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 vapor, 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 neighboring 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.

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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 Ntional 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 vapor. 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 vapor 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.

At the time of writing this post, there are a number of major fires impacting communities across Western Australia. The team at Perth Weather Live would like to take this opportunity to thank our brave fire fighting men and women, many of them volunteers, who are risking their lives to save others and their property. For information on the fires currently burning in Western Australia, please visit

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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 vapor 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.

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.

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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.
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


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.
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.

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.


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.

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

Adam Delves_18oct2014
Image© Adam Delves/PWL – Single pea size hail from 18th October 2014, Perth

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

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

10723383_10205033548137855_1015474873_n  10728764_10205033548417862_858974483_n
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.

Sun Halo

A halo is a ring of light surrounding the sun or moon. Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have colour.

Beazley02Sun Halo
Image Credit: ©Michael Beazley/PerthWeatherLive

Halos form when light from the sun or moon is refracted by ice crystals associated with thin, high-level clouds, like cirrostratus clouds, which are commonly found between 5-10kms in the troposphere.

There are several types of halos, but in this article we will look at the two most common ones seen in Western Australia.

The 22 degree Halo

A 22 degree halo is a ring of light 22 degrees from the sun (or moon) and is the most common type of halo observed and is formed by hexagonal ice crystals with diameters less than 20.5 micrometers (1mm = 1000 micrometers)
Light undergoes two refractions as it passes through an ice crystal and the amount of bending that occurs depends upon the ice crystal’s diameter.

A 22 degree halo develops when light enters one side of a columnar ice crystal and exits through another side. The light is refracted when it enters the ice crystal and once again when it leaves the ice crystal.

The two refractions bend the light by 22 degrees from its original direction, producing a ring of light observed at 22 degrees from the sun or moon.

The 46 degree Halo

A 46 degree halo is a ring of light observed 46 degrees from the sun or moon. Although they are less common than 22 degree halos, the process by which they form is similar.

What determines if a 46 degree halo or a 22 degree halo develops is the path of the light as it passes through hexagonal ice crystals. A 22 degree halo results from “in one side, out another side”, whereas a 46 degree halo results from “in one side, out the bottom“.
These ice crystals are hexagonal-shaped columns with diameters between 15 and 25 micrometers and have an appearance resembling tiny pencils.

A 46 degree halo develops when light enters one side of a columnar ice crystal and exits from either the top or bottom face of the crystal. The light is refracted twice as it passes through the ice crystal and the two refractions bend the light by 46 degrees from its original direction. This bending produces a ring of light observed at 46 degrees from the sun or moon.

Sometimes, you will notice that the inside of the halo appears darker. This is because light is deflected by the ice into a wide range of directions larger than 22°, but no light goes below that angle. This means that no light is added by the ice into the inside of the halo, at angles lower than 22°. Technically, the sky inside the halo is the same brightness as it would otherwise have been without the ice.

Glenn Casey01Sun Halo
Image Credit: ©Glenn Casey/PerthWeatherLive

MarkFinleySun Halo
Image Credit: ©Mark Finley/PerthWeatherLive

CraigEcclesSun Halo
Image Credit: ©Craig Eccles/PerthWeatherLive

CameronFisherMoon Halo
Image Credit: ©Cameron Fisher/PerthWeatherLive

Moon Halo
Image Credit: ©Daniel Pardini/PerthWeatherLive

The text and diagrams in this article are used with permission 
under non-commercial rights from the University of Illinois WW2010 Project.
All photos are copyright and remain the property of named photographer.

Sun Rays

Sun Beams, God Rays, Cloud Breaks, Jacob’s Ladder, Ropes of Maui… these are just some of the names that sun rays are known by. But their proper name is crepuscular rays. A fairly common optical phenomenon, crepuscular rays appear when sunlight is blocked by objects, causing a visible contrast between sunlight and shadow. Clouds, mountains, building and even trees can cause this to happen.

The name comes from their frequent occurrences during the crepuscular hours (those around dawn and dusk), when the contrasts between light and dark are the most obvious. Crepuscular comes from the Latin word “crepusculum”, meaning twilight.
Crepuscular rays visible in fog as the light passes through trees. Image credit: ©Matt Fricker

If the atmosphere contains particles that ‘capture’ or reflect the sun light (like very small water particles, smoke, dust and even airborne aerosols) this allows the sun light passing thought the atmosphere to become ‘visible’.
Crepuscular rays are usually red or yellow in appearance because particles in the air scatter short wavelength light (blue and green) much more strongly than longer wavelength (yellow and red) light. Also, because the light traveling through the atmosphere at this time of the day has to pass through as much as 40 times more air than it does when the sun is high in the sky, the filtering effect is more prominent. This process is known as Rayleigh scattering.

photo 1 photo 2 photo 3
Image credit: ©Matt Fricker

As you can see in the images above, crepuscular rays seem to radiate outwards from behind the object. This is in fact an optical illusion due to our perspective as observers.

Screenshot at Aug 19 16-08-52    Screenshot at Aug 19 16-08-28

It is a similar effect to what you see when you stand next to railway tracks or power lines. As you look along the lines toward the horizon, they appear to get narrower.

Image credit: unknown

Crepuscular rays are (almost) parallel. Interestingly, it is actually the shadows that get slightly narrower. This is due to an effect called umbra, and it is best explained in the following diagram.

The illustration on the left represents the situation we have on earth, with the sun as the source of light being much larger. This causes the darkest part of any shadow to gradually taper the further it gets away from the source. 
Image credit: unknown

Crepuscular rays can also form when the sun is high in the sky and light rays break through holes in the clouds.



Anti-Crepuscular Rays
Anti-crepuscular rays are similar to crepuscular rays, excepting that they are seen opposite the sun in the sky (the anti-solar point). Anti-crepuscular rays are most commonly seen at sunrise or sunset and are much less visible. This is due the amount of atmosphere that the sun light has to pass through.

Although anti-crepuscular rays appear to converge onto a point opposite the sun, the convergence is actually an illusion. The rays are in fact (almost) parallel, and the apparent convergence is due to the vanishing point at infinity.

To see anti-crepuscular rays, turn your back to the rising or setting sun and look towards the horizon.

Below are some great examples of anti-crepuscular rays captured by some of the PWL chasers.

Image credit: ©Cameron Fisher

Image credit: ©Jeff Miles – Near Esperance, Western Australia. See more of Jeff’s photography here.

Image credit: ©Grahame Kelaher – Grahame captured this image in Guam. See more of Grahame’s photography here.

We will add more images to this article as they become available.

**Parts of this article have been adapted and used with permission under Creative Commons Attribution ShareAlike License.