Cranbrook Institute of Science
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cranbrook Institute of Science clear sky chart from 2005-11-06 to 2026-03-03.

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% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	130268
	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 34%	131010
Transparency	Day	48% 7% 12% 20% 12% 1	133838
	Night	49% 6% 12% 21% 12% 1	135511
Seeing	Day	18% 14% 30% 21% 12% 4	17075
	Night	19% 11% 30% 28% 11% 1	26944

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	13% 16% 7% 15% 6% 5% 4 3 4 5% 23%	4691
	11th to 20th	Day	13% 14% 7% 17% 11% 9% 6% 4 4 4 11%	4710
	21st to end	Day	17% 15% 6% 12% 9% 5% 7% 3 5% 6% 15%	5736
February	1st to 10th	Day	14% 15% 4 11% 9% 6% 6% 3 4 5% 22%	1890
	11th to 20th	Day	13% 14% 5% 12% 8% 5% 5% 3 4 6% 24%	1977
	21st to end	Day	13% 16% 5% 11% 6% 5% 4 3 5% 6% 26%	1730
March	1st to 10th	Day	15% 12% 5% 10% 5% 4 4 3 5% 6% 30%	4628
	11th to 20th	Day	13% 10% 4 8% 6% 5% 5% 3 5% 7% 34%	4980
	21st to end	Day	20% 14% 4 8% 5% 5% 4 3 4 5% 29%	5465
April	1st to 10th	Day	16% 13% 4 7% 4 4 5% 4 5% 8% 30%	2315
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 5% 5% 7% 34%	2545
	21st to end	Day	18% 9% 3 7% 4 4 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 35%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 7% 33%	5861
	21st to end	Day	15% 6% 2 7% 4 4 4 3 5% 7% 42%	6433
June	1st to 10th	Day	10% 5% 3 7% 5% 5% 4 4 6% 9% 41%	2881
	11th to 20th	Day	10% 6% 2 7% 4 5% 6% 5% 6% 9% 40%	3000
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 6% 7% 10% 37%	2868
July	1st to 10th	Day	9% 5% 3 7% 5% 5% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 10% 45%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	6515
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	4234
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 8% 9% 38%	4174
	21st to end	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 9% 39%	4649
September	1st to 10th	Day	11% 7% 2 6% 5% 5% 6% 4 6% 8% 41%	2254
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 4 7% 8% 48%	2200
	21st to end	Day	15% 7% 2 8% 5% 5% 5% 4 6% 8% 36%	2200
October	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 3 5% 6% 44%	2136
	11th to 20th	Day	18% 10% 3 6% 5% 5% 5% 3 5% 7% 35%	2000
	21st to end	Day	19% 11% 4 7% 5% 4 5% 3 5% 7% 30%	2045
November	1st to 10th	Day	11% 10% 3 7% 5% 4 5% 4 6% 8% 37%	1852
	11th to 20th	Day	15% 12% 4 8% 7% 5% 5% 4 5% 6% 31%	1890
	21st to end	Day	18% 13% 5% 9% 4 5% 4 3 4 6% 28%	1753
December	1st to 10th	Day	13% 15% 7% 12% 6% 4 5% 4 5% 7% 23%	2680
	11th to 20th	Day	20% 20% 5% 9% 5% 4 4 4 5% 4 20%	2846
	21st to end	Day	27% 20% 7% 12% 4 4 3 2 3 4 13%	3948
January	1st to 10th	Night	14% 14% 6% 13% 6% 4 4 3 4 5% 25%	8232
	11th to 20th	Night	9% 16% 7% 15% 8% 7% 6% 4 5% 6% 16%	8280
	21st to end	Night	14% 15% 7% 11% 6% 5% 5% 3 4 5% 24%	8443
February	1st to 10th	Night	14% 15% 4 11% 8% 4 5% 4 5% 5% 25%	2730
	11th to 20th	Night	14% 12% 5% 11% 7% 5% 5% 4 5% 5% 27%	2525
	21st to end	Night	17% 13% 3 9% 5% 5% 4 3 5% 6% 30%	2076
March	1st to 10th	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 35%	5269
	11th to 20th	Night	11% 8% 3 7% 5% 4 5% 4 5% 7% 41%	4980
	21st to end	Night	16% 10% 3 8% 5% 4 4 3 4 5% 38%	5467
April	1st to 10th	Night	17% 10% 3 6% 5% 4 5% 4 5% 6% 35%	2101
	11th to 20th	Night	18% 8% 3 6% 4 4 3 4 6% 6% 38%	1803
	21st to end	Night	16% 8% 3 6% 4 4 4 4 5% 6% 40%	1800
May	1st to 10th	Night	22% 8% 2 8% 4 4 4 3 5% 6% 34%	4033
	11th to 20th	Night	19% 7% 2 7% 4 4 3 3 4 6% 40%	4032
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	4421
June	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 6% 8% 47%	1504
	11th to 20th	Night	11% 5% 2 6% 4 5% 4 4 7% 8% 45%	1400
	21st to end	Night	9% 6% 2 5% 4 4 3 3 6% 8% 50%	1523
July	1st to 10th	Night	8% 4 1 5% 5% 4 4 4 6% 10% 48%	3618
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 5% 9% 50%	3617
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 5% 8% 49%	4340
August	1st to 10th	Night	12% 5% 2 6% 3 4 3 3 6% 9% 47%	2915
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 5% 8% 49%	2875
	21st to end	Night	9% 5% 2 7% 4 4 4 3 5% 7% 49%	3246
September	1st to 10th	Night	10% 6% 2 4 5% 4 4 4 5% 6% 49%	2125
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 7% 57%	2200
	21st to end	Night	16% 6% 2 6% 4 4 4 4 5% 6% 44%	2200
October	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 5% 6% 48%	2262
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 5% 5% 38%	2400
	21st to end	Night	20% 9% 3 6% 5% 5% 5% 3 5% 6% 33%	2790
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 4 5% 7% 43%	2666
	11th to 20th	Night	15% 11% 3 8% 5% 5% 5% 5% 6% 6% 31%	2730
	21st to end	Night	18% 15% 5% 9% 4 3 4 4 4 6% 28%	2863
December	1st to 10th	Night	12% 14% 6% 13% 5% 5% 5% 4 5% 7% 25%	4720
	11th to 20th	Night	16% 20% 6% 11% 7% 4 4 3 4 6% 21%	5559
	21st to end	Night	27% 20% 6% 9% 4 4 4 3 3 4 16%	7265

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	63% 6% 11% 14% 5% 1	4692
	11th to 20th	Day	73% 6% 9% 6% 6%	4710
	21st to end	Day	69% 5% 11% 9% 6%	5736
February	1st to 10th	Day	63% 7% 9% 14% 7%	1890
	11th to 20th	Day	61% 5% 10% 16% 8% 1	1986
	21st to end	Day	59% 5% 12% 16% 7% 1	1730
March	1st to 10th	Day	55% 5% 14% 16% 8% 2	4628
	11th to 20th	Day	48% 6% 15% 22% 8% 1	4980
	21st to end	Day	57% 5% 12% 15% 11% 1	5465
April	1st to 10th	Day	51% 7% 12% 18% 11% 1	2315
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2545
	21st to end	Day	49% 6% 12% 18% 13% 1	2600
May	1st to 10th	Day	49% 6% 10% 20% 12% 3	5897
	11th to 20th	Day	47% 9% 11% 20% 12% 1	5865
	21st to end	Day	41% 6% 9% 21% 18% 5%	6447
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2881
	11th to 20th	Day	39% 8% 12% 23% 15% 2	3000
	21st to end	Day	39% 9% 12% 22% 16% 2	2868
July	1st to 10th	Day	38% 7% 13% 24% 16% 2	6291
	11th to 20th	Day	33% 7% 14% 27% 18% 1	6285
	21st to end	Day	34% 8% 13% 22% 22% 1	6330
August	1st to 10th	Day	37% 8% 14% 25% 15% 1	4221
	11th to 20th	Day	38% 10% 11% 30% 10% 1	4213
	21st to end	Day	39% 8% 12% 28% 12% 1	4648
September	1st to 10th	Day	38% 8% 15% 24% 13% 2	2254
	11th to 20th	Day	31% 8% 13% 32% 15% 1	2200
	21st to end	Day	43% 8% 13% 23% 13%	2200
October	1st to 10th	Day	31% 5% 12% 30% 20% 1	3474
	11th to 20th	Day	40% 5% 12% 26% 18%	3240
	21st to end	Day	45% 5% 13% 23% 13% 1	3282
November	1st to 10th	Day	45% 7% 13% 23% 12% 1	1852
	11th to 20th	Day	54% 5% 13% 19% 9% 1	1890
	21st to end	Day	58% 6% 12% 17% 7% 1	1753
December	1st to 10th	Day	60% 6% 17% 13% 4	2680
	11th to 20th	Day	66% 5% 11% 12% 6%	2842
	21st to end	Day	76% 4 7% 8% 5%	3948
January	1st to 10th	Night	60% 5% 10% 16% 7% 1	8233
	11th to 20th	Night	67% 5% 11% 10% 6%	8280
	21st to end	Night	62% 5% 10% 16% 7%	8443
February	1st to 10th	Night	60% 7% 10% 16% 7% 1	2730
	11th to 20th	Night	57% 6% 10% 18% 8%	2527
	21st to end	Night	55% 5% 13% 18% 8% 1	2076
March	1st to 10th	Night	49% 4 14% 21% 10% 1	5269
	11th to 20th	Night	39% 6% 15% 24% 15% 1	4980
	21st to end	Night	48% 5% 11% 19% 14% 2	5467
April	1st to 10th	Night	49% 6% 11% 21% 12% 2	2101
	11th to 20th	Night	44% 6% 11% 18% 18% 2	1803
	21st to end	Night	44% 7% 11% 19% 17% 3	1800
May	1st to 10th	Night	51% 6% 9% 17% 13% 4	4060
	11th to 20th	Night	44% 6% 10% 25% 11% 3	4033
	21st to end	Night	37% 7% 11% 23% 17% 5%	4429
June	1st to 10th	Night	34% 6% 11% 27% 18% 2	1504
	11th to 20th	Night	35% 9% 13% 25% 17% 2	1400
	21st to end	Night	33% 6% 13% 27% 21% 1	1523
July	1st to 10th	Night	30% 6% 15% 29% 16% 5%	3580
	11th to 20th	Night	31% 7% 13% 30% 17% 2	3572
	21st to end	Night	32% 8% 14% 21% 23% 2	4212
August	1st to 10th	Night	34% 7% 14% 28% 17% 1	2904
	11th to 20th	Night	33% 7% 11% 36% 12% 2	2902
	21st to end	Night	34% 7% 12% 33% 14% 1	3246
September	1st to 10th	Night	34% 8% 13% 28% 16% 2	2125
	11th to 20th	Night	27% 6% 12% 32% 21% 2	2200
	21st to end	Night	40% 7% 12% 28% 13% 1	2200
October	1st to 10th	Night	26% 4 10% 32% 28% 1	3704
	11th to 20th	Night	43% 4 11% 23% 18%	3899
	21st to end	Night	41% 4 12% 28% 14% 1	4503
November	1st to 10th	Night	41% 6% 12% 26% 14% 1	2666
	11th to 20th	Night	50% 6% 14% 20% 9% 1	2730
	21st to end	Night	57% 6% 12% 19% 6%	2863
December	1st to 10th	Night	59% 7% 16% 15% 4	4720
	11th to 20th	Night	64% 6% 12% 12% 5% 1	5562
	21st to end	Night	73% 4 9% 7% 6%	7265

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	23% 23% 36% 13% 4	330
	11th to 20th	Day	24% 22% 40% 12% 1 1	330
	21st to end	Day	28% 17% 36% 15% 3	363
February	1st to 10th	Day	23% 22% 36% 13% 4 1	330
	11th to 20th	Day	24% 21% 37% 17% 2	330
	21st to end	Day	22% 31% 37% 9% 2	273
March	1st to 10th	Day	27% 21% 37% 13% 3	356
	11th to 20th	Day	24% 15% 36% 20% 5% 1	500
	21st to end	Day	29% 16% 32% 17% 5% 1	550
April	1st to 10th	Day	22% 19% 34% 17% 7% 1	500
	11th to 20th	Day	26% 20% 30% 13% 7% 4	492
	21st to end	Day	24% 16% 32% 18% 8% 2	500
May	1st to 10th	Day	23% 17% 29% 20% 9% 3	503
	11th to 20th	Day	21% 12% 29% 22% 11% 5%	527
	21st to end	Day	15% 10% 26% 26% 16% 6%	573
June	1st to 10th	Day	12% 12% 32% 28% 12% 3	706
	11th to 20th	Day	13% 10% 34% 27% 14% 2	730
	21st to end	Day	12% 11% 32% 29% 12% 4	730
July	1st to 10th	Day	8% 9% 25% 30% 19% 9%	721
	11th to 20th	Day	11% 8% 30% 27% 17% 7%	718
	21st to end	Day	9% 5% 28% 29% 21% 8%	615
August	1st to 10th	Day	9% 5% 22% 29% 22% 12%	526
	11th to 20th	Day	11% 5% 23% 30% 20% 11%	519
	21st to end	Day	11% 6% 25% 27% 21% 10%	583
September	1st to 10th	Day	13% 7% 25% 25% 20% 10%	530
	11th to 20th	Day	10% 3 21% 25% 28% 13%	530
	21st to end	Day	17% 11% 26% 23% 17% 7%	530
October	1st to 10th	Day	17% 10% 28% 21% 17% 7%	530
	11th to 20th	Day	21% 15% 34% 22% 7% 1	530
	21st to end	Day	26% 20% 37% 13% 4	428
November	1st to 10th	Day	18% 23% 35% 16% 7% 1	330
	11th to 20th	Day	25% 24% 30% 16% 5% 1	330
	21st to end	Day	26% 21% 34% 14% 4	258
December	1st to 10th	Day	25% 23% 30% 14% 8%	220
	11th to 20th	Day	32% 25% 31% 10% 2	220
	21st to end	Day	30% 27% 31% 10% 1	334
January	1st to 10th	Night	22% 16% 37% 19% 4 1	1050
	11th to 20th	Night	25% 17% 34% 21% 3 1	1050
	21st to end	Night	22% 14% 34% 25% 4	940
February	1st to 10th	Night	23% 14% 36% 24% 3	840
	11th to 20th	Night	22% 16% 33% 26% 3	823
	21st to end	Night	25% 17% 38% 18% 2	692
March	1st to 10th	Night	21% 15% 34% 26% 5%	809
	11th to 20th	Night	18% 16% 38% 23% 5% 1	800
	21st to end	Night	22% 15% 30% 25% 7% 1	880
April	1st to 10th	Night	22% 15% 34% 25% 4	701
	11th to 20th	Night	22% 11% 33% 26% 7% 1	593
	21st to end	Night	19% 11% 32% 30% 8%	600
May	1st to 10th	Night	21% 8% 31% 31% 8% 1	600
	11th to 20th	Night	23% 6% 27% 32% 12% 1	591
	21st to end	Night	12% 5% 27% 38% 17% 2	650
June	1st to 10th	Night	12% 7% 27% 41% 13%	485
	11th to 20th	Night	12% 5% 27% 47% 10%	400
	21st to end	Night	9% 6% 26% 40% 18% 1	523
July	1st to 10th	Night	7% 3 21% 41% 24% 4	585
	11th to 20th	Night	9% 3 23% 38% 23% 3	582
	21st to end	Night	6% 4 21% 46% 22% 2	657
August	1st to 10th	Night	7% 4 20% 39% 28% 2	597
	11th to 20th	Night	8% 3 21% 37% 28% 4	594
	21st to end	Night	10% 4 19% 34% 30% 3	660
September	1st to 10th	Night	12% 6% 21% 32% 24% 5%	725
	11th to 20th	Night	10% 4 17% 35% 29% 5%	800
	21st to end	Night	17% 7% 23% 29% 20% 5%	800
October	1st to 10th	Night	16% 8% 27% 29% 17% 3	800
	11th to 20th	Night	22% 14% 29% 25% 8% 1	800
	21st to end	Night	24% 14% 33% 24% 5%	880
November	1st to 10th	Night	19% 13% 32% 24% 12% 1	820
	11th to 20th	Night	22% 17% 33% 22% 5% 1	840
	21st to end	Night	25% 17% 34% 18% 5%	840
December	1st to 10th	Night	24% 18% 34% 18% 6%	840
	11th to 20th	Night	24% 17% 37% 19% 3	942
	21st to end	Night	32% 14% 34% 18% 2	1155

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