L5 - UK Weather

UK weather is primarily influenced by low pressure systems known as extratropical cyclones or depressions, which are frequently associated with changeable weather patterns. These systems develop predominantly in the mid-latitudes, where cold polar air meets warm tropical air, leading to dynamic weather conditions. The study of mid-latitude weather systems began in earnest with advancements in synoptic weather observation techniques during the 19th century, allowing meteorologists to better track and forecast these complex systems.

Norwegian Cyclone Model

• Description: A theoretical framework illustrating the life cycle of extratropical storms. This model helps explain storm development and the various stages complex weather systems undergo.

• Life Cycle: These cyclones usually last between 2 to 8 days and exhibit distinct stages as they evolve:

  • A: Early Stage - Initial development of low pressure with a well-defined front.

  • B: Open Stage - The storm is fully developed with strong warm and cold fronts.

  • C: Occluded Stage - The cold front catches up to the warm front, and the storm begins to weaken.

  • D: Dissolving Stage - The cyclone dissipates as energy is exhausted and the fronts become less defined.

Development of Middle-Latitude Cyclones

• Low pressure systems develop primarily through vertical motion influenced by upper air dynamics, including jet streams that play a crucial role in shaping weather patterns.

• Surface winds converge at low pressure centers, leading to rising air, while at high pressure centers, winds diverge, causing descending air temperatures.

• Geostrophic Balance: In the upper levels of the atmosphere, where friction is negligible, winds follow a geostrophic balance that facilitates the development of cyclones. When an upper-level low coincides with a surface low, the cyclone may dissipate due to filling, indicating a loss of organization within the storm.

Weather System Dynamics

• Tilt of Weather Systems: Typically, weather systems exhibit a tilt westward with height due to the Coriolis effect, which causes the winds to curve. This tilt influences how the systems interact with surrounding air masses.

  • Region of upper-level divergence: Found above surface lows, this area promotes ascent, allowing for precipitation and storms to develop.

  • Upstream of troughs: Areas of convergence often lead to downward motion, reducing precipitation and stabilizing air.

• Intensity Mechanics: The strength of upper-level divergence compared to surface convergence is critical in determining cyclone intensity:

  • Strong upper-level divergence: Results in lower surface pressure, intensifying the cyclone.

  • Weak upper-level divergence: Allows surface pressure to increase, leading to weakened systems.

Planetary Waves

• It is common to observe 3 to 6 troughs and ridges, globally referred to as Rossby waves, which are instrumental in maintaining large-scale weather patterns and influencing storm tracks.

• These planetary waves provide an overview of cyclone development at various atmospheric levels, including sea-level pressure and 500mb heights, supporting forecasting efforts.

Surface Weather Features

• Cyclonic circulation sharpens temperature gradients, producing well-defined fronts that serve as boundaries between different air masses:

  • Front Types:

  • Warm front: Characterized by gradual precipitation from nimbostratus clouds, associated with steady rain over expansive areas.

  • Cold front: Typically brings intense precipitation and storm events, marked by towering cumulonimbus clouds.

Anticyclones

• High pressure systems known as anticyclones are characterized by subsiding air that promotes calm conditions and fewer clouds. These systems can persist for extended periods, leading to significant weather patterns such as clear skies and stable temperatures.

• Persistent anticyclones can cause extreme weather events, such as severe cold in winter or heatwaves during summer months, profoundly affecting local climates.

Blocking Events

• When anticyclones position themselves strategically, they can block the progression of mid-latitude cyclones, leading to significant and sometimes prolonged changes in weather patterns.

• Notable effects include extended cold winters or persistent heatwaves during summer as weather systems are stalled by these blocking patterns.

North Atlantic Oscillation (NAO)

• The North Atlantic Oscillation is a crucial driver of winter climate variability in the North Atlantic region, affecting numerous countries, including the UK. It describes the pressure variation between the subtropical high (Azores) and the subpolar low (Iceland).

  • Positive NAO: Characterized by a strong Azores high and a deep Icelandic low, leading to increased frequency of winter storms and warmer, wetter winters across Europe.

  • Negative NAO: Signifies weak pressure systems resulting in fewer storms and variable temperature distributions across the region.

Ensemble Weather Forecasting

• Forecasting weather accurately beyond a 10-day horizon remains challenging due to the chaotic nature of atmospheric conditions. This unpredictability necessitates advanced methods to improve forecast accuracy.

• Ensemble forecasting: A technique that produces varied probable future atmospheric states, enhancing the reliability of predictions. Historical example: Forecasts of London's temperature from the mid-1990s highlighted significant variances in outcomes due to uncertainty in atmospheric behavior.

Key Points Summary

• The UK’s weather is primarily dominated by extratropical cyclones, which have a typical life cycle of 2-8 days.

• Cyclogenesis relies heavily on the westward tilting of weather systems and the dynamics of upper-level divergence and surface convergence.

• Fronts serve as crucial indicators of weather conditions, delineating boundaries between warm and cold air masses across the UK.

• Anticyclones are associated with stable, calm weather and can create blocking patterns that significantly influence seasonal weather.

• The NAO is a key factor in determining storm frequency and temperature distributions, shaping the overall climatic patterns experienced in the UK.

Historical Context

• Records dating back to the founding of the Radcliffe Observatory in 1772 provide invaluable long-term meteorological data that correlate with climate change trends observed in the UK. These records notably illustrate warming patterns that have been particularly pronounced in recent decades, highlighting the impact of climate change on local and national weather patterns.