Black Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Creek Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	9% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	126413
	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 43%	123301
Transparency	Day	38% 9% 15% 18% 14% 6%	130137
	Night	36% 8% 15% 19% 15% 7%	127646
Seeing	Day	11% 7% 21% 25% 21% 15%	16727
	Night	11% 6% 20% 33% 24% 5%	26752

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	9% 6% 2 5% 3 3 2 2 3 6% 58%	4056
	11th to 20th	Day	18% 9% 4 8% 4 4 3 2 4 6% 38%	4060
	21st to end	Day	11% 6% 2 6% 4 3 3 3 5% 8% 49%	4535
February	1st to 10th	Day	15% 7% 2 6% 4 4 4 3 5% 6% 45%	2000
	11th to 20th	Day	13% 5% 2 5% 4 4 3 3 5% 6% 50%	1948
	21st to end	Day	12% 6% 3 5% 4 3 3 4 5% 6% 48%	1713
March	1st to 10th	Day	13% 7% 3 7% 5% 4 4 3 5% 8% 41%	4935
	11th to 20th	Day	7% 3 2 3 3 4 4 4 6% 8% 57%	4980
	21st to end	Day	11% 5% 2 5% 3 4 4 3 5% 7% 49%	5465
April	1st to 10th	Day	8% 4 2 6% 5% 4 5% 4 6% 8% 48%	2200
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 6% 6% 46%	2221
	21st to end	Day	5% 3 2 5% 4 5% 5% 4 7% 8% 51%	2456
May	1st to 10th	Day	6% 5% 2 6% 5% 5% 6% 4 8% 9% 43%	5862
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 5% 9% 43%	5861
	21st to end	Day	5% 3 2 6% 5% 5% 5% 5% 8% 12% 45%	6433
June	1st to 10th	Day	12% 6% 3 9% 7% 7% 7% 5% 8% 10% 26%	2600
	11th to 20th	Day	6% 5% 2 9% 8% 7% 9% 7% 9% 10% 29%	2600
	21st to end	Day	8% 6% 2 9% 8% 7% 7% 6% 9% 10% 28%	2600
July	1st to 10th	Day	7% 7% 3 11% 9% 9% 10% 8% 11% 12% 15%	5900
	11th to 20th	Day	4 4 2 8% 6% 7% 7% 7% 11% 16% 29%	5900
	21st to end	Day	4 4 2 8% 7% 7% 8% 7% 10% 14% 29%	6424
August	1st to 10th	Day	11% 7% 3 8% 6% 7% 8% 6% 9% 11% 24%	4234
	11th to 20th	Day	7% 5% 2 7% 7% 6% 7% 6% 9% 11% 34%	4113
	21st to end	Day	9% 7% 3 9% 8% 7% 8% 6% 9% 10% 24%	3950
September	1st to 10th	Day	11% 7% 3 10% 8% 8% 8% 6% 8% 10% 22%	2200
	11th to 20th	Day	13% 7% 3 9% 8% 7% 7% 5% 8% 8% 24%	2200
	21st to end	Day	12% 8% 3 9% 6% 6% 6% 4 5% 7% 35%	2200
October	1st to 10th	Day	13% 6% 3 8% 6% 5% 6% 4 6% 8% 34%	2180
	11th to 20th	Day	7% 5% 2 6% 4 4 4 3 4 7% 53%	1980
	21st to end	Day	8% 5% 2 6% 4 3 4 4 6% 7% 50%	2090
November	1st to 10th	Day	12% 5% 2 5% 5% 3 5% 4 6% 8% 46%	1957
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 6% 8% 44%	1991
	21st to end	Day	12% 6% 2 5% 5% 4 4 4 5% 6% 47%	1800
December	1st to 10th	Day	6% 4 1 5% 4 4 4 4 5% 7% 55%	2930
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 5% 7% 40%	3373
	21st to end	Day	14% 8% 3 8% 4 4 4 3 5% 6% 42%	4466
January	1st to 10th	Night	9% 6% 2 4 2 2 3 2 4 6% 59%	5889
	11th to 20th	Night	18% 8% 2 7% 5% 4 5% 3 6% 7% 34%	5900
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 8% 43%	6416
February	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 47%	2400
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 48%	2338
	21st to end	Night	11% 7% 2 6% 5% 4 4 3 5% 6% 49%	1915
March	1st to 10th	Night	16% 9% 3 6% 4 4 3 3 4 5% 41%	4918
	11th to 20th	Night	5% 5% 1 5% 3 3 4 3 6% 9% 55%	4980
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 8% 49%	5467
April	1st to 10th	Night	7% 6% 2 4 4 4 5% 4 6% 8% 50%	2200
	11th to 20th	Night	8% 6% 2 6% 3 3 3 3 5% 7% 54%	2124
	21st to end	Night	6% 4 2 4 4 4 5% 4 7% 9% 53%	1958
May	1st to 10th	Night	4 4 2 6% 5% 5% 5% 3 7% 10% 48%	4033
	11th to 20th	Night	10% 7% 2 5% 4 3 4 3 6% 11% 44%	4032
	21st to end	Night	4 3 1 5% 4 4 4 4 7% 12% 52%	4421
June	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 5% 10% 12% 33%	1800
	11th to 20th	Night	6% 5% 2 7% 6% 6% 6% 7% 9% 12% 34%	1800
	21st to end	Night	7% 5% 2 7% 6% 5% 6% 6% 9% 13% 34%	1800
July	1st to 10th	Night	8% 7% 3 11% 7% 7% 8% 7% 9% 11% 22%	4058
	11th to 20th	Night	3 4 2 7% 5% 6% 7% 5% 11% 14% 34%	4057
	21st to end	Night	5% 6% 3 8% 6% 7% 8% 7% 9% 11% 32%	4427
August	1st to 10th	Night	10% 6% 2 8% 6% 6% 7% 5% 9% 12% 28%	2915
	11th to 20th	Night	4 4 2 6% 4 5% 6% 6% 10% 12% 41%	2931
	21st to end	Night	7% 6% 3 7% 6% 5% 6% 6% 8% 12% 34%	3911
September	1st to 10th	Night	11% 8% 2 9% 6% 6% 7% 6% 8% 9% 30%	2200
	11th to 20th	Night	12% 8% 3 8% 6% 6% 6% 5% 7% 10% 30%	2200
	21st to end	Night	10% 7% 3 7% 4 5% 4 4 6% 11% 40%	2200
October	1st to 10th	Night	11% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	2218
	11th to 20th	Night	7% 4 2 6% 4 4 4 3 5% 7% 55%	2377
	21st to end	Night	8% 5% 2 6% 4 4 4 3 5% 8% 51%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 5% 4 3 6% 8% 49%	2341
	11th to 20th	Night	10% 6% 2 6% 5% 4 4 4 6% 6% 47%	2405
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 6% 47%	2600
December	1st to 10th	Night	9% 5% 2 5% 4 4 3 2 4 6% 56%	4250
	11th to 20th	Night	13% 9% 3 7% 5% 4 4 3 4 6% 44%	4822
	21st to end	Night	13% 9% 3 7% 4 4 4 4 5% 6% 40%	6490

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	29% 3 8% 16% 29% 16%	4057
	11th to 20th	Day	49% 3 4 12% 20% 12%	4060
	21st to end	Day	35% 4 9% 24% 17% 11%	4535
February	1st to 10th	Day	41% 5% 8% 14% 21% 10%	2000
	11th to 20th	Day	35% 5% 10% 15% 23% 12%	1958
	21st to end	Day	37% 5% 9% 17% 19% 12%	1713
March	1st to 10th	Day	42% 6% 10% 14% 19% 9%	4935
	11th to 20th	Day	25% 4 12% 19% 21% 19%	4980
	21st to end	Day	33% 6% 9% 23% 18% 12%	5465
April	1st to 10th	Day	33% 6% 11% 20% 18% 12%	2200
	11th to 20th	Day	37% 6% 9% 17% 22% 9%	2221
	21st to end	Day	29% 6% 12% 23% 22% 8%	2456
May	1st to 10th	Day	32% 9% 17% 25% 13% 4	5897
	11th to 20th	Day	36% 6% 14% 30% 12% 1	5865
	21st to end	Day	28% 9% 18% 28% 15% 2	6447
June	1st to 10th	Day	50% 9% 17% 16% 8% 1	2600
	11th to 20th	Day	44% 13% 21% 17% 5%	2600
	21st to end	Day	47% 12% 21% 14% 5%	2600
July	1st to 10th	Day	52% 19% 21% 6% 1	5835
	11th to 20th	Day	37% 17% 34% 11% 2	5831
	21st to end	Day	37% 17% 27% 18% 2	6240
August	1st to 10th	Day	48% 14% 25% 10% 2	4221
	11th to 20th	Day	37% 13% 24% 17% 8%	4152
	21st to end	Day	47% 14% 19% 18% 2	3950
September	1st to 10th	Day	53% 13% 17% 10% 6% 1	2200
	11th to 20th	Day	54% 11% 14% 13% 7%	2200
	21st to end	Day	46% 8% 13% 17% 13% 3	2200
October	1st to 10th	Day	51% 8% 9% 12% 14% 6%	3557
	11th to 20th	Day	26% 5% 7% 17% 34% 11%	3220
	21st to end	Day	24% 6% 8% 21% 34% 8%	3428
November	1st to 10th	Day	36% 5% 10% 21% 17% 11%	1957
	11th to 20th	Day	38% 6% 9% 16% 21% 10%	1991
	21st to end	Day	38% 4 8% 18% 23% 8%	1800
December	1st to 10th	Day	28% 5% 12% 25% 20% 11%	2930
	11th to 20th	Day	41% 8% 10% 15% 18% 8%	3370
	21st to end	Day	43% 5% 8% 19% 20% 6%	4466
January	1st to 10th	Night	28% 3 7% 14% 35% 13%	5889
	11th to 20th	Night	49% 4 7% 15% 19% 7%	5900
	21st to end	Night	39% 6% 10% 19% 16% 11%	6416
February	1st to 10th	Night	37% 5% 9% 16% 20% 12%	2400
	11th to 20th	Night	38% 5% 8% 20% 17% 12%	2339
	21st to end	Night	36% 4 10% 20% 17% 11%	1915
March	1st to 10th	Night	44% 5% 9% 15% 16% 11%	4918
	11th to 20th	Night	25% 4 14% 20% 19% 19%	4980
	21st to end	Night	33% 5% 10% 20% 18% 13%	5467
April	1st to 10th	Night	31% 7% 14% 20% 18% 10%	2200
	11th to 20th	Night	31% 5% 9% 24% 22% 9%	2124
	21st to end	Night	26% 7% 15% 25% 20% 7%	1958
May	1st to 10th	Night	29% 10% 21% 26% 12% 3	4060
	11th to 20th	Night	33% 7% 18% 27% 12% 1	4033
	21st to end	Night	23% 8% 20% 30% 18% 1	4429
June	1st to 10th	Night	39% 12% 22% 19% 7%	1800
	11th to 20th	Night	37% 14% 24% 21% 5%	1800
	21st to end	Night	38% 13% 24% 18% 5% 1	1800
July	1st to 10th	Night	50% 18% 24% 8% 1	4015
	11th to 20th	Night	33% 19% 34% 12% 2	4006
	21st to end	Night	38% 15% 27% 19% 2	4297
August	1st to 10th	Night	43% 15% 30% 10% 2	2904
	11th to 20th	Night	30% 16% 26% 21% 8%	2958
	21st to end	Night	40% 13% 25% 19% 3	3911
September	1st to 10th	Night	46% 13% 21% 13% 6% 1	2200
	11th to 20th	Night	46% 13% 17% 18% 6%	2200
	21st to end	Night	39% 8% 16% 21% 12% 3	2200
October	1st to 10th	Night	49% 9% 11% 13% 13% 6%	3621
	11th to 20th	Night	28% 5% 7% 18% 33% 9%	3876
	21st to end	Night	23% 5% 9% 22% 32% 9%	4120
November	1st to 10th	Night	33% 6% 12% 21% 17% 11%	2341
	11th to 20th	Night	36% 6% 9% 18% 19% 12%	2405
	21st to end	Night	38% 6% 9% 17% 21% 9%	2600
December	1st to 10th	Night	30% 4 11% 21% 18% 15%	4250
	11th to 20th	Night	41% 6% 10% 19% 15% 9%	4824
	21st to end	Night	43% 6% 11% 17% 17% 5%	6490

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	13% 15% 47% 18% 5% 1	300
	11th to 20th	Day	11% 10% 45% 24% 8% 2	300
	21st to end	Day	19% 18% 43% 16% 3	330
February	1st to 10th	Day	16% 11% 40% 23% 9% 1	300
	11th to 20th	Day	15% 17% 40% 19% 9%	300
	21st to end	Day	15% 11% 37% 24% 12% 1	312
March	1st to 10th	Day	13% 14% 34% 27% 12% 1	500
	11th to 20th	Day	13% 11% 35% 26% 12% 2	500
	21st to end	Day	13% 12% 32% 26% 15% 2	550
April	1st to 10th	Day	12% 13% 32% 24% 16% 3	500
	11th to 20th	Day	11% 11% 29% 27% 15% 6%	492
	21st to end	Day	4 7% 26% 30% 19% 14%	500
May	1st to 10th	Day	6% 7% 24% 30% 20% 12%	503
	11th to 20th	Day	9% 4 21% 28% 21% 16%	527
	21st to end	Day	9% 3 14% 23% 31% 20%	573
June	1st to 10th	Day	14% 3 11% 27% 25% 20%	530
	11th to 20th	Day	7% 3 13% 21% 28% 28%	530
	21st to end	Day	4 3 9% 25% 29% 30%	530
July	1st to 10th	Day	7% 4 7% 16% 29% 37%	525
	11th to 20th	Day	5% 1 4 14% 34% 41%	524
	21st to end	Day	9% 1 4 19% 32% 35%	576
August	1st to 10th	Day	11% 2 6% 18% 30% 33%	526
	11th to 20th	Day	8% 2 4 17% 30% 38%	519
	21st to end	Day	12% 3 7% 21% 30% 27%	583
September	1st to 10th	Day	12% 5% 9% 18% 27% 28%	530
	11th to 20th	Day	12% 4 10% 30% 26% 18%	530
	21st to end	Day	14% 5% 8% 24% 32% 18%	530
October	1st to 10th	Day	18% 6% 22% 27% 19% 7%	530
	11th to 20th	Day	8% 7% 18% 32% 28% 7%	522
	21st to end	Day	9% 6% 22% 38% 20% 5%	539
November	1st to 10th	Day	15% 6% 26% 27% 21% 5%	495
	11th to 20th	Day	16% 10% 25% 30% 15% 4	491
	21st to end	Day	12% 11% 38% 26% 11% 2	300
December	1st to 10th	Day	13% 12% 33% 24% 13% 5%	300
	11th to 20th	Day	17% 13% 32% 29% 7% 2	300
	21st to end	Day	16% 10% 31% 30% 9% 3	330
January	1st to 10th	Night	15% 12% 32% 33% 8% 1	800
	11th to 20th	Night	15% 11% 28% 35% 11% 1	800
	21st to end	Night	17% 12% 34% 30% 6%	880
February	1st to 10th	Night	15% 8% 30% 36% 11% 1	800
	11th to 20th	Night	15% 10% 32% 35% 7% 1	783
	21st to end	Night	13% 9% 34% 38% 6%	660
March	1st to 10th	Night	13% 10% 32% 37% 9%	800
	11th to 20th	Night	11% 11% 31% 34% 12% 1	800
	21st to end	Night	11% 9% 32% 36% 11% 1	880
April	1st to 10th	Night	10% 9% 31% 37% 12%	800
	11th to 20th	Night	12% 9% 30% 34% 14% 1	791
	21st to end	Night	7% 5% 26% 42% 18% 1	758
May	1st to 10th	Night	5% 3 21% 48% 21% 2	600
	11th to 20th	Night	6% 3 18% 41% 29% 3	591
	21st to end	Night	7% 3 8% 38% 37% 7%	650
June	1st to 10th	Night	11% 2 10% 35% 37% 6%	600
	11th to 20th	Night	7% 3 9% 28% 38% 15%	600
	21st to end	Night	9% 2 6% 23% 43% 17%	600
July	1st to 10th	Night	8% 2 4 22% 45% 19%	585
	11th to 20th	Night	6% 1 3 16% 52% 21%	582
	21st to end	Night	4 2 3 24% 51% 16%	657
August	1st to 10th	Night	9% 2 4 19% 50% 17%	597
	11th to 20th	Night	5% 2 4 17% 53% 19%	594
	21st to end	Night	12% 3 8% 21% 39% 16%	855
September	1st to 10th	Night	14% 3 9% 23% 40% 11%	800
	11th to 20th	Night	14% 2 7% 29% 38% 9%	800
	21st to end	Night	13% 4 8% 30% 37% 7%	800
October	1st to 10th	Night	14% 3 16% 34% 28% 5%	800
	11th to 20th	Night	8% 4 18% 37% 30% 2	793
	21st to end	Night	10% 4 20% 40% 24% 2	836
November	1st to 10th	Night	12% 4 24% 36% 22% 2	780
	11th to 20th	Night	12% 6% 22% 38% 21% 1	800
	21st to end	Night	14% 8% 25% 34% 18% 1	800
December	1st to 10th	Night	14% 8% 26% 32% 19% 2	800
	11th to 20th	Night	15% 9% 24% 36% 15% 2	800
	21st to end	Night	16% 8% 28% 35% 14%	880

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