Anomalous Propagation

Anomalous Propagation, also called anaprop (or more simply AP), is a phenomenon often seen on the weather radar and is something that many of our PWL viewers have asked about.
In very simple terms, AP refers to a false echo on the radar, which to the untrained eye, looks like rain. There are several reasons why the radar might show AP, but here in Perth the most common is due to something called temperature inversion. See our article on how the weather radar works here.

Under normal atmospheric conditions, the warmest air is found near the surface of the earth. The sun warms the surface of the earth (or ocean) and that warm air rises. The air gradually becomes cooler as altitude increases. In the evening, when the sun is low (or after it has set) the surface temperature can drop quite quickly. If the conditions are right, this creates a situation where there is a layer of cooler air sandwiched between the surface of the earth and the warmer air that rose earlier. This condition is known as temperature inversion.

Normally, when a radar signal encounters rain in the atmosphere, the signal bounces off the water particles and returns to the radar. ap.001

The following ‘normal’ radar image shows a line of thunderstorms approaching Port Hedland.


If there is an inversion layer however, the radar signal hits this layer of warmer air and is deflected downward towards the surface of the earth. The signal then bounces back and the radar reads this return signal as it would any other particle in the atmosphere. ap.002

The radar image below is from Broome, and shows the AP just off the coast. broome.001

Typically, when looking at a radar loop, you can tell if it is AP because it generally remains in the same location. Rain cells typically move in one direction.

Disclaimer: As you might appreciate, understanding AP, inversion layers and radar is a lot more complex than my simple explanation. If you are interested in the science behind AP you can look here for starters. Good luck!

Image credits: Radar images from BOM, illustrations by  Matt Fricker.



International Space Station

Whilst waiting for the summer storms to turn up, some of the PWL team like to head out and capture the many interesting things that can be see in the night sky. Anything from stars, comets, meteors and aurora’s are on the menu. But lets start with satellites…

Image Credit: NASA-International Space Station

The International Space Station (ISS) is a space station, or a habitable artificial satellite in low Earth orbit. It is a modular structure whose first component was launched in 1998. Now the largest artificial body in orbit, it can often be seen at the appropriate time with the naked eye from Earth. There a number of apps and websites that can tell you when and where the ISS will pass over (see below) and it often passes right over Perth.999938_10152174981729419_904539812_n     BGRIWDZCAAAeE3w
Image Credit: Commander Chris Hadfield / Perth on the Swan to the sea, Western Australia (@Cmdr_Hadfield – twitter)

The image above is just one of many that astronaut Chris Hadfield took as he orbited around the earth in the ISS.

The ISS travels at an average speed of 27,724 km/hour, and completes 15.50 orbits per day, with a with a minimum mean altitude of 330 km and a maximum of 410 km.

PWL Admin Grahame captured this image using a 12” Skywatcher on 22nd Jan 2014.
1601275_10152175080024419_1292464792_n copy
Image Credit: Grahame Kelaher / Perth Weather Live

The following image was captured by PWL Chaser Dan Searle on 22nd Jan 2014. You can see the ISS as it passes through the frame during a long exposure. 1558511_10203005664088286_231076188_n copy
Image Credit: Dan Searle / Perth Weather Live

You can find out about the ISS at the NASA site here.

You can check out the next best time to see the ISS in Western Australia here.

Happy spotting.



Roll Cloud or Shelf Cloud or Wall Cloud?

As the 2013/14 summer storm season gets under way, I though it would be good to have a look at some of the various types of cloud formations / thunderstorm features that you might see. In this post, we will look at the difference between Roll, Shelf and Wall clouds.

Roll Clouds
A roll cloud is a tube-shaped cloud and is a rare type of arcus cloud. They are nearly always completely detached from other cloud features and they appear to be “rolling” about a horizontal axis. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia. 640px-MorningGloryCloudBurketownFromPlane
Image used is shared under the Creative Commons Attribution-ShareAlike License.

However, similar features can be created by downdrafts in thunderstorms. This example is from PWL chaser Amery Drage and was taken in Jan 2010. You can clearly see that the cloud is detached from the cumulus cloud above it. 1456497_10202646561875994_2124245263_n
Image credit: ©Amery Drage / Perth Weather Live

One way that roll clouds associated with thunderstorms can form, is when the cool air in the forward flank downdraft at the front of the thunderstorm pushes out ahead of the storm, causing the shelf cloud to become detached from the storm. Known also as the gust front, it marks the leading edge of the rain-cooled outflow from the thunderstorm. If you see a well-defined roll cloud rolling toward you, prepare for strong and possibly damaging winds as it passes.

Shelf Cloud
A shelf cloud gets it’s name from the fact that it often looks like a shelf.  Also a type of arcus cloud, a shelf cloud is a low, horizontal wedge-shaped cloud, associated with a thunderstorm gust front. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud above it (and can sometimes be seen in non-thunderstorm cold fronts). Gust fronts can be very destructive and in the case below, the wind did a complete 180 degree turn in around 10 seconds, shifting from NE to SW as the gust front passed.
Image credit: ©Jordan Cantelo / Perth Weather Live

Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside can often appear turbulent, boiling, and wind-torn.  Shelf clouds form when rain cooled air rushes down to the ground within the thunderstorm and then spreads outwards as it hits the ground. When that cool air hits the ground and spreads out, it forces the warm and moist air upward. Another way to think about it is to picture a wedge of cool air lifting a blanket of warm air, like when you blow air under a sheet of paper.

A strong gust front can cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising fractus clouds. In a severe case there will be vortices along the edge, with twisting masses of scud that may reach to the ground or be accompanied by rising dust. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind squall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado.

The following image was sent in by PWL viewer Ben Niven and shows a classic shelf cloud over the Onslow Racecourse on 22nd February 2013.
Image credit: ©Ben Niven / Perth Weather Live

Wall Cloud
A wall cloud (sometimes called a pedestal cloud) is a large cloud formation that develops beneath the base of a sever thunderstorm or super-cell. It most often forms beneath the rain-free base part of the thunderstorm and indicates the area of the strongest updraft. Rotating wall clouds are an indication of a mesocyclone in a thunderstorm and most strong tornadoes form from these, but not all wall clouds rotate and not all tornadoes form from wall clouds. In this image, you can see the wall cloud ‘hanging’ down beneath the cloud base of a large thunderstorm in the wheatbelt, near Mukinbudin, Western Australia.
Image credit: ©Matt Fricker / Perth Weather live

The following image was captured just recently by PWL chaser Micheal Beazley. It shows a wall cloud very close to the ground. 1474603_10151997731635376_1007020455_nImage credit: ©Micheal Beazley / Perth Weather Live

Sometimes, if there is a lot of moisture in the system, a wall cloud will have a tail, which is a ragged band of cloud (fractus cloud) extending from the wall cloud toward the precipitation. in the following image, you can see a tail cloud forming under the wall cloud and to the right.
Image credit: ©Matt Fricker / Perth Weather live

Some wall clouds also have a band of cloud fragments encircling the top of the wall cloud where it meets the thunderstorm cloud base, which is commonly called a collar cloud. This classic super-cell image was was captured near Kalgoorlie on 17 March 2013 by PWL viewer Randal Webb, and features in the Perth Weather Live 2014 Calendar. You can clearly see the collar cloud bands in the middle section.
Image credit: ©Randal Webb / Perth Weather live

So next time you are out and you see a thunderstorm approaching, hopefully you will now be able to identify some of these storm features. But remember, thunderstorms can be dangerous and you must always take care to make sure you are not in harms way.

The information in this article is adapted and used with permission under Creative Commons Attribution ShareAlike License.



Willy-Willy, Dust Devil or Cockeyed Bob?

What you call them will depend on where in the world you come from. Willy-Willy, Dust Devil, Cockeyed Bob, Whirlwind and Sand Auger are just some of the names they go by. But how do they form? And are they the same as a tornado? Well, not really. Although they both have a rotating vortex, willy-willy’s are fair weather events. Tornadoes are formed in thunderstorms, super-cells or cold fronts. Water spouts are different again (see article here).
Willy-willy’s form when hot air near the ground rises quickly through a layer of cooler air above it. If conditions are just right, the rising air can begin to rotate.

As the air rapidly rises, the column of warmer air is stretched vertically and more warm air rushes in along the ground to the bottom of the newly forming vortex. As more warm air rushes in toward the developing vortex to replace the air that is rising, the spinning effect becomes further intensified and self-sustaining.  As the warm air rises, it starts to cool, losing its buoyancy. This cooler air then descends outside the core of the vortex. This cool air returning to the ground acts as a balance against the spinning warm-air outer wall and keeps the system stable.

dustdevil-jpgImage Credit: unknown

The spinning effect, along with surface friction, produces a forward momentum. Usually, willy-willy’s are very small and weak, often less than 1 metre in diameter with maximum winds speeds averaging about 70 km/h. Most dissipate less than a minute after forming. But occasionally,  they can reach a diameter of up to 90 metres with winds in excess of 100 km/h. If the conditions are right they can last for 20 minutes or more before dissipating. Willy-willy’s can suddenly just ‘disappear’ leaving the dust they were carrying to float to the ground. The following image shows a large willy-willy near Port Hedland. The stack on the left is 116m high!

1385801_588310904543459_1642485368_nImage Credit: ©Troy Bourne / Perth Weather Live

Interestingly, unlike tornadoes or cyclones, willy-willy’s will rotate either clockwise or anti-clockwise. Due to their size, the earth’s rotation has no effect of the direction of the spin and each direction occurs with equal frequency.

Even more interestingly, willy-willy’s have been observed on the surface of Mars! Check it out here.

1116518822_13678-4_dd_enhanced_486a-A496R1_brImage Credit: Courtesy NASA/JPL-Caltech

Here are some more images sent into us by PWL viewers:

1468555_614120531962496_1843613164_nImage Credit: ©Dean Thompson / Perth Weather Live.  This image was captured by Dean on his phone , not far from Roy Hill, in Western Australia’s Pilbara region..

960197_614319521942597_1045358762_nImage Credit: ©Name Supplied / Perth Weather Live. Another image from the North West of Western Australia.

This article is a modified version of and is shared under the Creative Commons Attribution-ShareAlike License. All images remain © of the owners.


Mini-Tornado Myth

If there is one thing that really frustrates storm chasers, it is the constant use of the term ‘mini-tornado’ by the media. To put it in simple terms… THERE IS NO SUCH THING AS A MINI TORNADO! Either it is a tornado or it is not. A willy-willy (or cock-eyed bob, dust devil, sand auger – depends where you come from) is not a tornado. A waterspout is not a tornado.

See related articles: Waterspouts, Willy-Willy’s

Recent events on the east coast of Australia have brought this issue into the spotlight again. See here.

So, in response to all this who-ha, I though I would share with you a very clever and funny post that I think sums it up brilliantly.  US based storm chaser, Bob Hartig, posted this on his page and it is shared here with his permission. The following is an extract from his page…

A true mini-tornado must meet the following standards:

• It is five feet tall or less. Of course, this implies an extremely low cloud base. You’d have to squat in order to get a decent photo.
• Width: Two feet or less.
• Human response: You feel a strong urge to say, “Awww, ain’t that cute!” You want to pet it and maybe even take it home with you and give it a nice bowl of debris.
• The synoptic conditions can be contained within five city blocks.
• Overshooting tops can be viewed from above by taking an elevator to the ninth floor.
• Damage (introducing the M Scale):

M0: Damage?
M1: No noticeable damage.
M2: No, there’s no stinking damage. Now go away.
M3: Okay, some damage now. Card houses knocked over unless securely glued together. Hair ruffled. That sort of thing.
M4: Now we’re talking damage. Well-built card houses scattered into a lawn-size version of 52-Card Pickup. Ill-fitting toupes snatched away. Nasty things happen when you spit into the wind.
M5: Inconceivable inconvenience. Securely glued card houses swept entirely away and lofted across the lawn. Well-gelled hair twisted into impressive new designs. You want to get out of the way of this baby.


So, there you go… Well said Bob.

By the way.. this is actually what a mini-tornado really looks like…




Contrails v’s Chemtrails

contrail_19I was in the process of putting together an article on the much debated Contrail v’s Chemtrail issue… but somewhere at about page 15 I stopped. Why? Because I couldn’t be bothered! (sorry if my honesty offends you).

So instead, here is a collection of links that I think will help to inform you. I make no apologies for sitting firmly in the contrail camp. Of course, you are entitled to your opinion.

These links will open a new page.


**NOTE**  I wouldn’t believe everything you read on Wiki type sites as they are open to being edited by anyone, but you can find some great articles in the reference section at the bottom of each article.



Iridescent Clouds

999585_10151941839080376_1343524199_n©Micheal Beazley/Perth Weather Live

PWL admin and storm chaser Micheal Beazley recently captured this great example of an iridescent cloud. Iridescent clouds appear when light is diffused by small water droplets or small ice crystals. Larger ice crystals produce halos (article coming soon).

When clouds (usually altocumulus,cirrocumulus or lenticular clouds) have small droplets or ice crystals of a similar size, the cumulative effect of light passing through them is seen as colors. In order for this to happen, the cloud must be very thin, so that the rays of light encounter only a very thin layer of water droplets or ice crystals. As in the image above, iridescence is mostly seen at cloud edges or in newly forming clouds , which tend to produce the brightest and most colourful iridescence. This is due mainly to the fact that in a newly forming cloud, the majority of the water or ice particles are all the same size.

The best way to see iridescent clouds is when the sun is behind another, thicker cloud or behind an object like a mountain or building.



The Smell Of Rain

P1060387We often see comments on the PWL page about how much people love the ‘smell of rain’… the distinctive scent which accompanies the first rain after a long dry period.  Have you ever wondered what that smell is? Well, it’s got a name… Petrichor.

The word comes from two Greek words, petros, meaning stone and ichor, referring to the fluid that flows in the veins of the gods in Greek mythology.

The term was first used in 1964 by two Australian researchers in an article that they wrote in a nature journal. In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, which is absorbed by clay-based soils and rocks. When it rains, the oil is released into the air. In another article some time later, they showed that the oil seems to delay seed germination and early plant growth. It seemed that the plants produced the oil in order to safeguard the seeds from germination during long dry spells or periods of drought.

I don’t know about you, but when I catch a whiff of the ‘smell of rain’… many childhood memories of growing up in the bush come flooding back.

This information is adapted and used with permission under Creative Commons Attribution ShareAlike License.  Image ©Matt Fricker Photography/Perth Weather Live


Rain Radar 101

A number of PWL viewers have asked us about how the weather radar works. To put it in simple terms, a radar is a device that sends and receives high frequency radio waves, also called microwaves. Imagine a lighthouse, but instead of a beam of light, a radar is a beam of energy. The signal is sent out and if it hits any particles in the atmosphere (rain, hail, ice, dust and even insect swarms) those particles ‘reflect’ the beam back towards the radar, which receives the signal. In many ways, it is like an echo. If you shout into a deep well, the sound waves will reflect of the walls and bounce back to your ears.

Different particles in the atmosphere reflect the signal in different ways, so the radar is able to determine how dense the particles are. Basically, denser particles reflect more signal back. This is then colour coded to illustrate what we might call ‘light’ or ‘heavy’ rain. In the example below, the Bureau of Meteorology uses a colour scale to indicate the estimated rain rate. radar01

It is also important to note that distance and even the curvature of the earth affect the accuracy of the radar’s results. As the following diagram shows, the further away from the radar source the beam travels, the higher up in the atmosphere it ‘sees’. This means that even though there might be heavy rain 200-300km away, it will only show up as light to moderate.


For a more detailed article on how the rain radar works, check out the Bureau of Meteorology’s page here.



Blue Clouds

Ever wondered why some storm clouds appear blue in colour?

blue cloudsThis image was taken on 25th August by PWL chaser Glenn Casey, near Lancelin, Western Australia.

 The moisture in a cloud has no colour. When the sunlight meets those droplets of water, it is scattered in all directions. That is what makes most clouds white, since white is the combination of all the visible colors of the light.

If the cloud is big enough, it ends up by scattering entirely the light of the sun and what appears to be a dark cloud is simply the shade of that cloud. When a cloud appears blueish, it is because the red frequency of the visible light spectrum, which is at the lowest end of the spectrum, is the first to be scattered. This process is called Rayleigh scattering. To put is simply, it appears blue because most of the red light has been filtered out.

It is also important to note that colour may also be affected by the camera’s white balance settings.