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 2024-12-02.

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% 3 8% 5% 5% 5% 4 6% 7% 33%	101228
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 36%	98393
Transparency	Day	47% 7% 12% 20% 12% 1	104842
	Night	47% 6% 12% 21% 12% 1	102919
Seeing	Day	18% 13% 30% 22% 12% 5%	15476
	Night	19% 11% 30% 28% 11% 1	25222

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% 18% 7% 10% 6% 5% 4 3 4 5% 22%	2527
	11th to 20th	Day	16% 16% 6% 12% 9% 8% 7% 4 5% 5% 12%	2530
	21st to end	Day	27% 20% 7% 10% 6% 4 4 2 4 4 12%	3093
February	1st to 10th	Day	14% 15% 4 11% 9% 6% 6% 3 5% 5% 22%	1710
	11th to 20th	Day	13% 13% 4 12% 8% 5% 5% 3 5% 6% 25%	1783
	21st to end	Day	14% 16% 4 11% 6% 5% 4 3 5% 6% 26%	1570
March	1st to 10th	Day	18% 15% 4 9% 5% 4 4 3 5% 6% 27%	3206
	11th to 20th	Day	13% 11% 4 8% 6% 5% 5% 4 5% 7% 31%	3480
	21st to end	Day	20% 12% 4 9% 5% 5% 4 4 5% 5% 28%	3815
April	1st to 10th	Day	15% 13% 4 7% 4 4 5% 4 6% 8% 31%	2199
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 5% 4 7% 34%	2416
	21st to end	Day	18% 9% 3 7% 4 4 5% 3 5% 7% 34%	2470
May	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 5% 6% 36%	4082
	11th to 20th	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 35%	4081
	21st to end	Day	11% 5% 2 6% 4 4 4 3 5% 8% 45%	4476
June	1st to 10th	Day	10% 5% 3 7% 5% 5% 4 5% 6% 9% 40%	2737
	11th to 20th	Day	10% 6% 2 7% 5% 4 6% 5% 6% 8% 41%	2850
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 36%	2725
July	1st to 10th	Day	10% 5% 3 8% 5% 5% 6% 5% 7% 9% 38%	4440
	11th to 20th	Day	8% 4 2 5% 4 5% 5% 4 7% 10% 45%	4440
	21st to end	Day	7% 5% 2 6% 6% 6% 6% 5% 7% 9% 41%	4529
August	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 5% 6% 8% 41%	4104
	11th to 20th	Day	10% 5% 2 6% 5% 6% 6% 5% 8% 8% 38%	4044
	21st to end	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 9% 38%	4507
September	1st to 10th	Day	11% 7% 2 6% 4 5% 6% 4 6% 8% 40%	2142
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 4 7% 8% 47%	2090
	21st to end	Day	15% 6% 2 8% 5% 5% 5% 4 6% 8% 36%	2090
October	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 3 5% 6% 42%	2029
	11th to 20th	Day	18% 10% 3 6% 5% 4 5% 3 5% 8% 34%	1900
	21st to end	Day	20% 11% 4 6% 5% 4 5% 3 5% 7% 29%	1943
November	1st to 10th	Day	11% 10% 3 7% 5% 4 5% 4 6% 7% 37%	1762
	11th to 20th	Day	15% 11% 4 8% 7% 5% 5% 4 5% 6% 30%	1800
	21st to end	Day	18% 13% 6% 9% 4 5% 4 3 4 6% 28%	1670
December	1st to 10th	Day	14% 16% 6% 12% 6% 5% 4 4 5% 6% 22%	1528
	11th to 20th	Day	18% 15% 5% 10% 5% 4 4 5% 5% 6% 24%	1769
	21st to end	Day	27% 20% 7% 12% 5% 4 4 3 4 5% 11%	2691
January	1st to 10th	Night	16% 15% 6% 11% 5% 4 4 3 4 5% 25%	4428
	11th to 20th	Night	14% 18% 7% 14% 6% 5% 6% 4 5% 5% 15%	4440
	21st to end	Night	23% 18% 8% 10% 5% 3 3 2 4 4 19%	4560
February	1st to 10th	Night	15% 16% 4 10% 8% 5% 5% 4 4 5% 24%	2470
	11th to 20th	Night	14% 12% 5% 10% 7% 5% 5% 4 5% 5% 27%	2279
	21st to end	Night	19% 12% 3 10% 5% 5% 3 3 5% 6% 30%	1884
March	1st to 10th	Night	15% 14% 4 8% 5% 4 3 3 5% 6% 35%	3646
	11th to 20th	Night	10% 8% 3 6% 5% 5% 6% 4 5% 8% 42%	3480
	21st to end	Night	15% 9% 3 9% 5% 4 4 4 4 5% 38%	3817
April	1st to 10th	Night	17% 10% 2 6% 5% 5% 5% 4 5% 6% 36%	1996
	11th to 20th	Night	18% 8% 3 5% 4 4 4 4 6% 6% 38%	1712
	21st to end	Night	17% 8% 3 6% 5% 4 4 4 5% 6% 40%	1710
May	1st to 10th	Night	19% 8% 2 7% 4 4 4 3 6% 7% 37%	2813
	11th to 20th	Night	16% 6% 2 7% 4 4 3 3 5% 7% 43%	2812
	21st to end	Night	10% 5% 1 5% 4 4 4 3 5% 8% 49%	3079
June	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 6% 8% 47%	1429
	11th to 20th	Night	10% 5% 2 6% 5% 4 4 4 7% 8% 45%	1330
	21st to end	Night	10% 6% 2 5% 4 4 3 3 6% 8% 49%	1447
July	1st to 10th	Night	9% 4 1 4 4 4 5% 4 6% 10% 49%	2530
	11th to 20th	Night	9% 4 1 4 3 3 3 4 5% 9% 54%	2530
	21st to end	Night	5% 4 2 5% 5% 5% 6% 6% 7% 8% 47%	3026
August	1st to 10th	Night	12% 5% 2 6% 3 4 3 3 6% 9% 47%	2825
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 5% 8% 48%	2785
	21st to end	Night	9% 5% 2 7% 4 4 4 3 5% 7% 49%	3146
September	1st to 10th	Night	10% 6% 3 5% 5% 4 4 4 5% 7% 48%	2019
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 7% 56%	2090
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 6% 44%	2090
October	1st to 10th	Night	14% 8% 3 5% 4 3 3 3 5% 6% 47%	2149
	11th to 20th	Night	19% 9% 3 6% 5% 4 4 3 5% 5% 37%	2280
	21st to end	Night	21% 9% 3 6% 5% 5% 5% 3 5% 6% 32%	2650
November	1st to 10th	Night	13% 8% 3 6% 4 4 4 4 5% 7% 44%	2536
	11th to 20th	Night	16% 11% 3 8% 5% 5% 5% 5% 6% 6% 31%	2600
	21st to end	Night	19% 14% 4 8% 4 3 4 4 5% 6% 28%	2726
December	1st to 10th	Night	17% 16% 6% 10% 5% 5% 5% 4 5% 5% 23%	2686
	11th to 20th	Night	15% 16% 5% 10% 6% 5% 5% 3 5% 6% 25%	3424
	21st to end	Night	29% 22% 6% 9% 4 4 4 2 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% 7% 11% 12% 6%	2528
	11th to 20th	Day	70% 9% 11% 6% 3	2530
	21st to end	Day	76% 6% 8% 6% 3 1	3093
February	1st to 10th	Day	63% 7% 9% 13% 7%	1710
	11th to 20th	Day	60% 5% 9% 16% 8% 1	1792
	21st to end	Day	59% 6% 11% 16% 7% 1	1570
March	1st to 10th	Day	58% 7% 12% 18% 5% 1	3206
	11th to 20th	Day	50% 8% 14% 19% 8% 2	3480
	21st to end	Day	56% 6% 13% 14% 9% 2	3815
April	1st to 10th	Day	51% 7% 12% 19% 11% 1	2199
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2416
	21st to end	Day	50% 6% 12% 17% 13% 1	2470
May	1st to 10th	Day	46% 7% 12% 23% 10% 2	4120
	11th to 20th	Day	44% 10% 13% 22% 9% 1	4085
	21st to end	Day	36% 7% 11% 18% 22% 6%	4489
June	1st to 10th	Day	38% 8% 12% 26% 13% 3	2737
	11th to 20th	Day	38% 8% 12% 23% 16% 2	2850
	21st to end	Day	40% 9% 12% 22% 15% 2	2725
July	1st to 10th	Day	41% 7% 14% 20% 16% 2	4371
	11th to 20th	Day	31% 7% 14% 31% 16% 1	4365
	21st to end	Day	36% 9% 14% 16% 22% 2	4386
August	1st to 10th	Day	37% 8% 13% 25% 15% 1	4091
	11th to 20th	Day	38% 10% 11% 30% 11% 1	4083
	21st to end	Day	40% 8% 12% 27% 13% 1	4506
September	1st to 10th	Day	39% 8% 15% 23% 13% 2	2142
	11th to 20th	Day	32% 8% 12% 32% 15% 1	2090
	21st to end	Day	44% 8% 13% 23% 12%	2090
October	1st to 10th	Day	32% 5% 12% 30% 20% 1	3367
	11th to 20th	Day	40% 5% 12% 25% 18%	3140
	21st to end	Day	45% 5% 14% 23% 12% 1	3180
November	1st to 10th	Day	45% 7% 13% 23% 12% 1	1762
	11th to 20th	Day	54% 5% 13% 18% 9% 1	1800
	21st to end	Day	57% 6% 12% 17% 7% 1	1670
December	1st to 10th	Day	63% 6% 12% 14% 4	1528
	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	61% 6% 12% 15% 5% 1	4428
	11th to 20th	Night	69% 7% 11% 9% 4	4440
	21st to end	Night	70% 5% 10% 12% 3	4560
February	1st to 10th	Night	61% 7% 10% 14% 7% 1	2470
	11th to 20th	Night	57% 6% 10% 18% 9% 1	2281
	21st to end	Night	55% 5% 13% 19% 8% 1	1884
March	1st to 10th	Night	51% 5% 13% 24% 7%	3646
	11th to 20th	Night	38% 8% 15% 20% 18% 1	3480
	21st to end	Night	48% 6% 11% 22% 11% 1	3817
April	1st to 10th	Night	48% 6% 10% 21% 12% 2	1996
	11th to 20th	Night	45% 6% 11% 18% 17% 2	1712
	21st to end	Night	44% 7% 10% 19% 16% 3	1710
May	1st to 10th	Night	45% 7% 11% 21% 13% 3	2840
	11th to 20th	Night	40% 7% 11% 27% 12% 2	2813
	21st to end	Night	32% 9% 11% 21% 21% 6%	3087
June	1st to 10th	Night	34% 7% 12% 28% 17% 3	1429
	11th to 20th	Night	34% 9% 13% 24% 18% 2	1330
	21st to end	Night	33% 6% 12% 27% 20% 1	1447
July	1st to 10th	Night	30% 5% 13% 31% 15% 6%	2490
	11th to 20th	Night	28% 6% 13% 34% 19% 2	2482
	21st to end	Night	30% 10% 17% 19% 21% 3	2929
August	1st to 10th	Night	34% 7% 13% 28% 16% 1	2814
	11th to 20th	Night	33% 7% 11% 36% 12% 2	2812
	21st to end	Night	35% 7% 12% 32% 14% 1	3146
September	1st to 10th	Night	34% 8% 13% 26% 17% 2	2019
	11th to 20th	Night	28% 6% 12% 31% 21% 2	2090
	21st to end	Night	40% 6% 11% 29% 13% 1	2090
October	1st to 10th	Night	27% 4 10% 31% 27% 1	3591
	11th to 20th	Night	43% 4 12% 23% 18%	3779
	21st to end	Night	42% 4 12% 28% 13% 1	4363
November	1st to 10th	Night	41% 6% 12% 26% 14% 1	2536
	11th to 20th	Night	51% 6% 14% 19% 9% 1	2600
	21st to end	Night	56% 6% 12% 19% 6%	2726
December	1st to 10th	Night	62% 7% 11% 14% 5%	2686
	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	24% 19% 41% 14% 1 1	270
	21st to end	Day	28% 15% 37% 16% 4	297
February	1st to 10th	Day	23% 23% 36% 13% 3 1	270
	11th to 20th	Day	24% 20% 39% 17% 1	270
	21st to end	Day	22% 31% 37% 8% 2	225
March	1st to 10th	Day	27% 21% 34% 15% 3	317
	11th to 20th	Day	23% 13% 37% 21% 5% 1	460
	21st to end	Day	28% 16% 32% 17% 5% 1	506
April	1st to 10th	Day	19% 19% 35% 18% 8% 1	460
	11th to 20th	Day	26% 19% 30% 14% 7% 4	452
	21st to end	Day	24% 16% 31% 18% 8% 3	460
May	1st to 10th	Day	23% 16% 29% 21% 9% 3	463
	11th to 20th	Day	20% 11% 29% 22% 12% 5%	487
	21st to end	Day	13% 8% 26% 28% 17% 7%	529
June	1st to 10th	Day	12% 11% 32% 29% 13% 3	657
	11th to 20th	Day	13% 10% 33% 27% 15% 2	680
	21st to end	Day	12% 11% 31% 29% 13% 4	680
July	1st to 10th	Day	8% 8% 24% 30% 20% 10%	671
	11th to 20th	Day	12% 7% 29% 27% 18% 7%	668
	21st to end	Day	9% 5% 27% 30% 22% 8%	569
August	1st to 10th	Day	10% 5% 23% 27% 22% 13%	486
	11th to 20th	Day	11% 5% 23% 29% 20% 11%	479
	21st to end	Day	11% 5% 24% 27% 22% 10%	539
September	1st to 10th	Day	13% 7% 26% 25% 21% 10%	490
	11th to 20th	Day	11% 3 22% 25% 27% 12%	490
	21st to end	Day	17% 11% 27% 22% 17% 7%	490
October	1st to 10th	Day	18% 10% 29% 20% 16% 7%	490
	11th to 20th	Day	21% 16% 32% 22% 8% 1	490
	21st to end	Day	26% 19% 37% 13% 5% 1	392
November	1st to 10th	Day	15% 25% 35% 17% 7% 1	300
	11th to 20th	Day	24% 24% 30% 16% 5% 1	300
	21st to end	Day	24% 22% 35% 14% 5%	235
December	1st to 10th	Day	23% 24% 32% 13% 8%	182
	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	25% 15% 33% 22% 4 1	950
	21st to end	Night	21% 13% 34% 27% 5%	851
February	1st to 10th	Night	23% 14% 37% 23% 3	760
	11th to 20th	Night	21% 16% 33% 26% 3	743
	21st to end	Night	25% 16% 37% 20% 2	628
March	1st to 10th	Night	20% 14% 34% 27% 6%	760
	11th to 20th	Night	18% 15% 38% 23% 5% 1	760
	21st to end	Night	22% 15% 30% 26% 7% 1	836
April	1st to 10th	Night	21% 15% 35% 26% 4	666
	11th to 20th	Night	22% 11% 34% 25% 7% 1	563
	21st to end	Night	20% 11% 32% 30% 8%	570
May	1st to 10th	Night	20% 6% 32% 31% 9% 1	570
	11th to 20th	Night	23% 5% 27% 33% 12% 1	561
	21st to end	Night	11% 4 26% 39% 18% 2	617
June	1st to 10th	Night	12% 5% 27% 42% 14%	461
	11th to 20th	Night	11% 4 26% 47% 11%	380
	21st to end	Night	9% 6% 26% 41% 18% 1	497
July	1st to 10th	Night	7% 3 21% 41% 25% 4	555
	11th to 20th	Night	9% 3 23% 39% 23% 4	552
	21st to end	Night	6% 4 21% 46% 22% 2	624
August	1st to 10th	Night	8% 4 20% 38% 29% 2	567
	11th to 20th	Night	9% 2 21% 37% 28% 4	564
	21st to end	Night	11% 4 19% 33% 31% 3	627
September	1st to 10th	Night	12% 6% 21% 33% 23% 5%	689
	11th to 20th	Night	11% 5% 18% 34% 27% 5%	760
	21st to end	Night	17% 7% 23% 29% 20% 5%	760
October	1st to 10th	Night	17% 8% 27% 28% 17% 3	760
	11th to 20th	Night	23% 14% 29% 24% 9% 1	760
	21st to end	Night	25% 14% 32% 24% 5%	836
November	1st to 10th	Night	18% 12% 31% 25% 12% 1	780
	11th to 20th	Night	22% 16% 34% 22% 6% 1	800
	21st to end	Night	25% 17% 34% 19% 5%	800
December	1st to 10th	Night	24% 16% 35% 18% 6%	768
	11th to 20th	Night	23% 16% 37% 20% 3	852
	21st to end	Night	31% 14% 34% 19% 2	1045

Page generated at 2024-11-29 10:43:09. Version ac07

Copyright © 2024 Attilla Danko.