It seems as though during the summer, the weather during an unsettled pattern is typically the same every day: Warm with a chance of an afternoon thunderstorm.

It’s hard not to wonder, though, why these thunderstorms flare up the greatest in the late hours of the day. How come we don’t see strong thunderstorms in the morning? Why does the best time to see t-storms always seem to be in the hours around sunset?

The simple answer comes down to the mechanics of thunderstorm formation, but to fully understand the reason, let’s break down how a thunderstorm is created in the first place.

Thunderstorms occur when there is a disturbance of air that begins rising. As it rises, water vapor within the air cools and releases heat. The air condenses, creating a cloud, which then grows until it forms storm clouds.

In the atmosphere, three steps govern how well thunderstorms can bubble up. The first is the air’s moisture content. Generally, a saturated atmosphere will cater better for development than a dry one. Here, the dew point is a super important value. Dew points need to be above 55 for thunderstorms to occur. This is why weather like this does not occur in the drier winter months.

Instability is the difference in the characteristics of an air parcel compared to the environment. For example, air near the ground can heat up much quicker than the air aloft. When this occurs, the lapse rate, or the rate at which air temperature falls with increasing altitude, increases. High lapse rates can allow for more air from below to travel aloft, furthering the charge for thunderstorm development.

Lift is the “spark” that allows this unstable air to quickly rise. Lift can come in several ways, including fronts, dry lines, orographically (lift along high terrain), or other smaller-scale mechanisms. Without lift, air can rise, but not to the level that can spawn strong storms. This is why it is so common to see thunderstorms riding ahead of a cold front- where there is a lot of lift.

With these three factors, it makes sense that thunderstorms occur in the summer- where there is ample moisture and heat in play throughout The Valley. But the question remains unanswered- how come these thunderstorms only occur in the later daylight hours?

 

 

Throughout the timeline of each day, temperatures go up and down thanks to the Sun and the area’s prevailing weather patterns. This is called the diurnal range. Typically, the cycle peaks in the later stages of the afternoon, when there is a balance between the sun’s position in the atmosphere and a maximum amount of heat emitted by the ground.

In the summer months, ample lift and moisture can be achieved at any point, whether that be at solar noon or the dead of night. When it comes to instability, it is this diurnal range that matters most. Remember, air near the ground heats up much quicker than air aloft, and throughout the day, the lapse rate gets larger and larger as more heat gets to the ground. This is why most days with this pattern will feature stable, calm mornings but suddenly turn stormy throughout the late hours!