A storm is a significant disturbance in the Earth's atmosphere, characterized by disruptive weather phenomena that substantially deviate from typical conditions. While often associated with strong winds and precipitation, the defining characteristic of a storm lies in its intensity and potential for adverse impacts. Storms are manifestations of the atmosphere's dynamic processes, driven by imbalances in temperature, pressure, and moisture.
When does "bad weather" become a storm?
The distinction between "bad weather" and a "storm" is primarily one of intensity and potential impact. While there's no single, universally agreed-upon threshold, a weather event generally escalates to the designation of a storm when it exhibits:
- Significant wind speeds: Winds reaching sustained speeds or gust thresholds that can damage property, vegetation, or pose a risk to life. For instance, in many meteorological contexts, winds exceeding a certain Beaufort scale rating (e.g., strong gale or higher) are considered stormy.
- Heavy precipitation: Rainfall rates that lead to flash flooding, significant snow accumulation causing travel disruption, or large hail accumulation that can damage crops and property.
- Associated hazards: The presence of thunder and lightning (indicating a thunderstorm), tornadoes, blizzards, or other severe phenomena that elevate the risk beyond mere inconvenience.
- Disruption to normal activities: When the weather forces widespread closures, travel advisories, power outages, or poses a direct threat to public safety, it's generally considered a storm.
Essentially, "bad weather" might be a strong drizzle, gusty winds, or light snowfall. A "storm" brings with it a sense of urgency, hazard, and the need for protective action.
Kinds of storms
Storms manifest in various forms, each with unique characteristics and underlying atmospheric drivers:
- Thunderstorms: Characterized by the presence of lightning and thunder, produced by cumulonimbus clouds. They often bring heavy rain, strong winds, and sometimes hail or tornadoes. They can be single-cell, multi-cell, or highly organized supercells.
- Tropical cyclones (hurricanes, typhoons, cyclones): Rotating storm systems that form over warm ocean waters, characterized by a low-pressure center (the "eye"), strong spiraling winds, and torrential rainfall. They're called hurricanes in the Atlantic and Northeast Pacific, typhoons in the Northwest Pacific, and cyclones in the South Pacific and Indian Ocean.
- Winter storms/blizzards: Dominated by heavy snowfall, strong winds leading to blowing and drifting snow, and often dangerously cold temperatures. A blizzard is a severe winter storm defined by specific criteria for wind speed and visibility.
- Nor'easters: A type of extratropical cyclone that forms along the East Coast of North America, often bringing heavy rain or snow, strong winds, and coastal flooding. The name derives from the strong northeasterly winds they typically produce.
- Dust storms/sandstorms: Meteorological phenomena common in arid and semi-arid regions, characterized by strong winds lifting large quantities of loose sand and dust from the ground, significantly reducing visibility.
- Tornadoes: Violently rotating columns of air extending from a thunderstorm to the ground. They're among the most intense and localized of all atmospheric storms, capable of immense destruction over short distances.
- Ice storms/freezing rain events: Occur when precipitation falls as rain but freezes upon contact with surfaces at or below freezing temperatures, leading to a dangerous accumulation of ice.
What qualifies as a named storm?
The naming of storms is primarily applied to tropical cyclones (hurricanes, typhoons, cyclones) and, in some regions, to significant winter storms or extratropical cyclones that are expected to have widespread and substantial impacts.
- Tropical cyclones: A tropical depression strengthens into a tropical storm when its sustained winds reach 39 mph (63 km/h). At this point, it's assigned a name from a predetermined list. This naming convention helps in clearly identifying and tracking these potentially devastating systems, facilitating communication among meteorologists, emergency services, and the public. Once sustained winds reach 74 mph (119 km/h), it becomes a hurricane, typhoon, or severe cyclonic storm, retaining its given name.
- Winter storms/extratropical cyclones (regional): In some regions, like the United States, certain entities (e.g., The Weather Channel) have begun naming significant winter storms to raise public awareness. However, this isn't a universally adopted practice by official meteorological agencies worldwide. Similarly, in Europe, some meteorological services collaboratively name significant windstorms that affect multiple countries. The criteria for naming these storms typically involve wind speeds reaching gale force or higher, and the expectation of significant impacts.
The primary purpose of naming storms is to enhance clarity, aid in tracking, and improve public communication regarding potentially dangerous weather events.
What causes storms to manifest?
Storms are the result of atmospheric instabilities and the dynamic interplay of several fundamental meteorological principles:
- Temperature and pressure differences: The Earth is heated unevenly, leading to temperature gradients. Warmer air is less dense and rises, creating areas of lower pressure, while cooler, denser air sinks, creating areas of higher pressure. Air naturally flows from high-pressure areas to low-pressure areas, creating wind. The greater the pressure difference, the stronger the wind.
- Moisture content: Water vapor in the atmosphere carries latent heat. When moist air rises and cools, the water vapor condenses into liquid water (clouds and precipitation), releasing this latent heat, which further fuels the upward motion of air and intensifies the storm.
- Coriolis effect: The Earth's rotation deflects moving air (and water) to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This effect is crucial in the formation of rotating storm systems like tropical cyclones and extratropical cyclones.
- Lifting mechanisms: For storms to develop, air must be lifted to higher altitudes where it cools and condenses. Common lifting mechanisms include:
- Convective lifting: Solar heating warms the ground, which in turn warms the air above it, causing it to rise. This is a primary driver of thunderstorms.
- Orographic lifting: Air is forced upwards as it encounters mountains or elevated terrain.
- Frontal lifting: Warmer, less dense air is lifted over cooler, denser air along weather fronts (boundaries between air masses of different temperatures and moisture content). This is a common mechanism for mid-latitude storms.
- Convergence: Air flowing horizontally from different directions meets and is forced to rise.
- Jet streams: Fast-flowing ribbons of air in the upper atmosphere, particularly the polar and subtropical jet streams, play a significant role in guiding and intensifying mid-latitude storm systems by creating areas of divergence and convergence that can enhance upward motion.
The combination and interaction of these factors create the specific conditions necessary for various types of storms to develop, grow, and dissipate.
