NZ Weather Forum

Hi Weetbix
No problem. When I joined Metservice as a technician back in the 80's we were sent to 'met school' and learnt quite a bit of this stuff. It's been a while though and I hope I've been accurate. If you want to improve your understanding further I'm sure anyone at Metservice would be happy to help plus there is a huge amount of information available online or in book form.
I was going to add that the process of air rising in one spot and then sinking in another is called convection. It happens on many scales, large and small. You can see convection in water when you heat some in a pot on a stove (air and water are both fluids and behave in the same way, ie they flow). There is sort of a circular motion whereby air rises forming a low, moves sideways at altitude, sinks forming a high, and then returns along the surface back to the starting point. The seabreeze in ChCh is a good example of a convective circulation on a local scale. The surface wind flows from the high (over the sea) towards the low (over the land). At a few thousand feet it is moving in the opposite direction. Google 'seabreeze' for diagrams.
The same process occurs on a global scale with air rising at the equator, moving north and south at altitude, and then sinking at higher latitudes. It then returns back towards the equator on the surface forming the trade winds. The is a band of low pressure at the equator where the air is rising and a band of high pressure at about 30deg north and south where the air is sinking.
The highs and lows you see on the TV weather map are on what is called the synoptic scale which refers to systems of roughly a few hundred kilometres across. They can be formed by large bodies of air rising and sinking over hot and cold surfaces, eg a low forming over the desert of Australia. They also form at the boundary between cold and warm air masses where the warm air is pushed up over the cold air.
Now the surface wind generated by these systems flows from high to low pressure, and this is obvious with the seabreeze. However the wind associated with the larger systems is deflected by an effect resulting from the rotation of the earth. This makes the wind on the TV weather map move parallel to the isobars and not straight from high to low pressure. If the earth didnt rotate the air would flow straight from a high to a low on the surface. The effect is called the Coriolis effect and is due to Newtons First Law, ie the air wants to keep moving in a straight line. It's an important but difficult concept.

That is an awesome reply Andy thank you very much for it. I think I have learnt more in your two posts than most of my reading of online materials

Would you be able to recommend any good websites or books for beginners? A lot of the sites I have seen start discussions at a higher level and assume knowledge of the basics, so I find them difficult to understand.

thanks again.

Any idea what creates such sharp clouds (arrows/circles) ?

On the 17/11/09 around 11:00am : And on the 18/11/09 around 10:00am :

Its out to sea,also its close to convection down south on the boundary of dry air,start of convection over the ocean out from canterbury????stab in the dark.
Also today is the 18th and its not yet 10am quite.[quote="Lacertae"]Any idea what creates such sharp clouds (arrows/circles) ?

On the 17/11/09 around 11:00am :
Terra_17-11-09_11am.jpg
And on the 18/11/09 around 10:00am :
Terra_18-11-09_10am.jpg/quote]

Oops ! I posted my message at 1am that's probably why I made such a silly mistake ...
Of course you should understand that the 17th = the 16th and the 18th = the 17th.

Both of those lines are associated with wind convergence zones which are triggering shallow convection.

I'm not at work at present so I don't have any access to maps or observations to confirm what has caused the convergence, but...

The first one may be the remains of a thunderstorm outflow, as I think there were some thunderstorms offshore Canterbury on Monday morning. If it's not that, it's probably an offshore southwesterly change.

The second one looks like a weak trough (southerly change) offshore converging with winds (maybe northerly or northeasterly) along the Canterbury coast.

Who makes these forecasts up? 110kmh winds rain warnings..all we had was less than an hour of moderate rain,winds were only fresh now tonight jim comes on saying benign weather,here now gusting 35 knots more than anything today.

Thanks for your answers !

DOES ANY ONE KNOW WHEN THE LAST FROST WAS IN HAMILTON?

I have October the 11th as getting down to 1.0°C which would have been a light ground frost if anything did occur.
September the 9th was last confirmed frost at -0.8°C by my records anyway...

0.7°C on the 10th of October and 0.2°C on the 20th of September and 0.1 on the 8th of October.
-1.4 on the 6th of October for the last "official" frost.

Many times on the forum,Canterbury people often report its still warm,dry,sunny temperature high etc when a SW has developed(when a change is meant to occur).Is it because the front has split in 1/2 because of the alps and the leading 1/2 has gone up the westcoast wheres is lagging an hour or two in Canterbury,it shows up on satellite charts often.The real change 2 hours after the front has gone.