A heat burst is a rare event and a dramatic weather changer. A heat burst is a special type of downdraft that, instead of producing refreshing cooler air, produces extremely hot and dry air. When air sinks, it warms adiabatically. In a normal downdraft, the air is cold enough that even though it warms while sinking it will still end up being colder than the air it is replacing. Also, a normal downdraft will often contain precipitation. The evaporation of precipitation as the air sinks contributes to the air not warming as much as it otherwise would. In the case of a heat burst, the downdraft begins higher aloft than a normal downdraft and typically at the stage when thunderstorms are dissipating. Evening and early night is a typical time they can occur. A heat burst will begin with air high aloft that rapidly accelerates toward the surface. Since storms are dissipating, this downdraft will not have much precipitation thus evaporative cooling will be much more limited as compared to a normal downdraft. The heat burst downdraft also starts high aloft, in the upper troposphere. These two factors (less moisture, a long distance to fall) allow a dramatic warming of the air as it sinks to the surface. One factor that makes heat bursts rare is that many times they don’t make it to the surface since the downdraft will lose negative buoyancy when it becomes warmer than the surrounding air. Thus, another factor in producing a heat burst is that the downward momentum must be extreme enough to allow it to reach all the way to the surface. A dramatic change in weather is in store for heat bursts that reach the surface. A heat burst reaching the surface can cause relative humidities to fall near zero and temperatures to increase to well above 100 F. The strongest heat bursts will cause surface temperatures to climb to between 100 F and 120 F. These conditions can persist for a couple of hours or so along with strong wind before the temperature returns back to normal. The strong wind combined with heat and extremely low relative humidity can quickly dry out vegetation. It is often dark outside when these events occur which makes them feel all the more unusual. |