Generally speaking, in a weather forecast issued in summer, the odds that thunderstorms are present are greater compared to another weather forecast, issued in winter.
And if you’re into the weather, you have likely already noticed that you’ll see more thunderstorms in late spring, summer and early autumn compared to other periods in the year.
Now, why is this the case? An interesting question, which we will try to answer in this article. For doing so, we’ll need to take a look at how thunderstorms grow in the troposhere, the lower part of the atmosphere where weather happens. Subsequently, once we know this, we can see through a diagram called Skew-T log-P how temperature greatly impacts how severe thunderstorms can become. Finally, we’ll be able to answer that particular question mentioned above:
Why are there more thunderstorms in summer than in winter?
Growth of thunderstorms in the troposphere
To answer this question, as said, we must take a look at general growth patterns of thunderstorms in the troposhere. Troposphere, you may ask? What’s that?
Very simple: it’s the lower part of our atmosphere where the weather happens. Our atmosphere itself has an altitude of tens of kilometres, but weather can only happen in the lowest 10 to (sometimes) 15 because sufficient water is present there.
In fact, airplanes often fly at cruising altitudes of 12 kilometres because they then avoid most weather – which improves the efficiency of flight.
Now, back to weather. From our article about the nascence of thunderstorms, we know that thunderstorms emerge out of regular cumulus clouds. These cumulus clouds form through condensation of water droplets because rising air cools down, and can contain less and less water once it cools. In the article, we also identified multiple ‘triggers’ for air to begin rising:
- Heat. Through thermal activity, i.e. heating of Earth’s surface by the sun, air can begin rising, because warmer air is less dense than colder air. Especially with thermal lows, which occur when it’s very hot at macro scale rather than micro scale, severe thunderstorms can occur.
- Dynamic trigger. Through a weather front, especially a cold front, which pushes warmer air upwards, air can begin rising – with convective activity as an end result. The same goes for so-called throughs and vores, which are extensions of low pressure areas, and where convergence of winds occurs, after air must rise.
- Orography. By consequence of mountainous terrain, air that is flowing in a particular direction is pushed “over” the mountain range. In those cases, air is also forced to rise. When this happens, air cools as well, with the possibility of condensation and storm formation.
Once condensation occurs, we know that a cumulus cloud forms – and that it can grow into a thunderstorm. But why does this happen once, but not another time? And, once a storm has initiated, why do some grow into monsters, while others remain relatively shallow and not-so-intensive?
To answer this, we must take a look at the vertical profile of the atmosphere.
Taking a look at the atmosphere: a Skew-T log-P diagram
The vertical profile of the troposhere can be captured in what is known as a Skew-T log-P diagram. Here is one such diagram. On the horizontal axis, you see the values for the parameters it plots (such as temperature as well as humidity related values). On the vertical axis, you see the altitude – expressed in hectopascals, i.e. air pressure. The latter makes sense because the higher you go, the less air presses onto you from above, and air pressure goes down. 100 hPa is often approximately 12 to 15 kilometres, depending on meteorological conditions.
Now, at the bottom – i.e., the 1000 hPa level – of the chart, you see a red line, hovering at approximately 27 degrees (you’ll have to follow the non-straight line to the axis). That’s the outside temperature at this particular location, somewhere near the Mediterranean Sea. The vertical movement of the red line indicates temperature with altitude – so, you see that outside temperature increases until approximately 900 hPa, after which it goes down.
At approximately 850 hPa, condensation can rise freely. That is: until that altitude, temperature keeps increasing with altitude – meaning that air that is rising won’t rise naturally, but needs to be pushed upwards. After this level, called the Level of Free Convection (LFC), air temperature falls fast enough so that the rising and cooling parcel of air is still warmer than the surrounding atmosphere (that’s why there is a difference between temperature increase and the LFC).
Air can then rise – and so can the storms! – until the Equilibrium Level (EL), which for our diagram seems to be even higher than 100 hPa, by following the non-straight lines to the top. Only at the EL, the rising air becomes cooler than the surrounding air. This means that our rising air parcel, and hence the storm, can grow to 12 to 15 kilometres – which could produce a severe thunderstorm!
Now, back to the question: why thunderstorms are more frequent during late spring and summer
In the previous section, we saw how severe convection can form if air can continue to rise – and how we can see this in a Skew-T log-P diagram.
This analysis allows us to answer that question as to why thunderstorms in summer are more frequent, and often more severe.
It’s truly simple: in summer, the differences between temperatures at lower altitudes (often +20 to +30 Celcius) and higher altitudes within the troposhere, are bigger than during winter.
When this happens, an air parcel that rises can rise quicker and reach higher altitudes – that is, the Equilibrium Level lies a lot lower, often around 3-4, sometimes 5 kilometers, instead of 12-15.
While storms therefore still appear on radar during winter, they cannot grow as high as the ones we see in late spring and summer – and that is why winter storms are often less severe.
In this article, we took a bit of a dive into why late spring and summer thunderstorms are more severe than winter ones. To answer this question, we first took a look at how thunderstorms grow in the first place. We saw that this happens through convection, which is triggered in any of three ways.
Subsequently, we looked at actual growth – and saw that an air particle, once rising, can keep rising until it reaches the Equilibrium Level. And given the fact that differences in temperature between lower and higher parts of our atmosphere are larger in summer than in winter, makes that once a thunderstorm appears, it is often more severe in summer.
I hope you’ve learnt something from this article! If you did, or if you have any questions or remarks, I would love to hear from you. Please feel free to leave a comment in that case, in the comments section below 🙂
Skew-T log-P diagram. (2004, December 12). Wikipedia, the free encyclopedia. Retrieved September 4, 2020, from https://en.wikipedia.org/wiki/Skew-T_log-P_diagram
How does a thunderstorm form? (2020, August 19). Mr. Weather. https://mister-weather.com/2020/08/19/how-does-a-thunderstorm-form/