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 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% 6% 2 6% 4 4 4 4 6% 8% 41%	101488
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	98155
Transparency	Day	39% 7% 13% 23% 16% 3	105048
	Night	40% 6% 12% 23% 16% 3	102683
Seeing	Day	19% 16% 30% 22% 10% 3	15156
	Night	18% 14% 30% 27% 9% 1	24472

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	15% 8% 3 5% 3 3 3 2 4 5% 48%	2517
	11th to 20th	Day	16% 9% 3 8% 5% 3 4 3 4 5% 41%	2620
	21st to end	Day	21% 10% 3 7% 4 3 4 3 4 5% 37%	3124
February	1st to 10th	Day	14% 8% 2 7% 4 3 4 4 5% 7% 41%	1710
	11th to 20th	Day	16% 7% 2 6% 4 3 4 4 5% 6% 42%	1832
	21st to end	Day	17% 10% 3 7% 4 4 3 3 4 6% 39%	1570
March	1st to 10th	Day	25% 7% 1 6% 3 3 3 3 4 6% 40%	3116
	11th to 20th	Day	9% 4 1 5% 4 4 5% 4 6% 7% 51%	3471
	21st to end	Day	20% 6% 2 6% 4 4 3 2 4 4 43%	4056
April	1st to 10th	Day	15% 8% 2 4 4 4 4 4 5% 7% 43%	2280
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 6% 7% 46%	2254
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 7% 7% 42%	2280
May	1st to 10th	Day	23% 9% 3 7% 4 5% 4 3 5% 7% 30%	3970
	11th to 20th	Day	24% 8% 2 7% 6% 4 5% 4 5% 6% 30%	4390
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	4837
June	1st to 10th	Day	10% 6% 2 5% 5% 5% 4 5% 8% 10% 40%	2660
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 8% 9% 41%	2660
	21st to end	Day	9% 5% 2 5% 3 4 4 5% 8% 10% 46%	2660
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 5% 9% 11% 38%	4440
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 9% 11% 38%	4440
	21st to end	Day	10% 7% 3 8% 6% 6% 6% 5% 8% 10% 31%	4699
August	1st to 10th	Day	17% 5% 2 6% 4 4 5% 4 8% 10% 35%	4053
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	3730
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 9% 45%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 4 4 6% 7% 44%	2280
	11th to 20th	Day	11% 6% 2 5% 4 4 3 3 6% 6% 49%	2240
	21st to end	Day	17% 8% 2 6% 4 4 4 4 4 7% 39%	2090
October	1st to 10th	Day	16% 6% 2 6% 4 3 3 3 5% 6% 46%	1919
	11th to 20th	Day	15% 5% 2 6% 2 3 3 2 5% 6% 51%	1900
	21st to end	Day	16% 5% 2 7% 3 4 4 3 3 5% 48%	2028
November	1st to 10th	Day	14% 5% 2 5% 3 4 3 3 5% 7% 52%	1761
	11th to 20th	Day	13% 6% 2 4 4 3 4 3 6% 7% 48%	1800
	21st to end	Day	14% 5% 2 5% 5% 4 4 3 4 7% 47%	1715
December	1st to 10th	Day	17% 8% 2 5% 4 4 5% 3 5% 7% 41%	1528
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 4 5% 45%	1906
	21st to end	Day	15% 7% 2 5% 5% 4 5% 4 6% 8% 40%	2772
January	1st to 10th	Night	17% 7% 2 4 3 4 4 3 4 5% 45%	4429
	11th to 20th	Night	19% 9% 2 5% 4 3 3 2 4 5% 44%	4337
	21st to end	Night	21% 8% 2 7% 4 3 4 3 4 5% 38%	4532
February	1st to 10th	Night	17% 7% 2 6% 4 4 4 4 5% 6% 41%	2470
	11th to 20th	Night	18% 8% 3 5% 4 3 3 3 5% 5% 42%	2233
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 5% 43%	1884
March	1st to 10th	Night	22% 10% 3 6% 4 3 3 2 3 6% 38%	3724
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 6% 49%	3484
	21st to end	Night	17% 10% 3 6% 4 4 3 2 3 5% 44%	3575
April	1st to 10th	Night	17% 8% 2 6% 5% 3 4 4 5% 6% 41%	1900
	11th to 20th	Night	12% 6% 2 8% 5% 5% 4 3 4 7% 43%	1875
	21st to end	Night	17% 6% 2 5% 4 4 4 3 6% 7% 44%	1900
May	1st to 10th	Night	18% 9% 4 8% 4 4 4 4 6% 8% 31%	2971
	11th to 20th	Night	23% 9% 3 7% 5% 4 6% 3 5% 6% 30%	2531
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 9% 43%	2738
June	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 6% 8% 43%	1520
	11th to 20th	Night	8% 6% 2 6% 5% 5% 5% 4 6% 10% 42%	1520
	21st to end	Night	8% 5% 2 6% 6% 4 6% 5% 8% 11% 41%	1520
July	1st to 10th	Night	7% 6% 2 9% 6% 5% 6% 5% 6% 9% 38%	2520
	11th to 20th	Night	10% 6% 3 6% 5% 4 4 4 7% 10% 40%	2520
	21st to end	Night	15% 8% 3 6% 4 4 6% 5% 7% 9% 33%	2850
August	1st to 10th	Night	19% 7% 2 7% 5% 4 5% 4 5% 9% 32%	2843
	11th to 20th	Night	11% 6% 3 6% 5% 5% 5% 4 6% 8% 42%	3097
	21st to end	Night	9% 6% 2 6% 4 3 4 4 5% 7% 52%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	1900
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 7% 54%	1938
	21st to end	Night	19% 7% 2 5% 4 4 4 3 5% 8% 39%	2090
October	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 6% 44%	2256
	11th to 20th	Night	15% 6% 2 4 3 2 3 3 4 6% 52%	2280
	21st to end	Night	15% 7% 2 5% 3 3 3 3 5% 6% 48%	2567
November	1st to 10th	Night	10% 5% 2 5% 4 2 3 4 6% 6% 52%	2537
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 6% 49%	2600
	21st to end	Night	11% 5% 2 5% 4 4 5% 3 6% 6% 48%	2680
December	1st to 10th	Night	18% 8% 2 6% 4 3 3 4 5% 6% 40%	2686
	11th to 20th	Night	17% 6% 3 6% 4 4 3 4 6% 7% 42%	3288
	21st to end	Night	15% 8% 3 6% 4 4 4 4 5% 7% 38%	4884

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	39% 4 10% 28% 15% 4	2518
	11th to 20th	Day	46% 5% 11% 22% 13% 4	2620
	21st to end	Day	50% 5% 11% 20% 11% 2	3124
February	1st to 10th	Day	42% 6% 14% 24% 13% 2	1710
	11th to 20th	Day	42% 5% 11% 24% 15% 3	1841
	21st to end	Day	47% 5% 9% 20% 14% 5%	1570
March	1st to 10th	Day	46% 4 9% 27% 12% 2	3119
	11th to 20th	Day	28% 7% 11% 31% 19% 4	3471
	21st to end	Day	44% 5% 7% 19% 19% 5%	4056
April	1st to 10th	Day	40% 6% 13% 19% 16% 5%	2280
	11th to 20th	Day	36% 6% 12% 24% 17% 4	2254
	21st to end	Day	39% 8% 13% 18% 18% 4	2280
May	1st to 10th	Day	52% 7% 12% 18% 9% 2	3982
	11th to 20th	Day	52% 9% 14% 16% 8% 1	4394
	21st to end	Day	33% 8% 13% 27% 17% 1	4837
June	1st to 10th	Day	36% 10% 14% 23% 15% 2	2660
	11th to 20th	Day	35% 11% 14% 24% 13% 3	2660
	21st to end	Day	31% 9% 17% 23% 18% 1	2660
July	1st to 10th	Day	37% 11% 21% 17% 13% 2	4370
	11th to 20th	Day	37% 8% 20% 23% 10% 2	4365
	21st to end	Day	45% 9% 13% 20% 13% 1	4549
August	1st to 10th	Day	42% 8% 17% 23% 8% 1	4040
	11th to 20th	Day	38% 8% 14% 27% 12% 1	3766
	21st to end	Day	32% 9% 14% 26% 17% 2	4180
September	1st to 10th	Day	37% 8% 14% 18% 20% 3	2280
	11th to 20th	Day	34% 6% 10% 24% 23% 4	2240
	21st to end	Day	45% 7% 10% 17% 18% 3	2090
October	1st to 10th	Day	35% 4 9% 19% 28% 5%	3192
	11th to 20th	Day	25% 3 9% 24% 29% 10%	3140
	21st to end	Day	26% 3 11% 29% 25% 6%	3318
November	1st to 10th	Day	33% 4 14% 28% 18% 4	1761
	11th to 20th	Day	35% 5% 11% 23% 24% 3	1800
	21st to end	Day	37% 5% 11% 30% 14% 3	1715
December	1st to 10th	Day	42% 6% 15% 22% 13% 1	1528
	11th to 20th	Day	42% 4 11% 28% 12% 3	1906
	21st to end	Day	40% 7% 15% 23% 14% 1	2772
January	1st to 10th	Night	40% 4 9% 27% 15% 4	4429
	11th to 20th	Night	44% 4 9% 25% 15% 2	4337
	21st to end	Night	48% 5% 8% 25% 13% 2	4532
February	1st to 10th	Night	42% 5% 14% 23% 14% 2	2470
	11th to 20th	Night	44% 4 8% 20% 19% 5%	2235
	21st to end	Night	43% 4 10% 22% 16% 5%	1884
March	1st to 10th	Night	49% 5% 9% 23% 12% 2	3734
	11th to 20th	Night	34% 7% 12% 26% 15% 5%	3485
	21st to end	Night	44% 6% 7% 17% 21% 6%	3575
April	1st to 10th	Night	43% 5% 9% 19% 19% 5%	1900
	11th to 20th	Night	40% 6% 10% 20% 19% 4	1875
	21st to end	Night	40% 7% 10% 19% 19% 5%	1900
May	1st to 10th	Night	51% 8% 10% 20% 10% 1	2978
	11th to 20th	Night	54% 8% 10% 17% 10% 2	2532
	21st to end	Night	37% 10% 15% 20% 16% 3	2739
June	1st to 10th	Night	38% 8% 13% 21% 19% 1	1520
	11th to 20th	Night	37% 9% 14% 26% 13% 2	1520
	21st to end	Night	35% 9% 15% 24% 15% 3	1520
July	1st to 10th	Night	40% 10% 19% 18% 11% 1	2480
	11th to 20th	Night	38% 8% 20% 26% 7% 1	2472
	21st to end	Night	46% 7% 17% 18% 12% 1	2759
August	1st to 10th	Night	48% 8% 17% 19% 8%	2832
	11th to 20th	Night	39% 7% 13% 28% 12% 1	3127
	21st to end	Night	30% 7% 11% 29% 21% 2	3476
September	1st to 10th	Night	39% 6% 12% 21% 19% 4	1900
	11th to 20th	Night	30% 6% 11% 25% 25% 3	1938
	21st to end	Night	44% 6% 9% 20% 18% 3	2090
October	1st to 10th	Night	38% 5% 10% 24% 21% 3	3763
	11th to 20th	Night	24% 3 12% 24% 29% 8%	3779
	21st to end	Night	24% 4 12% 30% 25% 5%	4227
November	1st to 10th	Night	32% 6% 11% 25% 21% 6%	2537
	11th to 20th	Night	39% 5% 9% 22% 21% 5%	2600
	21st to end	Night	35% 5% 12% 26% 19% 2	2680
December	1st to 10th	Night	45% 5% 12% 23% 14% 2	2686
	11th to 20th	Night	40% 6% 11% 21% 18% 3	3288
	21st to end	Night	43% 7% 15% 24% 10% 1	4884

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% 26% 30% 12% 4 1	180
	11th to 20th	Day	34% 28% 23% 12% 2	209
	21st to end	Day	28% 29% 26% 13% 2 1	297
February	1st to 10th	Day	28% 26% 32% 12% 3	270
	11th to 20th	Day	30% 25% 35% 8% 1	440
	21st to end	Day	30% 24% 30% 14% 2	382
March	1st to 10th	Day	23% 28% 28% 18% 3	460
	11th to 20th	Day	25% 28% 30% 12% 4 2	460
	21st to end	Day	25% 22% 35% 14% 4 1	506
April	1st to 10th	Day	24% 28% 33% 10% 4 1	460
	11th to 20th	Day	21% 20% 33% 19% 6% 2	452
	21st to end	Day	24% 18% 37% 15% 5% 1	460
May	1st to 10th	Day	27% 21% 28% 17% 6% 2	463
	11th to 20th	Day	21% 16% 35% 21% 6% 1	487
	21st to end	Day	16% 12% 25% 30% 14% 2	529
June	1st to 10th	Day	13% 12% 36% 24% 12% 2	490
	11th to 20th	Day	12% 11% 33% 29% 12% 3	490
	21st to end	Day	9% 11% 32% 32% 13% 2	490
July	1st to 10th	Day	11% 5% 30% 33% 17% 5%	485
	11th to 20th	Day	5% 4 27% 37% 22% 5%	484
	21st to end	Day	11% 5% 33% 32% 15% 4	532
August	1st to 10th	Day	12% 5% 29% 31% 18% 5%	486
	11th to 20th	Day	13% 5% 23% 34% 20% 5%	479
	21st to end	Day	9% 8% 28% 29% 15% 11%	539
September	1st to 10th	Day	17% 10% 24% 25% 15% 9%	490
	11th to 20th	Day	12% 7% 31% 27% 17% 7%	490
	21st to end	Day	23% 15% 27% 22% 8% 4	490
October	1st to 10th	Day	20% 12% 27% 25% 10% 6%	490
	11th to 20th	Day	19% 16% 28% 23% 11% 3	490
	21st to end	Day	18% 18% 31% 21% 10% 2	506
November	1st to 10th	Day	17% 15% 33% 21% 11% 3	395
	11th to 20th	Day	21% 22% 32% 19% 7%	400
	21st to end	Day	22% 20% 30% 21% 6% 1	315
December	1st to 10th	Day	28% 25% 31% 9% 7%	182
	11th to 20th	Day	33% 21% 28% 14% 3	180
	21st to end	Day	29% 18% 29% 17% 7% 1	198
January	1st to 10th	Night	21% 24% 33% 19% 3	760
	11th to 20th	Night	25% 21% 28% 23% 2	760
	21st to end	Night	18% 25% 37% 18% 1	836
February	1st to 10th	Night	21% 20% 35% 22% 2	760
	11th to 20th	Night	25% 27% 34% 13% 1	743
	21st to end	Night	24% 24% 35% 15% 3	628
March	1st to 10th	Night	20% 22% 34% 20% 4	760
	11th to 20th	Night	22% 23% 34% 16% 4 1	760
	21st to end	Night	21% 17% 39% 18% 5%	836
April	1st to 10th	Night	21% 23% 36% 16% 4	760
	11th to 20th	Night	17% 17% 36% 25% 4	751
	21st to end	Night	20% 14% 36% 25% 5%	760
May	1st to 10th	Night	19% 14% 36% 25% 6%	656
	11th to 20th	Night	18% 12% 34% 29% 7%	397
	21st to end	Night	16% 6% 27% 35% 16% 1	411
June	1st to 10th	Night	17% 8% 23% 40% 11% 1	380
	11th to 20th	Night	9% 6% 29% 43% 13%	380
	21st to end	Night	7% 4 32% 43% 14%	380
July	1st to 10th	Night	10% 4 23% 46% 16% 1	370
	11th to 20th	Night	8% 2 19% 45% 25% 2	368
	21st to end	Night	10% 3 24% 42% 20% 1	478
August	1st to 10th	Night	11% 3 18% 43% 23% 1	572
	11th to 20th	Night	11% 4 21% 43% 20% 2	750
	21st to end	Night	11% 4 20% 39% 22% 3	836
September	1st to 10th	Night	15% 5% 23% 37% 17% 3	760
	11th to 20th	Night	11% 4 21% 37% 23% 4	760
	21st to end	Night	22% 8% 25% 29% 15% 1	760
October	1st to 10th	Night	20% 9% 24% 28% 16% 4	760
	11th to 20th	Night	17% 14% 26% 29% 13% 1	760
	21st to end	Night	19% 14% 32% 25% 9% 1	836
November	1st to 10th	Night	15% 12% 37% 24% 11% 1	780
	11th to 20th	Night	19% 20% 30% 25% 6% 1	800
	21st to end	Night	17% 20% 33% 21% 9% 1	800
December	1st to 10th	Night	26% 17% 32% 16% 8% 1	768
	11th to 20th	Night	19% 19% 37% 19% 5%	760
	21st to end	Night	20% 18% 34% 25% 2	836

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

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