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 2024-01-13.

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	10% 6% 3 6% 6% 4 6% 6% 6% 8% 39%	74885
	Night	10% 5% 2 5% 5% 3 5% 5% 6% 9% 44%	74843
Transparency	Day	42% 8% 13% 17% 14% 6%	74608
	Night	36% 8% 15% 19% 15% 7%	74639
Seeing	Day	12% 6% 20% 24% 22% 16%	14289
	Night	12% 6% 20% 33% 25% 5%	24403

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	10% 6% 3 7% 5% 3 4 3 5% 7% 47%	1706
	11th to 20th	Day	9% 5% 3 6% 4 3 4 4 5% 7% 50%	1638
	21st to end	Day	13% 7% 3 6% 6% 3 4 4 4 6% 44%	1809
February	1st to 10th	Day	16% 7% 3 6% 5% 3 4 3 5% 6% 43%	1800
	11th to 20th	Day	12% 4 2 4 5% 3 4 4 5% 6% 51%	1748
	21st to end	Day	13% 6% 3 5% 5% 3 4 4 5% 7% 46%	1536
March	1st to 10th	Day	8% 4 2 4 5% 2 3 4 5% 8% 55%	1980
	11th to 20th	Day	9% 5% 3 4 4 3 5% 5% 6% 8% 49%	1980
	21st to end	Day	9% 5% 2 6% 4 4 4 4 4 7% 51%	2178
April	1st to 10th	Day	8% 4 2 5% 5% 4 5% 5% 6% 8% 48%	1980
	11th to 20th	Day	9% 5% 3 6% 6% 4 6% 6% 6% 6% 42%	1995
	21st to end	Day	6% 3 2 5% 5% 3 5% 5% 6% 8% 53%	2210
May	1st to 10th	Day	7% 3 2 4 5% 3 5% 5% 6% 8% 54%	2340
	11th to 20th	Day	9% 4 2 5% 7% 5% 7% 7% 7% 10% 38%	2301
	21st to end	Day	9% 4 2 6% 7% 4 7% 5% 7% 9% 39%	2531
June	1st to 10th	Day	12% 6% 3 7% 9% 6% 8% 7% 7% 10% 25%	2340
	11th to 20th	Day	6% 4 3 8% 9% 6% 8% 9% 8% 10% 29%	2340
	21st to end	Day	9% 6% 3 8% 9% 5% 7% 7% 8% 9% 29%	2340
July	1st to 10th	Day	9% 6% 3 8% 9% 4 8% 8% 9% 11% 24%	2340
	11th to 20th	Day	8% 7% 3 10% 9% 5% 9% 7% 9% 10% 23%	2340
	21st to end	Day	8% 5% 3 7% 10% 6% 8% 8% 8% 12% 25%	2547
August	1st to 10th	Day	9% 6% 3 8% 8% 6% 8% 8% 8% 11% 25%	2327
	11th to 20th	Day	11% 6% 4 8% 9% 6% 8% 8% 9% 10% 21%	2272
	21st to end	Day	12% 7% 4 8% 9% 6% 8% 6% 7% 9% 23%	2179
September	1st to 10th	Day	10% 6% 3 9% 9% 6% 9% 7% 9% 9% 23%	1980
	11th to 20th	Day	13% 7% 4 8% 9% 6% 8% 7% 8% 8% 23%	1980
	21st to end	Day	12% 8% 3 8% 7% 4 5% 6% 5% 7% 34%	1980
October	1st to 10th	Day	12% 6% 3 7% 7% 4 6% 6% 6% 8% 36%	1962
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 4 5% 7% 51%	1800
	21st to end	Day	9% 5% 3 6% 4 3 3 5% 6% 7% 49%	1980
November	1st to 10th	Day	12% 5% 2 5% 5% 2 4 5% 6% 8% 46%	1857
	11th to 20th	Day	10% 6% 3 6% 5% 4 4 4 6% 8% 45%	1892
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 5% 7% 46%	1710
December	1st to 10th	Day	10% 5% 2 5% 5% 3 4 5% 6% 8% 46%	1710
	11th to 20th	Day	13% 7% 2 5% 6% 3 4 5% 5% 6% 45%	2153
	21st to end	Day	15% 8% 3 8% 5% 3 4 3 4 6% 41%	3124
January	1st to 10th	Night	12% 7% 3 6% 5% 3 4 4 5% 7% 44%	2459
	11th to 20th	Night	11% 4 2 4 4 3 4 4 5% 8% 51%	2367
	21st to end	Night	15% 6% 3 5% 6% 3 4 5% 5% 7% 42%	2542
February	1st to 10th	Night	13% 6% 3 5% 5% 3 4 4 5% 6% 46%	2160
	11th to 20th	Night	11% 6% 3 5% 5% 3 4 4 4 6% 50%	2098
	21st to end	Night	12% 7% 2 5% 6% 3 4 4 5% 6% 46%	1718
March	1st to 10th	Night	8% 4 1 5% 5% 3 3 4 5% 8% 55%	1980
	11th to 20th	Night	8% 5% 2 5% 4 3 4 4 5% 9% 49%	1980
	21st to end	Night	8% 4 2 4 4 3 4 4 5% 8% 53%	2178
April	1st to 10th	Night	8% 5% 2 4 4 3 5% 4 5% 8% 51%	1980
	11th to 20th	Night	9% 6% 3 5% 4 2 4 4 5% 8% 51%	1910
	21st to end	Night	6% 4 2 4 3 2 4 5% 6% 8% 55%	1763
May	1st to 10th	Night	4 2 2 4 4 3 4 3 6% 10% 60%	1620
	11th to 20th	Night	6% 4 2 4 4 3 5% 5% 7% 11% 49%	1592
	21st to end	Night	6% 4 2 5% 6% 3 5% 6% 6% 11% 47%	1744
June	1st to 10th	Night	11% 5% 3 5% 6% 4 6% 7% 9% 12% 33%	1620
	11th to 20th	Night	6% 4 2 6% 6% 4 6% 8% 9% 12% 36%	1620
	21st to end	Night	7% 5% 2 5% 7% 4 6% 7% 8% 13% 35%	1620
July	1st to 10th	Night	7% 4 2 7% 6% 4 7% 8% 9% 12% 34%	1620
	11th to 20th	Night	4 5% 4 7% 8% 4 6% 6% 8% 11% 36%	1620
	21st to end	Night	4 5% 2 6% 7% 5% 7% 8% 8% 11% 37%	1771
August	1st to 10th	Night	7% 5% 2 6% 6% 4 7% 7% 9% 13% 34%	1611
	11th to 20th	Night	5% 5% 3 7% 6% 3 6% 8% 9% 13% 37%	1624
	21st to end	Night	9% 7% 3 7% 7% 5% 6% 7% 8% 10% 30%	2155
September	1st to 10th	Night	9% 7% 2 7% 7% 4 7% 8% 8% 10% 31%	1980
	11th to 20th	Night	12% 7% 4 6% 8% 5% 7% 6% 6% 9% 30%	1980
	21st to end	Night	9% 7% 3 6% 5% 4 4 5% 6% 11% 39%	1980
October	1st to 10th	Night	10% 6% 2 5% 6% 4 5% 5% 6% 8% 44%	1996
	11th to 20th	Night	6% 3 2 5% 5% 3 4 5% 5% 8% 54%	2160
	21st to end	Night	9% 5% 2 5% 5% 3 4 4 5% 8% 49%	2376
November	1st to 10th	Night	10% 5% 2 4 4 3 4 4 6% 8% 50%	2221
	11th to 20th	Night	10% 5% 2 5% 6% 3 4 4 6% 7% 48%	2284
	21st to end	Night	13% 5% 2 5% 6% 3 5% 4 5% 6% 45%	2470
December	1st to 10th	Night	11% 6% 2 5% 5% 3 4 4 6% 8% 48%	2470
	11th to 20th	Night	14% 7% 3 5% 4 3 3 4 4 6% 47%	3042
	21st to end	Night	15% 8% 3 6% 5% 3 5% 5% 5% 7% 40%	4532

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	38% 4 9% 19% 18% 12%	1707
	11th to 20th	Day	35% 4 8% 20% 23% 10%	1638
	21st to end	Day	43% 3 8% 16% 20% 10%	1809
February	1st to 10th	Day	43% 5% 9% 13% 20% 11%	1800
	11th to 20th	Day	34% 5% 9% 16% 24% 12%	1758
	21st to end	Day	38% 5% 10% 16% 20% 11%	1536
March	1st to 10th	Day	28% 5% 9% 21% 25% 11%	1980
	11th to 20th	Day	34% 5% 14% 20% 16% 11%	1980
	21st to end	Day	34% 5% 11% 21% 20% 9%	2178
April	1st to 10th	Day	35% 6% 11% 19% 18% 12%	1980
	11th to 20th	Day	41% 6% 9% 17% 20% 7%	1995
	21st to end	Day	29% 5% 12% 23% 22% 9%	2210
May	1st to 10th	Day	28% 6% 10% 23% 24% 9%	2340
	11th to 20th	Day	40% 9% 15% 19% 14% 3	2305
	21st to end	Day	40% 8% 13% 19% 17% 2	2531
June	1st to 10th	Day	51% 9% 17% 15% 7% 1	2340
	11th to 20th	Day	45% 13% 20% 17% 5%	2340
	21st to end	Day	48% 12% 21% 13% 6%	2340
July	1st to 10th	Day	51% 14% 20% 14% 1	2275
	11th to 20th	Day	52% 12% 23% 10% 3	2271
	21st to end	Day	48% 13% 22% 15% 2	2405
August	1st to 10th	Day	50% 13% 23% 12% 2	2314
	11th to 20th	Day	53% 14% 22% 10% 1	2272
	21st to end	Day	55% 12% 17% 13% 2	2179
September	1st to 10th	Day	53% 13% 17% 10% 6% 1	1980
	11th to 20th	Day	55% 12% 15% 12% 6%	1980
	21st to end	Day	48% 8% 13% 15% 13% 3	1980
October	1st to 10th	Day	44% 7% 10% 16% 13% 9%	1962
	11th to 20th	Day	34% 4 8% 21% 26% 7%	1800
	21st to end	Day	34% 6% 8% 18% 22% 13%	1980
November	1st to 10th	Day	36% 5% 11% 20% 17% 12%	1857
	11th to 20th	Day	39% 6% 9% 16% 21% 10%	1892
	21st to end	Day	40% 5% 8% 18% 24% 6%	1710
December	1st to 10th	Day	37% 5% 14% 22% 15% 6%	1710
	11th to 20th	Day	42% 5% 9% 17% 19% 9%	2150
	21st to end	Day	45% 3 7% 19% 18% 8%	3124
January	1st to 10th	Night	39% 4 9% 17% 20% 10%	2459
	11th to 20th	Night	33% 4 9% 20% 24% 10%	2367
	21st to end	Night	43% 6% 9% 14% 19% 11%	2542
February	1st to 10th	Night	39% 5% 10% 16% 18% 12%	2160
	11th to 20th	Night	37% 5% 8% 20% 18% 13%	2099
	21st to end	Night	38% 4 11% 20% 16% 10%	1718
March	1st to 10th	Night	28% 5% 11% 23% 21% 11%	1980
	11th to 20th	Night	33% 5% 16% 22% 15% 10%	1980
	21st to end	Night	30% 5% 11% 25% 18% 10%	2178
April	1st to 10th	Night	31% 7% 14% 19% 18% 10%	1980
	11th to 20th	Night	33% 5% 9% 24% 21% 7%	1910
	21st to end	Night	26% 6% 14% 26% 20% 7%	1763
May	1st to 10th	Night	23% 6% 12% 26% 25% 7%	1620
	11th to 20th	Night	27% 9% 22% 25% 13% 4	1593
	21st to end	Night	31% 8% 18% 24% 17% 2	1745
June	1st to 10th	Night	40% 11% 22% 20% 7%	1620
	11th to 20th	Night	35% 13% 25% 22% 4	1620
	21st to end	Night	38% 13% 25% 17% 6% 1	1620
July	1st to 10th	Night	39% 15% 27% 16% 2	1575
	11th to 20th	Night	40% 14% 27% 15% 4	1566
	21st to end	Night	33% 15% 30% 19% 3	1672
August	1st to 10th	Night	37% 16% 31% 13% 3	1600
	11th to 20th	Night	36% 16% 31% 16% 1	1624
	21st to end	Night	46% 14% 22% 15% 3	2155
September	1st to 10th	Night	44% 14% 22% 13% 5% 2	1980
	11th to 20th	Night	47% 14% 17% 16% 6%	1980
	21st to end	Night	40% 8% 16% 20% 13% 3	1980
October	1st to 10th	Night	37% 8% 13% 18% 15% 9%	1996
	11th to 20th	Night	29% 6% 9% 25% 25% 6%	2160
	21st to end	Night	33% 6% 10% 17% 22% 12%	2376
November	1st to 10th	Night	32% 6% 12% 20% 17% 12%	2221
	11th to 20th	Night	36% 6% 9% 18% 19% 12%	2284
	21st to end	Night	39% 6% 9% 18% 21% 7%	2470
December	1st to 10th	Night	35% 6% 15% 24% 15% 6%	2470
	11th to 20th	Night	40% 5% 9% 16% 20% 9%	3044
	21st to end	Night	44% 6% 11% 17% 17% 5%	4532

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	14% 14% 46% 19% 6% 1	270
	11th to 20th	Day	9% 8% 44% 27% 10% 2	246
	21st to end	Day	22% 16% 44% 14% 4	264
February	1st to 10th	Day	19% 13% 38% 20% 9% 1	240
	11th to 20th	Day	13% 14% 40% 22% 10%	240
	21st to end	Day	16% 8% 35% 28% 13% 1	254
March	1st to 10th	Day	11% 12% 33% 29% 14% 1	420
	11th to 20th	Day	13% 11% 32% 27% 14% 3	420
	21st to end	Day	12% 11% 31% 27% 17% 2	462
April	1st to 10th	Day	12% 14% 29% 24% 17% 3	420
	11th to 20th	Day	12% 12% 30% 25% 14% 7%	412
	21st to end	Day	5% 6% 28% 29% 17% 14%	420
May	1st to 10th	Day	7% 8% 24% 30% 20% 12%	423
	11th to 20th	Day	9% 3 18% 28% 24% 18%	447
	21st to end	Day	11% 3 10% 23% 32% 21%	485
June	1st to 10th	Day	16% 2 10% 24% 26% 22%	450
	11th to 20th	Day	9% 3 10% 21% 28% 30%	450
	21st to end	Day	5% 3 8% 23% 28% 32%	450
July	1st to 10th	Day	8% 3 5% 15% 29% 40%	445
	11th to 20th	Day	6% 1 3 13% 34% 42%	444
	21st to end	Day	10% 1 3 20% 32% 34%	488
August	1st to 10th	Day	10% 2 5% 19% 31% 34%	446
	11th to 20th	Day	10% 2 4 16% 29% 39%	439
	21st to end	Day	13% 3 6% 19% 29% 30%	495
September	1st to 10th	Day	11% 3 6% 18% 29% 32%	450
	11th to 20th	Day	13% 3 9% 29% 26% 21%	450
	21st to end	Day	13% 3 7% 24% 33% 19%	450
October	1st to 10th	Day	18% 4 22% 28% 21% 8%	450
	11th to 20th	Day	9% 5% 17% 32% 30% 8%	450
	21st to end	Day	9% 6% 22% 37% 20% 6%	495
November	1st to 10th	Day	15% 6% 27% 27% 20% 5%	455
	11th to 20th	Day	17% 11% 23% 31% 15% 4	452
	21st to end	Day	13% 9% 39% 27% 11% 1	270
December	1st to 10th	Day	13% 11% 31% 25% 14% 5%	270
	11th to 20th	Day	17% 13% 32% 30% 7% 2	270
	21st to end	Day	16% 9% 31% 32% 10% 3	297
January	1st to 10th	Night	16% 11% 31% 33% 8% 1	760
	11th to 20th	Night	14% 10% 26% 36% 12% 1	729
	21st to end	Night	17% 12% 34% 30% 7%	792
February	1st to 10th	Night	16% 8% 30% 34% 12% 1	720
	11th to 20th	Night	13% 10% 31% 37% 8% 1	703
	21st to end	Night	13% 9% 33% 38% 6%	592
March	1st to 10th	Night	11% 10% 31% 38% 9%	720
	11th to 20th	Night	12% 10% 30% 34% 13% 1	720
	21st to end	Night	11% 9% 31% 36% 13% 1	792
April	1st to 10th	Night	11% 9% 30% 38% 13%	720
	11th to 20th	Night	12% 9% 30% 33% 15% 1	711
	21st to end	Night	8% 5% 27% 41% 18% 1	683
May	1st to 10th	Night	5% 3 21% 47% 22% 2	540
	11th to 20th	Night	6% 2 16% 41% 32% 3	531
	21st to end	Night	7% 3 7% 36% 39% 8%	584
June	1st to 10th	Night	12% 2 10% 32% 39% 6%	540
	11th to 20th	Night	7% 2 8% 29% 39% 16%	540
	21st to end	Night	9% 2 7% 21% 43% 19%	540
July	1st to 10th	Night	8% 2 4 21% 45% 20%	525
	11th to 20th	Night	6% 1 3 16% 51% 22%	522
	21st to end	Night	4 1 3 25% 52% 15%	591
August	1st to 10th	Night	8% 1 3 19% 51% 18%	537
	11th to 20th	Night	6% 1 4 17% 52% 19%	534
	21st to end	Night	13% 2 8% 20% 40% 17%	769
September	1st to 10th	Night	13% 2 7% 24% 43% 12%	720
	11th to 20th	Night	14% 2 7% 28% 39% 10%	720
	21st to end	Night	13% 4 8% 30% 37% 8%	720
October	1st to 10th	Night	13% 3 15% 34% 29% 5%	720
	11th to 20th	Night	8% 4 17% 37% 32% 2	720
	21st to end	Night	11% 4 20% 39% 25% 2	792
November	1st to 10th	Night	13% 4 25% 35% 21% 2	740
	11th to 20th	Night	12% 6% 22% 38% 21% 1	760
	21st to end	Night	14% 7% 25% 35% 18% 1	760
December	1st to 10th	Night	14% 8% 25% 32% 19% 2	760
	11th to 20th	Night	15% 9% 24% 36% 15% 2	760
	21st to end	Night	16% 8% 26% 36% 14%	836

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