Holcomb Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holcomb Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	18% 11% 4 9% 5% 5% 5% 4 5% 6% 29%	137623
	Night	18% 14% 5% 10% 6% 5% 4 3 4 5% 26%	134403
Transparency	Day	37% 36% 7% 14% 6%	141093
	Night	42% 35% 6% 11% 6%	139022
Seeing	Day	26% 17% 29% 19% 8% 2	16686
	Night	26% 14% 26% 24% 9% 1	25689

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% 18% 7% 17% 8% 5% 4 3 4 4 15%	4661
	11th to 20th	Day	11% 16% 8% 19% 9% 6% 5% 3 4 4 16%	4650
	21st to end	Day	11% 16% 6% 13% 9% 8% 6% 3 5% 5% 19%	5124
February	1st to 10th	Day	14% 14% 4 14% 9% 6% 5% 4 4 5% 21%	1848
	11th to 20th	Day	12% 17% 5% 14% 7% 6% 4 3 4 5% 22%	2007
	21st to end	Day	16% 15% 5% 11% 7% 6% 5% 3 3 5% 24%	1730
March	1st to 10th	Day	21% 14% 5% 12% 6% 5% 4 3 4 4 22%	4576
	11th to 20th	Day	15% 17% 4 9% 6% 5% 4 3 5% 5% 28%	4759
	21st to end	Day	21% 13% 4 9% 4 4 4 4 4 4 29%	6102
April	1st to 10th	Day	22% 14% 3 8% 4 4 3 3 4 5% 30%	2520
	11th to 20th	Day	19% 11% 3 7% 6% 5% 4 3 3 5% 34%	2494
	21st to end	Day	23% 11% 3 7% 4 4 4 3 3 6% 33%	2693
May	1st to 10th	Day	28% 10% 3 8% 5% 5% 4 4 5% 6% 22%	7877
	11th to 20th	Day	20% 8% 2 7% 5% 5% 5% 4 5% 8% 31%	8221
	21st to end	Day	20% 9% 2 7% 5% 4 5% 4 6% 7% 31%	9060
June	1st to 10th	Day	23% 7% 3 6% 4 4 5% 5% 6% 8% 28%	2940
	11th to 20th	Day	16% 6% 1 5% 5% 4 5% 4 7% 9% 36%	2870
	21st to end	Day	21% 7% 3 6% 5% 5% 5% 4 6% 8% 31%	2800
July	1st to 10th	Day	12% 6% 2 7% 5% 6% 6% 5% 8% 10% 33%	6360
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 9% 37%	6360
	21st to end	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 41%	6926
August	1st to 10th	Day	16% 7% 3 7% 4 4 5% 4 6% 8% 34%	4563
	11th to 20th	Day	15% 7% 3 8% 5% 5% 4 4 6% 8% 35%	4031
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 9% 42%	4312
September	1st to 10th	Day	15% 6% 2 6% 3 3 3 4 5% 7% 46%	2400
	11th to 20th	Day	16% 6% 2 4 3 3 4 3 5% 6% 50%	2288
	21st to end	Day	16% 6% 2 6% 5% 5% 4 4 5% 7% 40%	2200
October	1st to 10th	Day	23% 8% 3 7% 5% 4 3 3 5% 6% 34%	2110
	11th to 20th	Day	20% 10% 3 6% 4 4 4 4 4 7% 33%	1946
	21st to end	Day	26% 14% 3 7% 4 4 4 3 4 5% 26%	1980
November	1st to 10th	Day	18% 15% 4 7% 4 5% 4 3 5% 5% 29%	1851
	11th to 20th	Day	18% 17% 4 9% 5% 5% 4 3 5% 5% 25%	1890
	21st to end	Day	19% 17% 6% 9% 5% 4 4 3 5% 4 22%	1680
December	1st to 10th	Day	17% 18% 8% 17% 8% 6% 4 3 4 4 11%	2680
	11th to 20th	Day	20% 15% 5% 11% 6% 5% 4 3 5% 4 21%	3066
	21st to end	Day	20% 16% 5% 12% 6% 5% 6% 4 4 5% 17%	4048
January	1st to 10th	Night	14% 21% 10% 18% 7% 4 3 2 3 3 15%	8234
	11th to 20th	Night	12% 20% 9% 19% 8% 6% 5% 3 4 3 12%	8235
	21st to end	Night	11% 14% 6% 15% 12% 7% 6% 4 4 4 16%	9057
February	1st to 10th	Night	14% 16% 6% 15% 8% 5% 5% 3 4 4 20%	2768
	11th to 20th	Night	13% 16% 6% 15% 8% 5% 5% 3 4 5% 20%	2497
	21st to end	Night	15% 18% 5% 12% 6% 4 4 2 5% 4 26%	2076
March	1st to 10th	Night	20% 16% 5% 11% 7% 4 3 3 3 4 23%	5484
	11th to 20th	Night	15% 18% 4 8% 5% 4 4 3 4 5% 28%	5403
	21st to end	Night	20% 18% 5% 10% 5% 4 3 3 3 3 27%	5072
April	1st to 10th	Night	23% 13% 4 8% 5% 4 4 3 4 5% 28%	2100
	11th to 20th	Night	26% 12% 4 7% 5% 4 3 2 4 4 29%	2075
	21st to end	Night	22% 11% 4 7% 3 3 4 2 4 4 34%	1926
May	1st to 10th	Night	27% 12% 4 8% 5% 4 4 3 5% 6% 21%	5063
	11th to 20th	Night	20% 10% 3 8% 5% 5% 5% 4 5% 6% 29%	4639
	21st to end	Night	21% 9% 3 7% 4 4 4 4 6% 7% 31%	5114
June	1st to 10th	Night	25% 7% 3 6% 4 3 4 4 5% 7% 32%	1680
	11th to 20th	Night	19% 5% 2 7% 5% 4 5% 3 5% 8% 36%	1640
	21st to end	Night	20% 8% 3 7% 4 4 4 4 7% 6% 34%	1600
July	1st to 10th	Night	13% 6% 3 7% 4 5% 6% 5% 8% 10% 34%	3600
	11th to 20th	Night	15% 6% 3 7% 5% 5% 5% 4 6% 7% 37%	3600
	21st to end	Night	12% 6% 3 8% 4 5% 5% 4 6% 8% 39%	3926
August	1st to 10th	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 36%	2586
	11th to 20th	Night	16% 7% 3 8% 5% 5% 4 4 4 7% 37%	2982
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 7% 42%	3586
September	1st to 10th	Night	15% 6% 3 5% 4 4 4 3 6% 7% 44%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 48%	2111
	21st to end	Night	18% 6% 3 5% 4 4 4 4 5% 6% 42%	2200
October	1st to 10th	Night	24% 7% 2 6% 4 4 5% 3 4 7% 34%	2285
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 5% 6% 32%	2452
	21st to end	Night	27% 12% 3 7% 4 4 4 3 5% 5% 26%	2860
November	1st to 10th	Night	21% 13% 3 7% 5% 3 3 3 5% 5% 30%	2667
	11th to 20th	Night	19% 15% 5% 9% 4 5% 4 3 4 5% 28%	2730
	21st to end	Night	20% 18% 6% 10% 4 4 4 3 5% 4 22%	2937
December	1st to 10th	Night	17% 28% 10% 13% 6% 4 4 2 3 3 10%	4720
	11th to 20th	Night	24% 16% 6% 11% 6% 4 4 3 3 4 19%	5348
	21st to end	Night	20% 18% 6% 12% 7% 6% 4 2 3 5% 17%	7150

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	60% 22% 7% 9% 2	4662
	11th to 20th	Day	59% 22% 5% 8% 4 1	4680
	21st to end	Day	55% 25% 5% 9% 7%	5124
February	1st to 10th	Day	28% 59% 4 6% 2 1	1848
	11th to 20th	Day	27% 58% 4 8% 3	2016
	21st to end	Day	26% 60% 3 5% 4 1	1730
March	1st to 10th	Day	49% 32% 4 6% 6% 3	4579
	11th to 20th	Day	43% 29% 10% 10% 8% 1	4759
	21st to end	Day	44% 29% 5% 9% 13% 1	6102
April	1st to 10th	Day	25% 59% 3 8% 4 1	2520
	11th to 20th	Day	24% 60% 2 8% 6% 1	2494
	21st to end	Day	23% 59% 4 8% 6%	2693
May	1st to 10th	Day	53% 25% 5% 12% 6%	7889
	11th to 20th	Day	42% 25% 11% 17% 5%	8225
	21st to end	Day	42% 24% 8% 20% 6%	9061
June	1st to 10th	Day	21% 60% 3 9% 6%	2940
	11th to 20th	Day	17% 61% 5% 11% 6% 1	2870
	21st to end	Day	19% 61% 5% 10% 5% 1	2800
July	1st to 10th	Day	35% 26% 12% 19% 8%	6290
	11th to 20th	Day	32% 26% 10% 23% 8%	6285
	21st to end	Day	30% 26% 8% 30% 6%	6728
August	1st to 10th	Day	32% 36% 10% 15% 5%	4549
	11th to 20th	Day	34% 37% 11% 16% 2	4070
	21st to end	Day	25% 36% 13% 15% 10%	4312
September	1st to 10th	Day	13% 58% 5% 16% 7%	2400
	11th to 20th	Day	14% 57% 6% 15% 8% 1	2288
	21st to end	Day	16% 58% 7% 13% 5% 1	2200
October	1st to 10th	Day	27% 35% 7% 23% 7%	3435
	11th to 20th	Day	31% 36% 9% 16% 8%	3149
	21st to end	Day	31% 36% 10% 20% 3	3180
November	1st to 10th	Day	30% 56% 4 7% 3	1851
	11th to 20th	Day	27% 54% 6% 8% 4	1890
	21st to end	Day	32% 55% 6% 5% 2	1680
December	1st to 10th	Day	56% 35% 5% 4 1	2680
	11th to 20th	Day	47% 31% 5% 13% 3	3066
	21st to end	Day	51% 27% 6% 10% 5%	4048
January	1st to 10th	Night	61% 21% 8% 7% 2	8234
	11th to 20th	Night	63% 23% 5% 6% 3 1	8280
	21st to end	Night	60% 23% 5% 6% 6%	9057
February	1st to 10th	Night	29% 59% 3 6% 2	2768
	11th to 20th	Night	27% 58% 4 7% 4	2499
	21st to end	Night	24% 59% 4 8% 5% 1	2076
March	1st to 10th	Night	49% 29% 5% 7% 9% 1	5494
	11th to 20th	Night	42% 29% 11% 9% 7% 1	5404
	21st to end	Night	49% 28% 3 9% 10% 1	5072
April	1st to 10th	Night	25% 59% 3 6% 6%	2100
	11th to 20th	Night	26% 60% 2 6% 6%	2075
	21st to end	Night	23% 59% 3 7% 7% 1	1926
May	1st to 10th	Night	53% 25% 7% 12% 3	5069
	11th to 20th	Night	45% 24% 8% 17% 6%	4640
	21st to end	Night	43% 23% 6% 17% 9%	5115
June	1st to 10th	Night	22% 59% 3 8% 8% 1	1680
	11th to 20th	Night	18% 61% 6% 8% 6% 1	1640
	21st to end	Night	19% 61% 4 11% 5%	1600
July	1st to 10th	Night	34% 25% 14% 19% 7%	3560
	11th to 20th	Night	36% 26% 8% 21% 9%	3552
	21st to end	Night	34% 26% 9% 23% 7%	3809
August	1st to 10th	Night	36% 36% 8% 14% 5%	2576
	11th to 20th	Night	35% 37% 11% 12% 5%	3009
	21st to end	Night	27% 36% 10% 16% 10% 1	3586
September	1st to 10th	Night	15% 58% 6% 13% 7% 1	2000
	11th to 20th	Night	15% 58% 6% 14% 7%	2111
	21st to end	Night	15% 58% 6% 12% 8% 1	2200
October	1st to 10th	Night	37% 35% 5% 16% 7%	3740
	11th to 20th	Night	32% 36% 7% 16% 9%	3988
	21st to end	Night	35% 35% 8% 17% 5%	4610
November	1st to 10th	Night	28% 54% 6% 8% 3	2667
	11th to 20th	Night	29% 54% 6% 7% 4	2730
	21st to end	Night	31% 55% 6% 5% 2	2937
December	1st to 10th	Night	57% 35% 4 4	4720
	11th to 20th	Night	51% 32% 4 7% 6%	5348
	21st to end	Night	54% 27% 4 9% 6%	7150

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	27% 23% 29% 18% 3	220
	11th to 20th	Day	31% 28% 30% 6% 3 1	220
	21st to end	Day	26% 23% 35% 12% 4	242
February	1st to 10th	Day	31% 30% 27% 10% 2	308
	11th to 20th	Day	27% 27% 34% 9% 2	495
	21st to end	Day	31% 21% 35% 11% 2	446
March	1st to 10th	Day	32% 19% 29% 16% 3	540
	11th to 20th	Day	29% 26% 28% 14% 4 1	540
	21st to end	Day	34% 22% 25% 12% 6% 1	594
April	1st to 10th	Day	37% 19% 24% 15% 5% 1	540
	11th to 20th	Day	35% 23% 20% 13% 6% 2	532
	21st to end	Day	30% 21% 28% 13% 6% 1	540
May	1st to 10th	Day	32% 18% 26% 15% 7% 1	543
	11th to 20th	Day	23% 16% 32% 22% 7% 1	567
	21st to end	Day	21% 18% 32% 20% 8% 1	617
June	1st to 10th	Day	27% 12% 32% 21% 8% 1	570
	11th to 20th	Day	19% 16% 33% 21% 9% 2	550
	21st to end	Day	23% 15% 32% 21% 8% 1	530
July	1st to 10th	Day	17% 10% 31% 29% 11% 3	525
	11th to 20th	Day	14% 9% 34% 29% 12% 2	524
	21st to end	Day	19% 7% 34% 30% 10% 1	576
August	1st to 10th	Day	15% 9% 28% 28% 17% 2	526
	11th to 20th	Day	18% 9% 24% 29% 14% 5%	519
	21st to end	Day	16% 14% 28% 26% 11% 5%	583
September	1st to 10th	Day	17% 14% 29% 23% 12% 5%	530
	11th to 20th	Day	16% 13% 20% 23% 20% 8%	530
	21st to end	Day	16% 14% 30% 22% 14% 5%	530
October	1st to 10th	Day	26% 13% 26% 17% 15% 4	530
	11th to 20th	Day	29% 16% 26% 20% 7% 1	500
	21st to end	Day	38% 16% 25% 17% 3	462
November	1st to 10th	Day	34% 18% 26% 16% 5%	425
	11th to 20th	Day	32% 25% 22% 14% 6% 1	430
	21st to end	Day	39% 21% 27% 12% 1	220
December	1st to 10th	Day	28% 25% 29% 17% 2	220
	11th to 20th	Day	31% 27% 31% 9% 1	220
	21st to end	Day	29% 26% 30% 12% 3	242
January	1st to 10th	Night	29% 20% 32% 16% 3	840
	11th to 20th	Night	31% 19% 29% 17% 4	840
	21st to end	Night	23% 18% 31% 24% 5% 1	924
February	1st to 10th	Night	23% 17% 34% 23% 3	840
	11th to 20th	Night	26% 19% 33% 19% 3	823
	21st to end	Night	27% 18% 29% 21% 5%	692
March	1st to 10th	Night	30% 16% 27% 23% 3	840
	11th to 20th	Night	28% 22% 25% 19% 5%	840
	21st to end	Night	32% 16% 25% 19% 7% 1	924
April	1st to 10th	Night	30% 15% 26% 24% 4	840
	11th to 20th	Night	33% 17% 22% 21% 7%	831
	21st to end	Night	27% 18% 27% 21% 7%	666
May	1st to 10th	Night	31% 11% 25% 25% 7% 1	563
	11th to 20th	Night	24% 9% 33% 24% 8% 1	414
	21st to end	Night	20% 9% 28% 33% 10% 1	455
June	1st to 10th	Night	26% 7% 26% 32% 9%	420
	11th to 20th	Night	20% 7% 28% 32% 12% 1	410
	21st to end	Night	22% 5% 26% 37% 10% 1	400
July	1st to 10th	Night	16% 3 23% 43% 15% 1	390
	11th to 20th	Night	12% 4 23% 42% 18% 2	388
	21st to end	Night	16% 6% 21% 42% 15%	438
August	1st to 10th	Night	16% 4 17% 41% 22% 1	398
	11th to 20th	Night	18% 4 20% 33% 22% 3	649
	21st to end	Night	16% 7% 23% 32% 21% 1	880
September	1st to 10th	Night	17% 8% 24% 34% 16% 2	800
	11th to 20th	Night	18% 8% 17% 30% 23% 4	800
	21st to end	Night	21% 9% 27% 25% 17% 2	800
October	1st to 10th	Night	28% 11% 20% 25% 15% 1	800
	11th to 20th	Night	31% 11% 23% 26% 8% 1	800
	21st to end	Night	32% 15% 27% 21% 5%	880
November	1st to 10th	Night	31% 17% 25% 20% 6% 1	820
	11th to 20th	Night	31% 16% 27% 17% 8% 1	840
	21st to end	Night	33% 20% 26% 17% 4 1	840
December	1st to 10th	Night	34% 19% 28% 14% 4	840
	11th to 20th	Night	29% 22% 28% 17% 4	840
	21st to end	Night	33% 17% 29% 17% 3	924

Page generated at 2026-06-13 02:31:38. Version ac07

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