Why Does Weather Channel Radar Continuously Load

Understanding the Images

Why is the radar showing no rain when it is raining where I am?

The effects of the curvature of the earth on weather radar.

1. The intensity of echoes tends to decrease with increasing distance from the radar. This is because:

a) The radar beam broadens with distance, thus decreasing the proportion of the beam which is filled with rain, which reduces the echo intensity;

b) The radar beam becomes further from the ground with distance (partly because of the Earth's curvature, and partly because the beam is angled upwards by a fraction of a degree), thereby missing the lower parts of the rain. A horizontal radar beam detects raindrops at a height of 1 kilometre above the Earth's surface from rain that is 100 kilometres away from the radar. It detects raindrops at a height of 3 kilometres from rain that is 200 kilometres away, and at a height of around 7 kilometres at a distance of 500 kilometres from the radar. In winter especially, the rain clouds can be below the radar beam at a distance of more than 200 kilometres from the radar, and hence the radar beam will overshoot the rain. As a result, the radar image will not show any rain even though at the ground level it may be raining at the time.

Due to the reasons described above, interpreting the radar images at extended ranges can be difficult and you should take great care in using these images. With practice, however, you will find them useful.

Check the Optimal radar coverage for your nearest radar.

c) The beam can lose power slightly when passing through very heavy rain, thus reducing the echo intensity further out from the radar.

Thus precipitation that is occurring some distance away from the radar might not show at all, or may show with a reduced intensity. The presence of significant echoes at large range probably indicates the presence of large amounts of rain at high levels above the ground (e.g. a thunderstorm). At these distances the radar echoes are likely to be reflections caused by ice rather than rain drops, where the relationship between reflectivity and rainfall rate is different.

2. The presence of mountains within the range of the radar can block part or whole of the radar beam, thus significantly reducing the echo intensity from rain on the other side of the mountains.

3. Because of the variations in rain closer to the radar site and in the refractive index of the air, attempts to correct for these limitations have not been very successful. Consequently the estimation of rainfall rates using the radar imagery should be used only as a very rough guide.

Why is there a difference between the rainfall measurements and the colours on the radar image?

The radar reflectivity is strongly dependent on the diameter of raindrops in the cloud not the amount of rain drops and therefore rainfall rates. Tropical maritime rainfall consists of very many moderate sized raindrops so that the reflectivity is much less than for similar rainfall rates in continental area rain clouds. The latter rain clouds typically consist of very large raindrops but much less in number.

Why does the radar show that it is raining when it is dry where I am?

The radar may sometimes detect faint echoes from non-precipitation targets such as aircraft, areas of smoke/ash from large fires, swarms of insects, flocks of birds or even the surface (when unusual atmospheric conditions bend the radar beam back down to the surface!). Permanent echoes are created when the radar beam reflects off ground features and buildings, normally within about twenty kilometres of the radar site, but mountain ranges further away can sometimes generate permanent echoes.

Check the radar site information to see if your nearby radar image is affected by any interference.

Remember that due to the Earth's curvature, the radar beam becomes higher above the ground the further it travels from the radar. Thus a weak echo may not mean that it is raining at the ground because under some circumstances light rain aloft can evaporate completely before reaching the surface.

The intensity of drizzle may be underestimated because of the lack of large droplets.

Why does one radar show less rain than another radar for the same location?

The main reasons for differences include the different distances and angles from the radar transmitters, the presence of topography and differences in the frequency and angle of the radar beam. Also the collection period of the two radars may well have been different.

Check the radar site information to see if your nearby radar image is affected by any interference.

Check the Optimal radar coverage for your nearest radar.

Why does the radar sometimes show echoes that don't look like rain?

Sometimes the refractive index of the air is such that the radar beam becomes "bent" and reflects the ground or ocean surface some distance away from the radar. This is known as Anomalous Propagation and occurs usually when there are strong temperature inversions present. Electronic processing of the returned signal usually detects the steadier reflections from ground clutter, but reflections from the waves on water are more likely to appear like true rain echoes.

Near sunrise and sunset the radar antenna momentarily scans the sun. On occasions this can be seen as a pencil line radiating out from the centre of the image in the direction of the sun.

Check the radar site information to see if your nearby radar image is affected by any interference.

Why do areas of widespread rain sometimes appear and disappear on the 512km composite image?

The 512km composite images are unique as they are a composite of all available radar images and sometimes several radars may contribute to the overall view. Some radars in the Bureau network are windfinding radars, which means that there are certain times of the day in which they do not provide input to the 512km composite radar views (click here for more information about the routine windfind time periods).

As a result, when widespread rain is evident in the region it can sometimes disappear from the radar image between one frame and the next. This happens because the nearby radar has switched to windfinding mode. Similarly rain can sometimes appear after the windfinding radar switches back to its normal mode. The Bureau's windfinding radar sites are shown on the radar home page map as blue squares.

Can the radar see smoke from a large fire?

Radar images may show areas of smoke from very intense fires. This type of phenomena is called Pyrocumulus. Very large fires may be distinguished by an elongated area of near-stationary echoes emanating from the source of the fire. See example from the 2009 bushfires in Victoria below.

What is the best distance that radars can see rain?

Radars images on the Bureau's website display the rainfall echoes from clouds approximately 3000m above the ground. Due to the curvature of the earth, the radar beams optimal range is from 5 to 200km for this 3000m level. Beyond 200km, the radar is displaying rainfall echoes from clouds at higher altitude in the atmosphere and the radar image may not reflect what is happening on the ground.

There are usually no echoes displayed very close to the radar because the radar beam does not scan directly above itself. This is commonly referred to as the "cone of silence".

The 512km composite images provide a more expansive coverage at a height of 3000 metres, by combining echoes from surrounding radars. However for remote radars it may not be possible to include other radars in the image and care should be taken when interpreting these images for distances greater than 200km away from the radar.

Check the Optimal radar coverage for your nearest radar.

Why does the radar image show a gap or clear area near the radar?

There are usually no echoes displayed very close to the radar because the radar beam does not scan directly above itself. This is commonly referred to as the "cone of silence".

How intense is the rainfall for each colour range on the radar image?

There is an approximate rainfall intensity for each colour range. What you observe at ground level may vary from the intensity values because the radar image is an estimate of rainfall droplets at around 3000 metres, and the showers or thunderstorms may be developing or decaying during that time.

Does the radar show snow?

In some circumstances, the radar may show clear weather when it is actually snowing.

Snow clouds may not show up on radar images because:

• snow clouds can be relatively low to the ground and not detected by the radar beam overhead; or
• mountains can block the radar beam so it can't detect what's happening on the other side.

It's important to check the weather before participating in outdoor activities in alpine areas. Check the Bureau's weather forecasts for the Australian Alps region and learn how to use MetEye for accessing detailed snow forecasts for all regions.

Using the Radar Viewer

Can I still get the simpler radar image without the new layers?

Yes, you can still get the latest single radar image. Just click on the label called "Single Images" located along the top of the Radar Viewer.

How can I get the radar to work on my mobile phone?

Radar images are available via the BOM Weather app.

Can I save the radar loop?

The radar viewer constructs the loops based on the individual images. The loops are not an animated gif. The Bureau cannot offer any solutions as to how to save the images as a loop.

Can you add lightning strikes to the radar images?

Lightning data is measured by a private company, and the Bureau does not have permission to present the data on its radar viewer.

Can you put more roads or rivers on the radar viewer?

The geographic information layers used by the Bureau are sourced from other government organisations. Any alterations or mistakes are the responsibility of those organisations.

What do I do if I found a location incorrectly labelled on the radar maps?

The locations used by the Bureau are sourced from the Geoscience Australia gazateer location database. Sometimes there are multiple coordinates to describe a geographical location and we may not have chosen the most appropriate option. We are more than happy to amend these. Let us know by submitting the feedback form.

Why don't you put arrows on the radar images to show where the rainfall is heading?

To determine which direction the rainfall echoes are heading, click on the loop option at the top of the radar viewer. The loop option will animate the images over the last 30 minutes. People using bookmarks to access the radar may need to update their bookmarks to take advantage of recently added features to the radar viewer. To update the bookmarks, please navigate to the relevant radar from the radar selection page.

What is UTC time?

UTC is an international 24 hour time and date standard (Coordinated Universal Time), which is the same as GMT (Greenwich Mean Time). The UTC times are displayed on all radar images above the legend. Local times are shown above the radar controls in the looping interface and underneath the legend for single images only.

Why isn't the 64km view available for most radars?

Only Doppler radars (denoted by a black diamond on the radar selection map) have the capability to display the rainfall echoes on a 64km view. Standard radars do not have the same characteristics of a doppler radar and therefore cannot monitor the wind speed or calculate rainfall accumulations.

How can we get radar coverage for our area?

The Bureau has been funded by the Government to upgrade its radar network and install a number of new radars to improve coverage over a greater proportion of Australia. The Bureau is constantly planning its radar network in accordance with the needs of its forecasting and severe weather warning requirements.

How do I calculate the speed that the rain is moving on the radar?

1. Turn on looping option
2. Identify a rainfall echo that you want to measure the speed for.
3. When the loop begins with the first image, move mouse over rainfall echo and click the mouse button to initialise the pointer coordinates (located on the panel to the right of the image)
4. Move and hold your mouse over the position of the rainfall echo for the last image of the loop.
5. The pointer coordinates should indicate how far the rainfall echo has moved (see third box on the panel)
6. To calculate the speed, simply multiply the distance travelled in 30 minutes by 2. This will give you the speed in kilometres per hour.
7. Example:- If a rainfall echo moves 30km between the first and last image of the loop over 30 minutes, then the resultant speed is 60 kilometres per hour.

What orientation are the Pointer coordinates based on?

The direction values are based on true north orientation.

Why do some radars not show images for a couple of hours each day?

The Bureau's radar network is comprised of two types of radar - Dedicated 24 hour weather watch radars and part-time windfinding radars. Part-time windfinding radars are used by the Bureau to track weather balloons to measure the wind conditions of the upper atmosphere. Upper atmosphere wind conditions are a vital piece of information to assist forecasters in preparing accurate forecasts.

Where can I get more information about radars?

The Radar Help section includes information about the radar viewer, radar images and the radar network. It is linked from a tab on the top right of the radar viewer.

Understanding Doppler Wind Images

Why is the wind speed on the doppler image different to the measurements?

Visit our About Doppler Images webpage for a detailed explanation.

How do I learn more about Doppler wind?

Visit our About Doppler Images webpage for more information.

How do I estimate the wind speed and direction from the doppler image?

Read about the techniques for estimating wind speed and direction, then follow our example for estimating the wind speed and direction.

Can I see a seabreeze.

An example of a wind change passing through Adelaide demonstrates how to identify this feature.

Understanding Rainfall Accumulation Images

How accurate are the rainfall accumulation values?

Visit our About Rainfall Accumulation Images webpage for a detailed explanation.

How do I learn more about Rainfall accumulation images?

Visit our About Rainfall Accumulation Images webpage for more information.

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Source: http://www.bom.gov.au/australia/radar/about/radarfaq.shtml

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