While we don’t have the capability to control the weather outside of the movies, we humans and our everyday activities really are able to indirectly influence the weather in ways that go far beyond our addiction to greenhouse gases.
Cities form heat islands:
They’re not exactly wrong when they call the capital of Georgia “Hotlanta.” Most populated areas generate heat simply by existing. The dense web of asphalt roads, concrete sidewalks, brick facades, and tar roofs are able to absorb a significant amount of heat from the daytime sun, even in the dead of winter. This human-made insulation, called the urban heat island effect, keeps city centers a tad hotter on hot days and a little less cool on cold days. The urban heat island effect is most noticeable during winter storms when air temperatures are hovering right around freezing, putting you right on the line between wet snow, an icy mix, or a cold rain. The artificial warmth from cities can influence the precipitation type in these storms, potentially lowering a city’s snow accumulations compared to its suburbs.
A 2011 study published in the Journal of Applied Meteorology and Climatology also found that the urban heat island effect can have a pronounced impact on thunderstorms that form over cities. The researchers studied 91 summertime thunderstorms that formed over Indianapolis, Indiana, and found that their research models could not replicate those thunderstorms without the influence of the urban area beneath the storms.
Jets create cirrus clouds:
The act of flying can create intricate patterns of clouds in the sky that wouldn’t have formed had we not perfected the art of air travel. The hot water vapour produced by the engine exhaust of a high-flying jet aircraft leave contrails, short for condensation trails, in their wake. Contrails can dissipate right away or linger for hours depending on upper-level humidity and winds. These man-made cirrus clouds are most common at high cruising altitudes, but places like Arctic and Antarctic get cold enough that contrails can form at or near ground level.
Crops jack up the humidity:
If cities can absorb the heat of the day and make it even hotter, you can imagine how the vast swaths of crops that blanket the countryside can also affect our daily weather. Instead of making it hotter, crops can make a humid day unbearable by sending moisture levels almost off the charts on a putrid summer’s day. Corn crops are notorious for pushing dew points — the temperature at which the air reaches 100 per cent humidity — up above 80°F in the middle of the summer, creating a heat index that soars far above 100°F. Compare that to a muggy day, which has a dew point around 70°F, or a comfortably dry day with a dew point in the low 50s.
The harvest can have the opposite effect, as seen in northern Oklahoma this summer when farmers harvested their winter wheat crops. The state’s Mesonet, a network of weather stations, found that the newly harvested areas were hotter and had a lower dew point than their cooler but muggier surroundings.
Nuclear power plants can trigger nuclear-effect snow:
Lake effect snow is a yearly phenomenon across North America’s Great Lakes, where bitterly cold air flows over the warm lake water, triggering convection that blows ashore as heavy bands of snow. The bands of snow are so intense that communities can see many feet of snow in one day, sometimes accompanied by thunder and lightning. It’s not only bodies of water that can cause this phenomenon. Nuclear power plants release large amounts of steam during the course of their operations, and on cold mornings when there’s enough moisture in the air, locations immediately downwind of a power plant’s steam stacks can experience “nuclear-effect snow,” which forms through similar means as lake effect snow. The phenomenon isn’t limited to just nuclear power plants, but they produce enough steam that it’s noticeable over a large area.
Paving increases the intensity of floods:
Our obsession with construction doesn’t stop at influencing temperatures. Paving over porous earth with relatively impervious materials like asphalt and concrete has also had a major impact on flooding during heavy rain events. Fewer places for rainwater to escape means that the sudden influx of water builds up in urban areas or runs off and inundates places that had never seen flooding before.
Urban density can amplify winds:
If you’ve ever walked down a city street on a windy day, you’ve probably noticed that it sometimes feels like you’re being buffeted by air shot out of an industrial fan instead of a regular windstorm. Dense building construction can amplify the winds and cause gusts to blow much faster than they would out in the open. This wind tunnel effect can cause serious damage, blowing out windows, knocking down trees, and sending dangerous debris hurtling toward the busy streets below.
The principle is the same as holding your thumb over the end of a garden hose to make the water spray out faster— the wind speeds up dramatically as it presses between the buildings. This is also why you should never take shelter underneath a bridge during a tornado.
