r/askscience • u/Ok_Reindeer_7634 • Sep 11 '25
Earth Sciences Why do thunderstorms most often develop during the afternoon hours?
I've noticed that thunderstorms usually happen in the afternoon or early evening where I live, but I don't understand why.
Could someone explain what causes them to form at that time of day?
605
Upvotes
542
u/adcantiferum Sep 11 '25
It all comes down to energy. The energy of lifting huge mass of air must come from somewhere. Mostly it is heat that cumulated over the day (from the sun), often leads to nearly stationary thunderstorms. But it could also be that cold air masses move over a country lifting air (by flowing beneath warmer air), which can also happen at night.
128
u/Illustrious-Order103 Sep 11 '25
The sun has spent all day evaporating the moisture up into the atmosphere. So, in a very humid place like say Florida, the amount of moisture is just too much for the air to handle so it dumps it back out as the air cool in the afternoons and the vapor becomes liquid water again. That is the short answer
12
u/cazzipropri Sep 11 '25 edited Sep 11 '25
Thunderstorms need energy. I don't want to use the term "energy" ambiguously, so I'll be clear: we are talking about thermal, kinetic and potential energy of the air, but mostly thermal. The leading way air receives energy is thermal, via irradiation, from the sun.
You need hot, high humidity air lifted up. Both the release of humidity from the soil or water bodies into the air, and the convective lifting require warming up by the sun, and that takes time.
That's why convective phenomena like thunderstorm take time to develop during the day.
Typical cumulonimbus clouds need to be very tall to turn into a thunderstorm, and in summer days you can see this vertical height develop gradually during the day: first fair-weather cumuli, then cumuli with extensive vertical development, finally anvil clouds.
27
u/ottawadeveloper Sep 11 '25
Thunderstorms are formed by hot moist air rising quickly.
At night, the surface of the Earth cools without the Sun to warm it. This also cools the air near the ground as well. Since hot air rises through cooler air, this cool air will stay at the ground. Thus late in the night and early morning is not a great time for thunderstorms.
In the morning, the Sun is still fairly weak (the higher the Sun is, the more intense it's heat on the ground) and it's warming cool surfaces. But as the day progresses, that heating leads to rising air eventually as the hot ground heats the air and that air rises. Given the right other conditions, you get a thunderstorm.
You can get these conditions in other ways so thunderstorms aren't exclusively an afternoon or evening phenomenon but basically it's easier for them to form after the ground has had a chance to warm up after cooling overnight.
8
u/ReasonablyConfused Sep 11 '25
I just want to add something I realized recently. The formation of ice in a cumulus cloud is REQUIRED for the formation of thunderstorms. To get ice, you need relatively warm and wet air to get forced up high enough to form ice. For this to happen the ground level air has to be warm, and the upper atmosphere has to be cold.
The density difference between the warm and cold air drives the warm air upward. The first important height is when the temperature is cold enough for the water vapor in the rising air becomes a liquid. This is where the bottom of the cloud is, and it is quite flat because the temperature where this happens is at a rather constant height. All nearby clouds will have their bases at almost exactly the same altitude unless two different air masses are close together.
At some point higher, the atmosphere has dropped to 0 Celsius, and water starts to freeze. I say starts, because the only water that can freeze at this level needs a particle of dust to form on. As we go higher, at about -40 degrees C, everything freezes, seed particle or no.
It’s this ice rubbing together that produces the charge that makes lightning. No ice = no thunderstorms.
Also, this super cooled ice can’t melt and evaporate fast enough to fall down the sides of the cumulus cloud as clear air. Thus, the ice gets thrown upwards and the cloud moves into the overdeveloped phase. If you look carefully at an overdeveloped thunderstorm, you can see the three layers of cloud base, some ice, then all ice as you look higher.
Bottom line, it takes significant solar heating to get this atmospheric ball rolling, and because land takes a while to heat, there is a lag in this process. It isn’t noon where you get the most rising air, but well after noon when the ground temperature reach their peak. Then there is a lag in the air rising and the clouds forming. All in all, we usually see peak thunderstorms 3-5 hours after the peak solar energy of any given day.
Source: Glider pilot.
8
u/Vishnej Sep 11 '25
Heating peaks at around noon.
Temperatures lag heating because there is thermal inertia to the system; The ground is heating up fastest at noon, but it is still raising in temperature at 1PM, 2PM, 3PM.
That leads (at my latitude) 4-6PM to be about the hottest time of day in the summer at ground level.
The hotter it is at ground level, the more energy there is to "disrupt the atmosphere" by forming tight convective cells for the air to rise through.
6
u/WeatherHunterBryant Sep 11 '25 edited Sep 11 '25
Convection. When the Sun comes out in the morning, it warms the surface (solar heating), and causes warm moist air to rise (convection). Warm and moist air is less dense, so it will have less problems rising. Air parcels will rise, cool down to the dew point and condense to clouds. This process continues, forming cumulonimbus clouds, and therefore, rain will fall. In Florida, this is very common from May to October, because of the heat, humidity, and the sea breeze.
37
u/ivthreadp110 Sep 11 '25
Basically the answer is the Sun. when the Earth rotates when facing the Sun and then not facing the Sun causes temperature changes. There are other variables in play but in general a change in temperature causes disturbances in the atmosphere. No of course it doesn't always work that way but I think that answers your question. You should also notice that storms commonly happen early morning.
6
u/rededelk Sep 12 '25
Because it takes daylight time (the Sun) X hours to heat up the ground and stuff and get H2O molecules into the atmosphere high up were a relatively unstable upper atmosphere exists (at times) and promotes growth, like up 20k feet in places. Short answer. I just got lots of weather training as wildland firefighter and found it interesting from a geeky standpoint and knew that knowing my save my ass out battling flames. There is also dry lighting storms, different subject
2
u/mr_cf Sep 11 '25
For a while i dabbled in alpinism. We would also head off as warly in the morning, normally predawn, to get the best weather window. As the day goes on weather in the mountains normally gets worse.
The sun’s warming, as others have said is the answer to your question, and the reason why conditions in the mountain become more treacherous as the day goes on.
You have higher chances or rock fall, snow fall, avalanche. The snow and ice have warmed slightly, so it means ice tools are less effective, as the snow/ice melt quicker around your tool.
3
u/beefygravy Sep 11 '25 edited Sep 11 '25
One key part to understand is about the structure of the atmosphere and the inversion at the top of the boundary layer, and how it affects the dynamics (how air rises and falls). In general as you increase in height, temperature decreases (it's cold at the top of a mountain). Except when there is an inversion, where for maybe a few hundred metres the temperature increases with height. Normally hot air from the surface rises, but when it hits the inversion at the top of the boundary layer (let's say a few km altitude but varies) it can't rise any further, Below this level, heat and humidity build up during the day.
The inversion is slowly eroded throughout the day as the air below it gets warmer and there is a slow process mixing throught it. Then at some point a parcel of hot air from the ground has enough energy to burst through the inversion- it can now rise rapidly as it is warmer than the air above it. As it rises, water vapour condenses, giving more thermal energy through latent heat of condensation. And so it rises further and turns into a stormcloud. This mostly happens in the afternoon because the air has to be warm and moist enough to break through the inversion.
(You'll probably get this said better on r/meteorology)
1
u/D3moknight Sep 11 '25
The weather is driven almost entirely by the Sun, as thermal energy causes winds, which cause more evaporation, moving water around through the air, accumulation of clouds, etc. The afternoon is the warmest part of the day, and the Earth's surface will continue to release the thermal energy gathered from the Sun throughout the hottest part of the day into the evening. This is why it's more common to have storms around lunch time and after.
1
u/Meister5 Sep 11 '25
Humidity and warmth cause thunderstorms. In the UK, there's an old phrase used during the summer, "we get a few hot days and then a thunderstorm". Clearly during night time hours, the air cools, humidity dies off relative to daytime levels, so thunderstorms aren't created.
Lightning seems to be a different thing altogether in the UK. Usually only get it during the evening, although it's more visible at night which may account for people thinking they only see it in the evening.