Hurricanes are large storms that originate in the tropics with sustained winds (winds averaged over a two-minute period) greater than 74mph. On the other hand, a mid-latitude cyclone (MLC) is a larger storm that forms in the middle and high latitudes.
MLC (top) source: wps.prenhall.com
Hurricane (bottom) source: wikimedia.org
Both of these storm systems get their fuel in very different ways. A hurricane gets its energy from the warm waters of the tropics from the latent heat of condensation. In technical speak; latent heat is the amount of energy used to change a substance from one phase of matter to another. For example, the phase change water goes through to turn into steam (a liquid to a gas). MLC’s derive their energy from horizontal temperature changes, such as a cold front passing over an area. If you look at a cross-section of a hurricane, there is a centralized column of air that is warmed from the surface as it lifts upwards. Thus, hurricanes are also known as warm-core lows. MLC’s, in contrast, are cold-core lows that will intensify with increasing height.
Cross section of a hurricane
Source: apollo.lsc.vsc.edu
A hurricane usually contains an eye, which is where sinking air occurs (hence the sometimes clear skies in the eye itself). In contrast, MLC’s have centers where air rises. Hurricane winds are the strongest at the surface, whereas the strongest winds produced by an MLC are aloft, typically found in the jet stream.
Source: patcegan.files.wordpress.com
Other differences can be seen on a surface chart; the more isobars (lines of equal pressure), the greater the change in pressure (pressure gradient) and the faster the winds will be. Also, hurricanes do not have the fronts (warm and cold) that are signature of an MLC.
The similarity between these two types of storms; both are surface areas of low pressure with winds moving in a counter-clockwise direction (if you are in the northern hemisphere).
Speaking of hemispheres, you can have an MLC in the southern hemisphere but a hurricane is solely a northern hemisphere storm (called a hurricane in the Atlantic, Caribbean, central and northeastern Pacific). In the northwestern Pacific, we end up with a typhoon, and in the Bay of Bengal (Indian Ocean) or the Arabian Sea…a cyclone.
