...wisps of personality passing on the zephyrs of time...©
Ephemerata Weather Radar FAQs Page
Click on a question to go directly to the answer or scroll down the page to work through them all.
Q. Why is there lots of browns and purples sometimes and not other? What are -dBZ values?
Q. How do I get on the scan zone email list?
Q. We had a whacking good thunderbanger last night but I didn't get an email. How come?
Q. Can I see tornados on the scan?
Q. Why is there lots of browns and purples sometimes and not other? What are -dBZ values?
A. We'll answer the second question first. The radar system is calibrated to report no detectable precipitation as 0 dBZ echo return. This does not mean there is no echo, but the strength of the echo at this level is used as the baseline to measure precipitation. Plus values above zero are echo return strengths of moisture that may result in some level of precipitation. Minus echo values are echo returns determined by the radar system to not be reflections associated with precipitation. Thin cloud, humidity, pollution, birds, aircraft and even temperature inversions, all contribute to these. These returns are reported as the beige and purple colours. Collectively, these returns are known as "clutter". For clarity of display, displays are often "decluttered" - these minus returns are removed from the presentation to focus on the plus values.
In clear air mode (VCP 31,32), all echo returns are reported, plus and minus, and are displayed. The Storm Predator software Ephemerata Weather Radar uses can suppress any echo return at will, and occasionally we suppress the minus values in clear air for better map clarity underneath. When the radar site switches to a precipitation mode, minus echo retuns are suppressed at the radar site, to prevent the heavier returns associated with the moisture from swamping the display.
When precipitation approaches, the radar site sometimes switches back and forth between clear air mode and precip mode in order to get a better picture of the approaching weather. Note the VCP number - you'll see it change with the display. If the radar site is playing with VCPs (i.e. modes), you know that there is weather approaching that is being analyzed.
Return to topQ. On clear evenings, the radar scan show broad areas of blues, growing as the evening progresses, yet there is not a cloud in the sky. There appears to be a lot of rain over the radar station, what's happening?
The smooth even areas of colour on clear windless nights is the radar site picking up a common phenomenon known as a radiational inversion. On clear quiet nights the air immediately above the ground cools faster than the air above it as longwave radiation from the ground gives heat off into space, cooling the air immediately above it. This leaves a stable (since cold air is heavier than warm air) evening air structure consisting of a layer of warmer air sitting over a layer of cooler air. This causes refraction and reflection of the radar beam resulting in the radar detecting the heavier, cooler air. This "anomalous propagation" is most pronounced over the radar site, making the radar site look like its in a perpetual shower all night. The inversion conditions can also produce "radiation fog" which may also appear to be on the radar as local areas of rain. Conversely, in the morning you'll see these areas recede as the sun rises and daytime heating commences. The western end of Lake Ontario regularly demonstrates this "ana-prop" on the radar screen.
Return to topQ. When I look outside, there is lots of heavy cloud, but nothing is showing on the radar. What gives?
A. The radar is not measuring cloud cover, it is measuring water content in the atmosphere. Above a certain amount, it produces a radar echo. Clouds, condensing water vapour in cooler air, may not have much water in them that can fall as rain. In Precipitation Mode, the radar suppresses low water content echos to concentrate on high content clouds. If there isn't at least a blue scan, then there is not much chance of rain from the cloud cover. In Clear Air Mode, if the lower echo levels aren't suppressed, the radar will show cloud, fog and haze.
Return to topQ. Why are there sometimes things missing from the web display, e.g., no radar display, missing legends or a message saying data is unavailable?
A. The automated web presentation by Ephemerata Weather Radar is a by-product of the Storm Predator software we use. The weather processor software we use doesn't just take images from NWS servers and throw them up on the web. The NWS servers supply raw data from the multitude of radar sites in a form that our processor software can access, download and process for our own purposes, and then take that data, assemble an image and upload it to the web as the scan display you see. If the data stream from the NWS is incomplete (radar site issues for example) or corrupt, our processor may or may not be able to use the data to complete an image to upload. We may then upload what we get, or wait till our polling cycles can obtain complete data from the NWS servers. There are dozens of radar sites reporting to the servers in a vast network. Phonelines go down, cable networks fail, servers burp, networks choke and packets collide, so occasionally we cannot get a complete dataset to process. If there is a radar site problem that we are aware of, we usually shift to an alternate site for data. Please note, our web system runs for the most part unattended, as it is fully automated (outside of trend commentary that we provide below the scans).
Return to topQ. Why are there sometimes only one scan, Scan A, and at other times several more than two, Scan B and the supplementals? Why are the loops not up all the time?
A. At present, the Storm Predator weather processor can only run as a single application on a single computer. To generate Scan B at the same time as Scan A with Storm Predator requires that a second computer be brought online to handle Scan B. We now generate Scan Bs with GRLevel3 software. While the Ephemerata Weather Radar Project has 5 computers in its LAN available, for practical purposes, at any one time only one or two may be in use. Presently, one computer is solely dedicated to the Ephemerata weather processor, and the system is mirrored on the other machines so that any number of them can be called into service as needed. Ephemerata Weather Radar has been undergoing considerable software and hardware development recently. We continue to evaluate supplementary software and dedicated computers to run the project, as well as alternate presentations of the scans. There may be as many as 5 scans on the main page, and we have introduced the Mosaics layout, available from the selector bar, which provides additional scan products if conditions warrant.
A consideration also has to be made in regard to the internet service provider's capability as well in terms of the traffic volume involved for both the data polling and the uploading. We add or subtract Scan B scans according to the need based on impending weather, in order to manage the bandwidth resource.
With regard to loops, at present, the internal uploader in the Storm Predator processor software can only upload one element per configuration,ie, either a current scan image or a current loop product, but not both simultaneously. By use of a third party uploader and the capacity of the program to create independent datafiles for current scan and loop, It is possible to have both current scan and loop going for both Scan A and Scan B, but it is very resource intensive requiring multiple computers and requires frequent monitoring of the internet connection for upload conflicts which may abort the link. We've recently added the US Great Lakes and Northeast regional scans as a standard feature of the main page to alleviate the load on hardware. The loops are very important for determining the directional trend of storm weather.
Return to topQ. How do I get on the scan zone email list?
A. If you reside in the GH-GTA or Southwestern Ontario and wish to receive an automated notification of severe weather within the zone, send us an email at the address on the radar scan page and request to be added to the list. There are two lists - one for those receiving emails on a full computer system, and one for those with mobile email capability like Blackberries. The full list option will send a copy of the current scan image for you to examine along with a note. The mobile alert option will simply send a text note. Due to expressed interest, I am also currently testing alerts for Southwestern Ontario. These will be generated by a separate computer. While the corresponding scan A won't be on the main page (subscribers will get a scan image in their email), the appropriate Scan Bs will up for review, should a subscriber receive an alert.
Return to topQ. We had a whacking good thunderbanger last night but I didn't get an email. How come?
A. The email alert system is set to provide an automated alert based on a number of factors related to the intensity of the storm. Not every storm will trigger an alert. While thunderstorms are always impressive, not all are severe storms. The alert system is set to warn for truly dangerous conditions but not so sensitive as to flood out emails for every banger. Once an alert is sent, the system will not send another one for an hour, unless conditions subside and re-develop, which can happen frequently in unstable air. You can expect approximately 3 alerts for a storm that maintains its strength across the zone. We monitor the response of the alert system regularly to reduce false alerts and over- or under-sensitivity.
Return to topQ. I got an email, but when I looked at the scan, I didn't see anything that looked like much of a storm anywhere.
A. In summer especially, convective storm cells (moist air rapidly rising due to ground heating) can pop up and develop quickly into small, but potent local storms. They may only cover a few city blocks, then dissipate. If these occur during the scan cycle, and the cell is potent enough, they'll trigger the alarm. They may only show as a small red spot somewhere in the zone. While small, there may be intense weather under the cell. If conditions are right, these cells can merge and grow with other cells forming bigger storms - an MCS - multi-convective system. Tornadic storms are frequently fairly small, but highly developed cells, which might be "rain-wrapped" in larger, less intense systems.
Return to topQ. Can I see tornados on the scan?
A. Sometimes, especially if they are large. Radar signatures of tornadic cells sometimes show a comma-shaped or hooked appearance that is diagnostic of a tornadic cell. Several different radar scans are used to look for tornados, most particularly the Storm Relative Velocity scan, which measures the wind patterns within the cells. In the right circumstances, SRV will show the characteristic trace of a "mesocyclone", weather-speak for a rotating air column within a storm cell, which may spawn one or more tornados if the conditions are right. The software we are running on Scan B at the present time is reporting Level III Doppler data, and will report conditions ripe for tornado development directly on the image. While tornados are not uncommon in southern Ontario, potent mesocyclones are relatively rare here, and most tornadic storms we see are limited, damaging to be sure, but fortunately, not very big. What we do see here frequently are the other damaging products of thunderstorms - lots of rain, gust fronts and microbursts. These are powerful wind gusts emanating from a well-organized thunderstorm, and can be as damaging as a small tornado. The damage pattern is usually linear, rather than the swirling pattern associated with a tornado.
Return to topQ. When I click on Scan B, I get a larger image, but its different than what's on the main page. How come?
The NEXRAD radar system produces many different kinds of radar scans, all designed to highlight certain features of the weather. For general monitoring purposes, two of the more commonly used scan "products" are Composite Reflectivity and Base Reflectivity.
Composite Reflectivity (image marked "CR") is a representation of the sum of all of the reflectivity scans up through the vertical column of atmosphere. A storm cell or cloud layer is not homogeneous up its vertical profile. Hail, rain and other features form with differing intensities at different places in the vertical column of air depending on the atmospheric conditions (temperature-wise, especially). Since the radar unit takes a "picture" of different layers as its dish antenna steps up in angle in rotation, it gets different results on each step. These layer results are important to the process of "interrogating", or interpreting, the storm structure. Composite Reflectivity takes the total of these layers and forms a view of the total echo response of the column of air "interrogated". EWR presents the Composite Reflectivity image on the main page as it represents the general weather overview.
Base Reflectivity products (image marked "BR1"), on the other hand, show the results for only one scan angle. The ability to scan at different angles is important; severe weather such as mesocyclone development (rotation of the core of the storm cell) and tornado genesis may not be evident in a composite scan since upper level storm features may hide lower level features in the Composite Reflectivity sandwich. EWR displays the 5° scan angle (lowest angle) Base Reflectivity product in the enlarged image in the B Scans, in order to provide a comparison to the composite reflectivity image, and show more of the storm structure close to the ground. This is especially important if conditions favour tornado development. If the B Scans are showing mesocyclone or TVS or hail icon pop-ups (see the "special note" linked to under the image for more information about these), its wise to keep an eye on the enlarged base reflectivity scan to see if diagnostic features of a rotating cell, like a "hook echo" may be being hidden by the rest of the storm in composite reflectivity.
You'll notice that the composite reflectivity scan lags by a little bit the base reflectivity scan in time and weather advancement. This is because the radar has to complete all of its scan cycles before it can report the composite scan, whereas the base reflectivity product is sent out as soon as each angle scan has been completed.
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