Topic: WU "Rain Rate" Nit

[LABob]

Shouldn't it be "Last hour of precipitation" or something? It seems to me kind of like if a navigation app labeled the distance you'd driven in the last hour as "speed".

[Scalphunter]

you can have various rain rates in an hour  where  last hour of precip would be the amount that  fell.   example I had  5 inches  of rain   falling  as  rain rate  yet in that  time  frame it amounted to  only .75  of an inch.


John

[LABob]

you can have various rain rates in an hour  where  last hour of precip would be the amount that  fell.   example I had  5 inches  of rain   falling  as  rain rate  yet in that  time  frame it amounted to  only .75  of an inch.


John

Exactly. It could rain 10 inches per hour for 3 minutes and then stop. WU would report the peak rain rate as 0.50 inches per hour.

[LFWX]

It depends on what your weather station software is sending.

For example, some versions of Virtual Weather Station send the instantaneous rain rate, while other versions send hourly rain (total that fell in the last 60 minutes).

[hankster]

If you are using Weather Display it calculates rain rate every second as the instantaneous rain rate with every bucket tip. This is than multiplied out and stored in the log file as inches/min and inches/hour.

The testtags.txt file will show that instantaneous rain rate that is calculated every second. So my website, which is updated every 10 seconds, it is not unusual to show this rain rate as high as 15 inches/hour (or more) during a heavy downpour. So what I can say is if it rained as hard as it is raining now for an hour we would get 15 inches.

[LABob]

It depends on what your weather station software is sending.

For example, some versions of Virtual Weather Station send the instantaneous rain rate, while other versions send hourly rain (total that fell in the last 60 minutes).

As far as I know, WU doesn't pay attention to anything but the reported amount of rain and calculates its "rain rate" based on the amount reported in the last 60 minutes. For example, your station would report today's total rain as:

6:00 - 0.00
6:05 - 0.50
6:10 - 0.50
6:15 - 0.50

And WU would report a rain rate of 0.50 inches per hour between 6:05 and 7:05.

[LFWX]

It depends on what your weather station software is sending.

For example, some versions of Virtual Weather Station send the instantaneous rain rate, while other versions send hourly rain (total that fell in the last 60 minutes).

As far as I know, WU doesn't pay attention to anything but the reported amount of rain and calculates its "rain rate" based on the amount reported in the last 60 minutes. For example, your station would report today's total rain as:

6:00 - 0.00
6:05 - 0.50
6:10 - 0.50
6:15 - 0.50

And WU would report a rain rate of 0.50 inches per hour between 6:05 and 7:05.

Wrong, WU can receive "rain rate".

I have attached 3 examples from stations near me, one reporting instantaneous rain rate, one reporting hourly rain rate, and one not reporting any type of rain rate.

[LABob]

Wrong, WU can receive "rain rate".

I have attached 3 examples from stations near me, one reporting instantaneous rain rate, one reporting hourly rain rate, and one not reporting any type of rain rate.

Turns out WU actually requires two rain data points as specified in their wiki:

rainin - [rain inches over the past hour)] -- the accumulated rainfall in the past 60 min
dailyrainin - [rain inches so far today in local time]

Weather software not uploading data in that format is ignoring WU's specifications. I suppose they have no way of validating it, though.

[nincehelser]

Wunderground calculates rain-rate themselves based on the rain you report in the last 60 minute rolling window.  (i.e. "rainin")

If you don't report "rainin", there will be no rain-rate plot.

Somewhere I read that wunderground did this to get some sort of rain-rate consistency among different station hardware. 

[LABob]

It's really just a question of semantics. "Rain rate" doesn't really describe the data WU is collecting or presenting. Rainfall in the last hour isn't really the rain rate unless the rain happened to be falling steadily for that whole hour, but when does that happen?

[nincehelser]

It's really just a question of semantics. "Rain rate" doesn't really describe the data WU is collecting or presenting. Rainfall in the last hour isn't really the rain rate unless the rain happened to be falling steadily for that whole hour, but when does that happen?

Wunderground isn't collecting a "rain rate".  They're using the "rainin" data and frequency of reporting to calculate a rain rate.

[hankster]

From Goddard Earth Sciences

RAINFALL RATE

Definition
Rainfall rate is a measure of the intensity of rainfall. It is measured by calculating the amount of rain that falls to the earth surface per unit area per unit of time.

----------------

From American Meteorological Scoiety
rainfall rate

A measure of the intensity of rainfall by calculating the amount of rain that would fall over a given interval of time if the rainfall intensity were constant over that time period.

The rate is typically expressed in terms of length (depth) per unit time, for example, millimeters per hour, or inches per hour.

[nincehelser]

So what about all those rain fall rates for events that last much shorter than an hour?

[hankster]

A measure of the intensity of rainfall by calculating the amount of rain that would fall over a given interval of time if the rainfall intensity were constant over that time period.

[ggsteve]

There is at least some vendor component to it.  Every time my Rainwise bucket tips for 1/100" of rain my rain rate graph shows a 6/100" spike.  My Davis station doesn't show this.  I have noticed that many of the different weather station brands reporting to WU have a different graphic "signature" if you will.  I can spot a Rainwise station just by looking at the WU graph.

[nincehelser]

There is at least some vendor component to it.  Every time my Rainwise bucket tips for 1/100" of rain my rain rate graph shows a 6/100" spike.  My Davis station doesn't show this.  I have noticed that many of the different weather station brands reporting to WU have a different graphic "signature" if you will.  I can spot a Rainwise station just by looking at the WU graph.

Exactly.  That's why wunderground is doing it they way they are doing it.  It isn't perfect, but it's probably the closest anyone is going to get.

[nincehelser]

A measure of the intensity of rainfall by calculating the amount of rain that would fall over a given interval of time if the rainfall intensity were constant over that time period.

And that time period is what?

[hankster]

A measure of the intensity of rainfall by calculating the amount of rain that would fall over a given interval of time if the rainfall intensity were constant over that time period.

And that time period is what?

Whatever period you want. It could be 10" per hour or .166" per minute or .003" per second. At the instant that reading was taken, they are all true (sort of). Any factor over an instantaneous reading is nothing more than calculating the rain rate for any time frame you choose.

Ahhhh, but there is the rub. To measure rain rate you need two readings over some period of time so even a single rain rate reading is nothing more than an average for rain that happened between those two readings. A tipping bucket rain gauge will never give you a true instantaneous rain rate reading.

[LFWX]

Wunderground isn't collecting a "rain rate".  They're using the "rainin" data and frequency of reporting to calculate a rain rate.

Sorry, but (in my experience) this statement does not match what WU is displaying. See the graphs I posted. Several stations experiencing the same storm, but showing different rain rates.

Additionally, I have used several versions of Virtual Weather Station through the years, all of them were reporting at the same interval, but the rain rate graph on WU changed from version to version...some were reporting a "rainin" that was hourly rain, some were reporting a "rainin" that was an instantaneous rain rate.

The "rainin" is supposed to be the rain that has fallen in the past 60 minutes...so nothing needs to be calculated (it's already a "per hour" number). If they were doing some type of calculation, the graph wouldn't be identical to the graphs Virtual Weather Station is creating. It's been my experience that they are simply plotting the "rainin" value as it is reported to them.



However, if WU really wanted to "calculate" a rain rate they should use the change in the "dailyrainin" over the last reporting interval.

Example:

10:00 pm dailyrainin = 0.25"
10:05 pm dailyrainin = 0.50"

...would give a rain rate of 3 inches per hour.
0.25 X 12 = 3 (0.25" in 5 minutes, with 12 "5 minute intervals" in one hour).

or to put it another way, since dailyrainin is a total, just as your car's odometer is a total.

10:00 pm odometer = 5 miles
10:05 pm odometer = 12 miles

...would give a speed of 84 miles per hour.
7 X 12 = 84 (7 miles in 5 minutes, with 12 "5 minute intervals" in one hour).

[nincehelser]

Wunderground isn't collecting a "rain rate".  They're using the "rainin" data and frequency of reporting to calculate a rain rate.

The "rainin" is supposed to be the rain that has fallen in the past 60 minutes...so nothing needs to be calculated (it's already a "per hour" number). If they were doing some type of calculation, the graph wouldn't be identical to the graphs Virtual Weather Station is creating. It's been my experience that they are simply plotting the "rainin" value as it is reported to them.

If they simply plotted the "rainin" value, then it would taper off as the rain stops instead of quickly dropping to zero.

I don't know exactly what wunderground's algorithm is, but there are clearly only two inputs (dailyrainin, and rainin), plus what they know of your reporting history.

 

[LFWX]

We recently had two wet days, so I carefully watched and compared my "Hourly Rain" (60 minute accumulated rainfall, "rainin") values against the "Precip. Rate" values on WU.


OBSERVATION:

The two values were identical! No manipulation of the data, or calculation of a rate, occurred.
The value does "taper off as the rain stops instead of quickly dropping to zero."
An accumulation of 0.01", with no other rain following, will show as a "Precip. Rate" of 0.01" for the next 60 minutes, even though it is not raining!


CONCLUSION:

The "Precip. Rate" is actually the raw "Hourly Rain" (60 minute accumulated rainfall, "rainin") data.




It's interesting (but not surprising) to note that WU labels the "Precip. Rate" as an accumulation (inches) and not as a rate (inches/hour, inches/minute, etc)