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 2024-01-15.

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% 8% 33%	74616
	Night	15% 10% 4 6% 6% 3 5% 4 5% 6% 35%	75146
Transparency	Day	48% 7% 12% 20% 11% 1	74326
	Night	49% 6% 12% 20% 11% 1	74957
Seeing	Day	18% 13% 30% 22% 13% 5%	14237
	Night	19% 11% 30% 28% 11% 1	24096

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	15% 14% 6% 10% 9% 4 5% 4 4 6% 21%	1517
	11th to 20th	Day	16% 14% 7% 11% 10% 4 5% 6% 5% 6% 16%	1472
	21st to end	Day	15% 18% 8% 9% 9% 4 7% 4 5% 5% 16%	1764
February	1st to 10th	Day	15% 14% 6% 9% 11% 4 7% 4 4 5% 20%	1620
	11th to 20th	Day	14% 12% 5% 11% 9% 5% 5% 5% 5% 6% 24%	1687
	21st to end	Day	15% 15% 6% 9% 8% 4 4 4 5% 6% 25%	1480
March	1st to 10th	Day	15% 11% 4 7% 6% 4 6% 4 7% 7% 28%	1844
	11th to 20th	Day	13% 10% 5% 8% 7% 3 4 5% 5% 7% 32%	1980
	21st to end	Day	19% 11% 5% 8% 7% 4 5% 4 4 6% 27%	2178
April	1st to 10th	Day	16% 11% 5% 6% 5% 3 5% 5% 6% 8% 30%	2083
	11th to 20th	Day	17% 9% 4 7% 5% 3 5% 5% 5% 8% 34%	2287
	21st to end	Day	19% 8% 3 6% 5% 3 5% 4 5% 6% 34%	2340
May	1st to 10th	Day	18% 8% 3 6% 7% 4 5% 4 5% 7% 33%	2340
	11th to 20th	Day	19% 8% 4 6% 5% 3 5% 6% 4 6% 35%	2301
	21st to end	Day	13% 5% 3 5% 5% 3 5% 6% 6% 9% 40%	2531
June	1st to 10th	Day	11% 4 3 6% 6% 4 5% 5% 6% 9% 41%	2592
	11th to 20th	Day	11% 6% 2 6% 5% 3 6% 6% 6% 9% 41%	2700
	21st to end	Day	10% 5% 4 6% 5% 3 6% 7% 7% 11% 37%	2582
July	1st to 10th	Day	7% 4 2 6% 5% 4 5% 6% 6% 8% 46%	2520
	11th to 20th	Day	10% 5% 2 5% 5% 3 6% 5% 7% 10% 42%	2520
	21st to end	Day	8% 5% 2 5% 5% 4 6% 7% 7% 9% 41%	2582
August	1st to 10th	Day	9% 5% 2 5% 6% 4 5% 7% 7% 10% 41%	2327
	11th to 20th	Day	10% 5% 2 6% 6% 5% 6% 7% 7% 8% 39%	2303
	21st to end	Day	10% 6% 2 6% 6% 3 5% 5% 6% 9% 42%	2563
September	1st to 10th	Day	12% 7% 2 5% 6% 4 5% 6% 6% 9% 38%	2030
	11th to 20th	Day	11% 5% 2 4 5% 2 5% 5% 7% 8% 45%	1980
	21st to end	Day	14% 6% 2 6% 5% 4 5% 5% 6% 9% 37%	1980
October	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 4 5% 7% 41%	1923
	11th to 20th	Day	19% 10% 4 6% 6% 4 5% 4 4 8% 32%	1800
	21st to end	Day	21% 11% 4 5% 6% 3 5% 4 5% 7% 29%	1841
November	1st to 10th	Day	11% 9% 4 6% 6% 4 5% 5% 6% 8% 37%	1672
	11th to 20th	Day	15% 11% 4 7% 7% 3 6% 5% 5% 6% 31%	1710
	21st to end	Day	19% 11% 6% 7% 5% 3 4 4 4 7% 29%	1587
December	1st to 10th	Day	15% 15% 7% 10% 8% 3 4 5% 4 6% 22%	1520
	11th to 20th	Day	18% 14% 6% 9% 5% 3 4 6% 5% 6% 24%	1769
	21st to end	Day	27% 19% 7% 11% 6% 3 4 3 4 5% 11%	2691
January	1st to 10th	Night	18% 12% 5% 9% 7% 4 5% 4 5% 6% 26%	2648
	11th to 20th	Night	18% 15% 7% 10% 8% 4 5% 4 4 5% 21%	2583
	21st to end	Night	13% 14% 7% 11% 7% 4 5% 5% 5% 6% 23%	2588
February	1st to 10th	Night	15% 16% 5% 9% 9% 3 6% 5% 4 5% 23%	2340
	11th to 20th	Night	15% 11% 6% 9% 8% 4 5% 5% 4 5% 27%	2155
	21st to end	Night	20% 12% 4 8% 6% 3 4 4 4 6% 30%	1776
March	1st to 10th	Night	15% 10% 4 7% 7% 4 4 4 5% 6% 34%	2115
	11th to 20th	Night	14% 9% 3 6% 5% 4 5% 5% 5% 7% 38%	1980
	21st to end	Night	18% 9% 4 7% 6% 4 5% 4 4 5% 35%	2178
April	1st to 10th	Night	16% 8% 3 6% 5% 4 5% 5% 6% 6% 36%	1891
	11th to 20th	Night	18% 8% 3 4 5% 3 4 5% 5% 6% 38%	1621
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 40%	1620
May	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 5% 6% 6% 35%	1620
	11th to 20th	Night	19% 5% 2 5% 4 3 4 4 5% 8% 40%	1592
	21st to end	Night	9% 5% 2 5% 4 3 5% 5% 5% 9% 46%	1744
June	1st to 10th	Night	11% 5% 3 4 4 3 6% 4 5% 9% 47%	1355
	11th to 20th	Night	10% 5% 2 4 5% 4 3 6% 6% 8% 46%	1260
	21st to end	Night	9% 5% 3 5% 4 3 4 4 6% 8% 49%	1370
July	1st to 10th	Night	7% 4 2 3 5% 3 5% 5% 5% 9% 53%	1440
	11th to 20th	Night	8% 4 1 4 5% 2 4 5% 5% 9% 53%	1440
	21st to end	Night	5% 4 2 4 4 3 5% 6% 7% 9% 50%	1736
August	1st to 10th	Night	8% 3 2 5% 4 3 4 5% 6% 10% 50%	1611
	11th to 20th	Night	7% 4 2 5% 5% 4 5% 5% 6% 8% 50%	1598
	21st to end	Night	9% 4 2 5% 4 3 3 4 5% 8% 52%	1790
September	1st to 10th	Night	11% 6% 3 4 5% 3 4 5% 5% 7% 47%	1912
	11th to 20th	Night	10% 5% 2 4 4 2 4 4 4 7% 54%	1980
	21st to end	Night	16% 5% 2 5% 5% 3 4 4 5% 6% 46%	1980
October	1st to 10th	Night	15% 8% 3 4 4 2 3 4 4 6% 46%	2035
	11th to 20th	Night	19% 9% 3 5% 6% 4 5% 4 5% 5% 36%	2160
	21st to end	Night	22% 9% 3 5% 6% 4 5% 4 4 6% 31%	2510
November	1st to 10th	Night	13% 7% 3 5% 5% 3 4 5% 5% 7% 44%	2406
	11th to 20th	Night	16% 9% 3 7% 6% 3 5% 6% 6% 6% 31%	2470
	21st to end	Night	19% 13% 5% 7% 5% 2 4 5% 4 6% 30%	2589
December	1st to 10th	Night	17% 15% 7% 9% 6% 3 5% 5% 5% 5% 23%	2660
	11th to 20th	Night	15% 15% 5% 9% 7% 4 5% 4 5% 7% 25%	3424
	21st to end	Night	29% 22% 6% 8% 5% 3 4 3 3 4 13%	4969

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% 8% 10% 12% 6% 1	1518
	11th to 20th	Day	68% 8% 12% 7% 4	1472
	21st to end	Day	70% 5% 8% 10% 5% 1	1764
February	1st to 10th	Day	66% 7% 9% 12% 7%	1620
	11th to 20th	Day	61% 5% 10% 16% 7% 1	1696
	21st to end	Day	60% 5% 11% 15% 7% 2	1480
March	1st to 10th	Day	54% 7% 14% 16% 7% 1	1844
	11th to 20th	Day	51% 6% 11% 19% 11% 3	1980
	21st to end	Day	58% 5% 11% 17% 7% 1	2178
April	1st to 10th	Day	52% 7% 12% 19% 11% 1	2083
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2287
	21st to end	Day	50% 6% 13% 16% 13% 1	2340
May	1st to 10th	Day	50% 7% 12% 20% 10% 2	2340
	11th to 20th	Day	48% 8% 11% 18% 13% 2	2305
	21st to end	Day	39% 7% 15% 23% 14% 2	2531
June	1st to 10th	Day	38% 7% 12% 26% 13% 3	2592
	11th to 20th	Day	39% 8% 13% 23% 16% 2	2700
	21st to end	Day	39% 9% 12% 23% 15% 2	2582
July	1st to 10th	Day	34% 8% 11% 25% 18% 4	2451
	11th to 20th	Day	36% 7% 14% 24% 17% 2	2445
	21st to end	Day	35% 10% 13% 21% 17% 3	2439
August	1st to 10th	Day	35% 11% 15% 26% 12% 1	2314
	11th to 20th	Day	38% 10% 14% 23% 13% 2	2303
	21st to end	Day	38% 8% 11% 25% 16% 2	2563
September	1st to 10th	Day	40% 8% 16% 22% 13% 2	2030
	11th to 20th	Day	33% 9% 13% 32% 13% 1	1980
	21st to end	Day	42% 8% 14% 24% 12%	1980
October	1st to 10th	Day	42% 7% 13% 24% 12% 1	1923
	11th to 20th	Day	51% 7% 13% 19% 9% 1	1800
	21st to end	Day	55% 6% 13% 18% 8%	1841
November	1st to 10th	Day	44% 7% 13% 22% 12% 1	1672
	11th to 20th	Day	53% 5% 13% 18% 9% 1	1710
	21st to end	Day	56% 6% 13% 17% 7% 1	1587
December	1st to 10th	Day	63% 6% 12% 14% 4	1520
	11th to 20th	Day	60% 7% 14% 14% 5%	1765
	21st to end	Day	77% 5% 9% 6% 3	2691
January	1st to 10th	Night	60% 6% 11% 17% 6% 1	2648
	11th to 20th	Night	66% 5% 12% 11% 5%	2583
	21st to end	Night	61% 6% 12% 15% 5% 1	2588
February	1st to 10th	Night	63% 7% 10% 13% 6% 1	2340
	11th to 20th	Night	58% 6% 10% 17% 9% 1	2157
	21st to end	Night	56% 4 12% 18% 8% 1	1776
March	1st to 10th	Night	50% 5% 12% 21% 10% 1	2115
	11th to 20th	Night	45% 7% 11% 21% 15% 2	1980
	21st to end	Night	53% 5% 9% 21% 10% 1	2178
April	1st to 10th	Night	47% 5% 11% 22% 13% 2	1891
	11th to 20th	Night	45% 6% 11% 18% 18% 2	1621
	21st to end	Night	44% 6% 11% 19% 16% 3	1620
May	1st to 10th	Night	48% 6% 13% 19% 12% 2	1620
	11th to 20th	Night	43% 6% 11% 18% 18% 3	1593
	21st to end	Night	33% 10% 14% 24% 16% 3	1745
June	1st to 10th	Night	35% 6% 12% 28% 17% 3	1355
	11th to 20th	Night	34% 9% 13% 24% 18% 2	1260
	21st to end	Night	33% 6% 12% 28% 20% 1	1370
July	1st to 10th	Night	28% 7% 13% 28% 21% 3	1400
	11th to 20th	Night	29% 7% 12% 32% 18% 3	1392
	21st to end	Night	27% 10% 17% 23% 20% 4	1639
August	1st to 10th	Night	29% 9% 14% 31% 15% 2	1600
	11th to 20th	Night	30% 9% 14% 32% 13% 3	1598
	21st to end	Night	31% 7% 13% 29% 19% 1	1790
September	1st to 10th	Night	35% 8% 14% 26% 16% 2	1912
	11th to 20th	Night	30% 7% 12% 31% 19% 1	1980
	21st to end	Night	38% 7% 12% 30% 13% 1	1980
October	1st to 10th	Night	40% 5% 10% 25% 19% 1	2035
	11th to 20th	Night	50% 6% 13% 19% 12% 1	2160
	21st to end	Night	54% 5% 11% 20% 9% 1	2510
November	1st to 10th	Night	40% 7% 13% 25% 14% 1	2406
	11th to 20th	Night	51% 6% 14% 20% 8% 1	2470
	21st to end	Night	55% 6% 13% 20% 6%	2589
December	1st to 10th	Night	63% 7% 10% 14% 5%	2660
	11th to 20th	Night	58% 8% 14% 13% 6% 1	3427
	21st to end	Night	76% 5% 10% 7% 2	4969

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% 24% 34% 14% 5%	270
	11th to 20th	Day	25% 18% 40% 15% 1 1	252
	21st to end	Day	25% 16% 39% 15% 4	264
February	1st to 10th	Day	25% 24% 38% 13% 1	240
	11th to 20th	Day	25% 20% 39% 14% 1	240
	21st to end	Day	23% 31% 37% 8% 2	198
March	1st to 10th	Day	27% 21% 34% 14% 3	284
	11th to 20th	Day	22% 14% 36% 22% 6% 1	420
	21st to end	Day	29% 16% 31% 18% 6% 1	462
April	1st to 10th	Day	19% 18% 34% 19% 9% 1	420
	11th to 20th	Day	26% 18% 29% 14% 8% 5%	412
	21st to end	Day	25% 16% 30% 18% 8% 3	420
May	1st to 10th	Day	24% 15% 28% 20% 9% 3	423
	11th to 20th	Day	21% 11% 30% 21% 12% 5%	447
	21st to end	Day	14% 9% 24% 29% 17% 7%	485
June	1st to 10th	Day	12% 10% 33% 29% 13% 3	608
	11th to 20th	Day	14% 9% 32% 27% 15% 3	630
	21st to end	Day	12% 10% 31% 30% 13% 5%	630
July	1st to 10th	Day	7% 7% 23% 32% 21% 10%	621
	11th to 20th	Day	11% 7% 28% 28% 18% 7%	618
	21st to end	Day	9% 5% 27% 28% 23% 8%	523
August	1st to 10th	Day	9% 4 23% 27% 24% 13%	446
	11th to 20th	Day	11% 5% 22% 29% 21% 13%	439
	21st to end	Day	12% 6% 24% 26% 22% 10%	495
September	1st to 10th	Day	14% 6% 24% 26% 21% 10%	450
	11th to 20th	Day	12% 3 22% 24% 27% 12%	450
	21st to end	Day	16% 11% 25% 23% 18% 7%	450
October	1st to 10th	Day	18% 9% 28% 19% 17% 8%	450
	11th to 20th	Day	22% 17% 32% 22% 7% 1	450
	21st to end	Day	26% 19% 38% 14% 3 1	356
November	1st to 10th	Day	14% 24% 35% 18% 8% 1	270
	11th to 20th	Day	22% 24% 30% 17% 6% 1	270
	21st to end	Day	24% 21% 37% 14% 5%	212
December	1st to 10th	Day	23% 24% 32% 13% 8%	180
	11th to 20th	Day	28% 26% 33% 11% 2 1	180
	21st to end	Day	26% 28% 33% 11% 2	272
January	1st to 10th	Night	22% 16% 36% 20% 5% 1	950
	11th to 20th	Night	26% 15% 33% 22% 4 1	923
	21st to end	Night	20% 13% 34% 28% 5%	806
February	1st to 10th	Night	24% 13% 38% 22% 3	720
	11th to 20th	Night	22% 16% 32% 26% 4	703
	21st to end	Night	26% 16% 37% 19% 2	592
March	1st to 10th	Night	20% 14% 33% 27% 6%	720
	11th to 20th	Night	18% 14% 38% 24% 5% 1	720
	21st to end	Night	22% 14% 30% 26% 8% 1	792
April	1st to 10th	Night	20% 14% 34% 27% 4	631
	11th to 20th	Night	21% 10% 35% 26% 8% 2	533
	21st to end	Night	20% 10% 32% 29% 9%	540
May	1st to 10th	Night	20% 6% 32% 31% 9% 1	540
	11th to 20th	Night	23% 5% 27% 33% 12% 1	531
	21st to end	Night	11% 3 26% 39% 19% 2	584
June	1st to 10th	Night	12% 5% 27% 44% 13%	437
	11th to 20th	Night	11% 4 25% 49% 11%	360
	21st to end	Night	9% 5% 25% 41% 19% 1	470
July	1st to 10th	Night	7% 3 20% 40% 26% 4	525
	11th to 20th	Night	9% 3 22% 39% 23% 4	522
	21st to end	Night	5% 3 21% 46% 22% 2	591
August	1st to 10th	Night	7% 3 19% 39% 29% 2	537
	11th to 20th	Night	8% 2 20% 37% 28% 4	534
	21st to end	Night	10% 4 19% 33% 31% 3	594
September	1st to 10th	Night	13% 5% 20% 34% 23% 5%	652
	11th to 20th	Night	11% 5% 19% 34% 26% 5%	720
	21st to end	Night	16% 7% 22% 29% 21% 5%	720
October	1st to 10th	Night	18% 8% 27% 28% 17% 3	720
	11th to 20th	Night	23% 14% 29% 24% 8% 1	720
	21st to end	Night	26% 14% 32% 23% 5%	792
November	1st to 10th	Night	18% 12% 30% 26% 13% 1	740
	11th to 20th	Night	21% 16% 34% 21% 6% 1	760
	21st to end	Night	24% 16% 34% 20% 6%	760
December	1st to 10th	Night	25% 16% 35% 18% 6%	760
	11th to 20th	Night	23% 16% 37% 20% 3	852
	21st to end	Night	31% 14% 34% 19% 2	1045

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