Happy #WeatherWednesday everyone!
You know what a cold front is, and normally can identify when one has passed. What about an “outflow boundary” passage? What is that? You may have heard the term. In most cases, they aren’t even noticeable to an observer. Today we will give a little background on the what, how and why.
An outflow boundary is a weather front that separates two different air masses, as does any front. This kind of front develops from storm complexes, as opposed to cold/warm/stationary/occluded fronts that result from ordinary mid-latitude cyclones (low pressure systems). The difference in air masses is also more subtle most of the time with OFBs.
An outflow boundary can behave similar to a cold front, a warm front, or a stationary front depending on the environment it’s in (i.e. surface wind velocity, strength of surface heating). They come into existence when a storm or complex of convection releases a “cold pool” where denser air is introduced into the ambient air and surges outward. The stormy air is colder and denser due to the cooling effect of rain, all thanks to latent heat absorption. This is a chemistry-level response when H2O changes phases, or diffuses/evaporates from a liquid to a gas. The phase change requires “work” at the molecular level and takes AWAY heat from its environment. And voila, rain-cooled air.
Once a storm cell dies, the remnant cold pool can vanish or it can survive hours to a day or two depending on the environment. The more time that passes, the smaller the temperature difference typically gets from one side of the OFB to the other.
Despite how subtle an OFB can be with its temperature gradient or wind direction/speed change, it can be the focus for newly developing convection. In volatile atmospheres it is always worthy to note where these boundaries exist. They are efficient at enhancing helicity and moisture convergence, a great recipe for thunderstorms. This is why meteorologists pay attention to them, even if they seem to travel “under the radar” 😉
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