Holmes Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Holmes Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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% 4 2 4 3 3 3 3 5% 7% 59%	105153
	Night	9% 5% 2 5% 3 4 4 3 5% 7% 54%	103110
Transparency	Day	27% 4 9% 20% 24% 17%	104913
	Night	31% 5% 10% 18% 20% 16%	102941
Seeing	Day	8% 9% 22% 33% 22% 6%	15553
	Night	9% 5% 14% 32% 32% 7%	24937

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	15% 8% 3 7% 4 4 4 3 6% 6% 40%	2744
	11th to 20th	Day	17% 7% 3 8% 6% 5% 5% 3 5% 6% 34%	2860
	21st to end	Day	19% 10% 3 7% 5% 4 5% 4 6% 6% 30%	3146
February	1st to 10th	Day	19% 6% 2 5% 4 3 4 3 5% 6% 44%	1710
	11th to 20th	Day	14% 6% 3 5% 5% 4 4 3 6% 7% 43%	1768
	21st to end	Day	15% 7% 2 6% 4 4 4 3 4 5% 45%	1570
March	1st to 10th	Day	16% 11% 3 9% 7% 6% 6% 4 5% 6% 27%	3384
	11th to 20th	Day	10% 6% 2 5% 3 3 4 3 5% 8% 53%	3490
	21st to end	Day	14% 8% 3 9% 6% 5% 7% 5% 6% 7% 32%	3882
April	1st to 10th	Day	14% 7% 2 5% 4 4 3 3 5% 7% 46%	2280
	11th to 20th	Day	9% 5% 2 5% 3 3 3 4 5% 8% 52%	2249
	21st to end	Day	6% 5% 2 5% 4 4 3 4 5% 7% 56%	2280
May	1st to 10th	Day	6% 3 1 2 2 2 2 2 4 5% 70%	3862
	11th to 20th	Day	3 2 1 4 3 3 3 3 5% 9% 64%	4121
	21st to end	Day	3 5% 1 5% 3 3 3 3 4 6% 65%	4813
June	1st to 10th	Day	4 4 2 4 3 3 3 3 5% 8% 60%	2660
	11th to 20th	Day	2 2 1 2 3 3 3 3 5% 7% 69%	2660
	21st to end	Day	2 2 1 2 3 2 3 3 3 7% 73%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 3 5% 82%	4389
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 7% 74%	4110
	21st to end	Day	1 1 2 2 2 3 2 4 9% 74%	4455
August	1st to 10th	Day	1 1 1 3 2 3 4 3 4 10% 67%	4091
	11th to 20th	Day	1 1 2 2 2 2 3 6% 81%	3791
	21st to end	Day	1 1 3 3 3 2 2 4 7% 74%	4180
September	1st to 10th	Day	1 2 1 2 2 2 2 2 2 6% 79%	2280
	11th to 20th	Day	3 3 1 5% 2 3 3 3 5% 8% 62%	2280
	21st to end	Day	3 2 1 2 2 3 2 2 4 5% 75%	2019
October	1st to 10th	Day	2 3 1 3 2 2 3 2 4 7% 71%	3180
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 8% 67%	3161
	21st to end	Day	4 4 1 3 3 3 4 4 5% 8% 61%	3448
November	1st to 10th	Day	8% 5% 2 4 3 3 4 3 5% 7% 55%	1937
	11th to 20th	Day	9% 6% 3 4 3 4 4 3 5% 8% 52%	1800
	21st to end	Day	13% 4 2 3 3 3 3 3 4 7% 54%	1651
December	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 5% 8% 44%	1520
	11th to 20th	Day	29% 9% 2 5% 4 4 3 3 5% 6% 30%	1928
	21st to end	Day	20% 9% 2 6% 4 4 4 4 6% 6% 36%	2794
January	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 5% 6% 45%	4192
	11th to 20th	Night	18% 8% 3 7% 5% 6% 6% 4 6% 6% 32%	4110
	21st to end	Night	20% 9% 3 7% 5% 5% 5% 4 5% 6% 32%	4521
February	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 5% 5% 47%	2470
	11th to 20th	Night	13% 6% 2 5% 4 4 4 5% 7% 8% 45%	2298
	21st to end	Night	12% 8% 2 4 3 3 3 3 5% 7% 50%	1884
March	1st to 10th	Night	18% 11% 3 7% 5% 5% 5% 4 5% 6% 31%	3477
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 5% 6% 56%	3490
	21st to end	Night	16% 10% 3 7% 5% 5% 4 3 5% 7% 34%	3769
April	1st to 10th	Night	11% 6% 1 4 4 4 4 4 5% 7% 48%	1900
	11th to 20th	Night	10% 5% 2 4 4 4 4 3 5% 6% 52%	1879
	21st to end	Night	5% 4 1 4 3 4 4 3 6% 10% 57%	1900
May	1st to 10th	Night	2 4 1 4 2 3 3 2 4 5% 69%	3098
	11th to 20th	Night	2 2 3 11% 5% 6% 6% 5% 6% 7% 47%	2775
	21st to end	Night	2 3 2 4 4 4 5% 4 6% 6% 61%	2762
June	1st to 10th	Night	3 3 2 4 3 3 5% 4 7% 8% 60%	1520
	11th to 20th	Night	1 2 2 3 3 3 5% 5% 7% 6% 64%	1520
	21st to end	Night	2 3 1 2 3 4 6% 4 4 5% 68%	1520
July	1st to 10th	Night	2 2 3 3 3 3 5% 78%	2566
	11th to 20th	Night	1 1 1 3 3 3 4 3 5% 6% 69%	2860
	21st to end	Night	1 1 5% 4 5% 7% 5% 6% 9% 55%	3109
August	1st to 10th	Night	1 3 1 4 4 4 5% 3 7% 9% 59%	2848
	11th to 20th	Night	1 2 2 2 3 3 4 8% 73%	3053
	21st to end	Night	1 2 1 4 3 4 4 3 4 6% 70%	3498
September	1st to 10th	Night	1 1 2 2 3 3 3 5% 7% 73%	1900
	11th to 20th	Night	3 2 1 6% 3 4 4 3 5% 8% 61%	1900
	21st to end	Night	2 3 1 3 2 3 2 2 4 6% 73%	2157
October	1st to 10th	Night	2 2 1 4 3 3 2 2 4 6% 72%	3800
	11th to 20th	Night	2 4 1 4 3 3 4 3 5% 6% 65%	3777
	21st to end	Night	5% 4 1 4 2 2 3 3 4 6% 65%	4121
November	1st to 10th	Night	7% 5% 2 4 3 3 3 3 5% 7% 58%	2360
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 6% 8% 53%	2600
	21st to end	Night	10% 5% 2 4 3 3 3 3 4 6% 56%	2678
December	1st to 10th	Night	15% 6% 2 5% 3 3 4 3 4 6% 48%	2660
	11th to 20th	Night	29% 9% 2 5% 4 3 3 3 5% 6% 32%	3276
	21st to end	Night	21% 8% 2 6% 4 4 4 4 6% 6% 36%	4862

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% 5% 13% 28% 10% 4	2744
	11th to 20th	Day	48% 6% 12% 15% 11% 8%	2860
	21st to end	Day	50% 5% 10% 16% 13% 5%	3146
February	1st to 10th	Day	40% 3 7% 20% 19% 10%	1710
	11th to 20th	Day	38% 5% 9% 20% 20% 8%	1777
	21st to end	Day	38% 5% 8% 24% 18% 7%	1570
March	1st to 10th	Day	50% 9% 10% 14% 13% 3	3384
	11th to 20th	Day	30% 3 14% 28% 13% 11%	3490
	21st to end	Day	44% 9% 16% 19% 11% 2	3882
April	1st to 10th	Day	35% 5% 15% 20% 21% 5%	2280
	11th to 20th	Day	29% 6% 14% 26% 19% 6%	2252
	21st to end	Day	28% 5% 14% 29% 21% 4	2280
May	1st to 10th	Day	18% 3 11% 28% 27% 13%	3862
	11th to 20th	Day	25% 5% 9% 29% 25% 7%	4124
	21st to end	Day	25% 5% 11% 29% 28% 3	4814
June	1st to 10th	Day	25% 6% 13% 26% 24% 6%	2660
	11th to 20th	Day	19% 4 10% 24% 31% 13%	2660
	21st to end	Day	19% 3 4 18% 31% 25%	2660
July	1st to 10th	Day	11% 2 5% 7% 18% 57%	4321
	11th to 20th	Day	16% 3 6% 9% 16% 51%	4041
	21st to end	Day	14% 3 4 10% 33% 36%	4315
August	1st to 10th	Day	18% 3 6% 15% 28% 30%	4076
	11th to 20th	Day	10% 3 3 12% 46% 24%	3828
	21st to end	Day	16% 3 4 13% 35% 29%	4179
September	1st to 10th	Day	15% 2 3 14% 28% 38%	2280
	11th to 20th	Day	23% 3 8% 22% 30% 14%	2280
	21st to end	Day	15% 2 6% 21% 37% 19%	2019
October	1st to 10th	Day	15% 2 6% 13% 29% 35%	3180
	11th to 20th	Day	17% 4 8% 25% 30% 16%	3161
	21st to end	Day	21% 2 7% 24% 29% 16%	3448
November	1st to 10th	Day	28% 4 8% 26% 24% 11%	1937
	11th to 20th	Day	30% 4 11% 24% 23% 9%	1800
	21st to end	Day	30% 2 10% 18% 27% 12%	1651
December	1st to 10th	Day	38% 4 10% 20% 17% 11%	1520
	11th to 20th	Day	52% 4 10% 13% 14% 6%	1928
	21st to end	Day	46% 4 12% 23% 12% 4	2794
January	1st to 10th	Night	36% 4 18% 26% 12% 4	4192
	11th to 20th	Night	47% 5% 15% 14% 11% 8%	4110
	21st to end	Night	49% 4 12% 15% 12% 7%	4521
February	1st to 10th	Night	38% 3 8% 18% 19% 14%	2470
	11th to 20th	Night	35% 5% 11% 19% 22% 8%	2300
	21st to end	Night	32% 4 11% 30% 17% 6%	1884
March	1st to 10th	Night	50% 6% 13% 14% 13% 3	3477
	11th to 20th	Night	30% 7% 9% 23% 20% 10%	3490
	21st to end	Night	44% 7% 15% 20% 11% 3	3769
April	1st to 10th	Night	34% 5% 15% 22% 18% 6%	1900
	11th to 20th	Night	31% 7% 11% 27% 16% 7%	1879
	21st to end	Night	24% 8% 13% 29% 21% 6%	1900
May	1st to 10th	Night	18% 3 14% 24% 26% 15%	3098
	11th to 20th	Night	42% 13% 8% 13% 16% 9%	2776
	21st to end	Night	23% 13% 13% 20% 28% 3	2762
June	1st to 10th	Night	25% 9% 10% 24% 23% 8%	1520
	11th to 20th	Night	23% 5% 8% 15% 29% 19%	1520
	21st to end	Night	25% 6% 4 13% 26% 27%	1520
July	1st to 10th	Night	14% 3 8% 5% 15% 55%	2526
	11th to 20th	Night	19% 5% 11% 8% 12% 45%	2806
	21st to end	Night	29% 6% 6% 8% 22% 28%	3011
August	1st to 10th	Night	26% 6% 10% 11% 24% 23%	2839
	11th to 20th	Night	16% 5% 7% 8% 35% 29%	3082
	21st to end	Night	20% 5% 8% 10% 34% 23%	3498
September	1st to 10th	Night	17% 5% 4 10% 28% 36%	1900
	11th to 20th	Night	25% 5% 5% 21% 29% 16%	1900
	21st to end	Night	18% 3 4 18% 35% 22%	2157
October	1st to 10th	Night	16% 3 6% 10% 21% 43%	3800
	11th to 20th	Night	20% 7% 7% 24% 23% 20%	3777
	21st to end	Night	22% 2 5% 22% 26% 22%	4121
November	1st to 10th	Night	26% 3 8% 25% 23% 14%	2360
	11th to 20th	Night	28% 4 11% 23% 24% 10%	2600
	21st to end	Night	29% 4 9% 20% 23% 15%	2678
December	1st to 10th	Night	36% 3 10% 19% 18% 13%	2660
	11th to 20th	Night	52% 3 11% 14% 16% 5%	3276
	21st to end	Night	46% 4 14% 18% 13% 5%	4862

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% 12% 22% 21% 16% 2	307
	11th to 20th	Day	17% 10% 23% 30% 17% 3	370
	21st to end	Day	16% 9% 23% 29% 19% 4	407
February	1st to 10th	Day	19% 14% 21% 23% 18% 5%	370
	11th to 20th	Day	17% 15% 26% 27% 13% 2	414
	21st to end	Day	12% 14% 29% 30% 14% 2	382
March	1st to 10th	Day	23% 14% 29% 23% 10% 1	460
	11th to 20th	Day	20% 14% 23% 24% 14% 4	460
	21st to end	Day	14% 15% 32% 26% 10% 2	506
April	1st to 10th	Day	17% 14% 25% 29% 12% 3	460
	11th to 20th	Day	10% 16% 29% 27% 14% 4	452
	21st to end	Day	5% 16% 29% 30% 13% 6%	460
May	1st to 10th	Day	9% 13% 31% 29% 14% 5%	463
	11th to 20th	Day	8% 9% 30% 31% 17% 5%	488
	21st to end	Day	6% 12% 24% 32% 19% 6%	529
June	1st to 10th	Day	5% 9% 24% 33% 23% 6%	490
	11th to 20th	Day	1 9% 28% 38% 20% 4	490
	21st to end	Day	2 3 19% 44% 26% 6%	490
July	1st to 10th	Day	2 1 16% 42% 32% 7%	486
	11th to 20th	Day	1 11% 46% 32% 10%	484
	21st to end	Day	1 11% 43% 36% 11%	531
August	1st to 10th	Day	3 1 17% 44% 28% 7%	487
	11th to 20th	Day	1 1 15% 42% 30% 11%	477
	21st to end	Day	2 11% 43% 33% 10%	539
September	1st to 10th	Day	2 3 10% 45% 29% 11%	490
	11th to 20th	Day	3 4 24% 37% 23% 8%	490
	21st to end	Day	4 5% 18% 33% 31% 9%	490
October	1st to 10th	Day	3 5% 22% 30% 30% 10%	490
	11th to 20th	Day	5% 6% 20% 29% 30% 10%	490
	21st to end	Day	7% 7% 21% 29% 28% 7%	539
November	1st to 10th	Day	8% 12% 19% 25% 27% 10%	473
	11th to 20th	Day	9% 9% 20% 28% 26% 9%	300
	21st to end	Day	13% 6% 21% 28% 26% 6%	231
December	1st to 10th	Day	17% 11% 15% 27% 23% 7%	180
	11th to 20th	Day	13% 15% 22% 21% 18% 11%	180
	21st to end	Day	17% 17% 24% 22% 14% 6%	198
January	1st to 10th	Night	17% 7% 17% 30% 24% 6%	760
	11th to 20th	Night	17% 5% 14% 32% 24% 8%	760
	21st to end	Night	18% 5% 13% 33% 26% 5%	836
February	1st to 10th	Night	18% 8% 14% 31% 24% 5%	760
	11th to 20th	Night	15% 8% 18% 34% 20% 6%	744
	21st to end	Night	14% 8% 21% 33% 21% 3	628
March	1st to 10th	Night	20% 8% 18% 29% 22% 3	760
	11th to 20th	Night	16% 8% 16% 29% 26% 5%	760
	21st to end	Night	14% 8% 20% 35% 20% 3	836
April	1st to 10th	Night	14% 6% 21% 34% 23% 3	760
	11th to 20th	Night	11% 10% 20% 32% 22% 5%	751
	21st to end	Night	6% 7% 27% 30% 23% 8%	760
May	1st to 10th	Night	6% 4 23% 35% 27% 5%	717
	11th to 20th	Night	5% 2 14% 36% 37% 7%	529
	21st to end	Night	3 7% 18% 38% 29% 5%	411
June	1st to 10th	Night	3 3 16% 41% 32% 4	380
	11th to 20th	Night	1 3 14% 46% 34% 2	380
	21st to end	Night	2 1 8% 40% 43% 6%	380
July	1st to 10th	Night	1 5% 35% 53% 6%	383
	11th to 20th	Night	1 3 30% 50% 16%	552
	21st to end	Night	1 4 26% 55% 13%	624
August	1st to 10th	Night	3 8% 33% 48% 7%	567
	11th to 20th	Night	1 1 9% 34% 46% 10%	703
	21st to end	Night	2 8% 30% 48% 12%	836
September	1st to 10th	Night	1 1 8% 31% 48% 12%	760
	11th to 20th	Night	4 1 10% 34% 42% 9%	760
	21st to end	Night	3 2 11% 29% 43% 13%	760
October	1st to 10th	Night	4 3 11% 33% 37% 12%	760
	11th to 20th	Night	6% 3 11% 28% 40% 11%	760
	21st to end	Night	7% 4 10% 32% 31% 16%	836
November	1st to 10th	Night	7% 6% 15% 27% 35% 10%	780
	11th to 20th	Night	11% 4 14% 33% 32% 7%	800
	21st to end	Night	11% 4 14% 37% 26% 7%	788
December	1st to 10th	Night	17% 4 15% 32% 25% 6%	760
	11th to 20th	Night	20% 6% 16% 33% 22% 4	760
	21st to end	Night	17% 10% 20% 30% 19% 3	836

Page generated at 2024-11-24 21:22:18. Version ac07

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