Black River Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black River Observatory clear sky chart from 2005-11-06 to 2024-01-14.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 9% 4 7% 6% 4 5% 5% 6% 7% 34%	74655
	Night	15% 10% 4 7% 6% 3 4 5% 5% 7% 34%	75095
Transparency	Day	48% 7% 13% 20% 11% 1	74366
	Night	49% 7% 13% 20% 11% 1	74905
Seeing	Day	16% 10% 29% 25% 14% 6%	14139
	Night	18% 9% 30% 29% 12% 2	23777

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 13% 6% 12% 7% 4 6% 5% 4 5% 23%	1517
	11th to 20th	Day	15% 17% 9% 11% 8% 4 5% 5% 5% 6% 17%	1473
	21st to end	Day	14% 16% 9% 10% 8% 5% 6% 5% 4 4 19%	1782
February	1st to 10th	Day	16% 14% 5% 12% 9% 5% 5% 5% 4 5% 20%	1620
	11th to 20th	Day	15% 13% 6% 10% 8% 4 4 5% 6% 6% 24%	1645
	21st to end	Day	15% 14% 7% 10% 7% 3 4 4 4 4 27%	1480
March	1st to 10th	Day	15% 11% 6% 9% 8% 4 4 4 4 6% 28%	1868
	11th to 20th	Day	12% 12% 6% 7% 7% 3 5% 5% 5% 6% 32%	1980
	21st to end	Day	19% 12% 3 11% 6% 4 5% 4 4 6% 26%	2178
April	1st to 10th	Day	16% 11% 5% 7% 6% 3 4 5% 5% 7% 30%	2060
	11th to 20th	Day	17% 9% 3 6% 6% 3 4 5% 6% 7% 35%	2312
	21st to end	Day	18% 9% 3 7% 6% 4 5% 4 5% 6% 34%	2340
May	1st to 10th	Day	16% 8% 3 8% 7% 4 5% 4 5% 8% 33%	2340
	11th to 20th	Day	19% 7% 3 5% 5% 4 5% 4 5% 6% 37%	2301
	21st to end	Day	11% 6% 3 6% 6% 3 5% 5% 6% 8% 41%	2603
June	1st to 10th	Day	10% 6% 3 6% 6% 3 4 5% 7% 9% 42%	2660
	11th to 20th	Day	10% 6% 2 5% 5% 4 5% 6% 5% 9% 43%	2700
	21st to end	Day	9% 5% 3 6% 6% 4 5% 6% 7% 9% 41%	2700
July	1st to 10th	Day	7% 4 3 4 5% 3 5% 5% 7% 9% 48%	2572
	11th to 20th	Day	9% 5% 2 5% 6% 3 5% 5% 7% 10% 43%	2520
	21st to end	Day	9% 5% 3 5% 5% 3 5% 7% 8% 10% 42%	2561
August	1st to 10th	Day	9% 5% 3 4 5% 4 6% 7% 8% 10% 39%	2327
	11th to 20th	Day	8% 5% 2 6% 6% 4 6% 5% 7% 10% 41%	2303
	21st to end	Day	11% 6% 2 6% 6% 3 4 5% 6% 8% 42%	2574
September	1st to 10th	Day	10% 7% 3 5% 6% 4 6% 6% 7% 7% 39%	2049
	11th to 20th	Day	11% 5% 2 5% 5% 3 5% 4 7% 8% 45%	1980
	21st to end	Day	15% 6% 2 6% 5% 4 4 5% 8% 9% 36%	1980
October	1st to 10th	Day	13% 7% 2 7% 6% 3 5% 5% 5% 7% 39%	1894
	11th to 20th	Day	18% 8% 3 7% 7% 4 5% 4 4 7% 32%	1800
	21st to end	Day	21% 11% 3 6% 7% 4 5% 4 5% 7% 29%	1867
November	1st to 10th	Day	14% 8% 3 7% 6% 4 5% 5% 6% 8% 33%	1672
	11th to 20th	Day	13% 10% 5% 8% 7% 4 5% 5% 6% 7% 30%	1668
	21st to end	Day	17% 12% 6% 8% 6% 3 4 5% 5% 6% 28%	1520
December	1st to 10th	Day	14% 13% 5% 10% 8% 4 6% 5% 5% 5% 24%	1520
	11th to 20th	Day	17% 14% 6% 8% 7% 4 4 5% 5% 7% 25%	1792
	21st to end	Day	26% 20% 7% 8% 6% 3 4 4 5% 5% 12%	2497
January	1st to 10th	Night	18% 11% 5% 10% 7% 4 5% 4 5% 7% 25%	2648
	11th to 20th	Night	18% 17% 6% 11% 7% 4 4 4 4 4 21%	2556
	21st to end	Night	13% 14% 8% 10% 8% 4 5% 5% 4 6% 23%	2574
February	1st to 10th	Night	15% 17% 6% 9% 8% 4 5% 5% 4 5% 22%	2340
	11th to 20th	Night	15% 13% 5% 9% 7% 4 5% 5% 5% 6% 27%	2199
	21st to end	Night	19% 11% 4 10% 8% 4 4 4 4 5% 27%	1776
March	1st to 10th	Night	14% 11% 5% 7% 7% 3 5% 5% 4 6% 32%	2091
	11th to 20th	Night	13% 8% 4 6% 6% 4 5% 5% 5% 8% 37%	1980
	21st to end	Night	18% 9% 4 8% 6% 3 4 4 4 5% 33%	2178
April	1st to 10th	Night	19% 9% 4 6% 5% 2 3 4 5% 8% 34%	1916
	11th to 20th	Night	19% 5% 4 6% 5% 3 4 4 5% 8% 38%	1597
	21st to end	Night	17% 8% 3 5% 5% 4 4 5% 6% 7% 36%	1620
May	1st to 10th	Night	15% 8% 3 7% 5% 4 4 6% 6% 7% 33%	1620
	11th to 20th	Night	17% 7% 2 5% 5% 3 5% 4 5% 6% 41%	1592
	21st to end	Night	11% 4 2 4 4 4 5% 5% 6% 9% 48%	1668
June	1st to 10th	Night	11% 5% 2 5% 4 3 4 6% 6% 8% 45%	1296
	11th to 20th	Night	10% 5% 2 5% 5% 3 5% 5% 6% 10% 45%	1260
	21st to end	Night	8% 5% 2 5% 5% 4 4 5% 5% 9% 48%	1260
July	1st to 10th	Night	7% 4 1 5% 4 3 4 4 6% 9% 54%	1383
	11th to 20th	Night	8% 5% 2 4 5% 3 4 5% 5% 9% 50%	1440
	21st to end	Night	6% 4 2 4 5% 4 5% 5% 7% 9% 49%	1753
August	1st to 10th	Night	7% 5% 2 5% 4 3 4 5% 7% 10% 48%	1611
	11th to 20th	Night	6% 3 3 4 5% 3 5% 6% 7% 9% 49%	1598
	21st to end	Night	8% 6% 2 4 5% 3 4 4 6% 7% 51%	1782
September	1st to 10th	Night	10% 6% 2 6% 5% 3 4 5% 5% 7% 45%	1889
	11th to 20th	Night	10% 5% 2 4 4 3 4 5% 4 6% 53%	1980
	21st to end	Night	15% 5% 2 5% 5% 3 4 4 5% 7% 44%	1980
October	1st to 10th	Night	14% 7% 3 6% 5% 3 3 4 5% 8% 43%	2065
	11th to 20th	Night	19% 8% 2 7% 6% 3 4 4 5% 6% 35%	2160
	21st to end	Night	23% 9% 3 7% 5% 3 5% 4 4 7% 29%	2488
November	1st to 10th	Night	13% 8% 3 6% 4 3 4 5% 5% 7% 41%	2406
	11th to 20th	Night	15% 10% 4 8% 6% 4 6% 5% 6% 7% 30%	2509
	21st to end	Night	19% 12% 4 7% 6% 3 5% 5% 5% 5% 30%	2660
December	1st to 10th	Night	16% 13% 6% 9% 7% 4 5% 4 5% 7% 23%	2660
	11th to 20th	Night	14% 13% 6% 10% 7% 4 5% 4 6% 7% 25%	3393
	21st to end	Night	26% 20% 6% 8% 6% 3 4 4 4 4 15%	5167

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	62% 7% 10% 14% 6% 1	1518
	11th to 20th	Day	68% 7% 12% 10% 4	1473
	21st to end	Day	67% 6% 8% 12% 6%	1782
February	1st to 10th	Day	67% 6% 10% 12% 5%	1620
	11th to 20th	Day	58% 7% 10% 16% 7% 1	1654
	21st to end	Day	61% 5% 11% 14% 8% 2	1480
March	1st to 10th	Day	57% 6% 13% 15% 9% 1	1868
	11th to 20th	Day	51% 7% 11% 17% 12% 2	1980
	21st to end	Day	58% 6% 9% 18% 7% 1	2178
April	1st to 10th	Day	52% 6% 13% 18% 10% 1	2060
	11th to 20th	Day	47% 6% 12% 18% 14% 2	2312
	21st to end	Day	50% 7% 14% 15% 12% 2	2340
May	1st to 10th	Day	48% 7% 12% 20% 11% 2	2340
	11th to 20th	Day	47% 7% 11% 19% 14% 2	2305
	21st to end	Day	40% 8% 13% 22% 15% 2	2603
June	1st to 10th	Day	38% 8% 14% 25% 14% 1	2660
	11th to 20th	Day	37% 7% 14% 24% 15% 2	2700
	21st to end	Day	38% 8% 14% 24% 15% 1	2700
July	1st to 10th	Day	31% 7% 13% 27% 18% 3	2503
	11th to 20th	Day	34% 9% 15% 26% 15% 1	2445
	21st to end	Day	34% 10% 15% 22% 16% 2	2419
August	1st to 10th	Day	35% 11% 17% 24% 11% 2	2314
	11th to 20th	Day	37% 9% 16% 25% 12% 2	2303
	21st to end	Day	38% 8% 13% 27% 13% 1	2574
September	1st to 10th	Day	40% 10% 15% 21% 12% 1	2049
	11th to 20th	Day	35% 7% 14% 33% 11% 1	1980
	21st to end	Day	43% 8% 15% 24% 10% 1	1980
October	1st to 10th	Day	44% 7% 12% 24% 13% 1	1894
	11th to 20th	Day	52% 6% 13% 20% 9% 1	1800
	21st to end	Day	55% 6% 14% 17% 7%	1867
November	1st to 10th	Day	47% 8% 13% 22% 9% 1	1672
	11th to 20th	Day	53% 7% 15% 16% 9%	1668
	21st to end	Day	56% 6% 13% 18% 5% 1	1520
December	1st to 10th	Day	61% 8% 14% 13% 4	1520
	11th to 20th	Day	58% 7% 14% 16% 5%	1788
	21st to end	Day	74% 7% 10% 7% 2	2497
January	1st to 10th	Night	59% 6% 12% 16% 7%	2648
	11th to 20th	Night	66% 6% 12% 11% 5%	2556
	21st to end	Night	61% 6% 12% 17% 4	2574
February	1st to 10th	Night	64% 6% 9% 13% 7% 1	2340
	11th to 20th	Night	58% 6% 10% 16% 9% 1	2201
	21st to end	Night	59% 6% 10% 17% 8% 1	1776
March	1st to 10th	Night	53% 6% 11% 21% 9% 1	2091
	11th to 20th	Night	45% 6% 12% 23% 13% 2	1980
	21st to end	Night	52% 6% 9% 21% 11% 1	2178
April	1st to 10th	Night	49% 5% 11% 21% 12% 2	1916
	11th to 20th	Night	45% 5% 10% 19% 18% 2	1597
	21st to end	Night	45% 7% 12% 18% 15% 4	1620
May	1st to 10th	Night	47% 7% 11% 21% 11% 3	1620
	11th to 20th	Night	42% 6% 10% 19% 18% 4	1593
	21st to end	Night	33% 8% 14% 25% 16% 4	1669
June	1st to 10th	Night	35% 7% 11% 27% 17% 2	1296
	11th to 20th	Night	33% 8% 14% 24% 19% 2	1260
	21st to end	Night	32% 8% 14% 25% 20% 1	1260
July	1st to 10th	Night	27% 5% 11% 29% 23% 4	1343
	11th to 20th	Night	29% 8% 13% 31% 16% 2	1392
	21st to end	Night	28% 10% 19% 23% 17% 3	1655
August	1st to 10th	Night	29% 9% 17% 30% 14% 2	1600
	11th to 20th	Night	29% 8% 17% 30% 12% 3	1598
	21st to end	Night	32% 7% 14% 29% 16% 1	1782
September	1st to 10th	Night	37% 7% 14% 26% 14% 2	1889
	11th to 20th	Night	31% 7% 13% 32% 17% 1	1980
	21st to end	Night	40% 7% 11% 29% 12% 1	1980
October	1st to 10th	Night	40% 7% 11% 25% 16% 1	2065
	11th to 20th	Night	49% 6% 14% 20% 10% 1	2160
	21st to end	Night	54% 7% 11% 19% 9% 1	2488
November	1st to 10th	Night	41% 7% 14% 25% 12% 1	2406
	11th to 20th	Night	51% 7% 15% 18% 8% 1	2509
	21st to end	Night	55% 6% 16% 17% 5%	2660
December	1st to 10th	Night	60% 7% 15% 14% 4	2660
	11th to 20th	Night	57% 7% 15% 15% 6%	3396
	21st to end	Night	72% 7% 11% 7% 3	5167

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 18% 37% 17% 7% 1	270
	11th to 20th	Day	21% 14% 43% 18% 3 1	249
	21st to end	Day	23% 15% 38% 18% 5% 1	264
February	1st to 10th	Day	23% 20% 43% 12% 2	240
	11th to 20th	Day	25% 17% 40% 17% 1	240
	21st to end	Day	20% 25% 40% 14% 2	198
March	1st to 10th	Day	24% 17% 39% 15% 4 1	308
	11th to 20th	Day	20% 15% 32% 24% 8% 1	420
	21st to end	Day	25% 13% 32% 24% 5% 1	462
April	1st to 10th	Day	18% 16% 36% 21% 9% 1	420
	11th to 20th	Day	24% 14% 27% 19% 10% 6%	412
	21st to end	Day	22% 10% 30% 25% 10% 3	420
May	1st to 10th	Day	19% 12% 30% 23% 13% 3	423
	11th to 20th	Day	20% 7% 28% 24% 15% 6%	447
	21st to end	Day	14% 7% 19% 33% 19% 9%	485
June	1st to 10th	Day	11% 5% 32% 32% 16% 4	590
	11th to 20th	Day	10% 7% 30% 31% 16% 5%	630
	21st to end	Day	11% 7% 26% 34% 18% 5%	630
July	1st to 10th	Day	7% 4 22% 31% 24% 12%	621
	11th to 20th	Day	10% 4 25% 34% 18% 8%	618
	21st to end	Day	7% 3 23% 34% 23% 10%	502
August	1st to 10th	Day	9% 4 16% 31% 25% 14%	446
	11th to 20th	Day	9% 4 18% 31% 23% 14%	439
	21st to end	Day	14% 5% 20% 24% 25% 11%	495
September	1st to 10th	Day	13% 6% 22% 26% 23% 10%	450
	11th to 20th	Day	10% 6% 18% 24% 27% 15%	450
	21st to end	Day	15% 10% 25% 23% 18% 9%	450
October	1st to 10th	Day	16% 10% 25% 24% 18% 7%	450
	11th to 20th	Day	18% 16% 35% 24% 6% 1	450
	21st to end	Day	23% 14% 40% 17% 5% 1	382
November	1st to 10th	Day	17% 16% 36% 22% 8% 1	270
	11th to 20th	Day	21% 21% 32% 18% 7% 1	250
	21st to end	Day	24% 19% 39% 14% 3	180
December	1st to 10th	Day	21% 19% 36% 17% 5% 1	180
	11th to 20th	Day	24% 21% 37% 15% 3 1	180
	21st to end	Day	27% 18% 35% 17% 3	218
January	1st to 10th	Night	23% 14% 37% 20% 5% 1	950
	11th to 20th	Night	25% 14% 34% 22% 5% 1	905
	21st to end	Night	17% 12% 36% 28% 7%	792
February	1st to 10th	Night	23% 12% 42% 21% 2	720
	11th to 20th	Night	22% 15% 34% 26% 5%	703
	21st to end	Night	23% 13% 40% 21% 3	592
March	1st to 10th	Night	19% 13% 35% 28% 5%	720
	11th to 20th	Night	17% 11% 37% 28% 6% 1	720
	21st to end	Night	23% 9% 30% 30% 8% 1	792
April	1st to 10th	Night	20% 13% 36% 25% 5%	664
	11th to 20th	Night	20% 10% 33% 26% 10% 2	533
	21st to end	Night	20% 7% 31% 32% 9% 1	540
May	1st to 10th	Night	18% 5% 29% 32% 13% 1	540
	11th to 20th	Night	19% 4 27% 36% 13% 1	531
	21st to end	Night	11% 4 23% 36% 24% 2	508
June	1st to 10th	Night	13% 4 24% 39% 19%	360
	11th to 20th	Night	13% 3 25% 41% 17% 1	360
	21st to end	Night	10% 3 20% 47% 19% 1	360
July	1st to 10th	Night	7% 1 16% 41% 30% 5%	468
	11th to 20th	Night	11% 1 17% 41% 25% 4	522
	21st to end	Night	7% 2 20% 45% 24% 2	591
August	1st to 10th	Night	8% 2 14% 40% 32% 3	537
	11th to 20th	Night	7% 2 17% 34% 37% 4	534
	21st to end	Night	11% 2 18% 35% 29% 4	594
September	1st to 10th	Night	11% 4 22% 35% 23% 5%	673
	11th to 20th	Night	11% 4 19% 35% 26% 5%	720
	21st to end	Night	16% 6% 21% 33% 20% 5%	720
October	1st to 10th	Night	16% 9% 26% 29% 18% 2	720
	11th to 20th	Night	23% 12% 30% 27% 8% 1	720
	21st to end	Night	26% 11% 35% 24% 4	792
November	1st to 10th	Night	18% 12% 31% 26% 12% 1	740
	11th to 20th	Night	19% 14% 38% 20% 7% 1	760
	21st to end	Night	25% 13% 35% 22% 6%	760
December	1st to 10th	Night	21% 14% 41% 18% 5%	760
	11th to 20th	Night	22% 15% 39% 20% 3	831
	21st to end	Night	30% 12% 34% 21% 3	1045

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