Weather in Hobart

Winds on the Derwent by Kenn Batt

The complex topography around the Hobart area makes for extremely tricky wind conditions over this extremely attractive sailing area. To the southwest through west to northwest lies a range of mountains extending from Mt Nelson (350 metres) to Mt Wellington (1460 metres) to Collins Cap (1098 metres). In the north through the east to the southeast lies Mt Direction (447 metres) to Mt Rumney (376 metres) to Droughty Point (150 metres). With such significant features so close to the shores of the Derwent, little wonder that sailing can be very frustrating and wind forecasting on the Derwent a forecaster’s nightmare.

The Derwent River drains not only water from Tasmania’s Central Plateau, but it also drains cold air. Temperatures fall overnight, particularly over the Central Plateau, significant areas of which are above 800 metres. Air over higher ground is naturally cooler and overnight, temperatures inland become markedly colder than those on the coast. As the air cools, it becomes denser and this denser air flows down slopes into the valleys. Like its liquid counterpart, this heavier colder air flows down the valleys into the sea. Hence in the morning, summer or winter, it is most frequent that winds flow down the Derwent Valley. This flow, known as a “katabatic wind or drainage flow”, can extend for a distance south of the John Garrow Light. The katabatic loses its influence over the winds, only when there is a strong (20 to 30 knots) SW through S to easterly gradient (900 metre) airflow. Thus, unless there is a 6 hPa pressure gradient or more across Tasmania from the SW, S or E, expect surface winds in the Derwent to come from the N or NW.  That’s the easy bit!

Once the sun comes out and warming commences, all sorts of different things have to be taken into consideration to determine which wind direction is going to be favoured in the Derwent.

Let’s first examine the northwest quadrant – N through W. As long as the gradient wind does not exceed 23 knots, we would normally expect the morning NW wind to decrease and a SE seabreeze to develop over the Derwent around noon local summer time. It could occur slightly earlier if it is warm and skies are clear inland, or later if it is cool to cold or it’s cloudy inland. If the maximum temperature inland is not expected to exceed 16 or 17C and it’s cloudy, the seabreeze could fail altogether and light NW winds will persist all day.

If the gradient strength is more than 23 knots, a seabreeze is unlikely and sailors can look forward to a lovely afternoon of shifting winds varying from very light to very gusty. The shape of the Central Plateau is such that it can sometimes “drag” air down from a considerable height to the surface over southeastern Tasmania. More often than not, the wind well above the surface is stronger than that lower down and consequently, air that is brought down to the ground level results in strong and shifty gusts.

I said above that in this circumstance, a seabreeze is unlikely, but it’s not impossible.

In summer, SE Tasmania can become very hot in a NW gradient stream, causing pressures to fall even more strongly than they do as a normal lee trough forms over eastern and southeastern Tasmania. On rare occasions, pressures fall so strongly that a small low develops in the lee trough, effectively forming in Storm Bay, resulting in fresh to strong S/SE winds developing over the general area.  In this situation, it is not unusual however, for the winds to continue from the NW over the Hobart area.

So much for northwesterlies!

Winds in the west to southwest quadrant can be also be very flukey. Because Mt Wellington acts as a giant cliff over Hobart in a west to southwesterly. We often observe mountain wave activity which result in very shift and gusty W/SW winds over most parts of the Derwent. When the wind speed exceeds 30knots at the top of Mt Wellington, one will often experience the effects of a rotor (situation where a mountain wave “breaks”

Figure 1 (From Wind, Waves and Weather, Tasmania)

and forms a closed clockwise circulation. See Figure 1). This is the situation where a light SE’ly is experienced over much of the Derwent (this sometimes is incorrectly termed a sea breeze). More intense rotors will see very light and shifty winds in the middle of the Derwent. And fresh and sometimes gusty W’ly flow on the far W and E sides of the River.

In south to southeast gradient wind situations, winds over the Derwent are generally more uniform, although some acceleration can be expected near the more prominent points and headlands.

In an easterly gradient situation, surface winds over the Derwent are quite frequently light in the morning and turn SE during the afternoon. Winds more often turn east to northeasterly during the afternoon under the influence of Coriolis force, which is caused by the Earth’s rotation.

The final quadrant, east to north, is again strongly affected by topography. Because of the steep sides of the Derwent Valley, northeast winds are generally uncommon on the River. We often observe light to moderate N/NW winds in the morning, while in the afternoon, the direction tends to veer into the east.

There are variations on all the above themes so obviously, take careful note of the forecast on the day. However, as I hope to have illustrated, there are so many variations possible and likely across the Derwent River, that forecasters have got little chance of describing them all. Indeed, would you be able to absorb a forecast that included every little nuance and shift?

Please note that tidal flows on the Derwent can play a very significant role in your racing strategy and tactics. Read the CSIRO report linked below and be extra careful when recent rain has fallen over the Derwent River catchment area.

References:

Bureau of Meteorology Wind, Waves and Weather. Tasmania. Commonwealth of Australia. 2000

Pendlebury. S.P. Notes Accompanying the Derwent River Estuary Wind Frequency Analyses. Bureau of Meteorology, 1987.

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