Collyer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collyer clear sky chart from 2005-11-06 to 2025-11-18.

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	7% 5% 2 5% 3 3 4 3 5% 7% 56%	135512
	Night	7% 5% 2 5% 3 3 4 3 5% 7% 55%	132188
Transparency	Day	28% 3 11% 27% 24% 7%	130376
	Night	29% 3 11% 26% 23% 8%	128344
Seeing	Day	11% 8% 29% 33% 16% 3	17049
	Night	10% 8% 34% 37% 10% 1	26938

By Month

Percentage Cloud Cover SW (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	8% 9% 3 6% 4 3 4 3 5% 7% 49%	3638
	11th to 20th	Day	5% 7% 3 9% 5% 4 4 3 4 6% 50%	3749
	21st to end	Day	4 7% 2 6% 4 3 3 3 3 6% 59%	4488
February	1st to 10th	Day	7% 8% 4 7% 5% 4 3 3 5% 7% 48%	1959
	11th to 20th	Day	5% 7% 3 7% 5% 4 5% 4 4 7% 49%	1949
	21st to end	Day	5% 6% 2 5% 3 4 4 3 5% 6% 58%	1650
March	1st to 10th	Day	6% 7% 2 4 3 3 3 3 4 5% 60%	4500
	11th to 20th	Day	6% 5% 2 5% 3 3 4 3 4 6% 59%	4936
	21st to end	Day	6% 6% 1 4 3 4 4 4 5% 7% 56%	5464
April	1st to 10th	Day	9% 5% 1 5% 3 3 3 3 5% 6% 57%	2393
	11th to 20th	Day	9% 7% 2 5% 4 3 4 4 6% 7% 49%	2386
	21st to end	Day	13% 7% 2 4 4 3 4 4 5% 7% 47%	2600
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 7% 51%	5900
	11th to 20th	Day	8% 3 1 4 3 3 3 3 5% 7% 59%	5862
	21st to end	Day	17% 6% 2 5% 4 3 4 3 5% 7% 44%	6446
June	1st to 10th	Day	5% 3 1 4 4 3 3 4 5% 8% 60%	2671
	11th to 20th	Day	6% 4 2 4 4 3 3 3 5% 8% 59%	2800
	21st to end	Day	4 4 1 4 4 3 4 3 6% 9% 58%	2800
July	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 6% 9% 53%	6339
	11th to 20th	Day	5% 3 1 3 3 3 4 3 6% 9% 59%	6265
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 10% 56%	6424
August	1st to 10th	Day	12% 5% 2 5% 3 4 3 4 6% 8% 49%	5801
	11th to 20th	Day	6% 4 1 5% 4 4 4 4 6% 9% 53%	5697
	21st to end	Day	14% 5% 2 5% 3 3 4 3 5% 7% 49%	5984
September	1st to 10th	Day	8% 4 1 4 3 3 3 3 4 6% 59%	2328
	11th to 20th	Day	7% 5% 1 4 3 3 3 3 5% 7% 60%	2200
	21st to end	Day	7% 4 2 4 3 2 3 3 4 6% 63%	2200
October	1st to 10th	Day	8% 2 1 2 2 2 2 2 3 5% 71%	3590
	11th to 20th	Day	5% 3 1 3 3 2 4 4 5% 6% 64%	3349
	21st to end	Day	7% 5% 1 3 2 2 2 2 4 6% 66%	3222
November	1st to 10th	Day	6% 4 1 4 3 3 3 3 4 7% 63%	1853
	11th to 20th	Day	5% 5% 2 4 3 2 3 2 4 6% 65%	1863
	21st to end	Day	9% 5% 2 3 3 3 3 2 4 5% 61%	1800
December	1st to 10th	Day	3 5% 1 4 3 2 2 2 3 6% 68%	2940
	11th to 20th	Day	4 6% 2 4 2 2 3 2 4 6% 65%	3391
	21st to end	Day	8% 12% 4 8% 4 3 4 3 4 6% 43%	4075
January	1st to 10th	Night	7% 8% 3 9% 5% 4 4 2 5% 6% 48%	6327
	11th to 20th	Night	3 5% 2 6% 4 4 3 3 5% 7% 59%	6226
	21st to end	Night	6% 7% 3 6% 3 3 3 3 4 5% 56%	6490
February	1st to 10th	Night	8% 9% 3 7% 4 4 4 3 5% 6% 48%	2459
	11th to 20th	Night	5% 7% 3 8% 5% 4 4 4 6% 8% 48%	2338
	21st to end	Night	5% 6% 2 6% 3 3 3 3 4 6% 58%	1980
March	1st to 10th	Night	7% 5% 1 3 2 2 3 3 4 5% 64%	5374
	11th to 20th	Night	6% 4 1 4 3 3 5% 3 6% 7% 58%	5026
	21st to end	Night	7% 6% 2 7% 4 5% 4 4 7% 9% 46%	5467
April	1st to 10th	Night	8% 6% 2 5% 4 4 3 4 6% 8% 52%	2004
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 51%	1962
	21st to end	Night	12% 5% 2 5% 2 4 5% 3 6% 7% 49%	1800
May	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	4080
	11th to 20th	Night	7% 4 2 4 3 3 4 3 4 7% 58%	4053
	21st to end	Night	17% 6% 2 4 3 3 4 4 6% 8% 44%	4451
June	1st to 10th	Night	5% 3 1 4 3 4 5% 4 7% 11% 54%	1723
	11th to 20th	Night	6% 3 1 5% 4 3 4 3 8% 10% 54%	1600
	21st to end	Night	5% 4 2 4 4 3 4 4 6% 11% 53%	1600
July	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 6% 9% 50%	3640
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 5% 7% 10% 43%	3701
	21st to end	Night	5% 3 2 4 3 4 4 4 7% 11% 53%	4449
August	1st to 10th	Night	9% 4 2 5% 4 4 5% 5% 8% 10% 44%	4011
	11th to 20th	Night	7% 4 2 5% 4 3 5% 4 7% 10% 49%	4135
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 7% 9% 42%	4993
September	1st to 10th	Night	6% 4 2 4 3 2 3 3 5% 7% 62%	2070
	11th to 20th	Night	7% 3 1 3 3 3 4 3 6% 6% 61%	2200
	21st to end	Night	8% 4 2 4 2 2 3 3 4 5% 63%	2200
October	1st to 10th	Night	6% 3 1 3 2 2 2 2 3 4 73%	3590
	11th to 20th	Night	5% 3 1 3 2 2 2 2 4 6% 68%	3757
	21st to end	Night	6% 3 1 3 2 2 3 2 4 5% 68%	4573
November	1st to 10th	Night	5% 3 1 3 2 3 3 3 4 6% 66%	2665
	11th to 20th	Night	5% 5% 1 3 3 3 3 3 4 4 67%	2692
	21st to end	Night	7% 5% 2 4 3 3 3 2 5% 6% 61%	2600
December	1st to 10th	Night	3 4 2 4 3 3 3 2 3 5% 67%	4250
	11th to 20th	Night	4 3 1 4 3 3 4 3 5% 7% 64%	4816
	21st to end	Night	9% 12% 4 7% 4 3 3 3 4 6% 45%	6886

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	37% 2 12% 28% 18% 4	3639
	11th to 20th	Day	37% 3 9% 22% 25% 4	3749
	21st to end	Day	30% 2 11% 26% 25% 6%	4488
February	1st to 10th	Day	37% 3 12% 26% 17% 5%	1959
	11th to 20th	Day	36% 2 14% 25% 18% 6%	1959
	21st to end	Day	28% 3 8% 26% 25% 10%	1650
March	1st to 10th	Day	27% 2 6% 19% 30% 16%	4500
	11th to 20th	Day	26% 3 6% 21% 29% 15%	4936
	21st to end	Day	28% 3 16% 39% 13% 2	5464
April	1st to 10th	Day	28% 3 11% 25% 24% 7%	2393
	11th to 20th	Day	33% 4 11% 26% 20% 6%	2389
	21st to end	Day	37% 3 11% 28% 18% 3	2600
May	1st to 10th	Day	34% 3 12% 27% 17% 7%	5900
	11th to 20th	Day	25% 3 11% 25% 24% 12%	5866
	21st to end	Day	39% 4 11% 22% 20% 3	6447
June	1st to 10th	Day	22% 3 20% 34% 17% 4	2671
	11th to 20th	Day	25% 4 12% 28% 24% 8%	2800
	21st to end	Day	23% 3 10% 30% 26% 8%	2800
July	1st to 10th	Day	28% 4 9% 27% 26% 6%	6271
	11th to 20th	Day	22% 3 18% 28% 25% 4	6190
	21st to end	Day	24% 4 13% 31% 22% 6%	6240
August	1st to 10th	Day	33% 4 13% 27% 20% 3	4220
	11th to 20th	Day	30% 5% 14% 25% 21% 4	4125
	21st to end	Day	23% 4 13% 27% 24% 9%	4311
September	1st to 10th	Day	27% 3 9% 28% 23% 11%	2328
	11th to 20th	Day	25% 3 10% 28% 28% 7%	2200
	21st to end	Day	24% 3 8% 30% 28% 7%	2200
October	1st to 10th	Day	20% 2 5% 21% 40% 13%	3590
	11th to 20th	Day	22% 3 9% 32% 24% 11%	3349
	21st to end	Day	22% 2 6% 27% 37% 8%	3222
November	1st to 10th	Day	23% 2 8% 30% 28% 9%	1853
	11th to 20th	Day	22% 2 7% 30% 32% 7%	1863
	21st to end	Day	27% 2 9% 26% 29% 8%	1800
December	1st to 10th	Day	20% 1 11% 34% 25% 9%	2940
	11th to 20th	Day	23% 2 8% 42% 24% 1	3389
	21st to end	Day	43% 2 8% 25% 18% 4	4075
January	1st to 10th	Night	38% 2 12% 28% 17% 3	6327
	11th to 20th	Night	27% 2 10% 23% 29% 8%	6226
	21st to end	Night	32% 2 10% 24% 22% 10%	6490
February	1st to 10th	Night	38% 3 13% 22% 18% 7%	2459
	11th to 20th	Night	34% 4 12% 26% 21% 3	2338
	21st to end	Night	28% 3 6% 25% 26% 12%	1980
March	1st to 10th	Night	24% 2 5% 20% 32% 16%	5374
	11th to 20th	Night	26% 2 7% 20% 27% 18%	5026
	21st to end	Night	35% 3 14% 36% 10% 2	5467
April	1st to 10th	Night	30% 5% 10% 25% 25% 6%	2004
	11th to 20th	Night	33% 3 11% 27% 19% 6%	1962
	21st to end	Night	33% 4 12% 27% 21% 3	1800
May	1st to 10th	Night	36% 4 13% 25% 17% 6%	4080
	11th to 20th	Night	27% 2 11% 28% 22% 9%	4054
	21st to end	Night	39% 4 15% 22% 16% 4	4452
June	1st to 10th	Night	24% 4 18% 33% 17% 4	1723
	11th to 20th	Night	25% 5% 15% 27% 23% 5%	1600
	21st to end	Night	26% 4 11% 29% 23% 8%	1600
July	1st to 10th	Night	31% 4 17% 21% 24% 3	3600
	11th to 20th	Night	35% 4 16% 21% 20% 4	3653
	21st to end	Night	25% 4 17% 27% 21% 7%	4320
August	1st to 10th	Night	33% 4 14% 25% 22% 3	2916
	11th to 20th	Night	32% 4 18% 25% 17% 3	2996
	21st to end	Night	27% 5% 14% 27% 19% 8%	3597
September	1st to 10th	Night	23% 4 11% 26% 25% 11%	2070
	11th to 20th	Night	25% 3 9% 27% 28% 9%	2200
	21st to end	Night	25% 2 7% 25% 31% 10%	2200
October	1st to 10th	Night	18% 2 5% 18% 44% 13%	3590
	11th to 20th	Night	19% 2 11% 23% 32% 13%	3757
	21st to end	Night	21% 2 7% 25% 36% 10%	4573
November	1st to 10th	Night	20% 2 9% 29% 28% 12%	2665
	11th to 20th	Night	21% 2 7% 29% 31% 10%	2692
	21st to end	Night	26% 2 9% 26% 26% 11%	2600
December	1st to 10th	Night	22% 2 9% 37% 24% 7%	4250
	11th to 20th	Night	21% 2 7% 38% 27% 5%	4817
	21st to end	Night	42% 2 9% 29% 16% 2	6886

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	10% 6% 39% 32% 10% 2	300
	11th to 20th	Day	14% 12% 33% 30% 9% 1	300
	21st to end	Day	8% 7% 44% 30% 10% 1	330
February	1st to 10th	Day	13% 10% 41% 27% 8%	300
	11th to 20th	Day	12% 9% 45% 24% 9% 2	300
	21st to end	Day	6% 16% 52% 21% 4	249
March	1st to 10th	Day	11% 12% 39% 31% 7% 1	300
	11th to 20th	Day	15% 12% 36% 26% 10% 1	300
	21st to end	Day	15% 10% 45% 23% 7%	330
April	1st to 10th	Day	13% 20% 38% 22% 5% 1	493
	11th to 20th	Day	14% 17% 40% 22% 6%	507
	21st to end	Day	18% 15% 35% 22% 8% 1	700
May	1st to 10th	Day	14% 9% 33% 29% 13% 2	703
	11th to 20th	Day	16% 9% 29% 31% 12% 3	725
	21st to end	Day	15% 10% 32% 30% 12% 2	789
June	1st to 10th	Day	5% 6% 26% 35% 20% 7%	730
	11th to 20th	Day	8% 4 31% 38% 17% 2	730
	21st to end	Day	6% 4 31% 41% 16% 2	730
July	1st to 10th	Day	5% 3 19% 42% 25% 5%	721
	11th to 20th	Day	6% 1 18% 43% 28% 5%	718
	21st to end	Day	5% 2 11% 45% 31% 6%	794
August	1st to 10th	Day	9% 2 13% 42% 29% 5%	726
	11th to 20th	Day	6% 3 18% 44% 26% 4	669
	21st to end	Day	9% 2 18% 43% 21% 7%	583
September	1st to 10th	Day	10% 3 19% 45% 19% 4	458
	11th to 20th	Day	5% 4 25% 38% 21% 6%	330
	21st to end	Day	11% 6% 24% 33% 20% 6%	330
October	1st to 10th	Day	18% 5% 21% 32% 19% 5%	330
	11th to 20th	Day	5% 11% 42% 28% 11% 4	330
	21st to end	Day	15% 12% 32% 27% 11% 2	363
November	1st to 10th	Day	12% 10% 35% 30% 9% 4	330
	11th to 20th	Day	11% 9% 40% 26% 12% 2	321
	21st to end	Day	16% 9% 30% 25% 17% 3	300
December	1st to 10th	Day	8% 8% 36% 32% 13% 3	300
	11th to 20th	Day	11% 13% 37% 28% 10% 1	300
	21st to end	Day	15% 16% 34% 28% 8%	330
January	1st to 10th	Night	10% 8% 34% 40% 8%	1000
	11th to 20th	Night	9% 9% 37% 35% 8% 1	1000
	21st to end	Night	8% 8% 42% 36% 6%	1100
February	1st to 10th	Night	12% 9% 41% 31% 6%	859
	11th to 20th	Night	10% 11% 43% 31% 5%	784
	21st to end	Night	8% 13% 44% 30% 5%	660
March	1st to 10th	Night	11% 14% 36% 34% 5%	800
	11th to 20th	Night	12% 12% 35% 34% 7%	620
	21st to end	Night	14% 13% 41% 28% 5%	660
April	1st to 10th	Night	11% 13% 44% 29% 3	600
	11th to 20th	Night	15% 15% 41% 25% 4	593
	21st to end	Night	15% 14% 34% 29% 8%	600
May	1st to 10th	Night	14% 9% 36% 34% 8%	600
	11th to 20th	Night	13% 8% 34% 35% 9% 1	591
	21st to end	Night	14% 11% 36% 30% 8%	650
June	1st to 10th	Night	6% 5% 35% 39% 14% 1	600
	11th to 20th	Night	8% 7% 41% 35% 9%	600
	21st to end	Night	7% 6% 34% 44% 9% 1	600
July	1st to 10th	Night	8% 3 26% 48% 16%	585
	11th to 20th	Night	5% 2 23% 50% 19% 1	582
	21st to end	Night	6% 2 18% 49% 25% 1	657
August	1st to 10th	Night	9% 2 17% 51% 21% 1	597
	11th to 20th	Night	8% 2 22% 52% 16% 1	594
	21st to end	Night	5% 2 24% 47% 21% 1	660
September	1st to 10th	Night	7% 3 27% 48% 14% 1	600
	11th to 20th	Night	9% 5% 32% 40% 14% 1	600
	21st to end	Night	12% 6% 27% 40% 15% 1	600
October	1st to 10th	Night	13% 9% 30% 36% 14%	600
	11th to 20th	Night	9% 9% 36% 36% 10%	710
	21st to end	Night	13% 10% 31% 33% 12%	1075
November	1st to 10th	Night	8% 9% 32% 43% 9%	1025
	11th to 20th	Night	9% 9% 37% 37% 8%	1036
	21st to end	Night	10% 9% 32% 37% 12% 1	1000
December	1st to 10th	Night	9% 8% 36% 39% 8%	1000
	11th to 20th	Night	10% 11% 35% 35% 8% 1	1000
	21st to end	Night	13% 9% 35% 36% 7%	1100

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