Complex sentence: A low pressure zone, that moves polewards or a high pressure zone that moves equator-wards, maintains or even accelerates the westerly flow; the local passage of a cold front may change that in a matter of minutes, and frequently does.
Split into simpler sentences: A low pressure zone, that moves polewards or a high pressure zone that moves equator-wards, maintains or even accelerates the westerly flow. The local passage of a cold front may change that in a matter of minutes, and frequently does.