As it says, the middle latitudes. Approximately 70°-40°S and 40°-70°N. Europe and the USA lie in the mid-latitudes.
Due to computational limitations, numerical models of the global atmosphere have a horizontal grid spacing of a few hundred km, and typically 20 levels covering the entire vertical depth of the atmosphere, well above the scale needed to resolve individual convective clouds, or the small-scale eddies that are responsible for frictional and heat flux transfers in the planetary boundary layer. Therefore, these and other unresolved processes must be parametrized; a physically and/or empirically based relationship must be derived to predict, for example, how much rain falls out of convective clouds, just by knowing the variables that the model can resolve, such as the large-scale wind, pressure, temperature and water vapour distributions.
These parametrization schemes can be unsatisfactory but are a necessary part of a comprehensive numerical model of the global atmosphere. Different parametrization schemes can be developed to represent the same processes; such schemes can give very different results when incorporated in a model.
The wind field can be decomposed into two parts: a rotational component and a divergent component.
The streamfunction shows the rotational part of the wind field. The (rotational part of the) wind blows along the streamfunction contours in the sense that if you have are looking along the direction of the wind then there are low streamfunction values to your left and high streamfunction values to your right. Hence a local maximum in the streamfunction field corresponds to the centre of a clockwise-rotating wind gyre, and a local minimum corresponds to an anticlockwise rotating gyre.
In the atmosphere, the rotational part of the wind is normally much larger than the divergent part of the wind, hence the streamfunction field can be conveniently used to show the wind. It is often easier to show contours of streamfunction than wind vectors; vector plots quickly get very busy.
Warning; even thought the divergent part of the wind is often much smaller than the rotational part, it is often the divergent part of the flow that governs how the flow will change, so it cannot be ignored!
Technically the band between the Tropics of Capricorn and Cancer (23.5°S to 23.5°N), but for our purposes the wider band from 30°S to 30°N is a good working definition.
The atmosphere can be divided into a set of layers lying one above the other, some of which are briefly described below.
Planetary boundary layer: Lowest 1 km or so. This is the layer of the atmosphere where the underlying surface (land or ocean) exerts an influence on the atmosphere, through surface friction and surface fluxes of radiation, heat and moisture.
Troposphere: Surface to approximately 15 km in the Tropics (only 8 km at the poles). This is the layer of the atmosphere where the weather is. The troposphere incorporates the planetary boundary layer and the free troposphere above that.
Stratosphere: Approximately 15 km to 50 km. This is where the ozone layer is (and the ozone hole over Antarctica during Southern Hemisphere spring). The boundary between the troposphere and the stratosphere is the tropopause.