The global circulations discussed in Chapter 12 are evident only after averaging the fluctuating winds that are actually measured (Note 13.A). In this chapter we consider some of the detail which was overlooked in focusing on year-long averages over the whole globe. In particular, we will consider winds on the scale of a thousand kilometres or so—the 'synoptic scale' (Table 1.1)—and look at day-by-day snapshots instead of long-term patterns.

The word 'synoptic' (i.e. 'together seen') implies that we observe a combination of measurements from a wide area—of the size of Australia, for instance. It is the scale of regional weather, uncomplicated by local effects like surface friction, slope or local differences of surface temperature. All the measurements refer to the same moment in standard time, the Greenwich Meridian Time (GMT), also known as Universal Time and signified by UTC or Z, e.g. 1300Z means 1 p.m. at Greenwich in London. Meteorological offices around the world use this convention, irrespective of local clocks.

Figure 13.1 shows mean sea-level pressure (MSLP) isobars over Australia at a particular moment, i.e. the map of pressures after each measurement has been corrected for elevation. It indicates the weather at that time, in three ways. Firstly, isobars at middle and high latitudes show the direction and strength of the geostrophic winds. The winds are shown blowing anticlockwise around the high-pressure regions (the highs) and clockwise around the lows, in accordance with Buys-Ballot's Rule (Section 12.2). Secondly, the MSLP reflects what is happening aloft, because it indicates the weight of air in the entire column above (Note 1.G). So changes of pressure demonstrate the overall inflows and outflows from the column at every level. Thirdly, mobile highs and lows determine most aspects of the weather, notably temperature, precipitation and cloudiness. For instance, lows or troughs on the MSLP map show where air is ascending and may form cloud, with rain as a possible sequel, while subsidence typically occurs where there are low-level highs or ridges.

The various conditions around a place of low pressure amount to what we call a cyclonic system. Likewise for an anticyclonic system associated with a region of high pressure. These systems are not to be regarded as independent

Figure 13 1 A typical pattern of sea-level pressures and therefore surface winds over Australia, showing the air masses involved and hence a front between mT and mP air masses, with different temperatures and wind directions.

whirls within an otherwise steady atmosphere; they interact with each other and are essential ingredients of the global circulations. It is only for convenience that we separate the various systems and their characteristics, to examine the life cycle of each system's formation, maturity and decay, which lies behind the local variabilities of weather.

equivalent potential temperature (Section 7.2) or an even more completely conserved variable called the potential vorticity, which is discussed in more advanced texts. It combines both air mass stability (Section 7.2) and vorticity (Note 12.K).

The concept of an air mass is used both near the surface and aloft. For instance, ozone concentrations are used to characterise stratospheric air masses. Here we will focus on ground-level conditions, where the air masses are separated by fronts (Section 12.3).

Air masses are considered in meteorology in much the same way as an air parcel in discussing stability and convection (Section 7.3). But air mass is a much larger body of air, whose movements and changes help us explain the weather.

The characteristics of an air mass are acquired when winds linger for a few days over a large uniform surface, like an ocean or extensive land areas more than about 500 km from the sea, such as a great desert. The required light winds occur where the atmosphere is slowly subsiding in the region of a high (Section 12.3), for example. Such regions are called source areas and give air masses their names (Table 13.1).

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