Holly Dome Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holly Dome Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	131018
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 44%	130151
Transparency	Day	39% 6% 12% 23% 16% 3	134561
	Night	39% 5% 11% 23% 17% 4	134694
Seeing	Day	20% 16% 30% 22% 9% 3	16705
	Night	18% 15% 30% 27% 9% 1	26119

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% 7% 2 6% 4 3 4 3 4 5% 49%	4661
	11th to 20th	Day	15% 8% 3 8% 4 3 3 2 3 4 49%	4847
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	5780
February	1st to 10th	Day	13% 8% 2 7% 4 4 4 4 5% 7% 41%	1890
	11th to 20th	Day	17% 8% 3 7% 4 3 4 4 5% 6% 40%	2031
	21st to end	Day	16% 10% 3 7% 4 4 3 3 4 6% 40%	1730
March	1st to 10th	Day	20% 5% 1 5% 3 3 3 3 4 6% 47%	4496
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 6% 6% 49%	4971
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 44%	5818
April	1st to 10th	Day	16% 8% 2 4 4 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	14% 6% 2 5% 4 3 5% 4 5% 7% 45%	2374
	21st to end	Day	14% 5% 2 6% 4 4 5% 4 7% 7% 42%	2400
May	1st to 10th	Day	22% 8% 3 7% 4 4 4 4 6% 8% 32%	5694
	11th to 20th	Day	23% 7% 2 6% 5% 4 4 4 5% 7% 32%	6310
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	6949
June	1st to 10th	Day	11% 6% 2 5% 5% 5% 4 5% 7% 9% 40%	2800
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 8% 9% 41%	2800
	21st to end	Day	9% 5% 2 5% 3 4 5% 5% 8% 10% 46%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 8% 11% 40%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 10% 12% 31%	6360
	21st to end	Day	9% 6% 2 7% 6% 5% 5% 5% 7% 10% 38%	6755
August	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 8% 10% 36%	4181
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	3850
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 45%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 7% 44%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 5% 6% 49%	2358
	21st to end	Day	17% 8% 3 6% 5% 4 4 4 4 7% 38%	2200
October	1st to 10th	Day	15% 5% 2 6% 4 3 3 3 5% 6% 46%	2020
	11th to 20th	Day	15% 5% 2 6% 3 3 3 2 5% 7% 50%	2000
	21st to end	Day	16% 5% 2 6% 3 4 4 3 3 6% 48%	2135
November	1st to 10th	Day	13% 5% 2 5% 3 4 2 3 5% 7% 53%	1851
	11th to 20th	Day	13% 6% 2 4 4 3 4 3 6% 7% 48%	1890
	21st to end	Day	14% 6% 2 5% 5% 4 4 3 4 7% 47%	1801
December	1st to 10th	Day	12% 5% 2 5% 3 4 5% 3 5% 8% 48%	2680
	11th to 20th	Day	20% 9% 2 5% 3 3 3 3 3 6% 44%	3066
	21st to end	Day	13% 7% 2 5% 4 4 4 4 6% 9% 43%	4048
January	1st to 10th	Night	12% 8% 2 5% 3 4 4 3 5% 6% 47%	8234
	11th to 20th	Night	15% 7% 2 6% 4 3 3 2 4 5% 50%	8021
	21st to end	Night	15% 7% 2 5% 3 3 4 3 4 5% 47%	8401
February	1st to 10th	Night	16% 7% 2 6% 4 4 4 4 5% 6% 41%	2730
	11th to 20th	Night	19% 8% 3 5% 4 4 4 3 5% 5% 42%	2474
	21st to end	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	2076
March	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 3 6% 46%	5389
	11th to 20th	Night	11% 9% 3 5% 4 3 4 3 4 5% 48%	4984
	21st to end	Night	15% 8% 3 5% 4 4 4 3 4 6% 44%	5113
April	1st to 10th	Night	17% 8% 2 6% 5% 4 4 4 5% 6% 40%	2000
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 3 4 7% 43%	1975
	21st to end	Night	16% 6% 2 5% 4 4 4 3 6% 7% 44%	2000
May	1st to 10th	Night	17% 8% 4 7% 4 5% 5% 4 6% 9% 30%	4247
	11th to 20th	Night	21% 9% 2 7% 4 4 5% 3 5% 7% 31%	3625
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 5% 8% 43%	1600
	11th to 20th	Night	9% 7% 2 5% 5% 5% 5% 4 6% 10% 41%	1600
	21st to end	Night	8% 4 2 6% 5% 4 6% 5% 8% 10% 42%	1600
July	1st to 10th	Night	9% 7% 2 8% 6% 5% 6% 5% 6% 9% 37%	3600
	11th to 20th	Night	11% 8% 3 8% 5% 4 5% 4 7% 11% 33%	3600
	21st to end	Night	11% 7% 3 6% 4 4 5% 4 7% 9% 40%	4093
August	1st to 10th	Night	19% 7% 3 7% 5% 4 4 4 5% 9% 33%	2934
	11th to 20th	Night	11% 6% 3 6% 5% 5% 5% 4 6% 9% 41%	3197
	21st to end	Night	9% 6% 2 5% 4 3 4 4 5% 7% 52%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 46%	2000
	11th to 20th	Night	10% 4 2 5% 4 3 3 3 5% 7% 53%	2040
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 8% 38%	2200
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 6% 45%	2375
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 4 6% 51%	2400
	21st to end	Night	15% 7% 2 5% 3 3 3 3 5% 6% 48%	2702
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 53%	2667
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 6% 49%	2730
	21st to end	Night	11% 6% 2 5% 4 4 5% 3 6% 6% 48%	2814
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	4720
	11th to 20th	Night	21% 6% 2 5% 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	16% 7% 3 6% 4 4 4 4 5% 7% 41%	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	38% 3 8% 27% 21% 3	4662
	11th to 20th	Day	42% 3 6% 22% 20% 7%	4878
	21st to end	Day	41% 4 10% 30% 13% 3	5780
February	1st to 10th	Day	41% 6% 14% 23% 14% 2	1890
	11th to 20th	Day	45% 4 10% 22% 15% 3	2040
	21st to end	Day	46% 5% 9% 22% 14% 5%	1730
March	1st to 10th	Day	38% 3 9% 24% 18% 7%	4499
	11th to 20th	Day	33% 6% 10% 28% 18% 6%	4971
	21st to end	Day	42% 5% 8% 16% 22% 7%	5818
April	1st to 10th	Day	40% 6% 13% 19% 16% 6%	2400
	11th to 20th	Day	37% 6% 12% 23% 17% 4	2374
	21st to end	Day	38% 7% 13% 19% 19% 4	2400
May	1st to 10th	Day	50% 7% 14% 18% 10% 1	5703
	11th to 20th	Day	50% 7% 11% 20% 11% 1	6314
	21st to end	Day	41% 7% 12% 23% 15% 1	6949
June	1st to 10th	Day	37% 10% 14% 22% 15% 2	2800
	11th to 20th	Day	36% 11% 14% 24% 13% 3	2800
	21st to end	Day	32% 9% 17% 23% 18% 1	2800
July	1st to 10th	Day	36% 10% 20% 19% 14% 2	6290
	11th to 20th	Day	42% 10% 21% 19% 7% 1	6285
	21st to end	Day	39% 8% 12% 22% 19% 1	6560
August	1st to 10th	Day	42% 8% 17% 24% 9% 1	4168
	11th to 20th	Day	38% 8% 14% 27% 11% 1	3886
	21st to end	Day	32% 9% 13% 26% 17% 2	4312
September	1st to 10th	Day	37% 7% 14% 19% 20% 3	2400
	11th to 20th	Day	34% 6% 10% 24% 22% 4	2358
	21st to end	Day	45% 7% 11% 17% 17% 2	2200
October	1st to 10th	Day	34% 4 9% 19% 28% 5%	3293
	11th to 20th	Day	26% 3 9% 24% 28% 10%	3240
	21st to end	Day	26% 3 11% 29% 25% 6%	3425
November	1st to 10th	Day	32% 4 14% 28% 18% 4	1851
	11th to 20th	Day	35% 4 11% 23% 24% 3	1890
	21st to end	Day	38% 5% 11% 29% 14% 3	1801
December	1st to 10th	Day	35% 5% 19% 24% 15% 3	2680
	11th to 20th	Day	44% 4 11% 28% 11% 3	3066
	21st to end	Day	38% 6% 14% 22% 19% 1	4048
January	1st to 10th	Night	38% 3 11% 25% 18% 5%	8234
	11th to 20th	Night	38% 3 7% 23% 20% 8%	8065
	21st to end	Night	39% 3 8% 28% 18% 3	8401
February	1st to 10th	Night	43% 5% 13% 23% 15% 2	2730
	11th to 20th	Night	45% 4 8% 20% 18% 4	2476
	21st to end	Night	43% 4 10% 21% 16% 5%	2076
March	1st to 10th	Night	41% 4 8% 23% 13% 10%	5399
	11th to 20th	Night	37% 6% 12% 24% 15% 6%	4985
	21st to end	Night	41% 5% 9% 17% 24% 5%	5113
April	1st to 10th	Night	44% 5% 9% 19% 19% 5%	2000
	11th to 20th	Night	41% 6% 10% 19% 20% 4	1975
	21st to end	Night	39% 7% 10% 19% 20% 5%	2000
May	1st to 10th	Night	50% 7% 13% 18% 9% 2	4254
	11th to 20th	Night	52% 7% 9% 20% 11% 1	3626
	21st to end	Night	45% 8% 11% 20% 14% 2	3927
June	1st to 10th	Night	39% 7% 13% 20% 20% 1	1600
	11th to 20th	Night	38% 9% 14% 26% 13% 2	1600
	21st to end	Night	34% 9% 15% 23% 15% 3	1600
July	1st to 10th	Night	42% 9% 16% 20% 12% 1	3560
	11th to 20th	Night	45% 9% 20% 20% 5% 1	3552
	21st to end	Night	39% 6% 16% 22% 16% 1	3973
August	1st to 10th	Night	48% 8% 16% 19% 8%	2923
	11th to 20th	Night	39% 7% 13% 28% 12% 1	3227
	21st to end	Night	30% 7% 11% 28% 21% 2	3586
September	1st to 10th	Night	39% 6% 12% 22% 19% 4	2000
	11th to 20th	Night	31% 6% 11% 25% 24% 3	2040
	21st to end	Night	45% 6% 9% 20% 18% 3	2200
October	1st to 10th	Night	37% 5% 10% 24% 21% 3	3882
	11th to 20th	Night	25% 3 12% 24% 29% 8%	3899
	21st to end	Night	25% 4 12% 30% 25% 5%	4362
November	1st to 10th	Night	32% 6% 10% 25% 21% 6%	2667
	11th to 20th	Night	38% 4 9% 21% 21% 6%	2730
	21st to end	Night	36% 5% 12% 26% 19% 2	2814
December	1st to 10th	Night	34% 4 14% 28% 17% 3	4720
	11th to 20th	Night	43% 4 12% 21% 16% 4	5348
	21st to end	Night	43% 5% 13% 20% 17% 2	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	29% 27% 29% 11% 4	220
	11th to 20th	Day	35% 29% 23% 11% 2	256
	21st to end	Day	29% 31% 25% 11% 2 1	363
February	1st to 10th	Day	28% 26% 32% 11% 2	330
	11th to 20th	Day	33% 25% 34% 8% 1	519
	21st to end	Day	29% 25% 30% 14% 2	446
March	1st to 10th	Day	25% 27% 28% 17% 2	508
	11th to 20th	Day	24% 27% 30% 13% 4 2	500
	21st to end	Day	24% 24% 34% 14% 3 1	550
April	1st to 10th	Day	26% 27% 33% 9% 4 1	500
	11th to 20th	Day	23% 21% 31% 18% 5% 1	492
	21st to end	Day	24% 17% 37% 15% 6% 1	500
May	1st to 10th	Day	27% 21% 29% 17% 5% 2	503
	11th to 20th	Day	22% 17% 34% 21% 6% 1	527
	21st to end	Day	18% 13% 25% 29% 13% 2	573
June	1st to 10th	Day	14% 13% 35% 24% 12% 2	530
	11th to 20th	Day	13% 12% 34% 28% 11% 3	530
	21st to end	Day	9% 11% 31% 33% 14% 2	530
July	1st to 10th	Day	11% 4 30% 33% 16% 5%	525
	11th to 20th	Day	5% 5% 27% 37% 22% 5%	524
	21st to end	Day	11% 5% 33% 32% 15% 4	576
August	1st to 10th	Day	12% 6% 29% 31% 17% 5%	526
	11th to 20th	Day	14% 5% 23% 33% 20% 6%	519
	21st to end	Day	9% 8% 28% 30% 15% 10%	583
September	1st to 10th	Day	17% 10% 25% 25% 15% 8%	530
	11th to 20th	Day	13% 6% 31% 27% 17% 6%	530
	21st to end	Day	22% 14% 29% 22% 8% 4	530
October	1st to 10th	Day	19% 12% 27% 24% 12% 7%	530
	11th to 20th	Day	21% 15% 28% 22% 11% 3	530
	21st to end	Day	18% 20% 31% 21% 9% 1	547
November	1st to 10th	Day	18% 16% 32% 21% 10% 3	425
	11th to 20th	Day	23% 21% 31% 19% 6%	430
	21st to end	Day	24% 20% 29% 20% 6% 1	341
December	1st to 10th	Day	30% 23% 32% 10% 5%	220
	11th to 20th	Day	33% 20% 30% 14% 3	220
	21st to end	Day	29% 20% 29% 15% 6%	242
January	1st to 10th	Night	21% 24% 32% 19% 3	840
	11th to 20th	Night	26% 23% 28% 22% 2	840
	21st to end	Night	19% 24% 37% 18% 1	924
February	1st to 10th	Night	22% 20% 35% 21% 2	840
	11th to 20th	Night	27% 26% 33% 13% 1	823
	21st to end	Night	23% 23% 36% 15% 3	692
March	1st to 10th	Night	20% 22% 34% 20% 4	809
	11th to 20th	Night	21% 22% 35% 17% 4 1	800
	21st to end	Night	21% 17% 40% 18% 4	880
April	1st to 10th	Night	21% 23% 36% 16% 4	800
	11th to 20th	Night	18% 18% 36% 25% 4	791
	21st to end	Night	19% 15% 36% 25% 5%	800
May	1st to 10th	Night	19% 14% 36% 25% 6%	690
	11th to 20th	Night	19% 13% 33% 29% 6%	418
	21st to end	Night	17% 6% 27% 34% 15% 1	433
June	1st to 10th	Night	17% 8% 25% 39% 11% 1	400
	11th to 20th	Night	11% 6% 28% 43% 12%	400
	21st to end	Night	8% 4 32% 43% 15%	400
July	1st to 10th	Night	10% 4 23% 46% 16% 1	390
	11th to 20th	Night	8% 2 19% 44% 25% 2	388
	21st to end	Night	9% 3 24% 42% 20% 1	504
August	1st to 10th	Night	11% 3 18% 44% 22% 1	603
	11th to 20th	Night	12% 4 20% 42% 20% 2	790
	21st to end	Night	11% 4 21% 39% 22% 3	880
September	1st to 10th	Night	15% 5% 23% 37% 17% 3	800
	11th to 20th	Night	12% 4 21% 37% 22% 3	800
	21st to end	Night	22% 8% 25% 30% 15% 1	800
October	1st to 10th	Night	19% 9% 24% 28% 17% 4	800
	11th to 20th	Night	18% 14% 25% 29% 13% 1	800
	21st to end	Night	19% 14% 32% 24% 9% 1	880
November	1st to 10th	Night	16% 12% 36% 24% 10% 1	820
	11th to 20th	Night	20% 21% 30% 24% 5%	840
	21st to end	Night	17% 20% 33% 22% 8% 1	840
December	1st to 10th	Night	25% 18% 33% 16% 7% 1	840
	11th to 20th	Night	21% 19% 36% 19% 5%	840
	21st to end	Night	21% 18% 33% 26% 2	924

Page generated at 2026-02-28 01:40:24. Version ac07

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