Extratropical Cyclones / Frontal Depression or Frontal Low
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth (outside the tropics) having neither tropical nor polar characteristics, and are connected with fronts and horizontal gradients in temperature and dew point otherwise known as “baroclinic zones”.
Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms.
Extratropical cyclones encompass a class of storms with many names. Although they are sometimes referred to as “cyclones”, this is imprecise; cyclone applies to numerous types of low pressure areas. The descriptor extratropical signifies that this type of cyclone generally occurs outside the tropics in the middle latitudes of Earth. The term mid-latitude cyclones may be used because of where they form; “post-tropical cyclones” if extratropical transition has occurred. Weather forecasters and the general public often describe them as “depressions” or “lows”. Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are often used as well.
Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition.
In the N (S) hemisphere, if a depression is approaching from the W and passing to the N (S) of the ship, clouds appear on the W horizon, the wind shifts to a SW (NW) or S (N) direction and freshens, the cloud layer gradually lowers and finally drizzle, rain or snow begins. If the depression is not occluded, after a period of continuous rain or snow there is a veer (backing) of the wind at the warm front. In the warm sector, the temperature rises, the rain or snow eases or stops, visibility is usually moderate and the sky overcast with low cloud.
The passage of the cold front is marked by the approach from the W of a thick bank of cloud (which however cannot usually be seen because of the customary low overcast sky in the warm sector), a further veer (backing) of wind to W or NW (SW) sometimes with a sudden squall, rising pressure, fall of temperature, squally showers of rain, hail or snow, and improved visibility except during showers.
The squally, showery weather with a further veer (backing) of wind and a drop in temperature may recur while the depression recedes owing to the passage of another cold front or occlusion.
If the depression is occluded, the occlusion is preceded by the cloud of the warm front; there may be a period of continuous rain mainly in front of and at the line of the occlusion, or a shorter period of heavy rain mainly behind the occlusion, according as the air in front of the occlusion is colder or warmer than the air behind it. There may be a sudden veer (backing) of wind at the occlusion.
Often another depression follows 12 to 24 hours later, in which event the barometer begins to fall again and the wind backs towards SW (NW), or even S (N).
If a depression travelling E or NE (SE) is passing S (N) of the ship, the winds in front of it are E and they back (veer) though NE (SE) to N (S) or NW (SW); changes of direction are not likely to be so sudden as on the S (N) side of the depression.