Weather

Weather is the state of the Earth's atmosphere at a specific place and time. It is usually described by looking at temperature, humidity, cloud cover, and stability. Most weather phenomena occur in the troposphere. This is the lowest layer of the planet's atmosphere, located just below the stratosphere. It is important to distinguish weather from climate. Weather refers to day-to-day conditions like precipitation and temperature. Climate is the term used for the average of these atmospheric conditions over much longer periods of time.

Weather is driven by differences in air pressure, temperature, and moisture between different locations. These differences often begin with the Sun's angle at a specific spot. Because Earth's axis is tilted relative to its orbital plane, known as the ecliptic, sunlight hits the surface at different angles. This tilt also causes the seasons. In the tropics, sunlight hits more directly, while at higher latitudes, the light spreads out and makes locations cooler. These temperature differences cause pressure differences. When a surface is hot, it warms the air above it. This causes the air to expand, which lowers its density and the resulting surface air pressure.

Air moves to balance these pressure differences. A horizontal pressure gradient moves air from higher pressure regions to lower pressure regions, which creates wind. As the air moves, the Earth's rotation causes a deflection of the airflow due to the Coriolis effect. On a large scale, the strong temperature contrast between polar and tropical air creates massive atmospheric circulations. These include the Hadley cell, the Ferrel cell, and the polar cell, as well as the jet stream. In the middle latitudes, weather systems like extratropical cyclones are caused by instabilities in the jet stream flow.

Temperature also changes with altitude. Higher altitudes are typically cooler than lower altitudes because most atmospheric heating comes from contact with the Earth's surface. However, sometimes an inversion occurs. An inversion is when the temperature actually increases with height. This can make mountaintops warmer than the valleys below. Inversions can lead to fog and can act as a cap that stops thunderstorms from developing. On a local scale, different surfaces like oceans, forests, or ice sheets can cause temperature differences because they have different levels of moisture or reflectivity.

Earth's weather is a chaotic system. This means that small changes in one part of the system can grow to have massive effects on the whole system. This instability makes weather forecasting difficult. While scientists use technology to predict the atmosphere, it is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead. We can see similar processes in space, too. Jupiter has a famous anticyclonic storm called the Great Red Spot. This landmark storm has existed for at least 300 years.

Weather is also a fundamental process that shapes the physical Earth through weathering. This process breaks down rocks and soils into smaller fragments and substances. During rain, water droplets absorb carbon dioxide from the air. This makes the rainwater slightly acidic, which helps the water erode surfaces. This released sediment and chemicals can then participate in further chemical reactions. Over time, geological forces can turn this sediment back into rocks or soils.

Weather has also played a direct role in human history. Extreme weather can cause populations to move or change the outcome of events. For example, the Kamikaze winds helped save Japan from a Mongol invasion in 1281. In 1565, a hurricane destroyed a French fleet, allowing Spain to conquer Fort Caroline. More recently, Hurricane Katrina redistributed over one million people across the United States. In the U.S., the National Weather Service tracks the impact of weather. In 2019, tornadoes caused over 3 billion dollars in crop and property damage.