A mid-latitude cyclone is a region of low pressure that often travels in connection with the jet stream. A mid-latitude cyclone is baroclinic which means it has fronts associated with it. There are three areas that they commonly develop which are land/sea boundaries, mountains and between air masses. Land sea boundaries can have a significant contrast in humidity and temperature. Mountains can disturb the wind flow creating waves in the wind flow. Air mass boundaries have contrasting temperatures over short distances. These processes can disturb the wind flow creating a region of relative low pressure. This region of low pressure with the counterclockwise flow can cause a further contrast in temperature from one side of the low pressure to the other (due to the fronts that are initiated). Thus, the temperature contrast along with a moisture source is the fuel for a developing mid-latitude cyclone. Once one develops, it can travel great distances (over continents and oceans).  The depiction below is of a mid-latitude cyclone with the associated fronts and surface isobars. The low pressure often moves in the direction that the warm front is located (more or less). The cold front and the colder air tend to follow behind the mid-latitude cyclone. Thus, as a mid-latitude cyclone approaches there can be above normal moisture and temperature followed by below normal temperatures as the mid-latitude cyclone moves away. The isobars “kink” at the frontal boundaries. This is due to the rapid contrast in temperature over a short distance (especially when associated with the cold front). This density contrast causes lifting that takes place in the vicinity of the frontal boundary (rising air lowers surface pressure). Thus, as a front approaches the air pressure decreases and the air pressure increases after the front has passed through. Also it needs to be factored what direction the low pressure is moving when analyzing air pressure changes.  The next diagram shows the general relationship between the position of the surface mid-latitude cyclone and the wind flow up at 500 mb. Upper level troughing is common behind a cold front since cold air is dense. It is also common to have high pressure at the surface with troughing aloft behind a cold front. This is because a surface location is below the heavy dense cold air while a location in the upper troposphere is above the heavy dense cold air layer. The fronts and mid-latitude cyclone tend to be located on the right side of the trough (a reason why clouds and precipitation are more common to the right side of a trough as compared to the left side).  All mid-latitude cyclones eventually weaken and dissipate. This can happen for a combination of the following reasons: loss of access to warm air, loss of access to moisture, occlusion of cooler/drier air around the low. Generally, the cold front moves faster than the warm front. Thus over time, the colder air will wrap all the way around the low. This will produce an occluded front. Typically a mid-latitude cyclone will weaken once it becomes occluded. An occluded low which no longer has any access to warm and moist air will typically continue to weaken and then dissipate.  |