Direct measures of climate

Since climate is largely a long-term view of weather, the measurable aspects of climate tend to be mostly the same as the measurable aspects of weather: temperature, precipitation, humidity, wind speed and direction, etc. Modern measurements of weather have been recorded for the past 100 to 150 years. Because climate is long-term, however, there are some additional factors that must also be considered. For example, one aspect of weather that meteorologists measure is precipitation or rainfall. A climate scientist might be interested in periods of drought (extended times of exceptionally dry weather), and would thus need to define how dry, and for how long the weather would need to be dry in order to be classified as a "drought". Here we will discuss some common measurements of climate.

Taking the Temperature

Average annual temperature is an obvious, basic indication of climate. Bangor, Maine is quite a bit cooler year-round than Miami, Florida. Averages don't tell the whole story, though. The degree of variation is important. As you probably know, large bodies of water can moderate temperature variations in nearby locales, whereas land-locked places can experience more extreme variations in temperature. This effect is important on both a daily basis and on a seasonal basis. For example, the difference in temperature between the day and the night is much greater in a desert environment, such as around Phoenix, than it is in a seaside city like San Francisco.

The average annual temperatures in Valdivia, Chile and in Beijing, China are nearly identical. However, the average temperature in the coldest month in Valdivia is 7º C (45º F), while the average in the hottest month is 16º C (61º F). In Beijing, the average temperature in the coldest month is -4º C (25º F), while the average in the hottest month is 26º C (79º F). The variation from winter to summer in seaside Valdivia is thus a modest 9º C (16º F), while the seasonal variation in inland Beijing is a more extreme 30º C (54º F).

Climate scientists must look at various aspects of temperature trends. How widespread is a heat wave or a cold spell? Does it effect a single city or state, or is its impact regional, or even global? How long does a temperature anomaly last? One cold winter doesn't signal the start of an Ice Age, but do 8 frigid winters out of 10 indicate a long-term cooling trend? An area might experience a rise or dip in its year-round average temperature; it might experience an increase in its summertime temperature but no change in winter weather, or it might feel a lowering of summer highs but a rise in winter temperatures. Likewise, Earth's overall average temperature could remain steady even as the tropics warmed and the polar regions cooled.

Climate scientists must thus decide how to spot trends that span both temporal and spatial scales. They need to decide how much the Earth must cool, over how large an area, and for how long of a time, for us to claim that the planet has entered an Ice Age. They must note that averages don't always tell the whole story; that extremes in certain seasons, during certain years, or at specific latitudes can indicate climate changes even if average values remain unchanged.

Climate scientists also need to pay attention to what they are measuring the temperature of. When speaking about weather, we normally think of measuring the air temperature. However, because water can act as a reservoir that stores heat for long times, sea surface temperatures (SST) and temperatures in the deep ocean can also be critical pieces of data when developing an overall picture of climate. To a lesser, though still significant, extent, soil temperatures, especially in high-latitude regions where permafrost is common, can be an important climate indicator. Even the atmosphere is not a single entity; temperatures in the lower troposphere can vary along different trends than do those in the stratosphere or higher layers.

Measuring Precipitation

The amount of precipitation that accumulates, mostly in the form of rain and snow, is another key ingredient of climate. As is the case with temperature, both the annual average and the seasonal variation in precipitation are important. Some regions have rainy seasons and dry seasons or monsoons that bring heavy rains for a few months out of the year. Other areas have more constant year-round levels of precipitation.

Mobile, Alabama has almost as much annual rainfall as does Bombay, India. However, in Bombay, nearly all of the rain falls during the four months of the summer monsoon.

Fog can be an important source of moisture in some areas, as is famously the case in ocean-side San Francisco. Deep snowpacks are significant sources of time-released water supplies. In the mountainous western half of Colorado, winter snowfall determines how much water flows into reservoirs during the spring-time melt and runoff.

The degree to which precipitation (or meltwater from spring thaws) comes in spurts determines the likelihood of flooding in many areas.

Climate scientists must also define and track certain trends in precipitation that are not factors in our day-to-day weather. Periods of drought or of excessive rain that lead to flooding are two such trends. El Niño events can bring heavy rains and flooding to much of South America, for example. Changes to the annual patterns of monsoons can alter the precipitation aspects of climate over large areas, as can changes in the number and severity of hurricanes and typhoons. Changes in the size of polar ice caps and glaciers, and in the amount of sea ice, are partially determined by snowfall amounts.

Measuring Wind

Wind is a third major measurable aspect of climate. When we measure wind we record both the speed and direction. As with temperature and precipitation, both average values and variations are important. Most locales have prevailing winds (winds from the general direction that the wind usually flows) that may change seasonally. Winds may also not be the same at the surface as they are at greater elevations. The jet streams, which are high-altitude winds, are one such example. They can play an important role in a region's weather and climate.

Winds are often part of extreme weather events. They can generate or be part of powerful storms such as blizzards, tornadoes, and hurricanes. Florida and other parts of the Atlantic and Gulf coasts of of the U.S. can be severely impacted by hurricanes in the late summer and early fall. The central plains of North America are similarly threatened by tornadoes each spring. Strong winds from powerful thunderstorms and supercells afflict many areas during the hot summer months.

But winds can also influence climate in less dramatic ways as they bring moist, dry, warm or cold air into a region, influencing precipitation and temperature trends in those areas. Coastal areas often have daily trends in their winds that are influenced by the differential heating of land and water. Such winds frequently influence climate as they carry moist air over land or dry air out to sea.

There are also many seasonal winds that are common in certain parts of the world. Examples include the chinooks in midwestern North America, the Mediterranean's Sirocco, the Santa Anas of Southern California, France's Mistral winds, and the monsoons of India. These winds influence regional climates as they draw characteristic levels of moisture and temperature into a locale. Such winds influence climate in specific ways, and can alter climate if they cease or are temporarily disrupted.

Climate scientists pay attention to long-term trends in wind patterns. Do tornadoes and hurricanes form consistently at the same times of the year and in the same places, or do they vary? Are the numbers and strengths of such storms remaining about the same from year to year, or are they changing? Are the annual cycles of prevailing winds, such as trade winds and monsoon winds, remaining consistent and predictable, or are they varying? Are winds in the upper reaches of the atmosphere, especially the jet streams, behaving as expected or are they fluctuating erratically?

Other Direct Climate Measures

Besides the three major measurable aspects of climate described above, there are numerous other quantities climate scientists can track. They include aerosols, atmospheric composition, cloud cover, humidity, and insolation.

Aerosols

Aerosols are tiny particles suspended in the air. Sources of aerosols include dust from deserts, soot from forest fires and smokestacks, volcanic ash, pollen, and sea salt. Aerosols can influence the amount of incoming sunlight that is reflected back into space and can serve as nucleation sites that allow ice crystals and raindrops to form.

Atmospheric composition

Although not directly a measure of climate, the atmospheric composition represents a series of quantities that do strongly influence climate and are measurable. Two well-known measures are concentrations of greenhouse gases (especially carbon dioxide and methane) and of stratospheric ozone.

Cloud Cover

Measurements of clouds, including the types of clouds, the fraction of a given area that is covered by clouds at a given time, and the percentage of time that a given location is cloudy are all important climate considerations. Ground-level clouds, in the form of fog, can also be tracked in terms of frequency of occurrence, geographic extent, moisture deposited as dew, and thickness (how little light reaches the surface).

Humidity

Humidity is the amount of moisture in the air. Humidity obviously influences precipitation; it also affects the capacity of a parcel of air to retain and transport heat.

Insolation

The amount of energy received by a given parcel of land (or water) from sunlight is called the insolation. Insolation varies by season and latitude, and by the degree of cloud cover. It can be measured in units of energy (Watts per square meter, for example), and influences temperature, evaporation, and chemical composition.