Horizontal Heavens Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horizontal Heavens Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	13% 6% 2 5% 5% 3 4 5% 5% 7% 46%	75043
	Night	14% 5% 2 4 4 2 3 4 4 7% 51%	75112
Transparency	Day	38% 5% 9% 20% 19% 9%	74768
	Night	34% 4 11% 20% 19% 12%	74907
Seeing	Day	15% 7% 22% 31% 21% 5%	13738
	Night	17% 7% 25% 35% 15% 2	23822

By Month

Percentage Cloud Cover SE (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	14% 6% 2 4 3 2 3 3 3 6% 53%	1707
	11th to 20th	Day	18% 8% 2 4 3 2 3 3 4 4 50%	1710
	21st to end	Day	16% 5% 2 4 3 2 3 4 3 6% 52%	2032
February	1st to 10th	Day	19% 6% 3 5% 4 3 3 3 4 5% 46%	1800
	11th to 20th	Day	19% 7% 3 3 4 2 4 3 4 6% 44%	1749
	21st to end	Day	19% 5% 3 4 3 2 2 3 4 5% 50%	1480
March	1st to 10th	Day	19% 8% 3 4 5% 3 4 4 4 6% 41%	1937
	11th to 20th	Day	15% 7% 3 5% 5% 3 5% 4 4 5% 45%	1980
	21st to end	Day	16% 6% 3 4 4 2 3 4 4 6% 47%	2178
April	1st to 10th	Day	15% 7% 3 7% 4 3 4 3 5% 6% 43%	1980
	11th to 20th	Day	21% 6% 2 4 4 3 3 4 4 5% 44%	1977
	21st to end	Day	14% 7% 3 5% 5% 3 4 4 4 6% 45%	2275
May	1st to 10th	Day	16% 6% 2 6% 5% 4 5% 5% 5% 6% 39%	2340
	11th to 20th	Day	14% 8% 4 6% 6% 4 5% 5% 5% 7% 37%	2302
	21st to end	Day	17% 6% 3 6% 5% 4 6% 6% 6% 7% 34%	2531
June	1st to 10th	Day	6% 4 2 5% 5% 3 7% 6% 6% 9% 47%	2340
	11th to 20th	Day	7% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	2340
	21st to end	Day	8% 6% 3 7% 6% 4 6% 6% 7% 9% 37%	2340
July	1st to 10th	Day	11% 6% 3 5% 6% 4 6% 7% 6% 8% 36%	2340
	11th to 20th	Day	5% 3 2 3 5% 3 5% 6% 7% 10% 49%	2340
	21st to end	Day	5% 3 2 4 4 3 6% 5% 6% 11% 50%	2546
August	1st to 10th	Day	2 2 1 3 3 3 5% 5% 5% 10% 61%	2288
	11th to 20th	Day	6% 4 1 5% 6% 3 4 4 7% 9% 51%	2123
	21st to end	Day	5% 4 2 6% 7% 5% 7% 6% 7% 10% 42%	2296
September	1st to 10th	Day	10% 5% 3 6% 7% 4 5% 6% 5% 6% 43%	1980
	11th to 20th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 8% 37%	1980
	21st to end	Day	8% 4 2 6% 5% 4 5% 5% 6% 7% 49%	1980
October	1st to 10th	Day	7% 4 2 5% 5% 3 5% 5% 4 7% 53%	1980
	11th to 20th	Day	14% 6% 2 4 4 3 4 4 5% 6% 50%	1880
	21st to end	Day	12% 4 2 3 3 3 3 4 4 5% 57%	1980
November	1st to 10th	Day	14% 7% 3 6% 5% 3 4 4 5% 6% 44%	1858
	11th to 20th	Day	13% 5% 3 5% 5% 2 4 4 5% 5% 49%	1758
	21st to end	Day	17% 7% 3 4 5% 3 4 3 4 5% 45%	1710
December	1st to 10th	Day	15% 6% 2 6% 5% 2 3 4 4 5% 48%	1710
	11th to 20th	Day	16% 8% 3 6% 4 2 3 5% 4 5% 45%	2161
	21st to end	Day	16% 6% 2 3 3 2 2 3 3 4 56%	3135
January	1st to 10th	Night	17% 5% 2 3 3 2 3 3 4 6% 53%	2458
	11th to 20th	Night	17% 5% 3 4 3 2 3 3 4 4 52%	2470
	21st to end	Night	18% 6% 2 4 3 2 3 3 4 6% 49%	2564
February	1st to 10th	Night	18% 7% 3 5% 3 2 3 2 4 6% 48%	2160
	11th to 20th	Night	20% 7% 2 4 3 2 2 3 4 5% 50%	2098
	21st to end	Night	21% 6% 2 4 4 2 4 4 4 5% 46%	1776
March	1st to 10th	Night	19% 6% 2 5% 5% 3 4 3 5% 6% 43%	2021
	11th to 20th	Night	17% 9% 3 5% 4 2 3 3 4 5% 46%	1980
	21st to end	Night	16% 6% 2 5% 3 1 3 4 4 8% 47%	2178
April	1st to 10th	Night	14% 6% 2 5% 5% 2 3 5% 5% 7% 46%	1980
	11th to 20th	Night	19% 7% 2 4 3 2 4 3 3 6% 46%	1928
	21st to end	Night	19% 5% 2 4 4 2 3 3 4 5% 48%	1684
May	1st to 10th	Night	15% 7% 2 4 4 3 3 4 5% 6% 46%	1620
	11th to 20th	Night	13% 6% 2 4 4 3 5% 3 5% 8% 45%	1593
	21st to end	Night	14% 5% 2 4 5% 3 5% 4 6% 10% 42%	1745
June	1st to 10th	Night	6% 3 1 4 3 2 4 3 5% 9% 60%	1620
	11th to 20th	Night	5% 3 1 4 4 2 5% 4 5% 9% 59%	1620
	21st to end	Night	9% 4 3 4 4 2 4 4 6% 9% 51%	1620
July	1st to 10th	Night	10% 5% 2 4 4 3 5% 7% 6% 10% 45%	1620
	11th to 20th	Night	3 2 1 3 4 2 4 4 6% 10% 60%	1620
	21st to end	Night	5% 3 1 3 3 2 4 4 6% 11% 57%	1772
August	1st to 10th	Night	2 2 1 2 3 1 3 4 6% 9% 69%	1646
	11th to 20th	Night	5% 4 2 3 4 3 5% 5% 6% 10% 53%	1778
	21st to end	Night	7% 4 2 3 4 4 5% 5% 6% 11% 50%	2060
September	1st to 10th	Night	8% 5% 3 5% 5% 2 5% 5% 5% 8% 50%	1980
	11th to 20th	Night	10% 6% 3 5% 5% 3 5% 5% 5% 9% 45%	1980
	21st to end	Night	9% 5% 3 4 3 3 4 5% 5% 8% 52%	1980
October	1st to 10th	Night	6% 4 2 5% 4 2 3 4 4 6% 58%	1980
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 5% 6% 52%	2060
	21st to end	Night	15% 5% 1 3 2 1 2 2 3 4 60%	2376
November	1st to 10th	Night	13% 7% 2 5% 4 2 3 3 3 5% 54%	2220
	11th to 20th	Night	12% 6% 2 4 4 2 3 4 4 6% 54%	2417
	21st to end	Night	19% 5% 2 4 4 3 4 3 4 6% 46%	2470
December	1st to 10th	Night	16% 5% 2 4 4 2 3 3 5% 6% 51%	2470
	11th to 20th	Night	18% 8% 3 5% 4 2 4 3 3 4 47%	3036
	21st to end	Night	19% 6% 2 3 2 1 2 3 3 5% 53%	4532

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	35% 3 4 19% 22% 16%	1708
	11th to 20th	Day	39% 3 5% 15% 21% 17%	1710
	21st to end	Day	36% 3 6% 13% 26% 15%	2032
February	1st to 10th	Day	43% 3 5% 12% 21% 17%	1800
	11th to 20th	Day	42% 3 7% 17% 18% 13%	1759
	21st to end	Day	39% 2 6% 17% 19% 17%	1480
March	1st to 10th	Day	45% 3 10% 13% 18% 11%	1937
	11th to 20th	Day	41% 4 6% 14% 19% 16%	1980
	21st to end	Day	38% 4 9% 17% 19% 12%	2178
April	1st to 10th	Day	42% 5% 7% 18% 17% 11%	1980
	11th to 20th	Day	44% 4 5% 17% 19% 12%	1977
	21st to end	Day	42% 4 5% 20% 18% 11%	2275
May	1st to 10th	Day	46% 5% 7% 16% 19% 7%	2340
	11th to 20th	Day	47% 6% 7% 18% 18% 4	2306
	21st to end	Day	47% 7% 10% 20% 15% 1	2531
June	1st to 10th	Day	31% 8% 11% 27% 19% 4	2340
	11th to 20th	Day	35% 5% 11% 27% 18% 4	2340
	21st to end	Day	41% 7% 11% 22% 15% 3	2340
July	1st to 10th	Day	42% 8% 13% 24% 11% 1	2276
	11th to 20th	Day	27% 7% 18% 28% 19% 2	2271
	21st to end	Day	25% 7% 19% 35% 13% 1	2405
August	1st to 10th	Day	20% 6% 15% 38% 20% 1	2274
	11th to 20th	Day	29% 7% 17% 31% 13% 4	2123
	21st to end	Day	35% 8% 15% 26% 12% 3	2296
September	1st to 10th	Day	41% 6% 9% 21% 16% 8%	1980
	11th to 20th	Day	41% 8% 11% 18% 17% 5%	1980
	21st to end	Day	33% 6% 10% 21% 18% 11%	1980
October	1st to 10th	Day	31% 5% 11% 19% 23% 10%	1980
	11th to 20th	Day	35% 4 9% 15% 21% 17%	1880
	21st to end	Day	31% 4 5% 15% 25% 20%	1980
November	1st to 10th	Day	41% 4 8% 19% 18% 10%	1858
	11th to 20th	Day	37% 4 7% 17% 21% 14%	1758
	21st to end	Day	43% 3 5% 16% 19% 14%	1710
December	1st to 10th	Day	39% 4 6% 20% 21% 9%	1710
	11th to 20th	Day	42% 5% 6% 10% 22% 15%	2159
	21st to end	Day	35% 3 5% 14% 29% 14%	3135
January	1st to 10th	Night	34% 2 6% 16% 23% 19%	2458
	11th to 20th	Night	38% 2 5% 17% 22% 17%	2470
	21st to end	Night	39% 3 4 14% 22% 18%	2564
February	1st to 10th	Night	40% 2 6% 13% 21% 18%	2160
	11th to 20th	Night	40% 3 6% 16% 19% 16%	2098
	21st to end	Night	42% 3 6% 14% 17% 18%	1776
March	1st to 10th	Night	43% 4 8% 15% 19% 11%	2021
	11th to 20th	Night	42% 4 8% 12% 17% 16%	1980
	21st to end	Night	38% 4 11% 16% 17% 14%	2178
April	1st to 10th	Night	39% 4 9% 18% 17% 14%	1980
	11th to 20th	Night	42% 4 8% 14% 18% 15%	1928
	21st to end	Night	41% 4 8% 19% 18% 10%	1684
May	1st to 10th	Night	40% 5% 9% 19% 17% 10%	1620
	11th to 20th	Night	38% 5% 10% 26% 16% 5%	1594
	21st to end	Night	38% 7% 14% 25% 14% 1	1746
June	1st to 10th	Night	23% 5% 16% 31% 22% 3	1620
	11th to 20th	Night	24% 5% 15% 31% 21% 5%	1620
	21st to end	Night	29% 7% 18% 27% 15% 3	1620
July	1st to 10th	Night	31% 10% 21% 27% 10% 1	1575
	11th to 20th	Night	19% 8% 21% 30% 19% 1	1566
	21st to end	Night	19% 8% 27% 35% 10% 1	1673
August	1st to 10th	Night	13% 6% 23% 40% 18% 1	1636
	11th to 20th	Night	26% 8% 21% 31% 11% 3	1778
	21st to end	Night	29% 9% 21% 27% 10% 3	2060
September	1st to 10th	Night	32% 7% 16% 22% 17% 7%	1980
	11th to 20th	Night	37% 8% 15% 22% 14% 5%	1980
	21st to end	Night	30% 6% 15% 20% 18% 11%	1980
October	1st to 10th	Night	27% 4 12% 25% 21% 12%	1980
	11th to 20th	Night	34% 3 9% 16% 19% 18%	2060
	21st to end	Night	31% 2 6% 16% 24% 21%	2376
November	1st to 10th	Night	35% 3 8% 19% 22% 13%	2220
	11th to 20th	Night	33% 4 9% 19% 20% 16%	2417
	21st to end	Night	40% 3 8% 16% 19% 14%	2470
December	1st to 10th	Night	36% 3 7% 20% 21% 12%	2470
	11th to 20th	Night	42% 3 5% 12% 19% 19%	3037
	21st to end	Night	36% 3 4 14% 29% 14%	4532

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	23% 16% 34% 19% 7%	270
	11th to 20th	Day	26% 14% 27% 23% 9% 1	270
	21st to end	Day	20% 18% 35% 22% 5%	294
February	1st to 10th	Day	32% 11% 32% 20% 5%	240
	11th to 20th	Day	28% 20% 34% 15% 4	240
	21st to end	Day	26% 20% 39% 15% 1	198
March	1st to 10th	Day	23% 15% 42% 17% 3	240
	11th to 20th	Day	19% 18% 38% 19% 7%	240
	21st to end	Day	21% 16% 38% 19% 5% 1	264
April	1st to 10th	Day	23% 13% 42% 20% 3	240
	11th to 20th	Day	24% 18% 32% 21% 5%	255
	21st to end	Day	19% 14% 37% 25% 5%	535
May	1st to 10th	Day	17% 13% 36% 27% 7%	603
	11th to 20th	Day	19% 8% 27% 34% 9% 2	625
	21st to end	Day	19% 5% 25% 36% 13% 1	679
June	1st to 10th	Day	8% 3 16% 41% 28% 5%	630
	11th to 20th	Day	9% 1 17% 46% 24% 3	630
	21st to end	Day	10% 3 11% 38% 30% 7%	630
July	1st to 10th	Day	11% 2 10% 37% 31% 10%	621
	11th to 20th	Day	5% 1 6% 36% 43% 8%	618
	21st to end	Day	4 2 4 35% 46% 10%	684
August	1st to 10th	Day	3 2 5% 38% 40% 13%	586
	11th to 20th	Day	7% 2 4 37% 39% 11%	437
	21st to end	Day	5% 1 5% 31% 39% 20%	415
September	1st to 10th	Day	10% 3 4 27% 41% 15%	270
	11th to 20th	Day	7% 1 5% 28% 44% 15%	270
	21st to end	Day	9% 2 8% 33% 36% 13%	270
October	1st to 10th	Day	6% 2 17% 36% 29% 10%	270
	11th to 20th	Day	13% 3 27% 35% 18% 4	270
	21st to end	Day	23% 9% 29% 22% 15% 2	297
November	1st to 10th	Day	20% 5% 26% 32% 12% 4	270
	11th to 20th	Day	22% 13% 24% 28% 9% 3	270
	21st to end	Day	27% 15% 31% 16% 9% 2	270
December	1st to 10th	Day	17% 8% 35% 28% 9% 3	270
	11th to 20th	Day	24% 12% 31% 25% 7%	270
	21st to end	Day	24% 9% 38% 23% 6% 1	297
January	1st to 10th	Night	20% 11% 30% 31% 7% 1	950
	11th to 20th	Night	22% 12% 30% 29% 7%	950
	21st to end	Night	23% 10% 29% 31% 6%	896
February	1st to 10th	Night	24% 12% 31% 27% 6%	720
	11th to 20th	Night	28% 11% 31% 27% 3	704
	21st to end	Night	28% 12% 39% 21% 1	592
March	1st to 10th	Night	22% 12% 37% 25% 4	581
	11th to 20th	Night	22% 10% 35% 29% 4	540
	21st to end	Night	20% 10% 38% 26% 5% 1	594
April	1st to 10th	Night	17% 11% 41% 28% 4	540
	11th to 20th	Night	22% 10% 37% 28% 4	533
	21st to end	Night	22% 7% 39% 22% 9% 1	540
May	1st to 10th	Night	19% 9% 39% 29% 4	540
	11th to 20th	Night	15% 4 36% 36% 8% 1	531
	21st to end	Night	17% 4 30% 38% 11%	584
June	1st to 10th	Night	7% 2 23% 44% 22% 1	540
	11th to 20th	Night	7% 1 31% 45% 15% 1	540
	21st to end	Night	8% 2 18% 46% 22% 4	540
July	1st to 10th	Night	13% 2 11% 44% 27% 4	525
	11th to 20th	Night	3 1 10% 54% 29% 2	522
	21st to end	Night	6% 1 9% 51% 30% 4	591
August	1st to 10th	Night	2 1 8% 52% 33% 4	537
	11th to 20th	Night	6% 2 7% 49% 31% 5%	534
	21st to end	Night	9% 1 5% 42% 38% 6%	594
September	1st to 10th	Night	9% 2 9% 40% 36% 5%	540
	11th to 20th	Night	12% 2 9% 37% 37% 4	540
	21st to end	Night	12% 2 12% 34% 36% 4	540
October	1st to 10th	Night	8% 3 12% 44% 27% 5%	540
	11th to 20th	Night	15% 4 22% 37% 21% 2	620
	21st to end	Night	17% 8% 22% 31% 19% 2	792
November	1st to 10th	Night	15% 6% 20% 40% 18% 1	740
	11th to 20th	Night	16% 7% 25% 37% 14% 1	897
	21st to end	Night	23% 9% 27% 30% 11% 1	950
December	1st to 10th	Night	20% 8% 27% 33% 10% 1	950
	11th to 20th	Night	25% 9% 26% 32% 7%	950
	21st to end	Night	22% 8% 29% 34% 7%	1045

Page generated at 2024-01-28 04:26:38. Version ac07

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