6.1 - Weather Systems
Objectives:
- To know the difference between high and low pressure systems.
- To understand the formation of an extra-tropical cyclone including the formation of warm and cold fronts.
- To understand the formation of a sea breeze.
- To know the fundamental weather patterns that affect Bermuda.
Weather is caused by changes in the atmosphere. The atmosphere, especially the lower section known as the troposphere, is a vast fluid that is in constant chaotic motion. This motion is caused by uneven heating, moisture content and the rotation of the Earth. Less dense air rises and denser air sinks. The density is determined by the temperature of the air and its humidity.
Warm air is less dense than cold air as the faster moving molecules tend to spread out, increasing the spacing between them. Humid air is less dense as some of the molecules in a certain volume of air have been replaced by water molecules. From chemistry class, we learned that water molecules have a relative molecular mass of 18, while nitrogen gas and oxygen have relative molecular masses of 28 and 32 respectively.
The uneven heating sets up large convection cells in the troposphere which were discussed in Unit 3. These produce the three main hemispheric wind patterns; the Tropical Trade Winds, the South Westerlies and the Polar Winds. The interesting mid-latitude weather systems occur due to the flexible and chaotic nature of the boundaries between these moving masses of air.
Note: as Bermuda lies in the northern hemisphere - this discussion will only consider this. All the directions are reversed in the southern hemisphere.
Warm air is less dense than cold air as the faster moving molecules tend to spread out, increasing the spacing between them. Humid air is less dense as some of the molecules in a certain volume of air have been replaced by water molecules. From chemistry class, we learned that water molecules have a relative molecular mass of 18, while nitrogen gas and oxygen have relative molecular masses of 28 and 32 respectively.
The uneven heating sets up large convection cells in the troposphere which were discussed in Unit 3. These produce the three main hemispheric wind patterns; the Tropical Trade Winds, the South Westerlies and the Polar Winds. The interesting mid-latitude weather systems occur due to the flexible and chaotic nature of the boundaries between these moving masses of air.
Note: as Bermuda lies in the northern hemisphere - this discussion will only consider this. All the directions are reversed in the southern hemisphere.
Schematic of high and low pressure systems over the US. Image: Met Office
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The defining features of mid-latitude weather systems are the concepts of HIGH and LOW pressure areas.
I tend to think of highs as being hills and lows as valleys, with the air flowing 'downhill' from high to low.
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Physics note: pressure is the force applied per unit area. The extra downwards force at a high pressure centre is caused by a) the extra weight of the denser air and b) its downward motion. Vice versa for the low pressure air. The atmospheric pressure is measured by a barometer. The unit of pressure used by meteorologists is the millibar. Standard atmospheric pressure is defined as \(1013\,\text{mbar}\).
On a weather chart the centres of the highs and lows are usually clearly labelled with large capital letters. The 'contour' lines are lines of equal pressure called ISOBARS. Wind blows from HIGH to LOW pressure - but not in a straight line. It would do, but the world is spinning. For most latitudes, the wind blows at an angle of approximately \(15^{\circ}\) to the isobars. The angle is a bit less further south and greater to the north.
On a weather chart the centres of the highs and lows are usually clearly labelled with large capital letters. The 'contour' lines are lines of equal pressure called ISOBARS. Wind blows from HIGH to LOW pressure - but not in a straight line. It would do, but the world is spinning. For most latitudes, the wind blows at an angle of approximately \(15^{\circ}\) to the isobars. The angle is a bit less further south and greater to the north.
Extra-tropical Cyclones
These are not hurricanes - see a later section. But rather the far larger versions that affect the mid-latitudes. The term cyclonic refers to the direction of rotation of the system, which is always anti-clockwise in the northern hemisphere. These systems are also known as depressions and form at the boundary between different air masses. The rotation is set up by the Coriolis Effect. The formation of them is known as cyclogenesis.
These are not hurricanes - see a later section. But rather the far larger versions that affect the mid-latitudes. The term cyclonic refers to the direction of rotation of the system, which is always anti-clockwise in the northern hemisphere. These systems are also known as depressions and form at the boundary between different air masses. The rotation is set up by the Coriolis Effect. The formation of them is known as cyclogenesis.
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The image on the left is the classic textbook diagram of an extra-tropical cyclone. There are two air masses colliding; the warm, humid air from the south and the cold, dry air from the north. The cold, dry air is denser and so sinks and undercuts the warmer, humid air which rises above it. As the warm, humid air rises it cools and the water vapour condenses to form clouds of water droplets. If these water droplets are suspended by the upflowing air for long enough they can form large raindrops. They fall out as rain (or snow/hail if the air is cold enough) when their weight exceeds the air's ability to suspend them. This happens if the droplet gets too large and/or the upflowing air speed decreases.
Cold fronts are shown on charts by blue triangles (icicles) and warm fronts by red semi circles. |
The meteorologist that figured all this out was a Norwegian called Bjerknes and he named the boundaries FRONTS after the battlefronts in warfare. The steeper cold front always moves faster than the warm front and usually catches it up. This forms an occluded front. Cold and occluded fronts usually bring foul weather.
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Another textbook diagram that shows the development of a storm system. Note the general path of the depression from west to north east. The upper level jet stream and other high pressure systems can affect the direction that the low pressure centre takes, which messes up the predictability somewhat. The heavy rainfall takes some of the energy out of the system and it eventually dissipates.
The faster the air moves and the more pronounced the difference in air masses, the faster and 'deeper' the low pressure will form. This leads to worse and more dramatic weather. Sometimes a new system can form towards the end of a long trailing cold front. These can turn violent very quickly and the process is known as 'explosive cyclogenesis'. The US sometimes calls them 'weather bombs'. A rapid drop in the barometric pressure usually is a sign that a storm system is approaching. |
Now onto real examples of some mid-latitude weather systems. Below is a screenshot from www.windy.com that clearly shows a classic low pressure system to the north of Bermuda. The island is experiencing a cold front passing through. The passage of the cold front brought the textbook weather - air temperature dropped, strong line of heavy rain and a shift in the wind direction from SW to NW. Just before the rain came the temperature dropped and the wind speed increased. This is due to a downdraft of cold air from the clouds. This noticeable temperature drop is a tell tale sign for mariners to very quickly prepare for heavy weather. If the convection is high enough, then there can be thunder and lightning as well.
Observations taken from Bermuda Weather Service website before and after the passage of the cold front. The Radar image shows the rainfall. Some indication of the speed of the front can be obtained from the radar sequence of images which are taken at 10 minute intervals. The wind and pressure data come from the mini-weather station on Pearl Island.
A satellite image taken of another cold front crossing the island during early March 2020. It takes more practice to interpret, but some key features can be identified.
ACTIVITIES
Boil a kettle of water. The 'steam' rising from the kettle is actually tiny water droplets condensing as the invisible water vapour condenses as it leaves the kettle. To see the effect more dramatically, put a plate in the freezer for 10 mins or so, then boil the kettle again but with the cold plate above the spout. You will quickly see water condensing on the surface. Also, notice that the cloud produced by the kettle is rising. The cloud droplets quickly evaporate back to water vapour and disappear. If you were to leave the kettle boiling you would fill the kitchen with clouds, the humidity reduces the rate of evaporation until it reaches 100% and the air is saturated. If the air is saturated, no more evaporation will take place and the clouds will get denser. You will notice that there is water forming on the walls and ceiling.
As an aside: you will also get shouted at by your parents!
Boil a kettle of water. The 'steam' rising from the kettle is actually tiny water droplets condensing as the invisible water vapour condenses as it leaves the kettle. To see the effect more dramatically, put a plate in the freezer for 10 mins or so, then boil the kettle again but with the cold plate above the spout. You will quickly see water condensing on the surface. Also, notice that the cloud produced by the kettle is rising. The cloud droplets quickly evaporate back to water vapour and disappear. If you were to leave the kettle boiling you would fill the kitchen with clouds, the humidity reduces the rate of evaporation until it reaches 100% and the air is saturated. If the air is saturated, no more evaporation will take place and the clouds will get denser. You will notice that there is water forming on the walls and ceiling.
As an aside: you will also get shouted at by your parents!
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Other Resources
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NOAA JETSTREAM - best weather educational resource ever.
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www.weather.org - list of weather resources
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