Virga

virga02Virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates  before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is often due to compressional heating, because the air pressure increases closer to the ground.

Daniel Pardini copy  Daniel PardiniImage credit: ©Daniel Pardini – The setting sun lights up virga over the northern suburbs of Perth, Dec 2015.

Virga can cause varying weather effects, because as rain is changed from liquid to vapour, it removes heat from the air. In some instances, these pockets of colder air can descend rapidly, creating a dry microburst which can be extremely hazardous to aviation.

Virga also has a role in seeding storm cells whereby small particles from one cloud are blown into neighbouring supersaturated air and act as nucleation particles for the next thunderhead cloud to begin forming. When virga is occurring, you will often see precipitation on the rain radar, but the ground will be dry.

virga01Virga from a decaying thunderstorm in the wheatbelt region of Western Australia.

This article is reproduced with permission and adapted under the Creative Commons Attribution-ShareAlike Licence. http://creativecommons.org/licenses/by-sa/3.0/
Article source: https://en.wikipedia.org/wiki/Virga

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14th November 2015

I don’t recall the exact location of many of these images. Somewhere between Quairading and York and Westdale in Western Australia. So rather than bore you with waffle… I’ll just let the pictures do the talking. From epic structure during the afternoon through to very lightning active nocturnal cells, this day had it all, including multiple fires due to lightning strikes. Enjoy.

Pyrocumulus Clouds

A pyrocumulus cloud is produced by the intense heating of the air from the earths surface. The intense heat induces convection, which causes the air mass above the heat source to rise. As the air rises it expands and cools. If the water content in the rising air mass cools to the dew-point temperature in the atmosphere around it (the temperature at which condensation forms) then cloud formation occurs. This is the same mechanism that causes thunderstorms to form on a hot day.
Amery Drage 01Image credit: ©Amery Drage. Large pyrocumulus cloud from a fire in the Kalbarri National Park, 13th March 2014. The base of this fire is between 50-60km away from the photographer. Notice how the winds at a higher altitude are pushing the top of the cloud over to the right.

Jordan Cantelo 01Image credit: ©Jordan Cantelo. Arial view of the Boddington fire, Feb 2015.

Phenomena such as volcanic eruptions, forest fires, and occasionally industrial activities can form of this type of cloud. The detonation of a nuclear weapon in the atmosphere will also produce a pyrocumulus, in the form of a mushroom cloud. Condensation of the moisture already present in the atmosphere, as well as moisture evaporated from burnt vegetation or volcanic steam, occurs readily on the particles of ash in the cloud. In reality, a pyrocumulus cloud is made up of smoke, ash and condensed water vapour. This is why they always have a greyish to brownish colour, with the tops looking more cloud like than the smoke plume closer to the ground.
Brayden Marshall 01Image credit: ©Brayden Marshall. View of the Boddington fire from the Perth metro area. The base of this fire is approximately 120km away from the photographers location.

craig eccles 01Image credit: ©Craig Eccles. Small pyrocumulus cloud starting to form. Notice how the majority of the smoke is blown to the right but the hot air rising straight up is forming a cloud as water vapour condenses.

Pyrocumulus clouds often contain strong updrafts, which can cause strong wind gusts at the surface. This effect can make an already dangerous fire even more so. As the hot air mass rises, fresh air is pulled into the base. Even a relatively small fire can create it’s own in-draft, which further fuels the fires capacity to burn.  A large pyrocumulus, particularly one associated with a volcanic eruption, may also produce lightning. A pyrocumulus which produces lightning is usually called a pyrocumulonimbus cloud, but not all pyrocumulonimbus clouds produce lightning.

There have also been many recorded examples of pyrocumulonimbus clouds producing rain. There are even a few examples where pyrocumulonimbus clouds, caused by forest fires, actually quenched the fire that spawned them.

Sam Kaye 01Image credit: ©Sam Kaye – Sam posted this photo into PWL on 3rd Feb 2015. It shows the pyrocumulus cloud from the fire near Boddington. Notice how the smoke turns to cloud at about the same altitude and the conventional clouds.

This article is reproduced and adapted with permission under the Creative Commons Attribution-ShareAlike Licence. http://creativecommons.org/licenses/by-sa/3.0/

Article source: http://en.wikipedia.org/wiki/Pyrocumulus_cloud

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2nd February 2015

Went out last night to watch the lightning over Perth from Champion Lakes in Kelmscott, Western Australia. I don’t usually like power lines in my shots, but I didn’t really have a choice on this occasion.

1st February 2015

This image was taken just outside of Gingin, Western Australia (you can see the town on the far left) and is made up of 8 separate images stitched together. After getting these shots, I had to get out of there and in front of it because this storm had dropped golf ball sized hail earlier. It chased me all the way to Bullsbrook.

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Just How Hot Is It?

Ever wondered why one day feels hotter or cooler than the next, even though the temperature is the same on both days? The answer is not as complicated as you might think. There is a formula for working out what the temperature of a given day might actually feel like.

The heat index (HI) is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature. In other words, how hot it feels.

The result is also known as the “felt air temperature” or “apparent temperature”. In Australia, the Bureau of Meteorology uses the term ‘feels like’.   For example, when the temperature is 32 °C with very high humidity, the heat index can be about 41 °C. That is why one day at 32 °C can ‘feel’ hotter or cooler than another day at 32 °C.

Screenshot at Jan 23 21-31-25
Screenshot from my personal weather station showing ‘feels like’ temperature. Note the high humidity.

But why? The human body normally cools itself by perspiration, or sweating. Heat is removed from the body by evaporation of that sweat. However, relative humidity reduces the evaporation rate because the higher vapour content of the surrounding air does not allow the maximum amount of evaporation from the body to occur. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. However, it is important to note that this effect is subjective. How one person ‘feels’ might be completely different to another person.  The ‘feels like’ temperature measurement system has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a heat index that relates one combination of temperature and humidity to another.

heat_index_chart
This table is from the U.S. National Oceanic and Atmospheric Administration, and has been adapted to reflect temperature in degrees Celsius.

To find the ‘feels like’ temperature, look at the Heat Index chart above. For example, if the air temperature is 36°C and the relative humidity is 65%, the heat index—how hot it feels—is 51°C.

The table below highlights the potential effects of heat on the human body.
27–32 °C   Caution: fatigue is possible with prolonged exposure and activity. Continuing activity could result in heat cramps.
32–41 °C   Extreme caution: heat cramps and heat exhaustion are possible. Continuing activity could result in heat stroke.
41–54 °C   Danger: heat cramps and heat exhaustion are likely; heat stroke is probable with continued activity.
over 54 °C   Extreme danger: heat stroke is imminent.

As air temperature is usually measured in the shade, it is important to note that exposure to full sunshine can increase heat index values by up to 8 °C.


This article is reproduced with permission under the Creative Commons Attribution ShareAlike Licence.   http://creativecommons.org/licenses/by-sa/3.0/

Source: http://en.wikipedia.org/wiki/Heat_index

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26-27th November 2014

My chase started late on Wednesday afternoon in Subiaco, as I had an appointment to have my injured hand checked out. The short version is that 10 days earlier, I tried cutting the end of my finger off with an angle grinder. But it is mending well and I was not going to let that stop me from chasing. I’m just happy it wasn’t my shutter finger.

So, with a fresh new bandage on, I set off through the northern suburbs of Perth, through peak hour traffic, with thunder, lightning and rain making things slower than normal. After 2 hours, I was finally on the open road and heading north. My plan was to head towards Wongan Hills and camp for the night. After a quick stop in Bindoon to grab some supplies, I headed up the Great Northern Highway, turning right onto the Calingiri Road. By this time, the western sky was on fire, probably due to the smoke in the air from fires around Gingin, which were caused by lightning strikes.
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From here, I headed NE towards Calingiri, then on to Wongan Hills, after stopping off to check out the Mount O’Brien lookout, which is 424 metres above sea level (making it the highest point in the region). The view is amazing, with a 360˚ panorama of the entire district. There were a few very distant storms to the east but I was able to get this image.
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It was now getting late, so I drove just south of Wongan Hills and found a quiet roadside parking bay to get some sleep. Time to test out my new bed arrangement, which is basically a large sheet of marine ply which sits across the folded down seats in my 4WD. My trusty camping mattress and sleeping bag made for a very comfy and peaceful sleep. The down side to sleeping in the car is that the sun wakes you up very early. After some brekky, I got on the road again and headed south to find a good spot to wait out the morning and see where the predicted storms might develop.

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By 9:30, it was clear that the convergence line running along the west coast was starting to move inland. This, combined with predicted temperatures in the low 30’s, meant that I would need to head north again. So I headed back along the road to meet up with fellow PWL chaser Grahame, who was waiting near Wongan Hills. After a few detours (to take some cloud development photos, and because one road I went down was a dead end even though the map said it wasn’t!) I finally met up with Grahame.

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Grahame had been watching a storm cell developing to the west of Wongan Hills, and once we met up, we headed west to intercept it. The plan was to get in front of it and leap frog our way down the Bindi Bindi Toodyay Road, stopping to take photos as we made our way south. The following images were taken at various spots between Yerecoin and Bolgart. We must of stopped half a dozen times. It was evident as we did this that this storm was picking up speed and strength.

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By now it was 1:30pm and we were just north of Bolgart. We pulled over and I got this short video which shows the updraft in the leading edge of the storm. It was also here that the storm began to change direction and head in a more SE’ly direction. As you can see, it was a fairly intense place to be standing and not something I would recommend doing if you don’t understand the risks (mainly due to lightning). Unfortunately the sound in this video was horrendous, so I had to drop it out.

As we left Bolgart, things started to get interesting. The wind picked up and as we were heading out of town on the Bolgart East Road, there were small tree branches flying everywhere and some fairly heavy rain. We went east about 10km’s and turned south onto a small side road so that we would be in front of the storm again. By this time there was a very strong gust front and it was hard to keep balanced whilst trying to shoot. I got some more video footage that shows the velocity. We also observed some cloud rotation in some scud on the SE corner of the cell.

From here, we made the call to head east to chase a new storm that had developed north of Dowerin. It was clear as we approached that this was a big system. In fact, we could see cells developing along a line that stretched north to south as far as we could see. This 360˚ panorama gives you some idea of what it is like to be surrounded by storms in almost every direction.

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(Click on the image to open in new window)

We stopped in Goomalling for fuel before proceeding east along the Goomalling Wyalkatchem Road. One feature that stood out to us was the leading edge of this storm, which you can see in the following image. This was going to be exciting.

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Normally, I don’t try to core punch storms, but sometimes in order to get in front of a system you have no choice. So through we went. As we got closer to Dowerin the rain really started to pick up. By the time we got into town, it was smashing down. The streets were flowing like rivers and the surrounding paddocks had water flowing off them. This is not ideal weather for this time of the year as many farmers are still harvesting. But you can’t control the weather and that is just part of life on the land. This short dash cam clip gives you some idea, but unfortunately it stopped recording as we passed through town. (I might be able to get some footage from Grahame as he had his GoPro running – will add later).

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At Dowerin, we dropped south 8km to Lake Dowerin, which is a salt lake that the road cuts in half, and we discovered that it had water in it. We found a safe spot to pull over and waited for the rain to ease. It was about 3pm when we got there and we stayed for a good hour watching the storm we had just passed through slide away to the SE. This was a good opportunity to set up a time-lapse (which I will post a link to once I process it) and take some photos of the storms around us.

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It was time to start thinking about our next move. Sunset was 3 hours away and there were more storms popping up further east. So the decision was made to push east towards Nungarin to find a good spot to set up and hopefully capture some epic sunset storm structure. I had been out this way late last year and I remembered that there was a salt lake NE of Nungarin which might make for an interesting foreground. We arrived at the lake just after 6pm and were greeting by a nice surprise. The lake had water in it. This made Grahame very happy because it meant we could get awesome reflections off the water of the distant lightning and sunset lit clouds, as you can see from the following images.

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As an added bonus we also got to shoot stars and storms, although they were very distant on the eastern horizon by now.

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By now it was was getting close to 9pm, so it was time to start the long trek back to Perth. We headed south to Merredin where we grabbed a late snack before hitting the road. This chase was the best so far this season, and hopefully a good sign of things to come. It was fun to meet up with Grahame and chase with him and I certainly learnt a few things from him during the day. You can check out his astro-photography page here.

So until next time…

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Cirrus Vertebratus

A type of cirrus cloud resembling a spinal column or fish skeleton, hence the Latin vertebratus for vertebrae-like.
Cirrus_vertebratus
Image credit: Wikipedia, © used with permission under creative commons license.

These clouds form at very high altitudes (usually between 6000m and 14000m) and often indicate that stormy weather is coming. They usually start out as a smooth band of ice crystals that is then blown by crosswinds to create the fine streaks to either side of the central column. They can be straight or curved, like the one I saw today (in the image below, which is a very poor quality phone image, sorry)IMG_3161Image credit: ©PWL/Matt Fricker taken on 16 Nov 2014

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Hail

Hail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted higher and higher into the thunderstorm until they freeze into ice. Once they become heavy enough, they start to fall. If the smaller hailstones get caught in the updraft again, they will collect more water on the way up.
hail_formation
As the stone gets higher and higher the water will freeze on the outside of the ice pellet, causing the hailstone to get bigger. Once the hail stone becomes heavy enough it will begin to fall. Depending on how many times this processes happens before the hailstone finally falls to the ground, will determine how big the hailstone will ultimately be.
hail-formation-large

In terms of diameter, the largest hailstone on record fell in Vivian, South Dakota, in the United States on 23 July 2010 (see image below). The stone measured 8 inches (20.32 cm) across, which is similar to the size of a small bowling ball. It likely was even larger when it fell, however, because it is believed to have melted somewhat before it was measured.

vivian_hail

Hailstones can fall from a height of 9000 m (30,000 feet) and approach the earth at speeds of as much as 193 km/h (120 miles per hour).

A hail stone shape is circular at smaller sizes and becomes more irregular at larger sizes.  Hail is generally compared to common, everyday objects when reporting size. The following chart will give you a rough idea of the size of hail and the estimated updraft speed required to carry it high into the thunderstorm. When the updraft is no longer strong enough to support the weight of the hail, it will fall to the earth.

Object         Size        Updraft Speed
pea              6mm         39 km/h
marble         13mm       56 km/h
20c              27mm       79 km/h
50c              32 mm      87 km/h
golf ball        44mm       103 km/h
egg              50mm       111 km/h
tennis ball     64mm       124 km/h
baseball        70mm       130 km/h
grapefruit      100mm     158 km/h
softball          114mm     166 km/h

Giant hail stones are usually more irregular in shape. This is because irregularities of a smaller size hail stone are exacerbated as the hail stone gets bigger. Also, smaller hailstones can merge onto a bigger hailstone. When this happens the hail stone will have bulges and will have a larger diameter in certain directions.

Below are some of the recent images we have received from PWL chasers and followers.

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Image© Daniel Pardini/PWL – Various sizes from 18th October 2014, Perth

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Image© Adam Delves/PWL – Single pea size hail from 18th October 2014, Perth

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Image© Carmen Mallard/PWL – Boyup Brook, 22nd October 2014.

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Image© Danica Justine/PWL – Large hailstone from Boyup Brook, 22nd October 2014

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Image© Blake Moore/PWL – Egg size hailstones from Boyup Brook, 22nd October 2014

To see more images like these, visit the Perth Weather Live Facebook page and like to receive regular updates, amazing photos and much more. You might like to check out these albums.

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