Q regarding weather radar

South African Weather Service.

Moderator: Moderators

User avatar
kb
Incipient Spin
Posts: 361
Joined: Wed Sep 07, 2005 4:14 pm
Location: FAGM
Has liked: 0
Been liked: 0

Q regarding weather radar

Unread post by kb » Wed Dec 30, 2015 9:54 pm

Question. If I look at the durban weather radar underberg and kokstad fall at 150 & 200 nm respectively. Would a storm cell in those areas show as accurately as they would closer to durban. Does the weather radar pick up the storm tops only, as line of sight would be affected by the miles of land and hills between durban and underberg? Meaning, would a storm cell on the radar be lot bigger than actual, and in that case it may not show at all unless it has enough vertical development. Hope that makes sense.
There are no stupid questions, only stupid people.
User avatar
tanglefoot
Fower Tousand
Fower Tousand
Posts: 4805
Joined: Wed Mar 04, 2009 11:05 am
Closest Airfield: FAGC
Location: Jhb
Has liked: 8 times
Been liked: 2 times

Re: Q regarding weather radar

Unread post by tanglefoot » Wed Jan 06, 2016 9:38 pm

kb wrote:Question. If I look at the durban weather radar underberg and kokstad fall at 150 & 200 nm respectively. Would a storm cell in those areas show as accurately as they would closer to durban. Does the weather radar pick up the storm tops only, as line of sight would be affected by the miles of land and hills between durban and underberg? Meaning, would a storm cell on the radar be lot bigger than actual, and in that case it may not show at all unless it has enough vertical development. Hope that makes sense.
As you are unlikely to get an official response from saws I will try to help.

Yes ,within 200nm the storm cells will show accurately. The red markings and arrows showing the direction of the storm are added manually and could change, or be wrong. These things are moving at 50km/h or more at times so are difficult to pin down.

The radar is definitely line of sight but, as far as I know, it picks up moisture over a relatively broad altitude, increasing with distance. Storm tips are often above 10000ft agl so pretty high and could be above the weather radar. The height above ground of the moisture is not possible to determine.

The biggest problem with weather radar is what lies behind the reflections. You can often see storm cell with what appears to be clear weather behind it. It is more likely that the radar can't penetrate beyond the storm cell.

Have a look at the spot graphs on the aeroclub pages. It will give you a better idea of the moisture content and potential cloudbase. It is a forecast but read in conjunction with the radar and other info is pretty good.

For the best available info just pm FAPE on avcom.
Wings are for fairies. Runways are for models...........................................
Don't get caught by Business Directory Scams. Facebook https://www.facebook.com/groups/55011156703/
Historical info on stopwbd.za.org
User avatar
kb
Incipient Spin
Posts: 361
Joined: Wed Sep 07, 2005 4:14 pm
Location: FAGM
Has liked: 0
Been liked: 0

Re: Q regarding weather radar

Unread post by kb » Tue Jan 12, 2016 8:20 pm

Thanks for the reply Tanglefoot. yes, all makes sense, I was just not sure how accurate the actual size of the storm would be shown at such a distance. I suppose 200nm with a increase in terrain elevation is small change compared to the altitude of the storm cells.
There are no stupid questions, only stupid people.
User avatar
tanglefoot
Fower Tousand
Fower Tousand
Posts: 4805
Joined: Wed Mar 04, 2009 11:05 am
Closest Airfield: FAGC
Location: Jhb
Has liked: 8 times
Been liked: 2 times

Re: Q regarding weather radar

Unread post by tanglefoot » Tue Jan 12, 2016 9:04 pm

kb wrote:Thanks for the reply Tanglefoot. yes, all makes sense, I was just not sure how accurate the actual size of the storm would be shown at such a distance. I suppose 200nm with a increase in terrain elevation is small change compared to the altitude of the storm cells.
I read an article online that said that the resolution at 200nm is around 1km x 1km, so a pretty big chunk of sky. "Ground clutter" is largely programmed out of the display but the radar is still line of sight and storms may be shielded behind high ground.

Treat radar info with circumspection. Even in the short term storm cells come and go faster than the pictures are refreshed on aviation.weathersa.co.za. If you feel like dodging thunderstorms then weathersa is not a good source :D

Even though it shows forecast directions of existing storm cells these change rapidly. It largely only tells you where the storm was at a specific time.

Google "weather radar". There is quite a bit of info out there.
Wings are for fairies. Runways are for models...........................................
Don't get caught by Business Directory Scams. Facebook https://www.facebook.com/groups/55011156703/
Historical info on stopwbd.za.org
FAPR
Wanna Fly
Posts: 4
Joined: Sat Jun 18, 2016 2:43 pm
Closest Airfield: FAWK
Has liked: 0
Been liked: 0

Re: Q regarding weather radar

Unread post by FAPR » Sun Jun 19, 2016 3:12 pm

kb wrote:Would a storm cell in those areas show as accurately as they would closer to durban?
A thunderstorm would often (but not always) appear larger the further away it is from the radar. What happens is the beam width increases the further away it travels from the radar, so much so that when the beam (which is now very wide at 200 km) is filled only partially by say, the top of the thunderstorm, the radar software will think the whole beam is filled, and therefore make it appear larger on the display. So yes, the storm is represented most accurately the closer it is to the radar. See the attachment of an example. The arrows point to the same storm but on different radars (Irene & Ermelo)
PFB.jpg

kb wrote: Does the weather radar pick up the storm tops only, as line of sight would be affected by the miles of land and hills between durban and underberg?
Considering thunderstorm tops reach on average 10 km, you will still see a good deal of the thunderstorm, anything from 3-4 km and above. So only the lower elevation scan of the beam will be blocked by the escarpment. Stratiform rain however might be missed completely since these occur at much lower altitude.
You do not have the required permissions to view the files attached to this post.
User avatar
tanglefoot
Fower Tousand
Fower Tousand
Posts: 4805
Joined: Wed Mar 04, 2009 11:05 am
Closest Airfield: FAGC
Location: Jhb
Has liked: 8 times
Been liked: 2 times

Re: Q regarding weather radar

Unread post by tanglefoot » Sun Jun 19, 2016 9:40 pm

At the end of July this information will no longer be available free of charge. Another tax on the GA pilot and in my opinion just one more tool for aviation safety available to all first world pilots (we are a first world country aren't we?) that we will no longer have access to. Soon the CAA will need to take responsibility for weather related aviation accidents. After all, it is their responsibility to create a safe environment for GA.
Wings are for fairies. Runways are for models...........................................
Don't get caught by Business Directory Scams. Facebook https://www.facebook.com/groups/55011156703/
Historical info on stopwbd.za.org
User avatar
kb
Incipient Spin
Posts: 361
Joined: Wed Sep 07, 2005 4:14 pm
Location: FAGM
Has liked: 0
Been liked: 0

Re: Q regarding weather radar

Unread post by kb » Mon Jun 20, 2016 12:24 pm

Thanks for the answers FAPE. clears it up abit.
There are no stupid questions, only stupid people.

Return to “SAWS”