Weather Fronts, Winds, and Thunderstorms Notes

Clouds and Fronts

• Warm Fronts:
  • Gentle lifting. Results in stratus clouds due to limited vertical development.
• Cold Fronts:
  • Steep lifting angle, promoting rapid lifting.
  • Leads to tall clouds like cumulonimbus.

Cloud Characteristics

• Cloud height drops, and coverage increases as a warm front approaches.
• Classic warm front scenario: clouds form far ahead of the frontal boundary.
• Cold fronts have a narrow area of cloud activity; rapid cloud formation and thickening may occur.

Winds and Isobars

• Using Weather Maps with Isobars:

  • Wind direction can be determined based on high and low pressure areas.
  • Winds blow from high to low pressure.
  • Example: If there's high pressure to the southeast of point B, there are southeasterly winds heading towards the low pressure.

• General Wind Patterns:

  • Air behind a warm front is drawn up, often with a southerly component (e.g., southeast).
  • Air behind a cold front is drawn in, with a westerly component.

• Exam Considerations:

  • Behind a warm front, expect winds from the south. Multiple-choice questions may require narrowing down options.

Cloud Types and Fronts (Lab-Specific)

• For lab purposes, limit cloud type choices.
• Warm Front Cloud: Nimbostratus.
• Cold Front Cloud: Cumulonimbus.
• Example Sequence: Nimbostratus (warm front), Cumulonimbus (cold front), then clearing after the cold front passes.

Interpreting Weather Scenarios on a Map

• Consider the temporal sequence: a warm front passes (clearing), followed by conditions ahead of a cold front (Cirrus, Cumulonimbus), then clearing again after the cold front.
• For exam clarity, shading on the map will indicate precipitation.
  • If rain is falling ahead of a cold front, expect cumulonimbus clouds.

Low Pressure Systems and Fronts

• Low pressure systems rotate counterclockwise in the Northern Hemisphere, driven by the jet stream moving west to east.
• The location of the front is most important to consider.
• Weather conditions move from west to east.
• Example: A cold front brings rain, modified by marine air masses.

Frontal System Dynamics

• The relative sequencing (warm front, cold front) remains consistent.
• The entire system rotates counterclockwise and progresses from west to east.
• Low pressure center rotation is the engine behind the weather system.

Occluded Fronts

• Expect heavy occluded precipitation.
• Warm air is lifted, moisture precipitates out, leaving cooler and colder air at the surface.

Jet Stream Influence

• The jet stream influences surface weather by steering low pressure systems.
• Polar jet stream divides air masses and guides their movement.

High Pressure Systems

• High pressure centers can act as anchors, deflecting the jet stream.
• Persistent high-pressure ridges can lead to extended periods of specific weather conditions (e.g., cold and dry).

Jet Stream Analogy

• The jet stream acts like a conveyor belt, carrying weather systems.

Rossby Waves and Jet Stream

• Mountains, temperature differences, and high-pressure zones can create bends in Rossby waves, influencing the jet stream.
• The jet stream is generally westerly with deviations.
• Coriolis effect influences the direction of jet streams in both hemispheres.

Air Mass Sequencing

• Polar air is in front of colder air.

Thunderstorm Geography and Drivers

• Thunderstorms are more frequent in areas with warm air, good surface heating, or frontal boundaries.
• Key drivers include surface heating for convective thunderstorms and cold fronts for frontal lifting thunderstorms.
• West Coast has fewer thunderstorms due to marine air influence.
• Denver experiences thunderstorms due to warming via air subsidence off the Rockies.

Thunderstorm Life Cycle

• Thunderstorms have three stages dictated by airflow: cumulus, mature, and dissipating.
• Classic convective thunderstorms last about an hour due to the cut-off of the warm air supply.

Cumulus Stage

• Dominated by warm air updrafts.
• Latent heat release is important as warm air rises and condensation occurs.
• Positive feedback loop: warm air rises, latent heat is released, and the cloud heats up.

Mature Stage

• Vertically developed cumulonimbus cloud.
• Updrafts continue to feed the cloud.
• Freezing height is reached.
• Droplets in the cloud grow until they're too heavy and start to fall.
• The more vigorous the lifting, the larger the droplets can be before they fall.
• Bellingham's light precipitation is due to limited vertical development.
• Cold air downdraft starts due to falling precipitation, cutting off the warm air supply.

Hail Formation

• Hailstones form as ice pellets get caught in updrafts and go up and down. This forms layers.
• The larger the hailstones, the more vigorous the convection.

Lightning and Thunder

• Occur in well-developed clouds with ice crystals.
• Charge separation occurs (positive and negative).
• Lightning is the equalization of charge.
• Lightning rapidly heats the air, causing expansion and thunder.
• Light travels faster than sound, creating the experience of seeing lightning before hearing thunder.

Dissipating Stage

• The cloud collapses as the warm air source is cut off.
• Lighter showers occur with a column of downdrafts.

Other Considerations

• Thunderstorms can occur at any time, especially with frontal lifting.
• Afternoon thunderstorms are classically convective.
• Storms in the dark are likely due to cold fronts.
• The cloud can evaporate if surrounding air is warm and dry.
• Condensation must occur for a storm to be declared a thunderstorm condition.
• Thunderstorms must go above the freezing point in order to be called a storm.
• Smaller thunder storms tend to be lower and not at the above freezing temperature when they are not getting much solar radiation, making them less severe.

Rainbows

• Occur when water vapor is at a 42-degree angle from the sun and scatters light.

Predicting Thunderstorms

• Requires knowing the rate of cooling in the atmosphere relative to the air mass.
• The air will rise as long as it's warmer than the surrounding air.
• Local lapse rate and atmospheric conditions must be considered.