Reading Synoptic Charts

So what do all the lines and symbols on these charts tell us?

Well, first of all remember that the charts are showing the nominal weather pattern at sea level.

The isobars connect areas of equal pressure with 4 Hpa spacing, and the closer together the isobars the windier it will be. The direction of the wind is generally parallel to the isobars; anticlockwise around a high pressure area, and clockwise around a low pressure area.

Important features to note are the frontal systems. This is where different air masses clash resulting in poor or hazardous weather conditions.

Cold fronts, the lines with blue triangles, represent the encroachment of cold air under a warmer moist air mass. This forces the warmer air mass to rise, creating cumuliform clouds that can result in light to heavy showers, as well as reduced visibility and lower ceilings during those showers. The passage of the front will create more intense, and possibly longer duration, shower activity depending on its speed.

After the the cold front passes the air is usually clearer; the wind direction having backed to the southerly quarter. If there is sufficient instability along the frontal zone, a line of active cumuliform clouds and Cb (thunderstorms) can form along the front.

Warm fronts, the lines with red semicircles, represent the encroachment of moist warm air over a cold air mass. Generally warm fronts move slower than cold fronts because the warm air mass is less dense than the cold air mass it has to rise over. Cloud ahead of the warm front is mostly stratiform and rainfall gradually increases as the frontal zone approaches. There is often low cloud preceding the warm front and then a clearance and rapid warming usually follows. If the warm air mass is unstable, CBs may be embedded among the stratiform clouds ahead of the front, and after the front passes showers may continue. Warm fronts are less well defined than cold fronts.

Occluded fronts, the lines with both blue triangles and red semicircles, represent areas where a cold front overtakes a warm front. A variety of weather is associated with an occluded front, with Cbs possible.  Isolated occluded fronts often remain for a time after a low pressure system has decayed. Cloudy conditions with lower visibility and patchy rain or showers are the result.

Less obvious perhaps than frontal systems, but equally important, are the overall airflows shown on the charts. For example, if there is a sustained airflow from the ocean, especially to the south, then the area of the country exposed to it will experience showery and, depending on the intensity of the flow, windy conditions.

Leeward of the mountain ranges will be dry and more settled because all the moisture has been removed from the air mass windward of the mountains. However, leeward areas can still be very windy. Take for example a strong southwesterly on the West Coast passing over the Southern Alps; it becomes an even stronger, turbulent and very dry northwesterly on the east coast of the South Island. This same effect can be experienced leeward of all of the mountain ranges in Aotearoa New Zealand.

How fast are the systems moving? Have a look at a series of prognostic charts to get a feel for how fast any given weather system is moving. Because we live in the mid-latitudes with so much ocean around us, the speed of the systems can vary greatly. A frontal system may pass through in less than an hour — or take the best part of a day. Similarly, anticyclones can sometime lock onto the country and be present for weeks, and low pressure areas, especially remnants of tropical cyclones can pass quite quickly.

Invercargill

Above mean sea level

TCu

Towering Cumulus

agl

above ground level

Cb

Cumulonimbus

Flight Information Service communications

ATS

Air Traffic Service

Royal New Zealand Air Force

in vicinity of aerodrome Thunderstorm

ATC

Air Traffic Control

VC

in vicinity of aerodrome

Probabilityxx%

TIL

Until

FM

From

Becoming

Temporarily

Routine air report from aircraft in flight

FIR

Flight Information Region

Internet Flight Information Service

VFR

Visual flight rules

Automatic Terminal Information Service

BWR

Basic weather report

New Zealand Flight Information Region

Aerodrome special routine meteorological report

Automatic aerodrome routine meteorological report

Aerodrome routine meteorological report

TAF

Aerodrome forecast

Area forecast

Significant meteorological information