How to Read the Charts

MSL Pressure/Thickness/Rainfall

These charts form the basis of the ?synoptic charts? that you see on TV Weather bulletins. The solid lines running across the chart are called isobars and connect lines of constant pressure. On every second isobar is written the pressure corresponding to that isobar. The isobars are drawn with a contour interval of 2hPa. Areas of closed isobars will be either high pressure centres or low pressure centres.

You can tell which is which by looking at the pressures on the isobars as they go into the centre - if they are decreasing it is a ?low?, if they are increasing it is a ?high?. The proximity of the isobars to each other is directly related to the strength of wind. Generally, isobars are closer together near low pressure centres and further apart near high pressure centres. It follows, therefore, that winds are stronger near lows and lighter near highs.

The thickness lines are the fainter, dashed lines running across the charts. These represent the physical thickness of the atmosphere between the 1000hPa level and the 500hPa level. This is normally around 5500m, but varies by up to 200m either side of this value.

As colder air is denser than warmer air, it is also thinner, so thickness values of cold air will be lower than those of warm air. Therefore, thickness is a quick way of getting an idea how warm or cold the atmosphere is over a certain point.

You will notice that thicknesses rise from south to north, reflecting the warmer air near the equator compared to near the poles. The 5760m line is shaded red and the 5400m line is shaded blue for ease of reference. Areas where colder (lower thickness) air advances northwards are known as ?upper troughs? and areas where warmer (higher thickness) air advances southwards are known as ?upper ridges?.

Generally, when an upper trough is passing an area, there is the potential for more unstable weather, and within upper ridges, the weather is generally more settled.

The final thing included on these charts is the rainfall prognosis. These are the coloured areas. The rainfall totals accumulate to every 24 hour multiple, so it is important to look at what time of day the model you are looking at refers to. Model rainfall prognoses are one of the least reliable model fields and should always be interpreted with caution.

Surface Temperature/Wind

Surface temperature is shown on this chart as coloured shadings. This is the temperature predicted by the model at screen height, or where temperatures are normally measured. The higher resolution models (Mesocast, MesoLAPS and LAPS) will do a better job of resolving coastlines, so you will see a more marked delineation between land and sea. Diurnal variations should also be very clear on these charts, with the land heating up and cooling down more than the sea.

Wind is shown on the charts as a wind stick. The wind sticks point from the direction that the wind is coming from. That is, the end of the wind stick with the barbs on the end is the direction the wind is coming from. Each barb represents 10 knots of wind. A knot is 1.85km/hr. Therefore, each full barb is roughly 20 km/hr. Each half barb represents 5 knots. A triangular barb represents 50 knots of wind.

Surface Dew Point/Wind

The dew point at screen level is shown in these charts as shaded contours. This is the best indicator of moisture at the surface, as it is independent of temperature, unlike relative humidity. It is easy to detect troughs, cold fronts and ?dry-lines? on these charts, as they show up as marked dew point discontinuities. As with most fields, the higher resolution models will do a better job of resolving land/sea differences.

The wind field is the same as the wind field shown on the Surface Temperature/Wind chart and should therefore be interpreted in the same way. It is overlaid on both the temperature and dew point charts because both variables, temperature and dew point, are advected by the wind. That is, a southwesterly wind will carry an airmass from the southwest, characterised by its temperature and dew point.

850hPa Temperature/Wind/Height

The temperature at the 850hPa level (approximately 1500m above sea level) is shaded in colour on these charts. This field is one of the best indicators of maximum temperature potential at a site. This is because the air will mix down from the 850hPa level to the surface and warm at a rate of 15C/100m. Therefore, add approximately 15C to this temperature get an idea of your potential maximum.

Also shown is the wind at this level. The wind is shown in the usual way with a wind stick. A full barb corresponds to 10 knots, a half-barb 5 knots, and a triangle 50 knots. These winds are important because they show the direction and speed that lower-level systems will tend to move. That is, if you are near the coast, if the 850hPa winds are onshore, you are more likely to receive showers than if they are offshore.

Finally shown on this chart is the height of the 850hPa level. While this is normally around 1500m, it does vary, and this is shown by the grey solid lines. When pressures at the surface are lower, the 850hPa height will correspondingly be lower. This means that there is less warming that mixed air will undergo when it descends from 850hPa, so surface temperatures will tend to be lower.

850hPa Relative Humidity/Wind

The relative humidity at the 850hPa level is shown on this chart in varying shades of green. Below 50% is not shaded, 50-70% is lightly shaded, 70-90% moderately shaded and greater than 90% heavily shaded. Generally, if it is less than 50%, skies will be clear, 70-90% will result in more cloud than clear sky and greater than 90% will normally result in some potential for precipitation.

The wind at the 850hPa level is also shown on this chart, exactly the same as on the 850hPa Temperature/Wind/Height chart. This is because temperature and humidity are both quantities that are advected by wind.

700hPa Relative Humidity/Wind

The relative humidity at the 700hPa level is shown on this chart in varying shades of green. Below 50% is not shaded, 50-70% is lightly shaded, 70-90% moderately shaded and greater than 90% heavily shaded. This level gives an indication of the extent of mid-level cloud. Values less than 50% will be free of mid-level cloud while 70-90% would indicate mid-level cloud and greater than 90% the chance of rain.

The wind at the 700hPa level is also shown on this chart, in the usual way. Winds at this level give a good indication of the steering flow for thunderstorms, in particular. That is, if storms are occurring in a particular area, the 700hPa wind speed and direction is a good estimate of the storm?s resultant speed and direction of movement. They also give some estimate of potential gusts from storms that occur. 

500hPa Temperature/Wind/Height

This chart shows the temperature at the 500hPa level in shaded colours. This temperature does not vary as much as that at 850hPa and is more representative of upper-level conditions. This will coincide closely with the thickness shown on the MSL Pressure/Thickness/Rainfall chart and is useful for seeing upper-level troughs and ridges.

The wind at the 500hPa level is also shown here, in the usual format. The 500hPa wind is not routinely used by forecasters on its own, however it is useful in combination with winds at other levels of the atmosphere when diagnosing wind shear. This is especially important in thunderstorm situations, when wind shear is critical in determining potential for severe weather.

The height of the 500hPa level is also shown here, by the solid grey lines. This is similar to the height of the 850hPa level shown on the 850hPa Temperature/Wind/Height charts. As with 500hPa wind, this is a parameter that is not routinely used by forecasters, certainly not anywhere near as often as the 850hPa height.

500hPa Vorticity/Height

This chart shows the vorticity at the 500hPa level, with areas of negative vorticity shaded in blue and positive vorticity shaded in red. Negative vorticity corresponds to cyclonic flow, and will normally be associated with some degree of uplift, which could translate to precipitation if sufficient moisture is present. Positive vorticity corresponds to anticyclonic flow, and normally leads to settled weather.

The height of the 500hPa level is also overlaid on this chart, once again in solid grey lines. This is identical to the 500hPa height shown on the 500hPa Temperature/Wind/Height chart. It is overlaid here because, normally, areas of negative vorticity will occur within upper troughs, which can easily be seen on the 500hPa height charts.

Total Totals Index/300hPa Wind

This chart shows the Total Totals Index, shaded for values above 40. This index is very useful for diagnosing thunderstorm potential in cold-air situations, as it is made up primarily of lower atmospheric parameters. Generally, a range of 45-50 indicates potential for cold-air showers if moisture is sufficient. 50+ indicates the chance of storms, while greater than 55 means cold air storms are likely.

Overlaid on this chart is the wind at the 300hPa level, shown in the usual format. These winds are indicative of the upper levels of the atmosphere, and are a good guide for locations of jetstreams. Look for bands of relatively strong winds. Areas where jetstreams are over-running regions of high instability (TT >50) may mean the potential for severe weather, due to increased wind shear.

Haines Index

The Haines Index is shown on this chart, in shaded colours. This index is designed to give a measure of how explosive any fires that occurred would become. It takes into account instability and dryness of the atmosphere. Areas of greater than 4-5 are deemed to be at great risk, because the atmosphere is both very dry and very unstable, a combination that would lead to explosive fires.

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