Peter and I were discussing the possibility of mounting a dark sky expedition between Christmas and New Years.
First we need to decide where to go. That is a tradeoff between how dark the sky is and the drive time to get to it.
I have previously compiled and posted a list of the darker dark sky public observing sights as listed in Clear Sky Clock but can't find it now that I need it. I only remembered two areas of the NE that have Bortle 1/2 skies: an area in northwestern Maine and the area of West Virginia between Spruce Knob and Green Bank. Both of these areas have some detractors. West Virginia is 7-9 hours away, not counting breaks, and, as I remember it, the Maine site is a similar distance, is hard to get too because it is in the middle of nowhere (maybe that is why it is dark) and doesn't look like there are a lot of resources (lodging, sustenance, cell phone coverage, WI-Fi).
[I just found the Dark Sky Finder that has a copy of the Bortle map that will make it easier to do this search in the future. I don't know what they are using for their data source but a quick look indicated some good data categories an the tabular data associated with each push pin. And I suspect there are different sites there than on other sites.]
So I "Googled" "dark sky sites" and came up with ObservingSites.com. Peter and I discussed some possible candidates. He is partial to sites in western PA where the conditions are Bortle 3-3+, while I am partial to the WV sites which are Bortle 1/2-2+.
Anyway in looking at this we found a county park near Grantsville, WV, in Bortle 1/2 country, that is supposed to have impressive skies and an old barn you can stay in for cheap. Peter will consider WV -- the difference in drive time is not that great when you consider the total drive time, and the price is right if it is available in the winter -- but is keeping his options open for PA. While I figure if we are going to a dark sky site we should shoot for the darkest one possible I recognize that a dark sky site is wasted on me since after the sky fills up with more than our six Central Park stars I am lost and can't even find bright things like the Big W or the Big Dipper. I am thinking mostly I will be appreciating how much we are missing in The City and trying to identify the familiar constellations.
And now I am remembering the cabin in the mountains on the margin of a Bortle 1/2-3 area where I spent weekends one semester in college. It looks like the cabin is still there (if I am remembering the geography from 30 years ago and am correctly translating it onto Google Earth and Maps), but I don't know if my classmate's brother still owns it and would let us use it. It is on the side of a hill so would have an obstructed western horizon and there is another hill across the valley to the west. The area is within the Manongahela National Forest and is very rural (when I was in college you could get fresh deer meet and moonshine there all year round). And we could always use it as a base to access other areas like Spruce Knob and Green Bank, and for side trips to tourist attractions.
To aide and abet our selection process I looked at the historical weather information in Clear Sky Clock. In the table beneath the chart there is sometimes a link to this data in the rightmost cell of the bottom table row. If you look at the listing of sites within a state the locations that have this data are indicated by a red star in the first column. There aren't many of them and, as in this case, they are probably not the site you want, but at least they give an indication of conditions this time of year around the state. WV and PA are not looking great.
Then I remembered dry day.com, a site that predicts the likelihood of a given day being dry 18 months in advance and locks in their forecast. They claim an 85% accuracy rate. I have used the site numerous times but have no idea how their predictions stack up against ground truth in NYC. So I decided to find out. I have looked up the forecast going forward and am going to compare it with the Central Park observed conditions for the remainder of the month. I will be looking at "average" cloud cover, humidity and precipitation recorded each calendar day
 |
 |
| Date | dry day.com | Central Park | LGA | |
|---|---|---|---|---|
| Condition | Risk of Precipitation | Reported Conditions | Observed Conditions | |
| 15 | Risky | 48% | Day: O Night: MC Precip: T RH: 68% | Day: O Night: MC Precip: T RH: 66% |
| 16 | Transition | 42% | Day: O Night: MC Precip: 0.00" RH: 48% | Day: MC Night: MC Precip: 0.00" RH: 48% |
| 17 | Transition | 48% | Day: O Night: O Precip: 0.11" RH: 89% | Day: MC Night: MC Precip: 0.00" RH: 54% |
| 18 | Risky | 58% | Day: C Night: C Precip: RH: 0.00" | Day: PC Night: PC Precip: 0.00" RH: 46% |
| 19 | Risky | 50% | Day: MC Night: O Precip: 0.00" RH: 54% | Day: MC Night: O Precip: 0.00" RH: 53% |
| 20 | Transition | 42% | Day: MC Night: O Precip: 0.01" RH: 64% | Day: O Night: O Precip: 0.01" RH: 62% |
| 21 | Dry | 25% | Day: O Night: O Precip: 0.11" RH: 89% | Day: O Night: O Precip: 0.08" RH: 83% |
| 22 | Dry | 40% | Day: PC Night: MC Precip: 0.06" RH: 60% | Day: MC Night: O Precip: 0.05" RH: 59% |
| 23 | Dry | 11% | Day: O Night: C Precip: 0.75" RH: 77% | Day: MC Night: PC Precip: 0.76" RH: 72% |
| 24 | Dry | 10% | Day: PC Night: C Precip: 0.00" RH: 4% | Day: PC Night: PC Precip: 0.00" RH: 52% |
| 25 | Transition | 42% | Day: MC Night: PC Precip: 0.00" RH: 57% | Day: MC Night: O Precip: 0.00" RH: 55% |
| 26 | Risky | 49% | Day: PC Night: C Precip: 0.00" RH: 56% | Day: PC Night: PC Precip: T RH: 52% |
| 27 | Transition | 40% | Day: MC Night: O Precip: 1.02" RH: 79% | Day: O Night: O Precip: 1.17" RH: 73% |
| 28 | Dry | 18% | Day: O Night: MC Precip: 0.00" RH: 59% | Day: MC Night: PC Precip: 0.00" RH: 57% |
| 29 | Transition | 43% | Day: PC Night: Precip: 0.00" RH: 46% | Day: MC Night: O Precip: 0.00" RH: 43% |
| 30 | Risky | 52% | Day: C Night: PC Precip: 0.00" RH: 65% | Day: MC Night: MC Precip: 0.00" RH: 61% |
| 31 | Risky | 70% | Day:MC Night: C Precip: 0.01" RH: 73% | Day: MC Night: PC Precip: 0.01" RH: 73% |
| Legend C = Clear PC = Partly Cloudy MC = Mostly Cloudy O = Overcast | ||||
I am comparing the dryday.com assessment against the automated observations from Central Park which may not be fair. I have long felt that the instruments at the Central Park weather station are optimists. When I was out this afternoon it was overcast but Central Park reported it as clear and tonight the GOES images show clouds blanketing the area while it is supposed to be clear at Central Park. So maybe I should add another column and also compare the La Guardia Airport observations. At least there are people there who might be looking out a window and notice clouds. Then I will be able to validate the two weather stations against each other and the dryday.com forecast against them.
Converting hourly observations of the current weather to Day and Night generalizations wasn't easy. I ended up loosely defining Day as 0751 to 1751 and Night as 1851 to 2351 and kinda averaging out what was reported over the period. It was hardest when the observations ranged between Clear and Overcast. I probably should have converted the text into octas and averaged them but even that would have had a lot of slop in it since a cloud cover description can span several octas and some of them overlap in their octa values.
I looked at what condition they assigned each day relative to their predicted Risk of Precipitation.
| Risk of Precipitation | Dry | Transition | Risky |
|---|---|---|---|
| 90%-100% | |||
| 80%-99% | |||
| 70%-79% | 1 | ||
| 60%-69% | |||
| 50%-59% | 3 | ||
| 40%-49% | 1 | 6 | 3 |
| 30%-39% | |||
| 20%-29% | 1 | ||
| 10%-19% | 3 | ||
| 0%-9% |
I looked at Risk of Rain compared about actual rainfall.
| Risk of Precipitation | Rain | No Rain |
|---|---|---|
| 90%-100% | ||
| 80%-89% | ||
| 70%-79% | 1 | |
| 60%-69% | ||
| 50%-59% | 3 | |
| 40%-49% | 6 | 3 |
| 30%-39% | ||
| 20%-29% | 1 | |
| 10%-19% | 1 | 2 |
| 0%-9% |
Lastly I compaired when it would rain against the risk condition they assigned each day.
| Rain | No Rain | |
|---|---|---|
| Dry | 3 | 2 |
| Transition | 3 | 3 |
| Risky | 3 | 3 |
Based on this short assessment I don't see this forecast being much good for astronomy or for anything else other than possibly as a CYA if you have to schedule an weather dependant event well in advance. You might as well flip a coin.
Looking at the reported conditions for Central Park and LaGuardia Airport I only found one instance where their cloud cover data was more than one "unit" in difference (25Dec11: Partly Cloudy vs Overcast) so my feeling that Central Park is overly optimistic remains a feeling. I have built a mirror to take cloud cover measurements . I tried it out at Bryant Park when I was doing solar observations on 26Dec11 and found that the clouds moved so quickly that it was hard to get an accurate count. Maybe I will have better luck taking pictures of the mirror and counting the clouds that way. My plan is to count the clouds at 51 minutes past the hour at Belvedere Castle and compare my assessment with the report of the Central Park automated weather station submitted at the same time.
