How to understand weather forecasting

All of Earth’s weather starts with heat from the sun that warms the air around the equator. Our atmosphere is technically about 600km high, but about 75 per cent of its mass is contained closest to us, in the troposphere where we go boating and fly airplanes.

The troposphere’s height varies between about 12km at the equator, to only about 6km at the poles. As this air is warmed in regions along the equator, it rises to the top of the troposphere, gaining density as it cools. It then falls back down to earth, moving northwards and southwards.

At sea level, this rising and falling air causes areas of convergence or divergence, depending on whether a low- or high-pressure system is at play. In an area with low pressure, the surrounding air is sucked in while, under areas of high pressure, falling air hits the earth’s surface and spreads out.

Now let’s add in the rotation of the earth which, in the southern hemisphere, causes this falling air to rotate counterclockwise, and rising air to rotate in a clockwise movement. In the northern hemisphere, the rotation is opposite. This tendency for air masses to rotate is caused by the Coriolis Effect, which increases the further you travel away from the equator.

Air moving from an area of high pressure to an area of low pressure over these large areas is called gradient wind. A Mean Sea Level Pressure (MSLP) analysis map, which joins areas of equal pressure using lines called isobars, gives us a general picture of the gradient wind direction and various wind strengths in any given area. 

Air only travels down these pressure gradients, from high pressure to low pressure at sea level, with no exceptions. Just as a ball can’t roll uphill, wind can’t blow up a gradient from a low to a high.

Morning land breezes and afternoon sea breezes are examples of local wind. These are caused by local temperature differences between land and sea, and don’t extend very far offshore, where the gradient wind will dominate.

Local wind might also be generated by thunderstorms and line squalls. Being local, short-lived and quite contained in one area, local wind can’t be represented on an MSLP weather chart.

All wind – gradient or local – creates waves. A critical factor determining the size of these waves is the distance over a body of water that this wind can act to build a sea state. This distance is known as its fetch and, as a rule, the longer the fetch, the bigger the waves.

Swell is independent of local wind effects, such as the wind that you feel on your cheek at the time. The wind that you can feel generates sea waves. These are short wave length, slower speed waves that, despite travelling more slowly, come with higher frequency and with steep faces that can present a greater threat to a boat in deeper water than swell waves.

The combination of sea and swell is referred to as sea state.

Thunderstorms often form after a hot and humid day. The BOM will only issue warnings about these when wind gusts of 90km/h or more are expected, or when it comes with large hail and possible tornadoes or waterspouts.

Despite their severity, thunderstorms are often short lived and their location hard to predict. Yachts should consider grounding their mast and stays to the water to mitigate the real risk of a lightning strike, and all boats should be well secured with loose items stowed below deck before one hits.

Different coastal regions of Australia experience weather effects that are characteristic to those areas. Across northern Australia, reliable southeasterly trade winds prevail for most of the year. These blow off the land in the northwest of WA, and onshore in northern Queensland.

Here’s what happens when high-pressure systems move into the Tasman Sea on their regular journey from west to east: they extend an elongation called a ‘ridge’ along the north Queensland coast that can accelerate the trade winds significantly, whipping around the tight isobaric curvature at this point. 

In the wet season (October to April), the equatorial lows move southwards, bringing hot and humid northerly winds associated with the monsoon trough, and increased water temperatures. The weather drivers these produce include daily afternoon thunderstorms, line squalls and tropical cyclones.

In southeastern Australia, cold fronts carried by the low-pressure systems to the south of the continent bring very regular bursts of short, sharp rain and wind events. That is, cold and dense air masses travelling westwards meet slower moving regions of warm air, wedging them upwards to release their moisture and creating unstable, blustery conditions.

Ahead of these cold fronts there will often be hot, strong winds blowing off the land from the northwest that rapidly increase before backing suddenly to the west as the front arrives. This weather driver is known for creating extreme fire risk throughout western Victoria and eastern South Australia ahead of a cold front arriving.

Off the NSW coast, line squalls known as Southerly Busters are common as these fronts move through. Off the southern Queensland and the entire NSW coast, East Coast lows can occur all-year-round. These form when an isolated cold-air mass in the upper atmosphere interacts with the warm East Australian Current.

East Coast lows can resemble a tropical cyclone on radar or satellite images, with wind speeds comparable to a Category 1 cyclone, but they are not related to cyclones at all and, in fact, form differently. The south side of these systems is the most violent. 

In mid-to-southwestern WA, the most famous weather influencer is the Fremantle Doctor. Don’t be fooled by the word ‘breeze’ as the Freo Doctor regularly exceeds 30 knots and can hit like a freight train.

Despite its nickname, this sea breeze extends well south of Fremantle and right up the coast past Geraldton. As a rule, the earlier it arrives, the harder it will blow. 

BOM’s online tool MetEye helps you visualise local weather observations and forecasts for any location in Australia, including wave forecasting. Go to: bom.gov.au/australia/meteye.

A FULL REVIEW OF ‘WATCHING THE WEATHER’ RAN IN THE FEBRUARY-MARCH (40.1) ISSUE OF CLUB MARINE MAGAZINE.