Mid-latitude weather systems
Due to a combination of increasing Coriolis Effect with increasing latitude and large differences in air temperature and air density over relatively short north-south horizontal distances (a concept known as a baroclinicity), the weather of the mid-latitudes is dominated by transient, often fast-moving, weather systems. These systems can be primarily described in terms of the prevailing air pressure pattern (figure 1).
Air pressure is dependent solely upon air temperature and air density (because of The Ideal Gas Law), and thus air always “feels” a force to move from high pressure to low pressure. The large contrasts in temperature and density which cause these pressure differences arise because of:
- An increased seasonal variation in the radiation and moisture fluxes as one moves away from the Equator
- The Coriolis Effect (which strongly influences how air moves)
- The unequal distribution of land masses and ocean surfaces across the globe – which alters the radiation and moisture fluxes.
The Ideal Gas Law
The Ideal Gas Law of Physics states that the pressure of a gas or a mixture of gases (such as air) is solely related to its temperature and density. Thus, if the temperature of air changes, so must either its density or pressure..
Coriolis Effect
An effect caused by the rotation of the Earth, whereby air moving across latitudes is deflected in a clockwise direction (in the Northern Hemisphere) or anti-clockwise direction (in the Southern Hemisphere).
Baroclinicity
The atmosphere is baroclinic when there are large changes in temperature, density and air pressure over horizontal distances (as happens frequently in the mid-latitudes). This is in contrast to the Tropics, where there are no great differences in temperature, density and air pressure horizontally (this is known as “barotropic” atmosphere – the opposite of baroclinicity).