Air Masses and Fronts

Air Masses and Fronts

• An air mass is a large body of air with relatively uniform temperature and humidity characteristics.
• Air mass source regions are areas where air masses originate, generally located over large, flat areas with uniform surface conditions.
• Favorable conditions for air mass source regions include consistent temperature and moisture characteristics.

Air Mass Classification

• Air mass classification is based on two factors: moisture content and temperature.
• Five air masses commonly occur in North America: maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP), and Arctic (A).
• A maritime tropical air mass is warm and humid, often located near the Gulf of Mexico and Southeast U.S.
• Maritime tropical air masses are known for frequent thunderstorms.

Fronts

• Fronts are boundaries between different air masses.
• Types of fronts include cold, warm, occluded, and stationary, each with distinct definitions.
• Symbols and colors represent each type of front on weather maps, indicating the direction they move.
• Front identification relies on wind direction shifts, temperature changes, dewpoint changes, and pressure minima.
• Frontal passage characteristics include changes in temperature, dewpoint, pressure, wind direction, and precipitation.
• Common precipitation phenomena are associated with each type of front; squall lines tend to occur at or ahead of cold fronts while warm fronts can bring sleet or freezing rain.

Mid-Latitude Cyclones

• A mid-latitude cyclone is a large-scale weather system that occurs in the middle latitudes.
• Cyclogenesis is the development or strengthening of a cyclone.
• The Norwegian Cyclone/Polar Front Theory explains cyclone formation in steps and describes the life cycle of a cyclone.
• The warm sector in a cyclone is important as it influences the storm's development and intensity.
• Cyclogenesis is typically favored on the leeward side of mountain ranges.
• A nor'easter storm is a type of cyclone that affects the northeastern coast of North America.
• A "bomb" cyclone is a mid-latitude cyclone that intensifies rapidly, with a central pressure drop of at least 24 millibars in 24 hours.

Surface Cyclone Intensification

• Upper-level divergence plays a role in surface cyclone intensification, including jet streaks.
• The relationship between the strength of upper-level divergence and lower-level convergence affects cyclone intensity.
• The ideal position of the upper-level trough relative to the surface cyclone enhances intensification.

Surface Anticyclone Intensification

• Upper-level convergence plays a role in surface anticyclone intensification.
• The relationship between the strength of upper-level convergence and lower-level divergence affects anticyclone intensity.
• The ideal position of the upper-level ridge relative to the surface anticyclone enhances intensification.

Thunderstorms and Lightning

• Three necessary ingredients for thunderstorm formation are moisture, instability, and a lifting mechanism.
• A fourth ingredient, wind shear, is required for severe thunderstorms.
• Ordinary thunderstorms have stages of development and don't last long due to a lack of sufficient wind shear or instability.
• Severe thunderstorms last longer than ordinary thunderstorms because of their favorable atmospheric conditions.
• A gust front is a boundary separating a mass of cool, descending air from a thunderstorm from warmer, ambient air.
• Key factors for hail formation in thunderstorms are strong updrafts and supercooled water.
• Squall lines are lines of thunderstorms, typically located ahead of fronts.
• Downbursts (microbursts) are localized columns of sinking air within a thunderstorm, causing an outward burst of damaging winds at the surface.
• Downbursts pose a threat to aviation due to sudden changes in wind speed and direction.
• The region with the most thunderstorms in an average year in the U.S. is in Florida.

Lightning

• Lightning is caused by the buildup and discharge of electrical energy in the atmosphere.
• Thunder is the sound caused by the rapid heating of air around a lightning channel.
• The formation process includes a stepped leader and return stroke.
• Negative charges are typically found in the lower part of a cloud, while positive charges are on the ground.
• The first visible sign of lightning is the stepped leader.
• Types of lightning include in-cloud and cloud-to-ground.

Tropical Cyclones (TCs)

• Tropical waves and disturbances form due to atmospheric instability and convergence.
• Tropical cyclones do not form along the equator due to the Coriolis effect, which is needed for rotation.
• A hurricane and a typhoon are the same type of storm, but they are named differently based on location.
• Tropical cyclones movement is caused by a combination of steering winds and the storm's own internal dynamics.
• A tropical depression is weaker than a tropical storm, and a tropical storm is weaker than a hurricane.

TC Structure

• The eye has the lowest pressure, sinking air, and partially clear skies.
• The eyewall has the strongest winds, rising air, and powerful thunderstorms.
• Outer spiral bands bring heavy rain/thunderstorms and sometimes tornadoes on landfall.
• Outflow is the divergence of air aloft, typically anticyclonic (clockwise).
• Inflow is the convergence of near-surface air.
• TC formation, intensification, and dissipation depend on water temperature, wind shear, moisture, and surface convergence.

Factors that Weaken TCs

• Factors that cause a TC to weaken include cooler water temperatures and increased wind shear.
• Environmental conditions that favor TC intensification includes warm water temperatures.
• Tropical cyclones differ from mid-latitude cyclones in their formation mechanisms, structure, and energy sources.
• The quadrant of a tropical cyclone with the strongest winds varies by direction of motion.
• A Hurricane Watch indicates hurricane conditions are possible, while a Hurricane Warning indicates hurricane conditions are expected.
• Storm surge is an abnormal rise of water generated by a storm, caused by the storm's winds pushing water toward the shore.

Climate

• Weather refers to short-term atmospheric conditions, while climate refers to long-term patterns and averages.
• Analyzing past climate change involves comparing past global temperatures to today's and using ice cores to understand past climate.
• The primary cause of current climate warming is anthropogenic greenhouse gas emissions.
• Areas of the globe that have warmed the most include the Arctic.
• Positive feedbacks amplify warming, while negative feedbacks dampen it.
• The water-vapor greenhouse feedback and the ice-albedo feedback are examples of climate feedback mechanisms.
• Aerosols are small particles suspended in the atmosphere that can affect the climate by reflecting sunlight or influencing cloud formation.

Volcanic Eruptions

• Large volcanic eruptions can affect the climate by injecting aerosols into the stratosphere.
• Sunspots and their cycles impact climate.
• Orbital variations (Milankovitch cycles) include tilt, eccentricity, and precession, and impact climate.
• Current climate change examples include sea level rise and warmer temperatures.
• Common greenhouse gases include carbon dioxide, methane, and nitrous oxide.
• The current concentration of carbon dioxide is about 420 parts per million (ppm).
• Greenhouse gases trap heat in the atmosphere.
• Most of the warming due to greenhouse gases has gone into the oceans.