Climate and Weather

Some Definitions of Climate and Weather

Winter Landscape with Skaters and Bird Trap. BRUEGEL, Pieter the Elder (b. cca. 1525, Breughel, d. 1569, Bruxelles). This is the most copied and imitated painting of the artist. The moral content of the subject: life is full with dangers both for the young skaters and the birds.

What is the difference between weather and climate? It is a matter of time scale. Weather is the day-to-day stuff: on Thursday it might be “good,” on Saturday it might be “bad.” The climate cannot be harsh on Thursday and mild on Saturday, because it is not measured in terms of days, but in terms of many years. The weather can change in an hour, say, when a cold front moves in, bringing Arctic air on a Christmas day, suddenly making it is freezing cold. Or, on a summer day, the barometer falls announcing the arrival of a low-pressure center and a rainstorm that is not far behind. In contrast, when we say “the climate has changed,” we make a statement about a long-term shift from cold to warm, or from dry to wet, or vice versa.

Weather is typically described by a weather map, showing lines of equal pressure (called isobars), boundaries between air masses (called fronts), and direction of motion. Climate is described in terms of overall temperature distributions, air pressure distributions, precipitation and cloud cover, and by seasons. Also, climate is described in terms of its variability between years (called the "interannual variability").

A well-known climate change was the arrival of the “Little Ice Age” in Europe, sometime in the 14th century. Winters became more severe and many summers were cool and wet in northwestern Europe. In the centuries following, the Thames River froze regularly, and the ice cover provided a place for a winter fair for the people of London. In Holland, the canals froze over, and the Dutch painter Pieter Bruegel (1520-69) made his famous pictures showing ice skaters within winter landscapes. In the second half of the 19th century the climate warmed again, and the "Little Ice Age" ended.

Weather and Weather Maps

An idealized model of cyclonic eddy circulation in North America. On weather maps, warm fronts are depicted by a red solid line with semi-circles that point towards the cooler air. Cold fronts are depicted by a solid blue line with triangles that point towards the cooler air. Cyclones are an area of low pressure (marked by the L) around which airflows counterclockwise on the northern hemisphere. The warm front is pushing northward and a cold front is pulling southward.

A commonly seen weather map in mid-latitudes describes a large cyclonic eddy, which forms when a tongue of warm, moist tropical air invades cold, dry polar air masses. The center of the cyclone is a region of low pressure (indicated by the letter L). There are two fronts: a warm front and a cold front. An observer on the ground sees first the warm front, as the cyclone moves north - eastward. Air pressure drops, temperature rises with the arrival of the tropical air, and so does humidity. Cloudiness increases and a rainstorm may follow. The reason for rain along fronts is that warm air surrounded by cold air must rise, and as it rises, it cools and loses its ability to hold water. The condensation of the water vapor within the rising air releases heat, so that the air warms and keeps rising till condensation stops. A few days later the cold front arrives, restoring cooler, drier weather. Cyclones are separated by high pressure centers (marked by an H on weather maps). Air moves clockwise around these H centers (anticyclonic circulation), on the northern hemisphere. The reason that the air moves in eddies, rather than in straight paths, because of the rotation of the Earth.

Climate and Regional Pattern Maps

Climate is typically described by the regional patterns of seasonal temperature and precipitation over 30 years. Average annual temperature, average rainfall, average cloud cover, and average depth of frost penetration are all typical climate-related statistics. The main expression of climate on land is in the vegetation and the soil type produced in each climatic region. The idea to classify climate by temperature, rainfall and vegetation was introduced by Wladimir Köppen in 1900. This classification of climates was later modified by Rudolph Geiger and is now generally accepted as follows:

Advancing cold fronts often bring rain.

Humid Tropics (A): Known for their high temperatures year-round and for their large amounts of perennial precipitation. These regions are found near the equator.
Arid Climates (B): Characterized by little precipitation and huge daily temperature range.
Humid Middle Latitudes Climates (C): Dominated by land/water differences. The eastern seaboard of the U.S. would be an example of this class, with cool winters and mild summers.
Continental Climate (D): Found in the interior regions of land masses of exceptional size (i.e. Omaha, in the middle of the U.S.A.). Total precipitation is not very high in amount, and seasonal temperatures vary greatly.
Cold Polar Climates (E) Areas covered by permanent ice and tundra. Here, average temperatures reach above freezing only about one third of the year.

A world map of Köppen climate types. See text for descriptions. Smaller case letters refer to subclassifications that are not discussed here.

As the picture illustrates, the ocean has a huge influence on the climate. Climates can be classified as "marine" or "continental" depending on distance to the ocean. Both the highest and the lowest temperatures appear in the interior, far from the moderating influence of the ocean. Note how the lines of climate zones tend to run parallel to the coast, especially in western North and South America, or from interior to exterior of the continent, as in Australia. Precipitation is highest where warm marine air has access to coastal mountain ranges, as for example in the coastal regions of Oregon and Washington.