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 2024-01-14.

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	8% 4 2 4 4 2 4 4 4 7% 57%	74690
	Night	9% 4 2 4 4 3 4 4 5% 7% 53%	75079
Transparency	Day	29% 4 9% 21% 23% 14%	74412
	Night	32% 5% 9% 19% 21% 14%	74880
Seeing	Day	9% 8% 22% 33% 22% 6%	14334
	Night	10% 4 14% 32% 32% 8%	23814

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	20% 6% 2 5% 4 3 4 5% 5% 8% 37%	1646
	11th to 20th	Day	13% 4 3 5% 4 3 4 4 4 7% 49%	1648
	21st to end	Day	15% 7% 2 5% 5% 2 5% 5% 5% 7% 42%	1782
February	1st to 10th	Day	19% 5% 2 4 4 2 3 3 5% 7% 45%	1620
	11th to 20th	Day	13% 5% 3 5% 5% 3 5% 4 5% 7% 46%	1673
	21st to end	Day	15% 6% 3 5% 4 2 5% 4 4 6% 45%	1480
March	1st to 10th	Day	17% 8% 3 6% 6% 3 5% 4 5% 8% 35%	1944
	11th to 20th	Day	17% 7% 3 6% 5% 3 5% 5% 5% 8% 36%	1980
	21st to end	Day	13% 5% 3 5% 5% 3 5% 5% 4 6% 46%	2210
April	1st to 10th	Day	15% 6% 2 4 4 3 4 4 5% 7% 45%	2160
	11th to 20th	Day	9% 5% 2 5% 4 2 3 4 4 8% 53%	2129
	21st to end	Day	6% 4 3 4 4 3 3 4 5% 7% 56%	2160
May	1st to 10th	Day	6% 5% 2 4 3 2 4 3 5% 8% 57%	2196
	11th to 20th	Day	5% 3 2 3 3 2 3 3 4 9% 62%	2325
	21st to end	Day	5% 6% 2 6% 4 3 4 4 4 8% 53%	2723
June	1st to 10th	Day	5% 4 2 4 3 2 4 4 5% 8% 60%	2520
	11th to 20th	Day	2 2 1 2 3 2 3 3 5% 7% 69%	2520
	21st to end	Day	2 2 1 2 3 1 2 3 3 7% 73%	2520
July	1st to 10th	Day	1 2 1 2 4 2 3 3 4 7% 72%	2502
	11th to 20th	Day	2 2 1 2 3 2 3 3 5% 8% 69%	2340
	21st to end	Day	1 1 1 2 2 2 3 3 4 9% 72%	2546
August	1st to 10th	Day	1 2 1 3 4 2 4 3 4 10% 66%	2328
	11th to 20th	Day	1 1 2 3 2 3 3 4 8% 73%	2165
	21st to end	Day	2 1 3 4 2 4 2 4 9% 69%	2376
September	1st to 10th	Day	2 2 1 2 2 1 2 2 2 6% 79%	2160
	11th to 20th	Day	2 3 1 4 3 3 3 4 6% 8% 63%	2160
	21st to end	Day	3 2 1 1 2 2 2 2 4 5% 74%	1914
October	1st to 10th	Day	3 3 1 3 3 2 3 3 4 6% 68%	1800
	11th to 20th	Day	5% 3 1 4 4 2 3 4 5% 7% 63%	1800
	21st to end	Day	7% 4 2 3 3 2 3 4 4 6% 62%	1980
November	1st to 10th	Day	9% 5% 2 4 4 3 4 4 4 7% 54%	1839
	11th to 20th	Day	9% 6% 3 4 4 2 4 4 5% 8% 52%	1710
	21st to end	Day	12% 4 2 3 3 3 3 4 4 8% 55%	1592
December	1st to 10th	Day	14% 6% 3 5% 5% 2 4 4 5% 8% 45%	1520
	11th to 20th	Day	29% 8% 2 5% 4 3 4 4 5% 6% 30%	1928
	21st to end	Day	20% 9% 2 5% 5% 3 4 5% 5% 7% 36%	2794
January	1st to 10th	Night	16% 6% 3 6% 5% 3 4 5% 6% 7% 40%	2516
	11th to 20th	Night	13% 6% 2 4 4 3 6% 5% 6% 7% 45%	2390
	21st to end	Night	15% 5% 2 4 4 3 4 5% 5% 7% 45%	2574
February	1st to 10th	Night	16% 6% 2 4 3 2 3 4 4 6% 49%	2340
	11th to 20th	Night	12% 5% 2 4 4 3 4 6% 6% 8% 46%	2173
	21st to end	Night	13% 7% 2 4 3 3 3 4 5% 7% 50%	1776
March	1st to 10th	Night	17% 8% 3 6% 4 4 4 5% 5% 5% 38%	2014
	11th to 20th	Night	15% 6% 3 5% 5% 3 4 3 6% 9% 42%	1980
	21st to end	Night	13% 6% 3 4 4 3 4 4 5% 8% 48%	2144
April	1st to 10th	Night	12% 6% 2 4 5% 3 4 5% 5% 8% 47%	1800
	11th to 20th	Night	10% 5% 2 4 4 3 4 4 4 7% 53%	1779
	21st to end	Night	5% 3 2 4 3 2 5% 3 6% 11% 57%	1800
May	1st to 10th	Night	4 5% 2 4 5% 3 4 4 5% 7% 58%	1760
	11th to 20th	Night	3 2 1 4 4 2 5% 5% 6% 11% 56%	1563
	21st to end	Night	3 4 3 5% 5% 4 4 6% 7% 8% 52%	1552
June	1st to 10th	Night	3 2 2 3 3 2 4 5% 6% 9% 60%	1440
	11th to 20th	Night	1 2 2 3 3 2 5% 6% 7% 7% 63%	1440
	21st to end	Night	2 3 1 2 4 3 6% 5% 4 6% 67%	1440
July	1st to 10th	Night	1 4 4 3 5% 4 5% 7% 67%	1452
	11th to 20th	Night	1 1 1 4 4 3 5% 5% 6% 8% 62%	1620
	21st to end	Night	1 2 1 5% 5% 4 8% 6% 6% 10% 52%	1773
August	1st to 10th	Night	1 4 2 5% 7% 4 6% 6% 6% 7% 52%	1611
	11th to 20th	Night	2 1 4 5% 2 5% 6% 5% 10% 60%	1733
	21st to end	Night	1 2 1 5% 5% 4 5% 5% 4 8% 61%	1980
September	1st to 10th	Night	1 1 2 2 2 4 4 4 8% 72%	1800
	11th to 20th	Night	3 2 1 5% 4 3 4 4 5% 9% 61%	1800
	21st to end	Night	2 3 1 3 2 2 2 2 3 6% 73%	2042
October	1st to 10th	Night	4 3 1 4 4 2 3 3 4 6% 67%	2160
	11th to 20th	Night	4 4 2 4 3 2 4 3 5% 7% 62%	2160
	21st to end	Night	6% 3 1 3 3 2 3 3 4 6% 64%	2376
November	1st to 10th	Night	7% 4 2 4 3 3 3 4 4 7% 58%	2239
	11th to 20th	Night	9% 4 2 3 4 2 4 5% 6% 8% 53%	2470
	21st to end	Night	8% 4 2 4 4 2 4 4 4 6% 57%	2584
December	1st to 10th	Night	15% 6% 2 4 4 2 4 4 4 6% 48%	2660
	11th to 20th	Night	29% 8% 2 4 4 2 3 4 5% 6% 32%	3276
	21st to end	Night	21% 7% 3 5% 5% 3 4 5% 5% 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	43% 5% 14% 20% 12% 6%	1646
	11th to 20th	Day	34% 5% 12% 18% 18% 14%	1648
	21st to end	Day	39% 6% 10% 13% 23% 10%	1782
February	1st to 10th	Day	39% 2 7% 21% 19% 11%	1620
	11th to 20th	Day	37% 5% 9% 20% 21% 9%	1682
	21st to end	Day	38% 5% 8% 23% 18% 8%	1480
March	1st to 10th	Day	44% 6% 9% 21% 15% 5%	1944
	11th to 20th	Day	44% 4 10% 19% 18% 4	1980
	21st to end	Day	36% 7% 14% 22% 17% 4	2210
April	1st to 10th	Day	36% 5% 15% 19% 21% 5%	2160
	11th to 20th	Day	29% 5% 14% 26% 20% 7%	2132
	21st to end	Day	28% 5% 13% 29% 21% 4	2160
May	1st to 10th	Day	25% 4 12% 26% 28% 5%	2196
	11th to 20th	Day	24% 3 13% 29% 23% 8%	2328
	21st to end	Day	33% 5% 15% 24% 20% 4	2724
June	1st to 10th	Day	26% 5% 13% 25% 24% 6%	2520
	11th to 20th	Day	19% 4 11% 24% 29% 13%	2520
	21st to end	Day	20% 3 4 18% 31% 23%	2520
July	1st to 10th	Day	19% 3 4 13% 32% 30%	2434
	11th to 20th	Day	22% 4 5% 14% 26% 28%	2271
	21st to end	Day	19% 3 4 16% 31% 27%	2405
August	1st to 10th	Day	24% 3 4 18% 29% 23%	2312
	11th to 20th	Day	17% 4 3 18% 29% 28%	2165
	21st to end	Day	23% 3 3 19% 28% 24%	2376
September	1st to 10th	Day	16% 2 3 14% 28% 37%	2160
	11th to 20th	Day	22% 3 8% 21% 30% 15%	2160
	21st to end	Day	15% 2 6% 22% 36% 18%	1914
October	1st to 10th	Day	18% 3 8% 20% 27% 23%	1800
	11th to 20th	Day	23% 3 10% 19% 23% 22%	1800
	21st to end	Day	26% 2 9% 22% 21% 20%	1980
November	1st to 10th	Day	29% 4 7% 26% 23% 11%	1839
	11th to 20th	Day	30% 4 11% 23% 23% 9%	1710
	21st to end	Day	29% 2 11% 19% 28% 12%	1592
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	41% 4 13% 22% 15% 6%	2516
	11th to 20th	Night	36% 4 12% 17% 16% 14%	2390
	21st to end	Night	36% 5% 13% 14% 19% 13%	2574
February	1st to 10th	Night	38% 3 7% 18% 19% 14%	2340
	11th to 20th	Night	34% 5% 10% 20% 23% 8%	2175
	21st to end	Night	32% 4 11% 29% 17% 7%	1776
March	1st to 10th	Night	46% 5% 11% 17% 15% 6%	2014
	11th to 20th	Night	39% 4 9% 22% 21% 6%	1980
	21st to end	Night	36% 5% 14% 21% 18% 6%	2144
April	1st to 10th	Night	35% 5% 15% 21% 18% 6%	1800
	11th to 20th	Night	31% 6% 11% 27% 17% 8%	1779
	21st to end	Night	24% 8% 12% 30% 20% 6%	1800
May	1st to 10th	Night	28% 5% 11% 22% 24% 11%	1760
	11th to 20th	Night	29% 6% 13% 22% 23% 7%	1564
	21st to end	Night	30% 10% 15% 20% 21% 4	1552
June	1st to 10th	Night	26% 9% 9% 24% 23% 9%	1440
	11th to 20th	Night	25% 5% 8% 15% 29% 19%	1440
	21st to end	Night	26% 6% 4 14% 26% 24%	1440
July	1st to 10th	Night	24% 5% 2 9% 26% 33%	1412
	11th to 20th	Night	24% 8% 5% 13% 20% 29%	1566
	21st to end	Night	36% 9% 4 11% 19% 22%	1674
August	1st to 10th	Night	38% 7% 3 14% 19% 19%	1602
	11th to 20th	Night	27% 5% 2 12% 26% 28%	1733
	21st to end	Night	30% 7% 3 12% 24% 25%	1980
September	1st to 10th	Night	18% 5% 3 11% 27% 36%	1800
	11th to 20th	Night	25% 5% 5% 21% 29% 17%	1800
	21st to end	Night	17% 3 4 19% 35% 22%	2042
October	1st to 10th	Night	23% 4 8% 16% 23% 27%	2160
	11th to 20th	Night	25% 5% 7% 20% 21% 23%	2160
	21st to end	Night	25% 3 6% 21% 23% 22%	2376
November	1st to 10th	Night	26% 3 8% 25% 22% 14%	2239
	11th to 20th	Night	29% 4 11% 22% 25% 10%	2470
	21st to end	Night	28% 4 9% 20% 23% 16%	2584
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% 9% 23% 29% 18% 3	350
	21st to end	Day	15% 9% 22% 30% 20% 5%	374
February	1st to 10th	Day	19% 14% 19% 23% 19% 6%	340
	11th to 20th	Day	16% 15% 27% 28% 13% 1	379
	21st to end	Day	12% 15% 29% 28% 14% 1	346
March	1st to 10th	Day	24% 13% 29% 23% 10% 1	420
	11th to 20th	Day	22% 13% 23% 25% 13% 4	420
	21st to end	Day	15% 15% 31% 27% 10% 2	462
April	1st to 10th	Day	18% 14% 26% 29% 12% 2	420
	11th to 20th	Day	11% 17% 31% 27% 13% 3	412
	21st to end	Day	5% 16% 29% 30% 14% 6%	420
May	1st to 10th	Day	9% 13% 29% 31% 14% 5%	423
	11th to 20th	Day	9% 10% 31% 30% 15% 5%	448
	21st to end	Day	7% 13% 24% 31% 18% 6%	485
June	1st to 10th	Day	6% 9% 24% 33% 23% 6%	450
	11th to 20th	Day	2 9% 27% 38% 20% 4	450
	21st to end	Day	2 3 20% 43% 27% 6%	450
July	1st to 10th	Day	2 1 17% 43% 32% 5%	446
	11th to 20th	Day	1 9% 48% 32% 9%	444
	21st to end	Day	1 11% 41% 37% 10%	487
August	1st to 10th	Day	3 1 19% 44% 28% 6%	447
	11th to 20th	Day	1 12% 43% 32% 12%	437
	21st to end	Day	2 11% 45% 34% 9%	495
September	1st to 10th	Day	2 4 10% 43% 30% 11%	450
	11th to 20th	Day	2 3 24% 38% 25% 8%	450
	21st to end	Day	4 5% 17% 34% 31% 9%	450
October	1st to 10th	Day	3 5% 23% 32% 27% 10%	450
	11th to 20th	Day	6% 5% 20% 29% 30% 11%	450
	21st to end	Day	7% 7% 20% 29% 29% 8%	495
November	1st to 10th	Day	8% 11% 18% 26% 27% 10%	435
	11th to 20th	Day	7% 7% 19% 29% 29% 10%	270
	21st to end	Day	9% 5% 22% 30% 28% 7%	214
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% 15% 32% 24% 8%	736
	21st to end	Night	18% 4 13% 33% 27% 5%	792
February	1st to 10th	Night	18% 7% 13% 31% 26% 6%	720
	11th to 20th	Night	14% 7% 19% 34% 20% 6%	704
	21st to end	Night	15% 9% 21% 32% 20% 3	592
March	1st to 10th	Night	20% 8% 18% 28% 23% 4	720
	11th to 20th	Night	16% 7% 16% 29% 26% 5%	720
	21st to end	Night	15% 7% 20% 35% 21% 3	792
April	1st to 10th	Night	14% 6% 19% 35% 22% 3	720
	11th to 20th	Night	11% 11% 21% 32% 21% 5%	711
	21st to end	Night	6% 6% 26% 31% 23% 8%	720
May	1st to 10th	Night	6% 4 22% 35% 28% 5%	680
	11th to 20th	Night	5% 3 14% 36% 36% 7%	501
	21st to end	Night	3 8% 18% 38% 29% 5%	389
June	1st to 10th	Night	3 3 16% 39% 33% 5%	360
	11th to 20th	Night	1 3 13% 46% 34% 3	360
	21st to end	Night	2 1 9% 40% 43% 5%	360
July	1st to 10th	Night	1 6% 36% 53% 6%	362
	11th to 20th	Night	2 3 29% 50% 17%	522
	21st to end	Night	1 1 4 26% 55% 14%	591
August	1st to 10th	Night	3 9% 34% 47% 7%	537
	11th to 20th	Night	1 1 7% 33% 48% 10%	665
	21st to end	Night	2 7% 30% 49% 12%	792
September	1st to 10th	Night	1 1 8% 30% 48% 13%	720
	11th to 20th	Night	3 1 10% 34% 43% 9%	720
	21st to end	Night	3 2 11% 28% 42% 13%	720
October	1st to 10th	Night	4 3 12% 33% 36% 12%	720
	11th to 20th	Night	6% 3 11% 28% 41% 11%	720
	21st to end	Night	7% 4 9% 32% 31% 17%	792
November	1st to 10th	Night	7% 5% 15% 28% 36% 10%	740
	11th to 20th	Night	11% 3 13% 33% 33% 7%	760
	21st to end	Night	9% 4 14% 39% 27% 7%	760
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 2024-01-11 10:31:57. Version ac07

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