3.4: Elements of Climate

People often confuse weather and climate; they are not identical. According to the American Meteorological Society (AMS), the weather is defined as the state of the atmosphere at some place and time, usually expressed in terms of temperature, air pressure, humidity, wind speed and direction, precipitation, and cloudiness. Meteorologists study the atmosphere, processes that cause weather, and the life cycle of weather systems. Climate is defined in terms of the average (mean) of weather elements (such as temperature and precipitation) over a specified period. (The World Meteorological Organization defines the typical time of time as 30 years). Climate also encompasses weather extremes for a particular place.

Scientists have developed a variety of ways for classifying climate. In the early 20th century, a German scientist named Vladimir Köppen developed one of the most widely used classification systems. The Köppen system categorizes climate into five main types, which can be further divided into subcategories.

Table 3.1: Basic Characteristics of the Köppen Climate Classification

Type of Climate	Characteristics
Tropical (A)	Average temperature of 18 °C (64.4 °F) or higher every month of the year, with significant precipitation (very humid).
Dry (B)	Evaporation exceeding precipitation with constant water deficiency throughout the year. Average temperature greater than 10 °C (50 °F)
Temperate (C) (Northern Hemisphere Mid-Latitude)	Humid and warm or hot summers, and mild or dry winters, with average temperatures between -3°C (27°F) and 18°C (64°F)
Continental (D) (Northern Hemisphere Mid-Latitude)	At least one month averaging below 0 °C (32 °F) (or −3 °C (27 °F)) and at least one month averaging above 10 °C (50 °F).
Polar (E)	Freezing winters and an average temperature of the warmest summer month below 10°C (50°F)

The planet’s climate has changed many times over Earth’s long geologic history. For example, over the past million years, Earth has experienced several glacial periods interspersed with interglacial (warmer) periods. The relatively constant and favorable interglacial period of climate experienced over the past 8,000 years has made human civilization’s advancement possible.

Figure 3.6: Köppen-Geiger Climate Subdivisions. Köppen-Geiger Climate Subdivisions Köppen and Geiger further divided each major division (A, B, C, D, E, F, H) into smaller groups based upon precipitation and temperature patterns. Image by NOAA is in the public domain.

Climate Change

Climate change refers to a significant and sustained (over decades or longer) change from one climatic condition to another. The term “global warming” refers to a specific kind of climate change in which Earth’s average temperature is increasing. Of growing concern is what is known as abrupt climate change. According to the National Oceanic and Atmospheric Administration (NOAA), abrupt climate change is a relatively new area of scientific research whose formal definition is still being developed, but it refers to a sudden, rapid transition from one climate state to another (over a period of years rather than centuries or millennia).

Meteorologists focus primarily on real-time (current) data to predict local or regional atmospheric conditions for the hours, days, or weeks ahead. Thus, weather prediction tends to be more local and relates to conditions in the immediate future from days to weeks.

Figure 3.7: Annual Temperatures During 1880-2018. Notice the Winter/Summer Patterns—& Then the Overall Dramatic Increase of The Patterns During the Past 20 Years. Image by NASA is in the public domain.

Climate scientists or climatologists, on the other hand, look at atmospheric conditions in terms of averages and trends (patterns) that have occurred over many decades, centuries, and millennia. Weather is variable but can be averaged over time to indicate climate trends. Therefore, climate scientists can use weather data plus proxy data, (reconstructed paleo climates) to help them identify previous trends and improve their predictions of future trends.

Meteorologists and climate scientists use similar tools. Weather balloons, satellites, uniquely designed airplanes, and radar and other ground-based data collection instruments (to measure wind speed, precipitation, air temperature, humidity levels, etc.) are all good examples. These methods and tools have enabled humans to collect reliable atmospheric data on a consistent basis since the mid-1800s. They have grown increasingly more precise and sophisticated over time, to such an extent that meteorologists can now consistently provide reasonably accurate near-term (1 week or less) weather forecasts.

Climate monitoring requires data covering all areas of the planet over a much longer time. Sophisticated Earth-observing satellites equipped with remote-sensing equipment circle the globe. With each pass, they can record sea surface and other temperatures, measure atmospheric gases and rainfall amounts, take visible and infrared photos of Earth’s surface, and calculate Earth’s outgoing infrared and reflected solar radiation.

Climate Change & Fires?

The year 2020 will be remembered for many things, not the least of which was a series of devastating fires around the globe that bear the fingerprints of climactic change. From Australia and South America’s Amazon and Pantanal regions, to Siberia and the U.S. West, wildfires set new records and made news year-round.

It was an especially bad year for wildfires on the U.S. West Coast. Five of California’s 10 largest wildfires on record happened in 2020, and the state set a record for acres burned. According to CAL FIRE, the state’s Department of Forestry and Fire Protection, more than 9,600 wildfires burned nearly 4.2 million acres through mid-December, causing more than 30 fatalities and damaging or destroying nearly 10,500 structures.

Figure 3.8: Smoke from numerous wildfires obscures much of California and Oregon on September 9, 2020. Image by NOAA is in the public domain.

It was the fire equivalent of a perfect storm. Record drought conditions across the Western United States over many decades prior and were followed by the hottest summer on record in the Northern Hemisphere. Poor forest growth management, increased powerlines to remote areas, unusually dry air, strong wind events, and an outbreak of summer thunderstorms in Northern California in August, and conditions were ripe for a dangerous fire season.

In recent decades, the U.S. West has warmed, and the frequency and severity of heat waves and droughts has increased. According to the National Oceanic and Atmospheric Administration (NOAA), temperatures in California have increased approximately 2 degrees Fahrenheit (1.1 degrees Celsius) since the beginning of the 20th century. This has dried out the air. Fire seasons are also starting earlier and ending later each year, while snowpack’s are shrinking, leading to earlier spring snowmelt and longer, more intense dry seasons.

Effect of General Circulation on Climate

A dominating factor in the weather of California is the semi-permanent high-pressure area of the north Pacific Ocean. This pressure center moves northward in summer, holding storm tracks well to the north, and as a result California receives little or no precipitation from this source during that period. In winter, the Pacific high retreats southward permitting storm centers to swing into and across California. These storms bring widespread, moderate precipitation to the state. Some of them travel far enough to the south to spread moisture beyond the Mexican border. When changes in the circulation pattern permit storm centers to approach the California coast from a southwesterly direction, copious amounts of moisture are carried by the northeastward streaming air. This results in heavy rains and often produces widespread flooding during the winter months.

There is another California weather characteristic that results from the location of the Pacific high. The steady flow of air from the northwest during the summer helps to drive the California Current of the Pacific Ocean as it sweeps southward almost parallel to the California coastline. However, since the mean drift is slightly offshore, there is a band of upwelling immediately off the coast as water from deeper layers is drawn into the surface circulation. The water from below the surface is colder than the semi-permanent band of cold water just offshore, which ranges from 25 to 50 miles in width.

The temperature of water reaching the surface from deeper levels varies from about 49°F in winter to 55°F. in late summer along the northern California coast, and from 57° to 65°F on the southern California coast. At 200 to 300 miles offshore, surface water temperatures range from 51° to 65°F in the north and from 60° to 67°F in the south. Thus, the water near the coastline is as much as 10°F colder during the summer than is the water farther west into the Pacific Ocean.

Comparatively warm, moist Pacific air masses drifting over this band of cold water form a bank of fog which is often swept inland by the prevailing northwest winds out of the high-pressure center. In general, heat is added to the air as it moves inland during these summer months, and the fog quickly lifts to form a deck of low clouds that extend inland only a short distance before evaporating completely. Characteristically, this deck of clouds extends inland further during the night and then recedes to the vicinity of the coast during the day. This layer of maritime air is usually from 1,500 to 2,000 feet deep, while above this layer the air is relatively warm, dry, and cloudless.

Extreme Weather

All-weather events that cause loss of life, disrupt normal human activities, and result in property damage appear extreme. It is a question of perspective: How do today’s severe weather events compare to severe weather events in the recent and distant past? The resolution of Global Climate Models can complicate making direct comparisons between past and present events. For example, since 1986 the global human population has grown by approximately 2 billion. Simply said, there are more people than ever living in formerly unpopulated or sparsely populated areas. Comparing death tolls, between recent and past events may not be the most meaningful indicator of a particular weather event’s intensity.

Nonetheless, the growing body of meteorological data indicates an increase in the number of extreme weather events occurring here in the United States since 1980, and the number of extreme events also appears to be rising worldwide.

2022-2023 California Floods

Periods of heavy rainfall caused by multiple atmospheric rivers in California between December 31, 2022 and January 25, 2023 resulted in floods that affected parts of Southern California, the California Central Coast, Northern California, and Nevada. The flooding resulted in property damage and at least 22 fatalities.

At least 200,000 homes and business lost power because of the storm and 6,000 individuals were ordered to evacuate. The floods were widely reported by media as an example of how climate change is increasing extreme changes in weather, especially cycles of precipitation and drought. Scientists interviewed by Los Angeles Times said that further study is needed to determine the connection and California has recorded similar events almost every decade since records started in the 19^th century. Other scientists have emphasized that floods were caused by ocean warming, directly related to climate change. Scientist Kevin Trenberth declared that "the interaction between the warming ocean and the overlying atmosphere (...) is producing these prodigious rainfalls that have occurred in so many places around the world recently". Climate change is intensifying the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions. It has been both predicted by scientists and observed in the last years and documented by the IPCC (International Panel for Climate Change 6th assessment report). Before the rains started, California had been in an extreme drought.

Figure 3.9: Precipitation Totals from December 26^th- January 10^th, 2023. Image by NOAA is in the public domain.