Storms and Extremes

Weather & Meteorology Fundamentals

Course 1 · Chapter 6 · Storms and Extremes

Most weather is ordinary, but a small share of it is violent, and that share causes most of the damage. This chapter looks at the main kinds of severe weather, what energy source drives each, and how the British Isles fit in: far less exposed than some regions, yet far from immune. Dates and figures are given from general knowledge and flagged where they should be checked.

Thunderstorms

A thunderstorm needs three ingredients: moisture near the ground, an unstable atmosphere in which a rising parcel of air stays warmer than its surroundings, and something to start the lift, such as a front, a hill or strong surface heating. The result is a towering cumulonimbus cloud (see Weather & Meteorology Fundamentals 3), which can reach the top of the troposphere.

Lightning

Collisions between ice particles inside the cloud separate electric charge, and the discharge between regions of opposite charge, or between cloud and ground, is lightning (check the details, since the exact charging process is still debated). The flash heats the air to a very high temperature (check), and the sudden expansion is the thunder.

Counting the distance

Light arrives almost instantly and sound takes about three seconds to cover a kilometre (check). Counting the seconds between flash and thunder gives a rough distance to the strike.

Hail

Strong updrafts carry raindrops up into freezing air, where they gather layers of ice and fall only when too heavy to be held up. The stronger the updraft, the larger the stones (check the largest recorded size).

Downbursts

Cold air dragged down by heavy rain can hit the ground and spread outward as a sudden damaging wind, which is why the gust ahead of a storm can be worse than the rain.

Tornadoes

A tornado is a narrow, violently rotating column of air in contact with both the ground and a parent cloud. The strongest form beneath rotating thunderstorms called supercells, and the central United States sees more than anywhere else, in the region nicknamed Tornado Alley (check). The damage is classed on the Fujita scale or its enhanced version, from 0 to 5, according to the destruction caused (check the scale and its origin). The British Isles gets tornadoes too, mostly weak ones, and they are often among the highest per area in the world though not the strongest (check this claim).

Waterspouts and funnels A funnel cloud is a rotating cone that has not reached the ground. A waterspout is a tornado over water. Neither should be mistaken for the dust devil, a harmless-looking column of whirling dust that forms on a hot, clear day under no storm cloud at all.

Tropical Cyclones

A tropical cyclone is a huge rotating storm that feeds on warm ocean water. It forms over seas warmer than about 26 or 27 degrees Celsius (check the threshold), away from the equator so that the Coriolis effect (see Weather & Meteorology Fundamentals 4) can set it spinning, and it dies quickly over land or cold water. The same storm is called a hurricane in the Atlantic and north-east Pacific, a typhoon in the north-west Pacific and a cyclone in the Indian Ocean and around Australia.

  • Structure: a calm, clear eye at the centre, surrounded by the eyewall, the ring of the fiercest wind and rain.
  • Scale: Atlantic hurricanes are rated on the Saffir-Simpson scale from category 1 to 5 by wind speed (check the scale and its thresholds).
  • Storm surge: the rise of sea level pushed ashore by wind and low pressure is often the deadliest part, and more dangerous than the wind itself.
  • Reaching Europe: remnants of Atlantic hurricanes sometimes arrive as deep depressions, no longer tropical, and can strengthen the autumn storms of north-western Europe (check examples).

Snow, Ice and Cold

Snow falls when the whole column of air is cold enough, or nearly so, that ice crystals do not melt on the way down. In the British Isles the margin is thin, so a small change in temperature turns snow into rain or sleet. Freezing rain falls as liquid and freezes on contact with cold surfaces, making glaze ice, which is far more dangerous than snow. Cold spells in Britain usually come when a blocking high (see Weather & Meteorology Fundamentals 5) draws air from the east or north.

The Great Storm of 1987 In the early hours of 16 October 1987, a rapidly deepening depression crossed the south of England and the English Channel, bringing the strongest winds in the south-east for many years and felling an estimated fifteen million trees (check the date, the figures and the death toll). It was memorable partly because the forecasts had underestimated the danger, and a widely retold story about a television forecaster's earlier remarks is often misreported (check the story before repeating it). The event led to improvements in forecasting and warnings, and to a greater use of the colour-coded warning system that followed in later years (check the details).

Extremes and Records

Records are exciting but need care, since they depend on the instrument, the site and the rules for accepting a measurement. The highest and lowest temperatures, the strongest gusts and the heaviest rainfall for a place are kept by national weather services and by the World Meteorological Organization, which checks the world records (check the current holders before quoting any figure).

Check the details Death tolls, damage costs, wind speeds and record values vary between sources and are often revised. Treat every figure in this chapter as approximate and confirm it before quoting it.

Exercises

Exercise 1

List the three ingredients a thunderstorm needs, and say how you could estimate how far away a lightning strike was.

→ Solution
Exercise 2

Explain why a tropical cyclone weakens quickly when it moves over land, and why the storm surge can be more dangerous than the wind.

→ Solution
Exercise 3

Say why a small change in temperature can turn British snow into rain, and why freezing rain is often more dangerous than snow.

→ Solution

Quick Reference

  • Thunderstorm: moisture, instability and a trigger for lift; cumulonimbus cloud
  • Thunder distance: about three seconds per kilometre between flash and bang
  • Tornado: a rotating column in contact with the ground, rated 0 to 5
  • Tropical cyclone: fed by warm sea; weakens over land; storm surge is the main killer
  • Snow: needs cold air through the whole column; freezing rain makes glaze ice
  • Great Storm: 16 October 1987, a rapidly deepening depression over southern England

Next chapter: Forecasting, which covers observation networks, satellites, computer models and ensembles.