Electric summer: Lightning to strike across NSW
The thunderstorm season has arrived and we are already seeing a number of major storm outbreaks strike NSW.
The first two days of December alone produced about 75,000 lightning strikes across the state. During major storm outbreaks there can be more than two million lightning strikes in 48 hours.
There are three types of lightning: cloud to ground, cloud to cloud and cloud to air. Most lightning strikes remain up in the clouds, however the type of lighting that humans need to worry about is cloud to ground lightning, which are the bolts reaching the earth's surface.
This type of lightning threatens not only industry but also poses a risk to property and occasionally life. It is estimated about five to 10 Australians die each year from being hit by lightning, while almost 100 are injured.
There are three ingredients for a summer thunderstorm:
A trigger: a process to initiate uplift from the surface
Instability: an unstable atmosphere which allows the rising air to keep rising through buoyancy; and
Moisture: enough water in the air so the rising air condenses into cloud.
Thunderstorms around Sydney are typically triggered by either an inland trough or the Great Dividing Range.
For example, a wind stream from the northeast is forced to rise over the Blue Mountains.
During a hot summer's day, the rising air may be warmer than the surrounding air a few kilometres above the surface, and therefore will keep rising to the top of the troposphere.
If the rising parcel of air is humid it will condense into a towering cumulonimbus cloud, i.e. a thunderstorm.
Following formation, a storm will then typically move east towards western Sydney. Even though winds at the surface could be blowing from any direction, the winds above the surface are almost always westerly.
Occasionally the storms don't make it all the way to the coastal suburbs of Sydney, weakened by the cooling sea breeze. As a result, western Sydney has slightly more storms each year than the coast.
Summer is the peak time of year for lightning storms in Sydney and NSW. The city on average sees about 20 days of thunder each year, with roughly half of those days occurring between December and February.
The total number of storms striking the Sydney Metropolitan region would be well above this figure considering your average thunder day has multiple storm cells and each of those cells would not necessarily move directly over the city - your average cell has a diameter of about 20km, much smaller than Sydney.
There are three factors that contribute to the amount of lightning and thunder in a storm:
Ice: the top of a storm is freezing cold, and as ice particles rub together, they charge up the top of the storm. For more ice we need more moisture rising from the surface, so a hot, sticky summer's day is perfect for lightning generation.
Powerful winds: strong winds at the top of a storm increases turbulence and violently mixes the precipitation to generate static electricity.
Dry air aloft: dry air enhances lighting because water molecules prevent the build-up of electrical charges.
Thunderstorms can be forecast. A forecaster will initially analyse weather maps to assess whether all three ingredients are met.
Once confirmed, the meteorologist will then study weather balloon data to determine whether or not the upper level winds are strong enough to carry the storms to Sydney and whether the sea breeze or other surface airstreams near the coast will prevent the storms from maintaining intensity on their trip across the Sydney Basin.
The balloon data is also used to assess the risk of severe weather from storms. For example, dry air aloft is an indication that storms will be gusty, while wind shear (the turning of wind with height), is an indication of supercell storms, the most dangerous type of storm.
In general, the most violent Sydney thunderstorms occur when the surface wind is a northerly and the wind a few kilometres up is a westerly.
Meteorologists and storm chasers use a radar to track individual storms, but you can easily calculate how far away a storm is just by counting the time between a lightning strike and thunder.
You will see a lightning strike almost instantaneously but sound travels far slower than light, so you won't hear the thunder until afterwards. Sound travels at 343m per second.
If you divide the time in seconds between lightning and thunder by three, it will give you the distance in kilometres. If you hear the thunder and see the lightning together, then the storm is within metres of you.
A La Nina summer, the first in nine years, offers the prospects of a stormy summer.
A La Nina phase of the Pacific increases rain across NSW and since a high proportion of summer rain comes from storms, we can expect lightning to be a common feature across the afternoon and evening sky.
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Originally published as Electric summer: Lightning to strike across NSW