Collins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collins Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	17% 8% 3 7% 4 4 4 4 6% 7% 36%	142928
	Night	18% 9% 3 7% 4 4 4 4 5% 7% 36%	138985
Transparency	Day	45% 6% 12% 21% 13% 2	146437
	Night	46% 6% 12% 21% 13% 2	143566
Seeing	Day	24% 16% 31% 19% 8% 2	17724
	Night	22% 12% 33% 26% 7%	26202

By Month

Percentage Cloud Cover NE (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% 9% 4 9% 6% 5% 5% 4 6% 6% 35%	4661
	11th to 20th	Day	15% 11% 4 8% 4 4 3 3 4 5% 39%	4650
	21st to end	Day	10% 9% 4 7% 5% 5% 4 3 5% 4 45%	5124
February	1st to 10th	Day	16% 10% 3 6% 4 5% 4 4 6% 6% 36%	1860
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 4 6% 6% 35%	2006
	21st to end	Day	14% 11% 3 8% 5% 5% 4 3 5% 6% 36%	1730
March	1st to 10th	Day	23% 12% 3 10% 5% 4 3 3 4 4 30%	4576
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 5% 5% 34%	5037
	21st to end	Day	27% 9% 3 6% 3 3 3 3 5% 7% 31%	5673
April	1st to 10th	Day	20% 12% 3 6% 5% 3 3 3 5% 5% 34%	2520
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 6% 7% 37%	2634
	21st to end	Day	22% 9% 3 6% 5% 3 4 4 5% 5% 34%	2730
May	1st to 10th	Day	28% 10% 3 7% 5% 4 4 3 5% 5% 26%	7639
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 5% 8% 32%	7739
	21st to end	Day	18% 7% 3 7% 5% 4 5% 4 6% 8% 33%	9060
June	1st to 10th	Day	22% 8% 2 6% 4 5% 5% 4 6% 8% 30%	3113
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 6% 7% 38%	3150
	21st to end	Day	23% 7% 2 6% 5% 4 4 4 6% 7% 31%	3096
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 8% 10% 32%	8280
	11th to 20th	Day	12% 5% 2 5% 4 5% 6% 5% 6% 9% 41%	8280
	21st to end	Day	14% 6% 2 5% 4 4 4 4 6% 9% 43%	8993
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 4 6% 8% 35%	4379
	11th to 20th	Day	15% 9% 3 8% 4 5% 4 4 6% 7% 35%	4227
	21st to end	Day	11% 6% 2 5% 3 4 4 4 7% 8% 45%	4553
September	1st to 10th	Day	11% 6% 1 6% 4 4 5% 4 5% 7% 48%	2338
	11th to 20th	Day	12% 5% 2 5% 3 4 3 3 5% 6% 53%	2200
	21st to end	Day	17% 5% 2 5% 3 4 4 3 5% 8% 44%	2200
October	1st to 10th	Day	18% 7% 2 7% 4 4 4 3 5% 5% 41%	2185
	11th to 20th	Day	20% 6% 2 6% 4 4 4 3 5% 7% 40%	1931
	21st to end	Day	23% 7% 3 7% 4 4 3 3 4 5% 36%	1980
November	1st to 10th	Day	17% 7% 2 5% 5% 4 4 4 5% 6% 41%	1851
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 6% 6% 41%	1890
	21st to end	Day	21% 8% 3 5% 4 3 3 2 3 6% 41%	1743
December	1st to 10th	Day	17% 10% 2 6% 4 5% 4 4 6% 7% 36%	2389
	11th to 20th	Day	19% 7% 2 5% 3 3 3 3 4 6% 44%	2688
	21st to end	Day	21% 11% 3 6% 4 4 5% 3 4 5% 33%	3823
January	1st to 10th	Night	12% 10% 4 9% 5% 5% 4 3 5% 6% 37%	8234
	11th to 20th	Night	14% 12% 4 10% 5% 5% 5% 5% 6% 7% 27%	8235
	21st to end	Night	11% 9% 3 7% 5% 5% 5% 4 5% 6% 39%	9057
February	1st to 10th	Night	15% 9% 4 8% 6% 4 5% 4 6% 6% 34%	2758
	11th to 20th	Night	12% 10% 3 8% 5% 5% 5% 3 5% 5% 39%	2495
	21st to end	Night	13% 11% 3 7% 5% 5% 4 4 4 5% 39%	2076
March	1st to 10th	Night	26% 11% 3 7% 5% 5% 3 3 4 4 29%	5484
	11th to 20th	Night	17% 11% 4 7% 4 4 5% 4 6% 7% 32%	5141
	21st to end	Night	25% 10% 2 6% 4 4 4 4 6% 6% 30%	5499
April	1st to 10th	Night	21% 13% 3 6% 5% 3 4 3 5% 4 34%	2100
	11th to 20th	Night	20% 7% 2 6% 6% 4 5% 4 5% 6% 35%	1935
	21st to end	Night	24% 9% 2 7% 4 4 4 3 5% 5% 34%	1890
May	1st to 10th	Night	29% 10% 3 5% 3 3 3 3 4 6% 29%	5259
	11th to 20th	Night	20% 8% 3 6% 4 4 5% 4 6% 7% 33%	5176
	21st to end	Night	17% 7% 3 6% 5% 4 5% 4 6% 8% 36%	5114
June	1st to 10th	Night	20% 8% 3 7% 4 5% 4 3 6% 8% 33%	1502
	11th to 20th	Night	15% 7% 2 6% 5% 5% 4 3 6% 9% 37%	1470
	21st to end	Night	22% 9% 2 6% 4 4 4 4 6% 8% 31%	1516
July	1st to 10th	Night	11% 8% 3 8% 5% 5% 6% 4 7% 8% 35%	4680
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 3 6% 8% 40%	4680
	21st to end	Night	14% 5% 2 5% 4 3 4 4 6% 9% 46%	5084
August	1st to 10th	Night	19% 8% 3 8% 5% 5% 6% 4 5% 6% 31%	2956
	11th to 20th	Night	15% 9% 3 6% 4 5% 5% 5% 6% 7% 35%	2909
	21st to end	Night	11% 5% 2 7% 4 5% 5% 3 5% 7% 45%	3341
September	1st to 10th	Night	14% 5% 1 5% 3 3 5% 3 5% 7% 49%	2060
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 52%	2200
	21st to end	Night	16% 7% 1 5% 4 4 4 3 5% 6% 44%	2200
October	1st to 10th	Night	18% 6% 3 6% 4 5% 4 3 4 7% 41%	2215
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 5% 7% 40%	2465
	21st to end	Night	24% 8% 2 6% 4 3 4 3 4 6% 35%	2860
November	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 5% 6% 42%	2667
	11th to 20th	Night	17% 6% 2 7% 5% 4 4 4 4 6% 42%	2730
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 5% 7% 38%	2873
December	1st to 10th	Night	17% 10% 3 7% 4 4 5% 4 6% 7% 34%	5016
	11th to 20th	Night	24% 7% 2 5% 4 4 4 3 4 6% 38%	5741
	21st to end	Night	21% 11% 3 5% 3 4 4 3 5% 6% 36%	7367

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	48% 5% 12% 23% 11% 1	4662
	11th to 20th	Day	47% 4 10% 26% 11% 2	4680
	21st to end	Day	41% 4 10% 20% 19% 5%	5124
February	1st to 10th	Day	47% 6% 10% 20% 14% 3	1860
	11th to 20th	Day	48% 6% 12% 19% 13% 3	2015
	21st to end	Day	48% 6% 10% 19% 12% 5%	1730
March	1st to 10th	Day	59% 4 7% 10% 14% 6%	4579
	11th to 20th	Day	50% 6% 14% 18% 11% 2	5037
	21st to end	Day	53% 4 6% 17% 16% 4	5673
April	1st to 10th	Day	52% 5% 8% 16% 15% 4	2520
	11th to 20th	Day	45% 7% 9% 22% 15% 2	2634
	21st to end	Day	51% 5% 8% 19% 13% 4	2730
May	1st to 10th	Day	58% 7% 11% 15% 7% 1	7651
	11th to 20th	Day	48% 8% 15% 19% 9% 2	7743
	21st to end	Day	46% 8% 11% 20% 13% 1	9061
June	1st to 10th	Day	50% 7% 12% 21% 10%	3113
	11th to 20th	Day	44% 7% 12% 20% 15% 2	3150
	21st to end	Day	50% 7% 12% 18% 10% 1	3096
July	1st to 10th	Day	43% 8% 14% 24% 10%	8210
	11th to 20th	Day	36% 8% 14% 25% 16% 1	8205
	21st to end	Day	37% 6% 12% 29% 15% 1	8795
August	1st to 10th	Day	45% 8% 14% 26% 8%	4365
	11th to 20th	Day	46% 8% 16% 22% 8%	4266
	21st to end	Day	33% 8% 15% 25% 18% 2	4553
September	1st to 10th	Day	34% 7% 14% 27% 18% 1	2338
	11th to 20th	Day	32% 5% 10% 29% 22% 2	2200
	21st to end	Day	38% 6% 12% 27% 17% 1	2200
October	1st to 10th	Day	49% 7% 11% 20% 12% 1	3562
	11th to 20th	Day	36% 5% 12% 29% 18%	3121
	21st to end	Day	38% 4 13% 24% 19% 3	3180
November	1st to 10th	Day	43% 5% 14% 21% 15% 2	1851
	11th to 20th	Day	42% 6% 11% 21% 17% 3	1890
	21st to end	Day	47% 4 15% 23% 10% 1	1743
December	1st to 10th	Day	46% 5% 16% 20% 12% 1	2389
	11th to 20th	Day	42% 5% 13% 18% 16% 6%	2688
	21st to end	Day	53% 5% 8% 18% 13% 2	3823
January	1st to 10th	Night	47% 5% 12% 23% 12% 1	8234
	11th to 20th	Night	54% 6% 12% 16% 10% 2	8280
	21st to end	Night	44% 5% 8% 22% 17% 5%	9057
February	1st to 10th	Night	49% 6% 11% 19% 12% 3	2758
	11th to 20th	Night	46% 5% 11% 20% 15% 3	2497
	21st to end	Night	47% 5% 9% 20% 15% 4	2076
March	1st to 10th	Night	58% 4 7% 12% 13% 6%	5494
	11th to 20th	Night	49% 7% 17% 18% 8% 2	5142
	21st to end	Night	54% 4 7% 15% 17% 3	5499
April	1st to 10th	Night	53% 5% 8% 16% 15% 3	2100
	11th to 20th	Night	48% 5% 10% 20% 15% 1	1935
	21st to end	Night	51% 5% 11% 16% 14% 3	1890
May	1st to 10th	Night	55% 6% 14% 14% 11% 1	5265
	11th to 20th	Night	48% 7% 13% 23% 8% 1	5177
	21st to end	Night	44% 8% 14% 22% 11% 1	5115
June	1st to 10th	Night	51% 5% 12% 23% 9% 1	1502
	11th to 20th	Night	41% 8% 15% 22% 12% 2	1470
	21st to end	Night	49% 8% 14% 18% 9% 2	1516
July	1st to 10th	Night	44% 8% 15% 24% 9%	4640
	11th to 20th	Night	40% 7% 14% 27% 12%	4632
	21st to end	Night	35% 6% 16% 31% 12%	4967
August	1st to 10th	Night	52% 8% 13% 20% 6%	2946
	11th to 20th	Night	44% 9% 17% 19% 10%	2936
	21st to end	Night	36% 8% 15% 24% 16% 2	3341
September	1st to 10th	Night	33% 7% 14% 29% 16%	2060
	11th to 20th	Night	33% 6% 10% 30% 19% 2	2200
	21st to end	Night	40% 6% 9% 28% 14% 2	2200
October	1st to 10th	Night	43% 6% 15% 22% 13% 1	3618
	11th to 20th	Night	35% 6% 11% 29% 18% 1	4014
	21st to end	Night	39% 5% 12% 27% 15% 2	4610
November	1st to 10th	Night	43% 5% 12% 24% 14% 1	2667
	11th to 20th	Night	43% 5% 11% 23% 16% 2	2730
	21st to end	Night	46% 5% 15% 19% 13% 1	2873
December	1st to 10th	Night	47% 6% 11% 25% 10% 1	5016
	11th to 20th	Night	48% 4 10% 18% 18% 2	5742
	21st to end	Night	50% 6% 10% 17% 16% 1	7367

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	29% 21% 27% 18% 5%	220
	11th to 20th	Day	32% 22% 27% 15% 3 1	220
	21st to end	Day	19% 28% 32% 19% 2	242
February	1st to 10th	Day	28% 19% 29% 21% 4	400
	11th to 20th	Day	26% 23% 35% 13% 3	514
	21st to end	Day	23% 22% 35% 16% 4	446
March	1st to 10th	Day	31% 20% 31% 14% 3	540
	11th to 20th	Day	28% 24% 32% 13% 3 1	540
	21st to end	Day	32% 21% 27% 13% 5% 2	594
April	1st to 10th	Day	36% 20% 28% 10% 4 1	540
	11th to 20th	Day	31% 24% 27% 12% 5% 1	532
	21st to end	Day	30% 21% 31% 12% 5% 2	540
May	1st to 10th	Day	37% 16% 28% 16% 2 1	543
	11th to 20th	Day	24% 19% 33% 17% 5% 2	567
	21st to end	Day	22% 19% 37% 17% 6%	617
June	1st to 10th	Day	24% 14% 31% 21% 9% 1	743
	11th to 20th	Day	22% 14% 31% 20% 12% 1	780
	21st to end	Day	25% 11% 29% 26% 6% 2	726
July	1st to 10th	Day	18% 12% 37% 25% 8% 1	565
	11th to 20th	Day	16% 10% 39% 21% 10% 3	564
	21st to end	Day	15% 10% 36% 25% 10% 4	620
August	1st to 10th	Day	12% 9% 33% 31% 13% 2	566
	11th to 20th	Day	15% 10% 29% 27% 15% 4	535
	21st to end	Day	13% 13% 32% 26% 11% 4	583
September	1st to 10th	Day	13% 12% 32% 25% 14% 3	530
	11th to 20th	Day	18% 10% 27% 22% 15% 8%	530
	21st to end	Day	17% 14% 31% 23% 12% 4	530
October	1st to 10th	Day	22% 14% 25% 23% 12% 3	522
	11th to 20th	Day	27% 14% 27% 19% 11% 2	420
	21st to end	Day	32% 12% 29% 19% 6% 1	462
November	1st to 10th	Day	27% 14% 31% 20% 7% 1	425
	11th to 20th	Day	29% 16% 30% 19% 5% 1	430
	21st to end	Day	29% 18% 27% 20% 5%	430
December	1st to 10th	Day	28% 18% 32% 16% 6%	246
	11th to 20th	Day	35% 20% 33% 9% 3	220
	21st to end	Day	35% 17% 26% 15% 7%	242
January	1st to 10th	Night	23% 14% 36% 24% 3	840
	11th to 20th	Night	25% 17% 34% 20% 4	840
	21st to end	Night	18% 17% 40% 21% 4	924
February	1st to 10th	Night	23% 16% 33% 25% 4	840
	11th to 20th	Night	21% 18% 39% 20% 2	823
	21st to end	Night	20% 15% 39% 23% 4	692
March	1st to 10th	Night	26% 16% 37% 20% 1	840
	11th to 20th	Night	24% 16% 34% 21% 5%	840
	21st to end	Night	25% 17% 32% 21% 4	924
April	1st to 10th	Night	30% 18% 30% 19% 3	840
	11th to 20th	Night	25% 15% 36% 20% 4	691
	21st to end	Night	31% 13% 33% 20% 3	630
May	1st to 10th	Night	27% 13% 31% 25% 4	630
	11th to 20th	Night	22% 12% 37% 23% 4	602
	21st to end	Night	21% 8% 35% 32% 4	455
June	1st to 10th	Night	24% 8% 32% 30% 7%	420
	11th to 20th	Night	16% 8% 30% 36% 9%	420
	21st to end	Night	25% 4 30% 35% 5%	420
July	1st to 10th	Night	17% 4 32% 37% 10%	410
	11th to 20th	Night	15% 6% 28% 39% 11%	408
	21st to end	Night	16% 3 28% 39% 14% 1	460
August	1st to 10th	Night	14% 5% 29% 40% 12%	591
	11th to 20th	Night	15% 7% 28% 37% 13% 1	605
	21st to end	Night	14% 8% 26% 37% 13% 1	726
September	1st to 10th	Night	16% 5% 34% 33% 13% 1	800
	11th to 20th	Night	16% 7% 27% 31% 17% 3	800
	21st to end	Night	20% 8% 30% 28% 12% 2	800
October	1st to 10th	Night	21% 9% 25% 30% 14%	800
	11th to 20th	Night	20% 10% 33% 26% 9% 1	800
	21st to end	Night	29% 13% 30% 23% 5%	880
November	1st to 10th	Night	24% 11% 32% 24% 7% 1	820
	11th to 20th	Night	22% 14% 32% 24% 7% 1	840
	21st to end	Night	24% 16% 33% 21% 6%	915
December	1st to 10th	Night	28% 12% 34% 21% 5%	950
	11th to 20th	Night	26% 17% 35% 19% 3	950
	21st to end	Night	28% 14% 35% 20% 3	976

Page generated at 2026-08-12 03:37:14. Version ac07

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