When it comes to weather—the condition of the atmosphere at any given moment—the factors of water, heat, and air movement operate in the city just as they do in the country. Some cities are famous for their weather, but most urban environments also show unique man-made changes.
♦ Wind and fog
San Francisco is famous for its sea fog, which happens when warm breezes, blowing in from the warm central Pacific, cool down over the cold offshore California Current just before they reach the San Francisco Peninsula. The fog forms because warm air is more likely to have a lot of water vapor in it. When that air is cooled, the humidity condenses out into clouds: fog is just a cloud at the surface instead of up in the air. Tourists are surprised to discover this sea fog is salty and irritating to the mucosal passages in the nose and throat. This is because water vapor needs something to condense around, and salt particles are plentiful in the air just above the waves and ocean spray.
Wind provides another sort of weather experience in Marseilles, France’s second largest city and largest commercial port. The nearby Mediterranean moderates temperatures here year-round, as do the surrounding hills. However, Marseilles sits directly in the path of the mistral, a strong, cold wind that forms further inland and to the northwest when there is an area of high pressure in the Atlantic between Spain and southeast France and an area of low pressure over the Gulf of Genoa in the Mediterranean Sea. Air always flows towards a zone of lower pressure, but the mistral is intensified, as it blows toward the Mediterranean, by the shape of the Rhone Valley, which channels it between the Alps and Cevennes and increases its velocity. By the time it reaches Marseilles, the mistral can be up to a Force 10 gale of cold, alpine air, and it may blow steadily for up to two weeks in winter.
♦ Lightning and rain
The city-state of Singapore has one of the highest rates of lightning activity on Earth. This seems odd, at first, because its weather changes very little over the course of the year, thanks to its location off the southern tip of Malaysia, just 1 degree north of the equator. None of the stormy weather patterns found in more northern latitudes trouble Singapore; it gets plenty of sunshine and is surrounded by lots of warm water, which makes it a very humid place as well.
This heat of the equatorial Sun and all that humidity actually are the main culprits behind Singapore’s frequent thunderstorms. The Sun evaporates water droplets at the sea surface and heats up the air just above the water. This warm, humid air then rises up into cooler air levels, where some of its moisture condenses into clouds and rain. A little heat is released by condensation, too, guaranteeing that the air will continue to rise even higher, cool more, and form even more clouds and rain. It turns into a cycle, forming thunderstorms.
Singapore gets, on average, over 170 thunderstorms a year. In comparison, Florida, which has the highest number of lightning-related deaths in the United States, gets fewer than 100 thunderstorms annually.
♦ Urban weather: Smog
While some cities have unique weather, all urban areas make their own weather to some extent. Probably the best known example is smog, a portmanteau word combining “smoke” and “fog”. Originally the word was used to describe the London murk around the turn of the 20th century. Like San Francisco’s fog, smog condenses around small particles in the air, but it isn’t pretty: a unpleasant, dirty mixture forms when those particles come from industrial smokestacks, traffic and residential areas.
Environmental legislation and other political and social changes greatly reduced this type of smog, but another kind was first reported during the 1950s. It is photochemical in nature, caused by the reaction that happens when sunlight hits airborne chemicals that arise from urban (and rarely, natural) sources. While the old type of smog was like a thick, black “pea-soup” fog, photochemical smog is a brown, acrid haze, and it hovers over many cities today.
♦ Urban weather: Wind
In any city that has a lot of buildings about three stories high or taller, an “urban canyon” effect happens that will increase the wind speed along certain streets and alleys, similar to the way France’s mistral wind accelerates as it runs down the Rhone Valley between two mountain ranges.
This effect may explain why Chicago is known as the Windy City when in fact it doesn’t even fall into the top ten windiest cities in the United States. Dodge City, Kansas, is actually the windiest city in the United States, with an annual wind speed of about 14 miles per hour, compared to Chicago’s annual average wind speed of a little over 10 miles per hour. However, Dodge City’s sole “skyscraper” is the Bank of America building, which is five stories tall. The city’s windiness comes from its location out on the prairie rather than the urban canyon effect. Chicago, on the other hand, has many skyscrapers that are clustered close together downtown. This creates, among other effects, a funnel that speeds up westerly winds at Madison and Jefferson streets. Anyone walking along the sidewalk there when this is happening will find it very easy to believe in Chicago’s super-breezy reputation.
♦ The urban heat island
The annual mean air temperature of a large city can be up to 5.4 degrees F warmer than its surroundings, but on a day-to-day basis this "heat island" difference is sometimes dramatic, up to 22 degrees F on a clear, calm night. Cities are warmer because open, moist land has been covered by hard surfaces that dry things out and absorb the sun’s heat, becoming 50 to 90 degrees F warmer than the air temperature. Dense traffic patterns, business, residential districts and industry add even more heat to the urban environment, and they also add air particles and water vapor.
The warm, moist air in and over a city rises, cools, and condenses into clouds and rain, just as tropical air at the equator does over Singapore. Wind and precipitation patterns are altered, and while no city distant from the equator can expect tropical weather from its urban heat island, some cities do show a rainfall increase of up to 51%, while precipitation rates downwind of cities are boosted anywhere from 56% to 116%, according to a NASA study reported in "The Guardian" in 2005.
♦ A mixed bag
San Francisco draws tourists with its mysterious, fog-shrouded streets. In Marseilles, they wait out the mistral and then enjoy the Mediterranean climate on days when it isn’t blowing, while in Singapore, citizens have learned to live with high humidity and always keep an eye out for lightning.
Most cities don’t have such dramatic weather changes to boast or complain about. Nothing good can be said about smog: it is a serious health problem and diminishes the quality of life. The rest of the ordinary urban weather experience is a mix of positives and negatives.
High wind speeds can cause expensive damage and injure people, but city winds also bring in cooler air during summer and blow away pollution. The urban heat island is dangerous and can be fatal during a heat wave. It leads to thermal water pollution from runoff and puts a heavy burden on our energy infrastructure, but it also lengthens the growing season and increases rainfall over what otherwise might be arid or even drought-stricken regions downwind.
As science learns more, perhaps some day we will be able to minimize the harmful effects of the urban environment and learn how to better manipulate its positive effects for the betterment of all people living in and near our 21st century cities.
Sources and more information:
“Designing Buildings for the Very Windy City.” Mark Meshulam (February 20), at http://chicagowindowexpert.com/windowtags/wind-speed
“Urban Heat Island,” Wikipedia (n.d.) at http://en.wikipedia.org/wiki/Urban_heat_island
“Basic Information: Heat Island Effect.” United States Environmental Protection Agency (February 9, 2009), at http://www.epa.gov/heatisld/about/index.htm
“The Mistral (wind).” Citizendia (2009), at http://www.citizendia.org/Mistral_(wind)
"Lightning Policy." Singapore American Community Action Council (n.d.), at http://www.sacac.sg/Sports/pdf/Safety%20lightning.pdf (Adobe Reader required)