Posted - 03 Jul 2009 : 12:29:29
| Wind Glorious Wind...
If you're into a sport that is powered by the wind, or want to avoid windy days then it pays to understand how wind works.
Wind is the movement of air molecules from one location to another. The perfect demonstration of wind is with a fully inflated car tyre. If you open the valve on the car tyre, air rushes out. Why? Because there is more air pressure in the tyre than there is in the atmosphere, and the atmosphere is always seeking to equalise pressure. Higher pressure pockets of air (i.e. inside the car tire) move to equalise lower pressure areas. So what causes these low pressure areas? Well, in the case of a seabreeze, it's caused by heat...
Cold sinks, heat rises. It's the concept that makes hot air balloons work, and why your vegies are at the bottom of the fridge. During a summer day, the land is heated by the sun, and this heat is absorbed by the surrounding air. Once this air meets a particular temperature, it begins to rise quite rapidly. As a result, a low pressure area is created at ground level. The sea doesn't heat up as quickly as the land, so the air temperature over the water is much less. As a result, this air is free to move sideways to occupy our new low pressure area. The result is a light to gale force wind which rushes in from the sea to fill the lower pressure area left by the hot air rising from the land. Seabreezes usually happen in spring and summer, when the difference in temperature between land and sea is greatest.
Seabreezes are the most amazing phenonenom. For many years I never really paid much attention to the wind. One day, we arrived (late) at the local river to do some water skiing. It was around 1pm, the water was glassy - perfect conditions. Then, me mate Mick says "Here comes the seabreeze...". Looking across the river we could see an approaching line of rough water. And then it hit us full on - the full glory of a 25 knot seabreeze.
What's even more amazing is that the wind can be travelling faster than the front of the seabreeze. It can be progressing inland at 25 km/h, but the actual wind can be blowing at 35 km/h! Sometimes the seabreeze can reach many kilometers inland, and other days it teases you by staying just offshore, coming in, and then going out again.
So to get a seabreeze, it needs to be hot inland. But too hot and you won't get one at all. To explain why this is the case, we need to understand high's and low's and the squiqqly charts they show on the telly called 'Synoptic Charts'.
You need to understand these if you want to have your own guess at what the wind may do. It also assists to understand what the weather man is talking about. Sometimes our local TV weather reports simply say '.. and an afternoon seabreeze'. About as useful as an ashtray on a motorbike.
So check this out:
From the title, it says MSL Analysis. This means it is a Mean (average) Sea Level Analysis. A slice of the atmosphere at sea level. The way the atmosphere changes at different levels is also extremely interesting, but beyond what we talking about here. The numbers indicate the air pressure, called the Barometric pressure. Barometric pressure is measured in hectopascals. Bigger numbers represent greater air pressure. The chart is generated by acquiring a huge number of readings from ships, sea based automatic weather buoys and land based weather stations. These data points are then processed using complex mathamatics to 'best guess' or 'interpolate' values for areas where no reading was available. The meteorologist then sharpens his best HB pencil and draws lines connecting areas of equal pressure (called isobars, or contours), draws a H in the middle of any Highs, and an L in the middle of any Lows. They also draw another chart called a prognosis which is what they estimate to be where the weather will go. It's always valuable to compare the analysis with the prognosis to get an idea of where, and at what speed the weather is travelling.
So high pressure areas rush to fill low pressure areas huh? If that was the case, then you would expect the wind to travel like this:
Bzzzzzzzt. Wrong! These are complex systems, and there's this other forces at work here, plus lets not forget that the world is 3D and this is only a horizontal slice. Wind actually travels pretty much in the direction of the isobar lines. And in the southern hemisphere they trave anti-clockwise around high's and clock wise around lows. And the whole weather pattern moves (generally) in a west to east (left to right) direction.
So the wind actually blows around the high cell like this:
But don't forget. This all gets totally ignored by local wind effects such as seabreezes along the coast in the afternoon.
This synoptic chart is a joy to a wind fanatic living on the coast of Western Australia because this pattern means wind - and lots of it - these cool winds also keep the maximum temperature down. In fact, the forecast for this pattern was:
TODAY'S MAX: 23 C (74 F)
Strong Wind Warning current.
S/SE winds 20/30kn in the morning, tending S/SW and strengthening
to 25/32kn in the afternoon.
The S/SE winds in the morning reflect the anti-clockwise direction of wind around the high, untill the afternoon when the thermal induced seabreeze kicks in and changes to an onshore S/SW. Bliss!
But that low cell sitting up in the north is what can ruin it all. These low pressure systems descend in a southerly fashion and bring with them hot and humid conditions. They are called troughs and generally spoil any chance of a seabreeze, as indicated with this chart and forecast:
TODAY'S MAX: 35 C (95 F)
E/NE winds 15/20kn, easing to 10/15kn by late morning. Winds shifting
NW/SW 10/15kn during the afternoon.
The capital city of Perth is going to be blasted with E/NE winds in the morning, brought in nice and fresh from the deserts, and these winds are toasty - a maximum of 35 degrees C, and only a mild, if any, seabreeze. But wait - that's a cold front approaching from the south west, and there's a good chance that this will push the trough inland and bring back the seabreezes.
Generally speaking the thing to look for when looking for strong seabreezes is a new high cell aproaching (following a previous high cell), or a tough moving inland and being replaced by a new high (extra windy!).
Well, unlike seabreezes, cold fronts are active mainly during winter and come at any time of day or night. Cold fronts do pass through during the other months, but there effect is usually not noticed by most. Like seabreezes, they come in all sorts of flavours from simply making the sky cloudy to full on roof removing gale force winds. Cold fronts, as the name suggests, is a line of cold air travelling along, generally west to east. As this cold air meets warmer air, the warm air rises, and you guessed, causes the cold air to rush in and fill the space. Thus, the actual 'front' of the front, called the 'squall line' is generally where most of the wind is.
Cold fronts are great when they come through, producing the locally famous 'Norwester'. Because the winds travel clockwise around a low pressure system, the first taste you get is Northerly, but more often than not North Westerly. As the cold front passes through these winds rotate around the compass, going from NW to W to SW, and maybe S. The S part can get quite chilly because it dredging all the cool air from the antartic regions. Not cold by UK standards, but enough to make your face numb. North Westerlies on the otherhand can be quite warm, relative to the usual winter temperatures. The time it takes for the NW to SW transition depends on the speed at which the front is travelling. Usually around a day, sometimes two.
Time to introduce another term - the pressure gradient. In the synoptic charts displayed above, the isobars have been relatively spaced out, because the difference in air pressure is quite small over a large area. When we get a large change in air pressure over a small area - rig up your small sails/kites - it's gonna blow.
Checkout this chart - one of the windiest days in the winter of '98. Blue arrows indicate wind direction. See how close the isobars are together. Also, remember that the entire frontal pattern moves from left to right (west to east), but the winds blow in the direction of the arrows. I hope you can grasp the concept!
TODAY'S MAX: 19 C (67 F)
Gale warning current.
W/NW winds 30/40kn this morning with possible squalls to 60kn in
thunderstorms. Winds tending W/SW 20/30kn by early afternoon and
easing to 15/20kn this evening.
One of the best things about a cold front is that extra commodity - waves. As any sailor knows, wind generates waves. The more wind, the more waves. If you've experienced a strong cold front, you know that they can get extremely windy, as a consequence the waves can get quite large.
Here's another less extreme (for Perth, anyways) cold front. In Perth it blew around 20-30 knots, whilst the south coast received 25-35 knots.
Stuff For Overseas Visitors...
Summer in Australia lasts from December through to the end of February, with the main windy bit being being December/January. Of course, it can be windy during October to November, there's just more chance of it happening in Dec/Jan. Spring and Autumn are not that windy due to the change of season. Not hot enough for seabreezes, and cold fronts aren't strong enough. (Generally, sort of)
If you come down during winter, then June/July is the peak of winter. Winter really is pot luck for wind. 1 to 2 days per week of winter action might be the norm. Sometimes, if we get a series of cold fronts one after another, it can blow all day and all night for 3 to 4 days.
Thanks from Kev - Forum and Site Admin