, condenses, and rains out
    2. warms and evaporates

23. Once the air sinks down the east side of the mountain, near San Jose, it is significantly ______.
    1. colder and wetter
    2. warmer and drier