Weather is the short-term (e.g., daily) condition of the atmosphere, and, if you watch the local news, then you are well aware of weather! The National Weather Service issues weather maps each day. Climate, on the other hand, is the average atmospheric conditions of a location or region over at least several decades.
1. Which statement is more about the climate, rather than the weather, of a town?
a. The average of the high and low temperatures measured yesterday was 19°C (66°F).
b. One centimeter (0.4 inches) of rain fell on 18 September 2010.
c. The high humidity this past week has been very unpleasant for working outside.
d. The average yearly snowfall is 122 centimeters (48 inches).
The two most common atmospheric variables that are shown in the context of climate are temperature and precipitation. One way you can visualize the different climates on Earth is to look at a map of climate types. The Köppen climate classification is based on the concept that native vegetation is the best expression of climate; thus, climate-type boundaries have been selected with vegetation distribution in mind. It combines mean annual and monthly temperatures and precipitation along with the seasonality of precipitation. As noted earlier, decades of temperature and precipitation data are needed to determine the climate — and thus climate type — of a location. Click on the Earth image below to view the Köppen climate classification in Google Earth. The image to the right of the Google Earth image is the climate-types legend; it is recommended that you open this on another monitor so you can quickly see to what climate type the colors in the Google Earth image correspond.
2. What is the climate type of Atlanta? Type “Atlanta” in the Search box and Google Earth will take you to Atlanta.
3. List a country, besides the United States, that has towns with the same climate type as Atlanta.
Click Climographs to open a file in Google Earth that has climographs for 20 cities in the Western Hemisphere extending from 3° S to 71° N. It may take a few minutes for all the climographs to load in Google Earth, so please be patient. A climograph is a chart showing both the average monthly temperature and precipitation of a place. Examine the climographs for the 20 cities and notice that the climaographs change when you move both latitudinally (e.g., south to north) and longitudinally (e.g., east to west). For example, Atlanta and Los Angeles are located at approximately 34° N, but Atlanta receives much more precipitation, especially during summer, and has a much larger difference in temperature between summer and winter, than does Los Angeles.
4. During what season (i.e., winter, spring, summer, or fall) do tropical (A) climates receive the most rainfall?
5. How does the temperature range (i.e., difference in temperature between hottest month and coldest month) generally change when moving from the Equator to the Arctic Circle and what is the primary cause of this change?
6. During what season do Ciudad Bolivar (Aw), Belize City (Am), Mexico City (Cwb), Winnipeg (Dfb), Fairbanks (Dfc), and Barrow (ET) receive a large proportion of their precipitation?
7. During what season do Los Angeles (Csa), San Francisco (Csb), and Vancouver (Cfb) receive a large proportion of their precipitation?
Click on the Earth image below to see the terrestrial biomes for each of the locations and notice how that the biomes are generally related to the climate types. A biome is a major ecosystem.
8. What is the biome of Atlanta?
9. What is the biome of Iquitos?
10. What is the biome of Fairbanks?
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The circulation of Earth’s atmosphere is the major control of both temperature and precipitation for the climate types, and the latitude — and thus the quantity of incoming solar radiation — is a major control of temperature. Two important precipitation-producing features on Earth are the intertropical convergence zone (ITCZ) and mid-latitude wave cyclones (i.e., cold fronts and warm fronts). The ITCZ, which is huge and generally doesn’t go away, stays near the equator and is responsible for the precipitation in the A climate types. Mid-latitude wave cyclones, which only appear for up to a week or so, are outside the tropics and travel west to east across the globe. All climate types except for tropical climates are affected by mid-latitude wave cyclones.
To see the ITCZ and mid-latitude wave cyclones in action, watch the animation below created with the Community Climate System Model (CCSM) and the National Center for Atmospheric Research (NCAR). It has an hourly time step during a typical year. Cloud cover is shown in white and areas of precipitation are shown in orange. The month and hour are shown in the upper right of the animation. Focus on June-August and December-February.
11. What feature brings a lot of rainfall to the Caribbean in the summer (June-August)?
12. What features were more likely to traverse across the Canada in the summer (June-August) — and produce rainfall — as opposed to traversing across Canada in the winter (December-February)?
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It is important to understand a distinction that only involves climate (and not weather) : the difference between climate variability and climate change. Climate variability is the year-to-year swings in a climate variable, such as temperature. Therefore, the term interannual variability is often used in place of climate variability. We have already looked at how some volcanic eruptions (e.g. Mount Pinatubo in 1991) are explosive enough to inject SO2 and particulates into the lower stratosphere. Since the materials can stay in the stratosphere for years – reflecting sunlight away from the Earth – these eruptions can lead to anomalously low surface temperatures one to two years after they have taken place. The picture below shows data for optical density from the year after Pinatubo erupted (1992) until two years later. The dark blue following the eruption showed just how much light the materials ejected from Pinatubo was blocking.
Another source of climate variability are the El Niño and La Niña events that we hear about in the news some times. An El Niño is characterized by warming of the east-central tropical Pacific Ocean (through a mechanism partly illustrated by the picture on the left below), which can cause anomalously high surface temperatures at the global scale – such as happened in 1997 – 1998 (depicted in the satellite image on the left below), while a La Niña is characterized by cooling of that same region (through a mechanism partly illustrated on the right below), which can cause anomalously low surface temperatures at the global scale — such as happened in 2010.
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Climate change, as defined by the Intergovernmental Panel on Climate Change (IPPC), is a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period – typically decades. The key parts of that definition are that it is a statistical variation over an extended period. Stock markets go up and down, but show trends over time; over different periods of time, those trends change (from the upward trend of a bull market to the downward trend of a bear market). Likewise, weather changes over short periods of time, but demonstrates patterns called climate; and climates will shift over longer priods of time resulting in climate change. Usually, there must be a consistent shift over three or more decades in a variable such as temperature to label what is happening as a climate change. We already have witnessed a dramatic example of climate change that occurred over thousands of years when you saw a major increase in temperature of approximately 8 °C from the Last Glacial maximum 21,000 years ago to the beginning of the present interglacial period 10,000 years ago.
Click on the image below view in Google™ Earth the extent of ice and other types of land cover during the Last Glacial Maximum. Also click Climographs so you can view the 20 locations on the globe.
13. Based on the climographs for the near-present situation, what do you think was the climate type for the location of Atlanta 21,000 years ago?
14. And what do you think the biome was 21,000 years ago for the location of Atlanta?
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You are now going to examine climate change that has occurred from 1979 to the present by looking at temperature data at the surface and throughout the lower troposphere. Surface temperature is measured with thermometers, whole lower-troposphere temperatures are estimated using data from satellites.
15. Why were 1992 and 1993 relatively cool years?
16. Why was 1998 a relatively warm year?
17. What was the general trend in both the surface and lower-troposphere temperature from 1979 to the present?