17.S: Summary

Water and Humidity

Water, a critical element of weather, exists in three phases: solid, liquid, and gas. Transitions between these states involve significant energy exchange. Phase changes include freezing, melting, evaporation, condensation, deposition, and sublimation. These processes either absorb or release latent heat, affecting the surrounding environment. For instance, condensation releases heat, warming the environment, while evaporation absorbs heat, cooling the environment. This energy exchange helps regulate Earth's temperature and distribute energy, particularly through evaporation at the equator and condensation in storms like hurricanes. Humidity, the amount of water vapor in the air, influences weather patterns. Relative humidity is the percentage of water vapor present compared to the maximum the air can hold. At 100% relative humidity, air reaches the saturation at the dew point temperature, leading to condensation and precipitation. Temperature controls the air's capacity to hold water vapor, with warmer air holding more moisture than colder air.

Clouds, Fog and Precipitation

Clouds form when water vapor condenses into tiny droplets of liquid or ice, significantly impacting weather by blocking solar radiation, absorbing ground-emitted warmth, and serving as a source of precipitation. Cloud formation requires air to reach its dew point, either through temperature decrease or humidity increase, and typically occurs in unstable air conditions where air rises, cools, and condenses. Clouds are classified based on their altitude and appearance, including low-level clouds like stratus and cumulus, middle-level clouds such as altostratus, and high-level clouds like cirrus. Additionally, various types of fog and precipitation including rain, snow, sleet, freezing rain, and hail, form under specific atmospheric conditions involving temperature changes and moisture content. Fog forms when humid air cools near the ground, while precipitation requires cloud formation in the atmosphere. Different precipitation types depend on moisture, temperature variations and the presence of condensation nuclei.

Air Masses and Weather Fronts

An air mass is a large body of air with uniform temperature and humidity characteristics, which it acquires from its source region. These masses are moved by high-level winds and influence the weather of the regions they move over, potentially causing storms when they encounter areas with different characteristics. Air masses are classified by their temperature and humidity, such as maritime tropical (mT) for moist, warm air, and continental polar (cP) for dry, cold air. When two air masses meet, they form a weather front, which is a boundary that can lead to various weather phenomena, including precipitation and storms. There are different types of fronts—cold, warm, occluded, and stationary—each causing distinct weather patterns. Cold fronts, for instance, bring storms and clear skies after passing, while warm fronts lead to gradual weather changes and precipitation. Occluded fronts, where a cold front overtakes a warm front, and stationary fronts, where air masses do not move, also significantly impact weather conditions.

Thunderstorms

Weather occurs daily, but storms are not as frequent. Storms vary based on factors like air composition, origin location, and season. Thunderstorms are common, with 14 million occurring annually worldwide. They form when warm ground temperatures cause air to rise, cool, and condense, progressing through three stages: cumulus, mature, and dissipating. During the cumulus stage, updrafts create cumulus clouds, which grow until precipitation falls, leading to the mature stage characterized by strong winds, heavy rain, lightning, and possibly hail. The storm weakens during the dissipating stage as downdrafts dominate. Severe thunderstorms can last for hours, causing significant damage with high winds, flooding, and tornadoes. Lightning, a result of accumulated energy in cumulonimbus clouds, heats and expands the air, creating thunder. In the U.S., thunderstorms kill about 200 people and injure 550 annually, mainly from lightning strikes.

Tornadoes

Tornadoes, also known as twisters, are powerful phenomena generated by severe thunderstorms. Formed when rising air in a thunderstorm is replaced by surrounding air, creating a funnel, tornadoes can last from seconds to hours. With average wind speeds of 177 km/h (110 mph) and ground speeds of 45 km/h (28 mph), they typically travel around 25 km (16 miles) before dissipating. Despite affecting small areas, tornadoes can cause significant destruction, primarily due to flying debris, resulting in an average of 90 fatalities annually in the United States. Tornadoes often form where maritime tropical (mT) and continental polar (cP) air masses meet, particularly in spring. A notable event, the Super Outbreak in April 2011, consisted of over 300 tornadoes across 15 states in three days. Tornado predictions are imprecise, though meteorologists can forecast potential danger areas. Tornado intensity is measured on the Enhanced Fujita Scale, ranging from EF0 (gale) to EF5 (incredible), based on wind speed and damage caused.

Cyclones

Cyclones are intense storms characterized by rotating winds around a low-pressure center, moving counterclockwise in the Northern Hemisphere. They come in two types: mid-latitude cyclones, which cause winter storms in middle latitudes, and tropical cyclones, known as hurricanes, typhoons, or tropical cyclones depending on their location. Mid-latitude cyclones form at the polar front and are major sources of winter weather in the U.S. and Europe. Tropical cyclones originate in warm tropical waters, gaining energy from warm sea surfaces and low wind shear. Anticyclones, on the other hand, are high-pressure systems with clockwise rotating winds in the Northern Hemisphere, typically bringing fair weather. Cyclones, especially hurricanes, are categorized by the Saffir-Simpson Scale based on wind speed and potential damage. Hurricanes can travel vast distances, causing widespread destruction through wind, rain, and storm surges, as seen in events like Hurricane Katrina and Hurricane Sandy. As climate change progresses, there is concern that hurricanes will become more powerful, extend their reach, and lengthen their season.

El Niño and La Niña

Weather and climate, often confused, are distinct concepts. Weather refers to the atmospheric conditions at a specific place and time, including factors like temperature, air pressure, humidity, wind speed, precipitation, and cloudiness. Climate, on the other hand, represents the average of weather elements over a long period, typically 30 years, encompassing long-term patterns and extremes. Significant climate patterns include the El Niño-Southern Oscillation (ENSO), which affects global weather. El Niño events, characterized by warm, nutrient-poor water in the eastern Pacific, bring heavy rains to South America and drought to Southeast Asia, while La Niña events, with unusually cold water, bring the opposite effects. Both phenomena impact global atmospheric circulation, influencing temperatures and precipitation worldwide, with far-reaching consequences such as droughts, flooding, and shifts in fish populations.

References

All section summaries were generated by ChatGPT (version 3.5) and edited by N. Ikeda.

Author: OpenAI. Located at: https://chatgpt.com/. Accessed on: July 23, 2024.