3.3: Seasonal Air Temperature Variations

Seasonal Variations

Just as air temperatures vary throughout the day, they also vary seasonally. The discussion below focuses on daily average high temperatures and their seasonal variation. Still, you could apply these same ideas to overnight low temperatures as well and get the same general results. A location's seasonal air temperature range can be determined by subtracting the coldest average annual temperature from the warmest average annual temperature. For example, San Jose, California, has the warmest average temperature in August at 82°F and in January at 60°F. yielding a 22°F range (Figure 3.3.1, and Table 3.3.1).

Figure \(\PageIndex{1}\): Average monthly high temperatures for San Jose, California. (CC-BY-SA 4.0; Alicia Mullens)

Table 3.1.1: Average monthly high temperatures for San Jose, California.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Temperature (°F)	60	63	66	70	75	80	82	82	81	76	66	60

A location's seasonal temperature variation depends on four main factors, often referred to as the "controls" on seasonal air temperature variations:

1. Latitude
2. Proximity to Ocean vs. Land
3. Elevation
4. Ocean Currents

Let's investigate each of these a little further:

Latitude

The latitude of a location has two main effects on its temperature. The first one is on the intensity of the incoming sunlight. From the previous section, recall that when the sun is higher in the sky, the intensity of incoming solar radiation is higher. As a result, locations closer to the equator see warm temperatures, while locations further from the equator see colder temperatures. The other important impact comes from the length of the day: locations near the equator experience nearly constant day length year-round, while locations further from the equator see greater and greater variation. In fact, locations poleward of 66.5°N/S have at least one full 24-hour day of light and one full 24-hour day of darkness per year, with the frequency rapidly increasing until you get to the pole, which sees six consecutive months of daylight followed by six straight months of darkness. Putting these two concepts together, the main takeaway from the discussion of latitude's impact on temperatures is that locations near the equator experience more consistent temperatures, while those farther from the equator experience greater temperature variation. This is illustrated in Figure 3.3.2 and Table 3.3.2, which shows the average monthly high temperatures for Honolulu (latitude 21°N) vs. Anchorage (latitude 61°N).

Figure \(\PageIndex{2}\): Average monthly high temperatures for Honolulu, Hawaii, and Anchorage, Alaska. Honolulu’s temperatures remain steady between about 77 and 84 degrees year-round. Anchorage’s temperatures start near 24 degrees in January, rise to about 66 degrees in July, then drop back to around 25 degrees in December. (CC-BY-SA 4.0; Alicia Mullens)

Table \(\PageIndex{2}\): Average monthly high temperatures for Honolulu, Hawaii, and Anchorage, Alaska.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Honolulu Temperature (°F)	78	77	77	79	81	82	83	84	84	83	81	79
Anchorage Temperature (°F)	24	27	34	45	56	64	66	64	56	42	29	25

Honolulu, which is closer to the equator, experiences minimal temperature variation (only 7°F between summer highs and winter lows). In contrast, Anchorage, which is closer to the north pole, sees significantly more (a difference of 42°F between summer highs and winter lows). It should be noted that both cities are coastal cities located at/near sea level. As you will see below, both factors do indeed influence seasonal temperature variations, but in this case, the primary difference between Honolulu and Anchorage is latitude.

Proximity to Ocean vs. Land

Have you ever been to the beach on a bright, warm day and tried to walk on the sand barefoot? It's very likely that the sand you were walking on felt very hot to your feet, and that probably left you running for the cooler ocean water. That dichotomy (warmer sand vs. cooler ocean water) has nothing to do with one receiving more/less heat than the other; assuming they are right next to each other, both are exposed to the same amount of sunlight. The big difference between the two is specific heat, the amount of heat required to raise an object's temperature by 1°C. Water has a specific heat that is, on average, five times higher than that of most land cover (soil, concrete, sand, etc.), and as a result, water requires substantially more heat to increase its temperature than land does. Consequently, water does not warm up or cool down as much as land does throughout the day AND throughout the year. During summertime, ocean waters remain relatively cool, providing a pleasant respite from hot coastal temperatures, while in wintertime, ocean waters remain relatively warm, preventing coastal regions from experiencing the biting cold common inland. The general rule is that coastal areas experience less seasonal temperature variation than inland areas. An excellent example of this is shown in Figure 3.3.3 and Table 3.3.3, which compares the monthly average high temperatures in Long Beach, California, and Dallas, Texas. Both cities have similar latitudes and elevations, but Long Beach is near the coast while Dallas is far inland.

Figure \(\PageIndex{3}\): Average monthly high temperatures for Long Beach, California vs. Dallas, Texas. Dallas starts around 58 degrees in January, rises to about 98 degrees in July and August, then drops back to 62 degrees in December. Long Beach begins near 68 degrees in January, peaks around 84 degrees from July through September, and returns to 68 degrees in December. (CC-BY-SA 4.0; Alicia Mullens)

Table \(\PageIndex{3}\): Average monthly high temperatures, Long Beach, California, and Dallas, Texas.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Long Beach Temperature (°F)	68	70	70	74	75	79	84	84	84	49	73	68
Dallas Temperature (°F)	58	62	72	79	85	94	98	98	91	81	69	62

Long Beach stays relatively mild year-round (with average high temperatures of 68°F in winter and 84°F in summer). At the same time, Dallas experiences significantly more variation (with average high temperatures of 58°F in winter and 98°F in summer). These averages only tell part of the story, too; winter weather in Dallas can be notorious for icy conditions, biting cold, and even blizzards, while Long Beach experiences nothing of the sort. Summer weather in Dallas isn't much better, with weeks of temperatures above 100°F, while Long Beach may experience a few days (if that) a year with such extreme heat. Their respective proximity to the ocean vs. the land is the main reason for such differences.

Elevation

Suppose you live in a tropical location, such as Hawaii, or a relatively mild climate, such as much of California. At sea level, in both climates, snow is rare, if not nonexistent (especially in Hawaii), but that doesn't mean either state never experiences it. In fact, the snowy weather typical of winter is common at higher elevations in both states. In Hawaii, snow is mainly restricted to the highest peaks of the Big Island's volcanic mountains (such as Mauna Loa), while in California, snow is common at higher elevations in the Sierra and Cascade mountains (and to a lesser extent in the transverse ranges of Southern California). This brings us to our third main factor on seasonal temperature variations: Elevation. All else being equal (latitude, proximity to the ocean, etc.), a location at a higher elevation will be cooler year-round than one closer to sea level. Figure 3.3.4 and Table 3.3.4 compare the average monthly high temperatures for Sacramento to South Lake Tahoe, California. Both cities are inland and at nearly the same latitude, but Sacramento has an average elevation of about 100 feet, while South Lake Tahoe has an average elevation of about 6,200 feet.

Figure \(\PageIndex{4}\): Average monthly high temperatures for Sacramento vs. South Lake Tahoe. Sacramento’s temperatures start at around 57 degrees in January, rise to about 95 degrees in July, then fall to 56 degrees in December. South Lake Tahoe’s temperatures begin near 42 degrees in January, peak around 78 degrees in July and August, and drop back to 42 degrees in December. (CC-BY-SA 4.0; Alicia Mullens)

Table \(\PageIndex{4}\): Average monthly high temperatures for Sacramento, California, and South Lake Tahoe, California.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
Sacramento Temperature (°F)	57	63	68	74	82	89	95	93	89	79	65	56
South Lake Tahoe Temperature (°F)	42	44	48	55	62	71	78	78	71	61	49	42

If you notice, both cities have very similar seasonal temperature variations (38°F in Sacramento and 36°F in South Lake Tahoe), but South Lake Tahoe is, on average, about 15-20°F cooler than Sacramento. This difference is purely due to their elevation differences: the lower-elevation Sacramento is warmer year-round, while the higher-elevation South Lake Tahoe is cooler year-round.

Ocean Currents

The final factor that controls a location's seasonal temperature variation is the ocean current it is next to. This factor obviously has less influence the further inland you go, but it can have a substantial impact on locations near the coast. While the details of ocean currents will be saved for a later chapter, this discussion will consider two types of ocean currents: warm ocean currents that originate near the equator and carry warm ocean waters towards the poles, and cold ocean currents that originate closer to the poles and carry cold ocean waters towards the equator. These ocean currents are an essential component of Earth's energy budget and play a key role in balancing Earth's temperature differences. Cold ocean currents are generally found along the west coasts of continents (such as the California Current along the west coast of North America).

In contrast, warm ocean currents are usually found along the east coasts of continents (such as the Gulf Stream along the east coast of North America). In the United States, the West Coast is located next to the cold California Current, while the East Coast is located next to the warm Gulf Stream. Hawaii, in the central Pacific, is still subject to the eastward-flowing equatorial current. Still, the temperature differences caused by this current are generally less significant than the temperature differences caused by the warm and cold currents that parallel extensive coastlines.

Figure 3.3.5 and Table 3.3.5 show the influence of ocean currents on two coastal cities with similar latitudes and elevations. San Diego, California, is located next to the cooler California current, while Savannah, Georgia, is located next to the warmer Gulf Stream. It should be noted that, for a reason that will be discussed in a later chapter (specifically, both locations are dominated by mid-latitude westerlies that blow from west to east in the wintertime), the differences between San Diego and Savannah are more noticeable in summer than in winter.

Figure \(\PageIndex{5}\): San Diego’s temperatures stay relatively steady, starting around 67 degrees in January, peaking near 78 degrees in August, and ending at 66 degrees in December. Savannah’s temperatures start around 62 degrees in January, rise to about 92 degrees in July, then decline to 64 degrees in December. (CC-BY-SA 4.0; Alicia Mullens)

Table \(\PageIndex{5}\): Average monthly high temperatures for San Diego, California, and Savannah, Georgia.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sept	Oct	Nov	Dec
San Diego Temperature (°F)	67	66	67	69	69	72	76	78	77	75	71	66
Savannah Temperature (°F)	62	65	71	78	84	89	92	91	86	79	70	64