When does bad weather become Extreme Weather?
In the UK, we're used to bad weather, we get it all the time. And we love to talk about it (or so the stereotype goes). The weather is indeed... variable, perhaps making it more interesting than say the predictable weather of some of our European cousins. But our weather is actually pretty tame compared with some other part of the world. In fact, we are blessed with a moderate climate that throws up relatively few extreme weather events. This is due to a few factors: being an island and our mid-latitudes location play a big role in keeping our weather stable, and the North Atlantic Current keep winter temperatures about 10 degrees warmer than the would be if it were to 'turn off'.
We do experience some extreme weather though, sometimes a hurricane gets a little lost in the Atlantic and heads our way:
Hurricane Gordon: 2006
As an island, we also experience heat waves, droughts, floods, cold snaps, snow storms and high winds. We typically suffer a few tornadoes a year, although we have a long way to go before we reach the levels seen in tornado alley. Some of these events are clearly extreme, although we still don't have a good definition for what 'extreme weather' is.
What is Extreme Weather?
Definition 1: outside of normal conditions
There are a few different answers depending on which definition of an extreme weather event you take. The first is that on average
5% of weather is extreme (according to the NOAA). That is, events that happen on average less than 5% of the time should be classed as extreme. Whilst this is a useful working definition, it suffers from one major drawback: the amount of extreme weather events will never increase above 5%. Clearly this is a problem if we want to look at whether these events are increasing or decreasing in frequency! So this definition works better when thinking about weather systems, as opposed to thinking about how weather events fit into the large climate systems. This definition of extreme weather does avoid seemingly nonsensical claims like
"Extreme events will be the norm" by John Prescott though!
Definition 2: Outside of predefined limits
An alternative way of defining an extreme weather event, as taken from
section 1.3.3 in this paper on extreme events, is to pick a value for a variable that you are monitoring, and define a maximum or minimum for that variable which is 'extreme' when averaged over a particular time period. According to the paper for example, a typical maximum value for temperature averaged over a month is 25°C. Any time that the temperature exceeds this is classed as an extreme weather event. Although the particular limit is somewhat arbitrary and will be location dependent, this has the advantage of allowing you to easily track the increase or decrease in the frequency of extreme weather events over time.
Closing remarks
All extreme weather events are interesting in their own right. However, some are not easily modellable by climate models. This is because the models are not run at a high enough resolution, either spatially or temporally, to capture the processes involved. A good example of this would be tornadoes; it is hard to capture an event that happens over ~100s of metres/half an hour if your model is running with a grid size of 10km and once per hour (although this may change!). So in the next few posts I'll be focussing on extreme weather events that are more amenable to being modelled over yearly timespans such as droughts, heat waves and cold snaps.
EDIT: Added a heading for each definition
