How atmospheric lapse rates are used to determine stability for storms
If you stepped outside across Central Missouri today, you immediately felt it: summer has arrived in full force. With temperatures climbing deep into the 90s and heat index values near the 100-degree mark, the atmosphere feels heavy, humid, and oppressive.
But while the weather on the ground seems quiet for now, a subtle, invisible shift happening miles above our heads is setting the stage for a volatile Thursday afternoon and evening. To understand the severe weather threat heading our way, there is an important metric we are tracking known as a lapse rate.
In simple terms, a lapse rate is just a measure of how quickly the air temperature drops as you go higher into the sky. On a standard, quiet summer day, the atmosphere cools down at a steady, predictable pace, roughly 6.5 degrees Celsius for every 1 kilometer you climb. On Thursday, however, that normal "speed limit" is going out the window.
While the afternoon sun continues to bake the ground here in Mid-Missouri, a dynamic blast of unseasonably cold air is moving into the mid-levels of the atmosphere, about 10,000 to 18,000 feet above us. This creates what is called a steep lapse rate. Tomorrow, temperatures aloft will plunge by as much as 8 degrees Celsius per kilometer.
Why does this mathematical rate matter to you? Think of a bubble of hot, humid surface air like a hot air balloon. A hot air balloon rises because the air inside it is warmer and lighter than the air surrounding it.
When the upper atmosphere is exceptionally cold compared to the baking ground, that temperature gap stretches wide open. The moment a developing storm tries to push air upward, that air hits the steep lapse rate and acts like it just stepped on an atmospheric trampoline. Instead of a gentle, slow rise, the air violently rockets upward into the stratosphere at speeds exceeding 60 to 80 miles per hour. This violent upward conveyor belt is what meteorologists call a storm's updraft.
These super-charged updrafts are the ultimate breeding ground for severe, large hail. When a storm's vacuum cleaner is that powerful, it can hold heavy chunks of frozen ice suspended in the upper, freezing layers of the storm for minutes at a time. The ice circles the storm's core, growing thicker and heavier with each pass, until gravity finally wins out.
Because of the steep lapse rates flashing across our forcecasting models for Thursday evening, our primary hazard with these storms will be large hail and damaging winds, although tornadoes will still be possible due to sufficient wind shear.
The atmospheric cap, which is a layer of warm air acting as a lid on the storm energy, is expected to hold for most of Thursday morning and early afternoon. However, as a cold front moves into Central Missouri during the late afternoon and early evening hours, that lid will pop.
Expect rapidly developing thunderstorm development during this time. Because these storms will have the green light to grow rapidly, make sure you have a way to receive severe weather warnings before you head home for the evening, and secure any vehicles or outdoor property that could be vulnerable to falling ice.
