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DID YOU KNOW? PART 16

Welcome to Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. Tonight's question is about the sea-breeze and it leads us into much more interesting topics later this week.


Waking up to glassy North Queensland waters, you go out on the boat but come back to something much different. Image www.fishingcairns.com.au

Did You Know:

"I live in North Queensland and I love taking my little boat out to the reef when it's flat. I leave in the morning and the sea is like glass, and it stays that way until I get close to the shore in the afternoon. As I get closer to the boat ramp in the afternoon the wind increases and soon the waves become quite choppy. It always makes getting out of the water much harder than getting into it. Why does this happen? "


ANSWER

You have encountered the sea breeze.

Want to know more? well read on

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A sea breeze is a local scale wind driven by temperature differences between land and sea. Let's go back to our three key weather rules in case we forgot what they were:


1 - TEMPERATURE DIFFERENCES DRIVE PRESSURE DIFFERENCES

2 - PRESSURE DIFFERENCES DRIVE WINDS

3 - WINDS DRIVE THE WEATHER

(Rule 1 is technically density differences drive pressure differences but we use temperature instead of density for ease of understanding) During Northern Australia's Spring and Summer (those are the seasons where the sea breeze is strongest and widest), the land heats up incredibly quickly through the day, but the ocean around that land remains a lot cooler throughout the day. This creates a strong thermal gradient between the edge of the ocean and the edge of the land. The warm air is less dense so it rises, this creates lower air pressure on the surface of the land. We know air will flow from HIGH to LOW air pressure so the cooler/denser (remember these two properties of the sea breeze for later topics) air from the ocean tries to rush in at the surface and fill the gap produced by the rising air over the ground. This generates the sea breeze. Above the land's surface the rising air creates an area of HIGHER pressure at a uniform height level relative to the ocean so the airflow well above the surface will reverse and once again flow from higher pressure to lower pressure.



The sea breeze brings with it cooler air for the coast and helps to trigger thunderstorm activity on unstable and moist days further inland - more on that part of the sea breeze later in the week. The entire sea breeze circulation system in Northern Australia is typically about 120km in horizontal distance and about 1000m-2000m in vertical distance and while the vertical distance remains fairly uniform the horizontal distance can vary significantly day to day due to thermal gradients (making the sea breeze extend from as little as 1km to as far as 300kms inland) and location to location due to topography (mountain ranges over 1000m high can stop a sea breeze dead in its tracks, while flat land can allow the sea breeze to extend a lot further inland). The sea breeze begins late in the morning and peaks around mid to late in the afternoon before collapsing by around 7-9PM local time. If the South-Easterly trade winds (or the synoptic scale environmental winds) are strong then the sea breeze may not be able to form at all. But on those days, you probably shouldn't be out on the water in a small vessel anyway.


On the ocean you will generally feel the sea breeze begin to affect your boat about 15-20km offshore and then the choppy waves begin just a couple of kilometres closer to the coast after you feel the first winds begin. That's why on the reef the water could be glassy, but as you head towards home, the winds increase and the waves chop up. The wind is likely to be at its strongest within a couple of kilometres of the coast and the waters at their choppiest right along the coastline where the wind driven waves is greatest and this coincides with the slope of the foreshore. In our Spring and Summer months, It is not uncommon to see winds sustained at 15-20 knots, gusting to 25-30knots and waves up to 1.5m, making what looked like an ideal calm day to go out on the water in your tinny, potentially dangerous by the afternoon.


The strong sea breeze of Northern Australia's Spring and Summer months is a hazard that can catch new boaties out in the region and something computer models have only started to model with some accuracy in the past decade. Unfortunately wave modelling remains well behind wind modelling and while you may be able to anticipate the wind strength by looking at a computer model, wave heights in those locations within a kilometre or two of the coastline are something that remains very poorly modelled via global weather models for local scale features like the sea breeze. So be careful not to trust wave models near the coastline on days where a strong sea breeze is expected. If you're not sure about the seabreeze strength on that day, use Google maps to try and find a boat ramp that is sheltered from any onshore winds (just be aware Northern Australia also has large tides so some of those sheltered ramps may not be accessible on low tides).

We take a look at why sea breezes can start thunderstorms on Wednesday, and why those same sea breezes kill thunderstorms on Thursday.



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