In like a Lion, out like a Lamb

The expression ‘In like a Lion, out like a Lamb’, is recorded as far back as the 1600’s but is thought to be much older.

It is used to describe the tendency of the month of March to begin with stormy weather yet end with calm, after all March marks the start of meteorological spring and is a month of major changes in our weather cycles.

Radiation and vortices

There are many factors which determine the cycles the British climate goes through in a year, in March the main ones in play are the increase in the day-length and changes in direction of the polar stratospheric vortex.

The increase in day-length, therefore the amount of solar radiation we receive in a day, which increases by over 2 hours from the 1st to the 31st of March, brings with it warmer weather, an average temperature rise of 9° to 12°Celsius, if this is associated with high pressure over the British isles this can hold off cold weather from the north.

The Polar Stratospheric Vortex is a weather system which sits to the north of the jet stream during the winter months, it brings winter storms to the British isles from over the Atlantic to the West, laden with moisture which can precipitate as snow. This system is held in check by the undulations of the jet stream which dictate the part of the isles to which a storm might be directed.

Beast from the East

This year (over the next week or so as I’m writing this) the vortex is predicted to be forced south towards the British isles, leading to a period of very cold weather, predicted by the Met Office to be as low as -15 °C in some areas.

This is because of a phenomenon known as Sudden Stratospheric Warming (SSW) where the upper layer of the atmosphere, the Stratosphere, about 10 miles up, is warmed and denser, cooler weather is forced down. This process is much too complex to describe in detail here, there are factors in play such as ‘Rossby waves’, massive atmospheric waves which break up the top of the polar vortex and weaken it, and others, which all interact to shape the weather that we experience here on the ground.

This weakening of the Westerly polar vortex allows colder air from the East to reach further West, and if the boundary of these conflicting weather systems meets over us here in the British isles it can cause winter storms of a ferocity we are un-accustomed to, these storms have been given the nickname ‘the beast from the east’.

The last time we experienced a ‘beast from the east’ was in March 2018, when storm Emma brought with her winds of 70mph, temperatures of −14.2 °C and 22 inches of snow, which led to an estimated 17 deaths and over £1 billion of damage to property.

Out like a Lamb

The good news is that these weather systems usually don’t last very long before the vortex weakens and moves back home to the north, allowing drier, and calmer weather to assert itself by the end of the month, thus ‘out like a Lamb’, hopefully this happens sooner, rather then later, and this beast is more of a kitten then a Lion!

A B-H