Weather Fronts and Thunderstorms Exam Prep Notes

Cold Fronts vs. Warm Fronts

• The primary difference between cold and warm fronts is the direction of travel.
  • Cold Front: Cold air displaces warmer air.
  • Warm Front: Warm air displaces colder air.
• If a cold front reverses direction, it becomes a warm front.
• To determine a front type, identify the location of relatively warmer air.

Density Differences

• Colder air is denser than warmer air.
• Cold Front Approach:
  • The denser cold air displaces and lifts the warm air, creating a sharp boundary.
  • The slope of the cold air mass is steep, forcing rapid uplift of warm air.
  • This leads to quick cooling and condensation, causing cloud formation and precipitation close to the front.
  • The onset of a cold front is characterized by rapid cloud formation, precipitation, and a temperature drop over a short time.
• Warm Front Approach:
  • Warm air rides over the colder air, leading to cooling and condensation aloft.
  • Cloud formation occurs far ahead of the surface warm front.
  • Friction causes the cold air mass to be dragged along, resulting in a gentler slope compared to cold fronts.
  • Condensation can occur up to hundreds of miles ahead of the surface warm front.
  • Cloudiness and precipitation often precede surface temperature changes.
  • The telltale signature of a warm front is cloudiness well in front of the warm front.

Air Mass Characteristics

• Key air masses:
  • Maritime Polar (MP)
  • Continental Polar (CP)
  • Maritime Tropical (MT)
• Maritime tropical air masses contain the most water vapor and energy.

Weather Patterns

• Weather is typically found along both cold and warm fronts and ahead of warm fronts.
• Regions influenced by maritime tropical air can experience widespread precipitation due to the air being warmer and pushed north over cooler land, making it unstable.
• Before a warm front passes, cloudiness and precipitation can occur before changes in surface temperature.
• Cold fronts have a more abrupt impact with rapid weather changes because of the rapid uplift of warm air.
• Warm fronts depend on the stability of the cold and warm air masses.
• Unstable air (conditionally unstable) accelerates upward once condensation begins, leading to tall cloud formation and thunderstorms.
• Stable air results in less thick cloud layers and reduced condensation because the air isn't forced upward further.
• Saturation vapor pressure decreases with temperature, leading to more water condensing in colder conditions.

Factors Affecting Front Steepness

• Cold fronts are steeper than warm fronts due to friction.
• As cold air moves along the ground, friction with the surface and warm air causes it to burrow underneath, resulting in a steep temperature transition and abrupt updrafts.

Frontogenesis

• Frontogenesis: the development and evolution of frontal boundaries (cold and warm fronts).
• Process:
  • A stationary front exists between cold and warm air masses.
  • Westerly and easterly winds cause friction and shear, distorting the boundary.
  • The boundary rotates counterclockwise due to wind shear.
  • The stationary front breaks into a cold front and a warm front.
• The cold air pushing southward is denser than the warm air moving northward.

Occluded Fronts

• An occluded front occurs when a cold front catches up to a warm front.
• The warm air is lifted off the surface as the cold air pushes under it, leading to its dissipation.
• An occluded front marks the beginning and end of frontogenesis, as the warm air no longer contacts the surface.
• Without the temperature gradient, the weather event dissipates.
• The purpose of frontogenesis is to eliminate temperature gradients between air masses.

Key Takeaways on Frontogenesis

• When one front type runs into another, it is the beginning of the end of the fronts.
• Cold fronts typically behind warm fronts can catch up, lifting the warm air off the surface.
• Frontogenesis occurs to resolve temperature gradients between air masses.

Extreme Weather and Fronts

• The region between maritime polar (mP) and maritime tropical (MT) air masses is a prime location for strong weather activity.
• Extreme weather events associated with fronts include thunderstorms and tornadoes.
• Hurricanes are not associated with fronts, as they form in tropical regions with uniform air masses.

Thunderstorms

• Thunderstorms: rapid, effective updrafts of unstable, humid air leading to the formation of cumulonimbus clouds.
• Characteristics: strong precipitation and lightning.
• Frequency: most frequent in tropical regions; in the US, most frequent in the Southeast.

Convective Thunderstorms (Single-Cell)

• Powered by solar heating of the Earth's surface on hot, sunny days.
• Process:
  • Heated surface warms the air, causing it to expand and rise.
  • Water vapor condenses as the air rises, leading to cumulonimbus cloud formation.
  • Precipitation cools the ground, stopping surface heating.
  • The thunderstorm dissipates as updrafts weaken.
• Cycle: formation, maturation, dissipation, lasting about one to two hours typically.

Hazardous Conditions in Thunderstorms

• Strong updrafts and downdrafts lead to increased surface winds.
• Downdrafts: sinking air due to falling rain and cooling, can cause wind direction changes.
• Downdrafts and wind shear are dangerous to aviation; airports may shut down during thunderstorms.

Requirements for Thunderstorm Formation

• Heat, sunlight, humid air, instability, and a mechanism for uplift.
• Thunderstorms at night are usually associated with fronts and forced lifting, leading to more severe conditions.

Areas of Frequent Thunderstorms

• Florida Peninsula: most thunderstorm days due to air mass convergence from the Gulf of Mexico and the Caribbean.
  • The Florida Peninsula warms faster than surrounding waters, causing air to rise and draw in moist air from both sides.
• The intrusion of mT air causes storm convergence.

Severe Thunderstorms (Supercell)

• Characteristics: hailstones (one inch or larger), tornadoes, or sustained surface winds around 60 mph (100 km/h).
• Severe thunderstorms are typically associated with significant weather patterns like frontogenesis, not daily convective pop-up storms.
• Key requirements
  • Large scale: Supercell thunderstorms are much larger and require more energy, often spanning tens of miles in diameter and reaching altitudes of up to 60,000 feet.
  • Significant hail: Hail is a defining characteristic of severe thunderstorms; supercells sustain an environment conducive to the formation and preservation of large hailstones.
  • Tornadoe: Capable of producing tornadoes. The size and complexity of the cloud reflect the intensity of the updrafts, capable of generating violent tornadoes.
  • Fronts or triggers: Often require a pre-existing front, boundary, or trigger to initiate and focus on the updrafts to produce a supercell. Large scale weather pattern that organizes, structures and focuses the energy releases.

Mesocyclones and Tornadoes

• Mesocyclone: a rotating column of air, several miles wide, usually associated with supercell thunderstorms. It serves as the precursor to tornado formation, requiring wind shear to initiate the spin.
• What is Wind shear?
  • Wind shear: The difference in wind direction and speed at different heights is essential for creating rotation within a storm. This wind shear can cause a rolling pin effect in the air.
• How do Tornadoes Form?
  • Rolling to Vertical: The low rotation of the rolling pin then becomes tilted or lifted upwards during deep convection, forming a vertical vortex.
  • Elongation and Acceleration: The mesocyclone which is now vertically longer, grows narrower because of the effect of conservation of angular momentum, causing the rotating wind speed to increase further.

Significance of Pressure Gradient Force (PGF)

• Dramatic pressure changes. A 10% pressure change is commonly seen between a tornado and the air that surronds it.

Classifying Tornadoes - The Fujita Scale

• Measurements: An F5 is an ultimate tornado.

Distribution of Supercells and Tornadoes

• They do not form everywhere.
• Supercells are severe and create strong weather such as tornadoes. They have two ways that they spawn.
  • Frontogenesis, strong areas/regions of origin.
  • Hurricanes, hurricanes be strong and generate them. Primarily, they are over water; they can form over land also, but more uncommon. If a hurricane forms over water, then it is called a waterspout, but is still a tornado.
• Most occur counterclockwise. 1% of the time clockwise, but do not have to because are not affected by the Coriolis Effect, due to small formation.

Lightning

• Lightning: electricity resulting from the discharge of electrons.

• What is it?

  • Discharge of of electro-static energy.

• What makes it?

  • Charge differences from the ground to the sky.
  • Voltage potential is the difference of potential energy during lightning.

• To separate charge, there needs to be two things.

  • Electrons, everything is built of it.
  • Affinity is required.

• What does air do?

  • Air has a different affinity of potential than the ground, and clouds. This is how movement happens.

• What are these differences.

  • The ground where you have the charge has the potential to give it to a cloud.
  • Because it is touching a cloud that contains water. Which the cloud, water, and air. Creates a cycle.

• How does that transfer happen?

  • The movement of air happens because the physical process is friction. Friction causes different potential to the ground and cloud.