Typical Weather MET

While these are quite sweeping generalisations, the following hold true most of the time:

In a high pressure, air is generally descending. If it is descending, pressure is increasing, so temperature is increasing, so the temperature is moving away from its dew point and condensation/cloud will not occur. This lends itself to settled conditions, no cloud or precipitation, but possibly poor visibility as the sinking air may cause a subsidence inversion, trapping solid particles beneath it (haze). Because there is no cloud, the sun's rays can reach the earth during the day and terrestrial radiation can escape at night, meaning a large diurnal temperature variation. The increased surface heating during the day may cause thunderstorms in the afternoon if certain conditions are met. The increased surface cooling at night may cause fog or mist, especially on long winter nights.

In a low pressure, air is generally ascending. If it is ascending, pressure is decreasing, so temperature is decreasing, so the temperature is moving towards from its dew point and condensation/cloud will frequently occur. This lends itself to unsettled conditions, with extensive cloud and precipitation. Because there is widespread cloud, the sun's rays cannot reach the earth during the day and terrestrial radiation is returned to the earth's surface by night, meaning a small diurnal temperature variation.