Thunderstorms are common across most of North America and in many other parts of the world during the summer months, and more often than not, they occur during the afternoon and early evening hours. Why is it that storms are more common during the heat of the day compared to the overnight and morning hours?

The Role of the Sun in Thunderstorm Development

During the cooler months, we associate heavy cloud cover with precipitation. But during the summer months, as counterintuitive as it sounds, sunshine is actually a key driver when it comes to thunderstorm development.

The reason is that heating of the lower levels of the atmosphere relative to the air above the surface is necessary for moist air parcels to rise vertically and condense into towering cumulus clouds – which can eventually develop into thunderstorms. This process is known as convection.

Strong solar radiation is the quickest way to heat up the earth's surface, which will then allow moist air parcels to rise into cooler air aloft in the form of updrafts. The greater the rate of decrease in temperature with altitude (referred to as the lapse rate), the more unstable the atmosphere is, and the stronger the updraft. 

Why The Atmosphere Is Usually More Stable At Night

When it's been dark all night long, and when temperatures are at their minimum values during the morning hours, there is not enough heating at the surface alone to initiate the process of convection. 

Solar radiation that is absorbed by the earth's surface during the daytime hours "escapes" back into the atmosphere at night once the sun disappears – provided that cloud cover is minimal, as the presence of heavy cloud cover overnight can limit the amount of solar radiation that "escapes".

As a result of the loss of solar radiation, temperatures cool at a much faster rate near the surface overnight compared to the air aloft. This has a stabilizing effect on the atmosphere as the rate of temperature decrease with height is much lower.

In addition, as the air in the lower levels of the atmosphere cools overnight, it becomes more dense and sinks toward the surface. This effect can be magnified in mountainous areas as well as mountain slopes accelerate the rate of cool/dense air heading downslope.

As a result of the sinking cooler air, temperature inversion layers (where the temperature warms with rising altitude) typically develop on clear nights with calm winds, further stabilizing the atmosphere.

This is the opposite of the daytime hours, when sunshine results in warmer air near the surface, which is less dense than the cooler air aloft, making it easier for air parcels to rise vertically.

Why The Atmosphere Becomes Less Stable During the Afternoon

Solar radiation and associated daytime heating have a cumulative effect over the course of the day. So while the angle of the sun is typically highest around noon, the sun still remains strong enough through the duration of the afternoon for temperatures to continue warming up at the surface.

It can also take some time for minor temperature inversions in the mid-level of the atmosphere (known as a cap) to erode, which can delay thunderstorm development until around noon and sometimes much later.

This is why thunderstorms are most common during the afternoon hours and early evening hours, as this coincides with the warmest temperatures at the surface, and thus the greatest rate of temperature decrease with altitude (daily temperature differences between day and night are less as you gain altitude) which contributes to higher instability.

When exactly thunderstorms develop on any given afternoon are based on a number of other factors, such as moisture, pre-existing cloud cover, orographic lift, and any atmospheric "features" such as fronts or shortwaves that can trigger thunderstorms. 

It's also not uncommon for thunderstorm activity to persist past sunset. This typically occurs when clusters of strong thunderstorms have developed during the late afternoon and evening hours, and they maintain enough "juice" to continue well into the evening before the loss of solar radiation eventually causes them to weaken.

How Overcast Cloud Cover Can Suppress Thunderstorm Development

Sometimes, widespread flat cloud cover in the mid to upper levels of the atmosphere will arrive over a large area and can persist well into the day – if not all day.

This type of cloud cover (as opposed to scattered vertically growing cumulus) acts as a barrier to solar radiation reaching the surface, and limits the amount of heating needed for the atmosphere to destabilize.

As a result, flat overcast clouds typically prevent thunderstorms from developing. If cloud cover eventually erodes during the afternoon hours, then thunderstorms could develop in the hours to follow. But if the cloud cover persists all day, then it can lead to a "bust" in which thunderstorms do not develop at all.

Why Do Thunderstorms Occasionally Happen Late At Night Or During The Morning?

As is often the case in meteorology, there are usually exceptions to every rule of thumb. And it is true that sometimes thunderstorms happen during the overnight and morning hours. These instances are less predictable and can more easily catch you off-guard if you're outdoors.

The main driver of "off-hour" thunderstorm activity is elevated instability. This occurs when temperatures are cooling near the surface, but when a layer of unstable air exists in the mid to upper levels of the atmosphere, originating above the surface.

In cases of elevated instability, typically a trigger such as a trough or a shortwave is needed for thunderstorms to develop and the cloud bases of these storms are typically higher compared to surface-based thunderstorms.

Cool outflow winds from prior thunderstorms can also act as an overnight thunderstorm trigger when elevated instability is present.

These types of storms are always more difficult to predict, as often it takes just the right interaction between a "trigger" and the presence of elevated instability for thunderstorms to develop in the off-hours, and models often do not handle these situations well.

Therefore, it's always a good idea to pay attention to the skies if you're hiking or climbing early in the morning, and keep an eye out for lightning flashes and vertically building cumulus clouds, as opposed to flat cloud layers which typically indicate stability. 

Alan Smith, Meteorologist