Weather

• 1. Introduction

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  In 1281, a fleet sent by Kublai Khan sailed to invade Japan, and the wind destroyed it. The Japanese called those winds the Kamikaze. Weather had intruded directly into history, sparing a nation. This is the everyday subject we rarely stop to define. Weather is the state of the Earth's atmosphere at a specific place and time, described through temperature, humidity, cloud cover, and stability. It is the day-to-day reality of rain, snow, fog, and wind. Almost all of it unfolds in the troposphere, the lowest layer of the atmosphere, just below the stratosphere. How does this thin layer of air come to drive tornadoes, hurricanes, and the slow grinding of rock into soil? Why can forecasters never see more than about two weeks ahead? And why does weather appear not only on Earth, but on Jupiter, on Neptune, and in the empty space between planets?

• 2. The Engine Of Air

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  Air pressure, temperature, and moisture differences from one place to another set weather in motion. These differences trace back to the Sun's angle, which varies by latitude. The farther a place lies from the tropics, the lower the sun angle, spreading sunlight over a greater surface and cooling that spot. The strong temperature contrast between polar and tropical air gives rise to the largest atmospheric circulations. These include the Hadley cell, the Ferrel cell, the polar cell, and the jet stream. In the middle latitudes, weather systems such as extratropical cyclones arise from instabilities of the jet stream flow. In the tropics, different processes drive systems like monsoons and organized thunderstorm clusters. Because Earth's axis is tilted relative to its orbital plane, called the ecliptic, sunlight strikes at different angles through the year. In June the Northern Hemisphere tilts toward the Sun, so light falls more directly than in December. This effect causes the seasons. Surface temperature differences in turn create pressure differences. A hot surface warms the air above it, which expands, loses density, and lowers the surface pressure. The resulting horizontal pressure gradient pushes air from high to low pressure, creating wind. Earth's rotation then deflects that airflow through the Coriolis effect. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters shift the distribution of solar energy, a long-term influence named the Milankovitch cycles.

• 3. Inversions And Instability

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  Higher altitudes are typically cooler than lower altitudes, because most atmospheric heating comes from contact with the Earth's surface while radiative losses to space stay mostly constant. This produces the adiabatic lapse rate. Sometimes the pattern reverses and temperature increases with height, a phenomenon known as an inversion. An inversion can make mountaintops warmer than the valleys below. It can spawn fog and act as a cap that suppresses thunderstorm development. On local scales, different surfaces drive temperature differences, since oceans, forests, ice sheets, and human-made objects differ in reflectivity, roughness, and moisture content. The atmosphere is a chaotic system. Small changes to one part can accumulate and magnify into large effects across the whole. This instability is why weather is harder to predict than tidal waves or eclipses. Even in theory, useful day-to-day predictions cannot reach more than about two weeks ahead, an upper limit on how good forecasting can ever get.

• 4. Sculptor Of The Surface

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  Weathering breaks rocks and soils into smaller fragments and then into their constituent substances. As rain falls, the water droplets absorb and dissolve carbon dioxide from the surrounding air. That makes rainwater slightly acidic, sharpening the erosive power of water. The released sediment and chemicals then enter chemical reactions that reshape the surface further, including acid rain, while sodium and chloride ions settle as salt in the seas and oceans. Over time, geological forces can compress that sediment back into other rocks and soils. Through this slow cycle, weather stands among the fundamental processes that erode and remake the planet.

• 5. A Force In Human History

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  In 1565, a hurricane destroyed the French fleet and ended French claims to Florida, letting Spain conquer Fort Caroline. Extreme weather has redrawn populations and steered events again and again. More recently, Hurricane Katrina scattered over one million people from the central Gulf coast across the United States, the largest diaspora in the country's history. The Little Ice Age brought crop failures and famines to Europe. During the Grindelwald Fluctuation, from 1560 to 1630, volcanic forcing events seem to have driven more extreme weather, with droughts, storms, and unseasonal blizzards, and the Swiss Grindelwald Glacier expanded. The 1690s saw the worst famine in France since the Middle Ages. Finland endured a severe famine in 1696-1697, during which about one-third of the Finnish population died. In the present-day United States, the National Weather Service compiles an annual report of fatalities, injuries, and damage costs to crop and property, drawing data from offices across the 50 states and the territories. As of 2019, tornadoes had the greatest human impact, with 42 fatalities and over 3 billion dollars in crop and property damage.

• 6. Reading The Sky

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  Human beings have tried to predict the weather informally for millennia, and formally since at least the nineteenth century. Forecasting collects quantitative data about the current state of the atmosphere, then projects how it will evolve. Once an all-human craft based on barometric pressure, current conditions, and sky condition, it now relies on forecast models. Human input still matters, since a forecaster must pick the best model using pattern recognition, teleconnections, and knowledge of model performance and biases. Forecasts lose accuracy as the range increases, undermined by chaos, the heavy computation needed to solve the atmospheric equations, errors in measuring initial conditions, and gaps in understanding. Ensembles and model consensus help narrow the error and choose the most likely outcome. End users vary widely. Weather warnings protect life and property, temperature and precipitation forecasts guide agriculture and the commodity traders who follow it, and utility companies use temperature forecasts to estimate demand. The tropics behave differently. The vertical direction is perpendicular to Earth's axis of rotation at the equator, while at the pole the axis and the vertical align, so rotation shapes circulation more strongly at high latitudes. Clouds and rainstorms in the tropics arise more spontaneously and are harder to forecast, yet temperature there is easy to predict, because it changes little.

• 7. Bending The Weather

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  In 2008, China fired 1,104 rain dispersal rockets from 21 sites in Beijing to keep rain away from the opening ceremony of the Summer Olympic Games on the 8th of August. Guo Hu, head of the Beijing Municipal Meteorological Bureau, confirmed the operation's success. Rainfall of 100 millimeters fell in Baoding City of Hebei Province to the southwest, while Beijing's Fangshan District recorded 25 millimeters. The wish to control weather runs throughout human history, from ancient rituals meant to bring rain for crops to the U.S. Military's Operation Popeye, which tried to lengthen the North Vietnamese monsoon to disrupt supply lines. The most successful deliberate efforts involve cloud seeding, used to disperse fog and low stratus at major airports, to boost winter mountain precipitation, and to suppress hail. The evidence for these techniques' efficacy is inconclusive. Far clearer is the inadvertent kind. Industrial emissions of sulfur dioxide and nitrogen oxides produce acid rain that harms freshwater lakes, vegetation, and structures. Anthropogenic pollutants cut air quality and visibility. Greenhouse gases drive climate change expected to alter the frequency of droughts, extreme temperatures, flooding, high winds, and severe storms. Heat from large metropolitan areas has been shown to affect nearby weather even at distances as far as 1600 kilometers.

• 8. Beyond Earth

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  Jupiter's Great Red Spot, one of the most famous landmarks in the Solar System, is an anticyclonic storm known to have existed for at least 300 years. Studying weather on other worlds has helped scientists understand it here, since the same physical principles play out at different scales and in different chemical compositions. The Cassini-Huygens mission to Titan found clouds of methane or ethane that drop rain of liquid methane and other organic compounds. Earth's atmosphere holds six latitudinal circulation zones, three per hemisphere, while Jupiter's banded face shows many, Titan has a single jet stream near the 50th parallel north, and Venus has a single jet near the equator. On giant planets the lack of a surface lets wind reach enormous speeds, with gusts up to 600 metres per second, about 2100 kilometers per hour, measured on Neptune. Neptune receives only a fraction of the solar energy Earth does, yet its weather is far more intense, a puzzle for planetary scientists. The strongest planetary winds yet discovered blow on the extrasolar planet HD 189733 b, thought to carry easterly winds moving at more than 9600 kilometers per hour. Weather is not confined to planets. The Sun's corona is constantly lost to space, creating a very thin atmosphere throughout the Solar System, and the mass it ejects is the solar wind. Inconsistencies in that wind, along with coronal mass ejections, form space weather with features like pressure and wind. Those ejections have been tracked as far out as Saturn, and where the solar wind meets Earth's atmosphere it can paint spectacular aurorae and disrupt electricity grids and radio signals.