A Historical Explanation of Redding’s Multiple Climates

Redding has a long history of temperature and precipitation records dating back to 1875. However, the observation site has been moved several times. Because the precipitation and minimum temperature can vary dramatically over the area, it is hard to compare the data from the different sites.

The intent of this paper is to trace the history of weather observations in the Redding area, and explain, where possible, why the station was moved. It will then discuss how temperature and precipitation vary across the greater Redding area.

The earliest observation site is not known, but it was probably at the home of L. F. Bassett who was known to be the observer from 1892 to 1911. Mr. John H. Buick an agent for Mammoth Copper Mining Company, made observations from 1912 to 1920, first at a site on the east side of the city about halfway between the Post Office and the river, and then at his north side home about 500 feet south of the river.

In the summer of 1922 the station was moved to the courthouse yard where it remained until April 1929. Mr. George Albro was the observer during this period. A First-Order Weather Bureau Office was established at the National Bank Building in July 1928 and observations were taken concurrently with the courthouse until April 1929. At that time the Weather Bureau Office was moved south to Benton Field and the downtown stations were closed. The Weather Bureau operated at the airport with a roof exposure of instruments until the Redding Weather Bureau Office was closed in August 1944.

After the Weather Bureau was closed the climate program was taken over by Mr. Fay Conkling at his residence, three-fourths of a mile northwest of the post office. In 1947 the instruments were moved to the airport again. Mr. Raymond W. Bogart was the observer until June 1948 and Mrs. Alma J. Hinds from then until February 1949. The station was moved to Fire Station Number 2, at the corner of Placer and West streets, in March 1949.

During the ’60’s and ’70’s the population expanded eastward, and more people begin to move into the Palo Cedro and Bella Vista areas. At this time the published climatic data for Redding, based on the Fire Station 2 and Benton Field data, showed that Redding had a 278-day growing season, with the average date of the last spring freeze Feb. 24.

Based on the existing climate data one agricultural publication even sited Redding as the northernmost point in the U.S. where citrus could be commercially grown. As will be explained later, the areas east and south of the Redding observation site are colder and have shorter growing seasons.

By the early ‘70s the Shasta County Farm Advisor was receiving complaints, from people whose plants and trees had suffered frost damage, that the Redding climate data was not representative of their area. The Farm Advisor began to petition the National Weather Service to relocate the observation site to a location where the minimum temperatures were more representative of the local area.

In 1979 Fire Station No 2 was moved to its current location (as of 2007) at the corner of Placer and Buenaventura, and there was no room for the weather observation equipment at the new station. The climate station was relocated to Fire Station No 4 on South Bonneyview in May 1979.

In 1986 the National Weather Service opened a 24-hour-a-day full service office at the Redding Airport. At this time the climate observation program was moved from Station 4 to the NWS office at the airport.

In the 1996 the National Weather Service closed the Redding office and the aviation and climate observation program was taken over by automated equipment at the airport. While the move to Station 4 and the airport provided colder temperature readings, it provided less representative precipitation readings.

In some parts of the U.S. people joke about their weather by saying “if you don’t like it wait a minute…” The Redding area weather could be described by the statement, “If you don’t like the weather here go to the other side of town”.

In mountainous areas topography can cause several different microclimates in a relatively small area. This is significant in the north end of the Sacramento Valley because of the population density of the area and the effect weather can have on the Interstate 5 corridor. The following is a brief explanation of how temperatures and precipitation vary over the area from Anderson northward to Shasta Dam.

There are three tables for reference. The first is the Redding Climatological Summary for 1931-1960, when the observations were at Benton Field and old Fire Station 2. The second is the Redding Climatological Summary for 1971-2000 for the Redding Airport Station and the third is the Shasta Dam Climatological Summary for 1971-2000. These summaries were retrieved from the National Climatic Data Center (NCDC).

Temperature
The tables show the airport hotter than the downtown site. This is probably due to where the instruments were sited. The airport site is very exposed and receives the maximum amount of available sunshine and also has mostly asphalt and dry ground around it.

The downtown site probably experienced some shading and might have had some irrigated land in the vicinity. During the hottest part of day the air is rising and is prone to be unstable, this thermal mixing makes the maximum temperatures less variable across the greater Redding area than minimum temperatures. The hottest areas will be those exposed areas away from water or irrigated land.

Redding averages about 40 days a year of 100 degrees or greater. The average temperature difference between Shasta Lake City and Redding, in the summer, is about 2 to 4 degrees. The hottest days will be those that have light north to northeasterly winds during the entire day. This is because these winds are foehn winds. This means the wind is blowing from a higher to a lower elevation, and the descending air is being warmed by compression. Other foehn type winds are the Santa Ana winds in southern California and the Chinook winds in the Rockies.

Minimum temperatures can vary greatly across the greater Redding area. As was noted above, when then weather station was moved in 1979 one the main reasons was to obtain a more representative minimum temperature reading. As the surface of the earth cools at night, it also cools the air in contact with it, a process that is called radiation cooling. It occurs every night in which skies are relatively clear and winds not strong or gusty. This cooler air then sinks and runs down the slopes and canyons similar to water.

The cooler air begins to fill up the valleys and forms a “pond” or “lake” of cooler air that deepens during the night, reaching its maximum depth about sunrise.

The warmer air that had been in the valley is lifted upward as cooler, denser air slides under it. This puts a layer of warmer air over cooler air and forms a temperature inversion. Where this warmer air layer intersects the surrounding hillsides is called a “thermal belt”.

The warmest and driest conditions will be in the thermal belt. Thermal belt conditions have been documented around the world and in many areas agricultural crops are planted in the thermal belt area because of the longer growing season. A very pronounced thermal belt can be found in the mid slope locations of west Redding.

The tables below show that on the average, nights are warmer at Shasta Dam than the Redding Airport. In fact, the average Shasta Dam minimum temperatures are very similar to the 1931-1960 Redding data. This may be due to being near the top of the thermal belt but is more than likely caused by the down canyon flow that occurs at night in Sacramento River drainage. This keeps the air mixed at the dam and prevents a radiation type inversion from forming.

During the winter the minimum temperature difference between the upper westside of Redding and the lower sheltered areas to the east can be 10 or more degrees. This is why the old temperature data for Redding was not representative of the lower sheltered elevations of Redding and in the Anderson and Cottonwood areas.

When freezing temperatures occur by the process described above, it is called a radiation frost or freeze. During radiation freezes the sheltered lower elevations will have colder temperatures and longer duration freezing conditions than the thermal belt areas of west Redding.

A less frequent but much more destructive type of freeze is called an advection freeze. This occurs when a polar or arctic air mass moves southwestward from central Canada into Northern California. The air mass is very cold and dry and these events are usually accompanied by strong north to northeast winds.

The last significant advection freeze occurred in December 1990. The Redding Airport recorded a low temperature of 17 degrees and went more than 24 hours with the temperatures below freezing. During this period low-lying wind sheltered areas reported single digit temperatures. When advection freezes occur the thermal belt is non-existent due to the strong winds.

Precipitation
Precipitation occurrence, duration, and amounts can change rapidly and radically across the area from Redding to Shasta Dam.

Average annual precipitation at Red Bluff is just over 23 inches. It increases to 33 ½ inches at the Redding airport and over 63 inches at Shasta Dam. This is because southerly winds precede storms as they move toward the west coast. As the air is brought northward the narrowing of the northern Sacramento Valley causes convergence or squeezing of the air mass and the topography of the Redding area forces the air mass upward. This accentuates the precipitation generating process of the incoming storm system and causes precipitation amounts to increase as you move northward toward Shasta Lake.

Annual rainfall amounts increase approximately 3 to 5 inches for every 100-foot elevation increase between Anderson and Shasta Dam. At times this topographic rain enhancement can be very dramatic and cause very heavy rainfall in one area with little or no precipitation to the south.

The most dramatic example of this occurred on August 14, 1976. A stationary frontal system lay east to west between Redding and Shasta Lake City with southerly winds blowing up the Sacramento Valley into the front. A large thunderstorm formed on the frontal system during the afternoon and remained stationary until it dissipated at sunset. Flooding occurred in several sections of northern Redding.

An Army Corp of Engineers team investigated the storm and concluded that some areas of northern Redding received 10 to 12 inches of rain during the afternoon. This event is still considered the greatest 24-hour rainfall, for the month of August, to ever occur in California. The official rain gauge at Fire Station 2 recorded 2.84”.

Snow, while not common in Redding, is also usually caused by the topographic effects of the north valley. During the late fall through early spring, cold dry air masses will move over northern California after frontal systems pass through the state. As the next storm approaches the state the southerly winds will increase and scour out the cold air. However, sometimes because of the wind direction and depth, and stability of the existing air mass, some of the cold air will remain trapped in the north end of the valley and in the Sacramento River Canyon from Shasta Lake to Mt. Shasta City.

If you could see this air mass it would look like a glacier coming out of the Sacramento River Canyon and extending southward into the Redding area. Precipitation with the incoming winter storms will start out as snow aloft, but normally turns to rain as it falls into the warmer air near the ground. However, as the precipitation falls into the cold-trapped air described above, this melting does not occur and the precipitation remains as snow to much lower levels.

This phenomenon occurs quite often during the winter in the Sacramento River Canyon north of Redding and usually makes it southward to Shasta Lake City and north Redding at least once each winter. In rare cases trapped cold air snowfalls have occurred as far south as Chico. The demarcation line between the trapped cold air and warmer air to the south is defined not only by temperature by also by light north winds in the trapped cold air and strong southeast to southwest winds to south of it. As the south winds hit the trapped cold air they are forced aloft and this increases the intensity and amount of precipitation occurring in the cold air. It is not uncommon to see snowfalls of over an inch an hour. One of the largest snow storms to hit the Redding area occurred in December 1968, with snow depths of 23 to 24 inches measured in downtown Redding.

This is very general summary of the climate of the Redding area. With a denser observation network the microclimates of the area could be better defined.

The following generalizations can be made about the climate between Anderson and Shasta Dam:

• During the summer the hottest temperatures will be those exposed areas away from water or irrigated land. On the average, maximum temperatures will be 2 to 4 degrees cooler in Shasta Lake City than south Redding and Anderson.
• The coldest temperatures will usually be in those wind sheltered low-lying areas. There is a well pronounced thermal belt along the mid slope areas west and north of Redding. The areas in this thermal belt will have less chance of frost and longer growing seasons.
• Annual precipitation will increase 3 to 5 inches for every 100-foot increase in elevation from Anderson northward to Shasta Dam.

Technical review and input provided by John Snook, Meteorologist in Charge USFS Predictive Service Unit, Redding, Ca.

-The Old Forecaster

This article was re-printed courtesy of the Shasta Historical Society. It first was written by Chris Fontana in 2007 for the Shasta Historical Society, which published it that same year in the Shasta Historical Society’s “Covered Wagon”.

Chris Fontana, aka The Old Forecaster, started his career as a meteorologist with the National Weather Service (NWS) in Sacramento. After four years as a weather officer in the U.S. Air Force, Chris transferred to the NWS office in Redding, where he was the meteorologist in charge from 1976 until the office closed in 1996. From 1996 until his 2005 retirement, he was the meteorologist in charge of the U.S. Forest Service Predictive Service Unit. Chris and his wife Jane have been married over 40 years and he works as a window covering installer in her business, Westwood Window Coverings.

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