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 2025-02-21.

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% 4 4 4 3 5% 7% 46%	109195
	Night	15% 6% 2 5% 3 3 3 3 4 6% 50%	109640
Transparency	Day	37% 5% 9% 21% 19% 10%	113026
	Night	35% 4 10% 19% 20% 12%	113936
Seeing	Day	15% 8% 22% 30% 20% 5%	15135
	Night	17% 7% 25% 35% 15% 1	25213

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	16% 9% 2 5% 3 3 2 2 3 5% 50%	4077
	11th to 20th	Day	16% 6% 2 4 2 2 3 3 5% 5% 53%	4080
	21st to end	Day	26% 7% 2 5% 2 2 2 2 4 6% 43%	4879
February	1st to 10th	Day	19% 7% 3 6% 3 4 3 2 4 5% 45%	2000
	11th to 20th	Day	18% 7% 2 5% 3 3 4 3 5% 6% 45%	1949
	21st to end	Day	18% 6% 2 4 3 2 3 3 4 5% 50%	1570
March	1st to 10th	Day	16% 8% 2 6% 4 3 4 2 4 6% 43%	3370
	11th to 20th	Day	21% 11% 3 8% 5% 5% 4 3 4 4 33%	3480
	21st to end	Day	16% 8% 2 5% 3 3 3 3 5% 7% 45%	3814
April	1st to 10th	Day	14% 7% 2 7% 4 3 4 3 5% 6% 44%	2090
	11th to 20th	Day	21% 6% 2 6% 4 3 2 4 4 5% 42%	2089
	21st to end	Day	14% 7% 2 6% 5% 4 4 3 5% 6% 44%	2402
May	1st to 10th	Day	24% 9% 2 7% 4 5% 4 3 5% 6% 31%	4082
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 5% 6% 43%	4082
	21st to end	Day	12% 6% 2 6% 4 5% 6% 5% 7% 9% 37%	4477
June	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 6% 9% 47%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 4 7% 9% 44%	2470
	21st to end	Day	7% 7% 3 7% 5% 6% 5% 5% 7% 9% 38%	2470
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 6% 8% 46%	4120
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 5% 7% 11% 47%	4120
	21st to end	Day	9% 6% 2 7% 5% 6% 6% 5% 7% 10% 38%	4465
August	1st to 10th	Day	1 2 1 2 3 3 3 3 5% 10% 67%	4022
	11th to 20th	Day	4 2 1 4 3 3 3 3 7% 11% 58%	3726
	21st to end	Day	4 3 2 6% 5% 6% 6% 5% 7% 10% 46%	4030
September	1st to 10th	Day	9% 6% 3 7% 7% 5% 5% 4 5% 6% 43%	2090
	11th to 20th	Day	11% 6% 2 7% 6% 5% 6% 4 6% 8% 38%	2090
	21st to end	Day	8% 4 2 6% 4 5% 5% 3 6% 7% 50%	2090
October	1st to 10th	Day	7% 4 2 6% 4 4 4 4 4 6% 55%	2090
	11th to 20th	Day	13% 6% 1 4 3 3 4 3 4 6% 51%	1984
	21st to end	Day	12% 4 2 4 3 3 3 3 3 5% 58%	2090
November	1st to 10th	Day	14% 7% 3 6% 4 4 3 3 5% 6% 44%	1958
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 5% 50%	1850
	21st to end	Day	16% 7% 3 4 4 4 3 3 4 6% 45%	1800
December	1st to 10th	Day	26% 8% 3 7% 3 2 2 3 3 5% 39%	2940
	11th to 20th	Day	17% 8% 2 6% 4 3 3 3 4 5% 44%	3391
	21st to end	Day	15% 6% 2 4 2 3 2 2 3 4 59%	4488
January	1st to 10th	Night	20% 7% 2 4 2 2 2 2 3 5% 51%	5888
	11th to 20th	Night	12% 3 1 3 2 2 3 2 4 4 64%	5900
	21st to end	Night	29% 6% 2 5% 3 2 3 2 4 5% 41%	6140
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 4 5% 48%	2400
	11th to 20th	Night	20% 7% 2 4 3 2 3 2 4 5% 49%	2338
	21st to end	Night	20% 6% 2 4 4 3 3 3 4 5% 46%	1884
March	1st to 10th	Night	18% 9% 2 6% 4 4 3 3 4 6% 41%	3484
	11th to 20th	Night	22% 12% 2 7% 4 3 4 3 4 4 35%	3480
	21st to end	Night	18% 8% 2 6% 3 3 3 3 4 6% 44%	3817
April	1st to 10th	Night	15% 6% 2 6% 4 3 3 4 5% 7% 46%	2090
	11th to 20th	Night	19% 8% 2 5% 3 3 3 3 3 5% 45%	2036
	21st to end	Night	20% 6% 2 5% 4 3 3 2 4 4 47%	1777
May	1st to 10th	Night	27% 10% 2 5% 3 3 3 3 3 5% 37%	2823
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 49%	2823
	21st to end	Night	14% 7% 2 4 4 3 4 4 6% 8% 43%	3090
June	1st to 10th	Night	6% 4 1 5% 2 3 3 3 5% 9% 59%	1710
	11th to 20th	Night	6% 3 1 4 4 4 4 3 5% 8% 58%	1710
	21st to end	Night	9% 5% 2 4 3 3 4 3 7% 9% 51%	1710
July	1st to 10th	Night	6% 4 1 4 3 3 4 5% 6% 10% 53%	2850
	11th to 20th	Night	3 4 1 5% 4 4 5% 4 7% 10% 53%	2850
	21st to end	Night	11% 9% 2 6% 4 4 5% 4 6% 9% 41%	3098
August	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 7% 74%	2913
	11th to 20th	Night	3 3 1 3 2 3 4 3 6% 11% 60%	3113
	21st to end	Night	5% 4 1 3 3 4 4 3 6% 10% 54%	3632
September	1st to 10th	Night	9% 5% 2 5% 4 3 5% 4 5% 8% 49%	2090
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 5% 8% 45%	2090
	21st to end	Night	8% 5% 2 5% 3 3 5% 4 6% 8% 52%	2090
October	1st to 10th	Night	6% 4 1 5% 4 3 3 3 4 6% 60%	2090
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 5% 6% 54%	2175
	21st to end	Night	15% 5% 1 4 2 2 2 2 3 4 60%	2508
November	1st to 10th	Night	14% 8% 2 5% 4 2 3 2 3 5% 52%	2340
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 6% 55%	2545
	21st to end	Night	18% 5% 1 4 3 3 3 3 4 7% 47%	2600
December	1st to 10th	Night	24% 8% 2 5% 4 3 3 2 4 5% 39%	4250
	11th to 20th	Night	22% 9% 2 5% 3 2 2 2 3 4 47%	4816
	21st to end	Night	18% 6% 2 4 2 2 2 2 3 4 56%	6490

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% 2 4 20% 23% 12%	4078
	11th to 20th	Day	34% 1 5% 15% 24% 20%	4080
	21st to end	Day	45% 2 4 10% 25% 13%	4879
February	1st to 10th	Day	43% 3 4 12% 20% 17%	2000
	11th to 20th	Day	41% 3 7% 18% 18% 13%	1959
	21st to end	Day	39% 2 6% 17% 19% 16%	1570
March	1st to 10th	Day	43% 3 7% 15% 21% 11%	3370
	11th to 20th	Day	54% 6% 5% 9% 16% 9%	3480
	21st to end	Day	38% 4 10% 16% 20% 12%	3814
April	1st to 10th	Day	42% 5% 7% 17% 17% 12%	2090
	11th to 20th	Day	44% 4 5% 16% 19% 11%	2089
	21st to end	Day	43% 5% 5% 20% 18% 10%	2402
May	1st to 10th	Day	53% 5% 8% 14% 14% 5%	4120
	11th to 20th	Day	41% 6% 8% 18% 21% 7%	4086
	21st to end	Day	40% 7% 12% 29% 11% 1	4489
June	1st to 10th	Day	31% 8% 11% 27% 19% 4	2470
	11th to 20th	Day	35% 5% 11% 26% 19% 4	2470
	21st to end	Day	40% 6% 11% 23% 16% 4	2470
July	1st to 10th	Day	33% 7% 15% 37% 7% 1	4056
	11th to 20th	Day	28% 6% 17% 31% 16% 2	4051
	21st to end	Day	37% 8% 16% 29% 9%	4325
August	1st to 10th	Day	15% 5% 11% 41% 28% 1	4008
	11th to 20th	Day	20% 5% 23% 38% 12% 2	3762
	21st to end	Day	30% 9% 16% 25% 17% 2	4029
September	1st to 10th	Day	41% 5% 9% 20% 16% 9%	2090
	11th to 20th	Day	40% 7% 11% 18% 18% 5%	2090
	21st to end	Day	32% 6% 10% 21% 18% 13%	2090
October	1st to 10th	Day	18% 3 7% 12% 24% 36%	3480
	11th to 20th	Day	24% 3 8% 19% 24% 22%	3276
	21st to end	Day	28% 3 5% 17% 33% 14%	3428
November	1st to 10th	Day	41% 4 8% 18% 18% 12%	1958
	11th to 20th	Day	36% 3 7% 17% 21% 16%	1850
	21st to end	Day	41% 3 6% 17% 19% 14%	1800
December	1st to 10th	Day	50% 3 4 19% 15% 10%	2940
	11th to 20th	Day	42% 5% 7% 11% 21% 14%	3389
	21st to end	Day	32% 3 4 16% 28% 18%	4488
January	1st to 10th	Night	37% 2 5% 18% 23% 15%	5888
	11th to 20th	Night	26% 1 4 18% 29% 22%	5900
	21st to end	Night	48% 2 3 10% 21% 15%	6140
February	1st to 10th	Night	41% 2 6% 12% 21% 18%	2400
	11th to 20th	Night	40% 3 5% 17% 19% 16%	2338
	21st to end	Night	41% 3 5% 15% 18% 17%	1884
March	1st to 10th	Night	45% 4 7% 15% 19% 10%	3484
	11th to 20th	Night	53% 5% 8% 11% 14% 10%	3480
	21st to end	Night	41% 4 10% 15% 19% 10%	3817
April	1st to 10th	Night	39% 4 8% 18% 16% 15%	2090
	11th to 20th	Night	42% 4 8% 14% 18% 15%	2036
	21st to end	Night	42% 4 8% 19% 17% 10%	1777
May	1st to 10th	Night	51% 4 8% 16% 13% 7%	2850
	11th to 20th	Night	35% 6% 11% 22% 19% 8%	2824
	21st to end	Night	38% 7% 13% 28% 11% 3	3099
June	1st to 10th	Night	23% 5% 16% 32% 21% 3	1710
	11th to 20th	Night	25% 5% 15% 30% 21% 5%	1710
	21st to end	Night	29% 7% 17% 28% 16% 3	1710
July	1st to 10th	Night	23% 9% 25% 36% 6% 1	2805
	11th to 20th	Night	25% 8% 20% 31% 15% 1	2796
	21st to end	Night	38% 8% 21% 25% 7%	2999
August	1st to 10th	Night	11% 4 15% 47% 22% 1	2903
	11th to 20th	Night	19% 6% 31% 33% 9% 2	3143
	21st to end	Night	25% 11% 21% 26% 16% 2	3632
September	1st to 10th	Night	32% 7% 15% 21% 17% 8%	2090
	11th to 20th	Night	37% 8% 14% 21% 15% 5%	2090
	21st to end	Night	29% 6% 14% 20% 18% 13%	2090
October	1st to 10th	Night	17% 2 8% 15% 21% 37%	3480
	11th to 20th	Night	23% 3 9% 16% 30% 20%	3609
	21st to end	Night	27% 2 6% 18% 33% 15%	4120
November	1st to 10th	Night	37% 3 8% 18% 21% 13%	2340
	11th to 20th	Night	32% 4 8% 19% 20% 18%	2545
	21st to end	Night	39% 3 8% 17% 19% 15%	2600
December	1st to 10th	Night	49% 2 6% 16% 16% 11%	4250
	11th to 20th	Night	44% 2 5% 10% 21% 18%	4817
	21st to end	Night	34% 2 3 14% 29% 17%	6490

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	24% 17% 34% 19% 6%	300
	11th to 20th	Day	26% 17% 27% 22% 8% 1	300
	21st to end	Day	22% 19% 33% 21% 5%	330
February	1st to 10th	Day	32% 11% 31% 20% 5%	300
	11th to 20th	Day	26% 21% 36% 13% 3	300
	21st to end	Day	25% 19% 42% 14%	225
March	1st to 10th	Day	23% 16% 41% 17% 3	270
	11th to 20th	Day	19% 18% 40% 18% 6%	270
	21st to end	Day	21% 19% 37% 18% 4 1	297
April	1st to 10th	Day	23% 14% 40% 20% 2	270
	11th to 20th	Day	23% 19% 33% 20% 5%	287
	21st to end	Day	19% 13% 37% 25% 5%	582
May	1st to 10th	Day	18% 13% 36% 26% 6%	653
	11th to 20th	Day	19% 9% 29% 33% 9% 2	675
	21st to end	Day	18% 6% 27% 35% 12% 1	734
June	1st to 10th	Day	9% 3 17% 41% 26% 5%	680
	11th to 20th	Day	10% 2 18% 45% 23% 3	680
	21st to end	Day	10% 3 12% 38% 30% 7%	680
July	1st to 10th	Day	10% 2 10% 37% 31% 10%	671
	11th to 20th	Day	6% 1 7% 37% 42% 8%	668
	21st to end	Day	5% 2 6% 34% 44% 9%	739
August	1st to 10th	Day	2 2 4 38% 40% 13%	633
	11th to 20th	Day	6% 2 4 40% 38% 10%	477
	21st to end	Day	5% 1 6% 31% 39% 19%	454
September	1st to 10th	Day	10% 4 7% 28% 37% 13%	300
	11th to 20th	Day	6% 1 5% 30% 44% 14%	300
	21st to end	Day	8% 2 9% 36% 34% 12%	300
October	1st to 10th	Day	5% 2 17% 36% 29% 10%	300
	11th to 20th	Day	12% 2 26% 38% 18% 4	300
	21st to end	Day	21% 9% 28% 23% 15% 3	330
November	1st to 10th	Day	21% 6% 27% 31% 11% 4	300
	11th to 20th	Day	20% 13% 27% 27% 10% 3	300
	21st to end	Day	25% 15% 33% 16% 9% 2	300
December	1st to 10th	Day	21% 8% 34% 26% 8% 3	300
	11th to 20th	Day	24% 12% 32% 24% 7%	300
	21st to end	Day	23% 10% 36% 22% 7% 1	330
January	1st to 10th	Night	21% 11% 30% 31% 7% 1	1000
	11th to 20th	Night	21% 13% 31% 29% 7%	1000
	21st to end	Night	24% 10% 29% 30% 6%	945
February	1st to 10th	Night	26% 11% 30% 27% 6%	800
	11th to 20th	Night	30% 11% 31% 26% 3	784
	21st to end	Night	27% 12% 40% 20% 1	628
March	1st to 10th	Night	23% 12% 37% 24% 4	613
	11th to 20th	Night	23% 10% 35% 28% 4	570
	21st to end	Night	21% 11% 38% 25% 4	627
April	1st to 10th	Night	17% 12% 39% 27% 4	570
	11th to 20th	Night	23% 10% 36% 27% 4	563
	21st to end	Night	23% 7% 38% 22% 8% 1	570
May	1st to 10th	Night	21% 9% 38% 28% 4	570
	11th to 20th	Night	15% 5% 37% 35% 8% 1	561
	21st to end	Night	17% 6% 31% 36% 11%	617
June	1st to 10th	Night	8% 2 24% 43% 22% 2	570
	11th to 20th	Night	8% 2 31% 45% 14% 1	570
	21st to end	Night	8% 2 18% 46% 22% 4	570
July	1st to 10th	Night	12% 2 11% 44% 27% 3	555
	11th to 20th	Night	4 1 10% 54% 29% 2	552
	21st to end	Night	7% 1 9% 50% 29% 4	624
August	1st to 10th	Night	2 1 8% 51% 34% 4	567
	11th to 20th	Night	6% 2 6% 51% 30% 5%	564
	21st to end	Night	8% 1 5% 42% 38% 6%	627
September	1st to 10th	Night	10% 2 10% 40% 34% 5%	570
	11th to 20th	Night	12% 2 8% 38% 36% 3	570
	21st to end	Night	11% 2 13% 35% 35% 4	570
October	1st to 10th	Night	8% 3 12% 45% 28% 5%	570
	11th to 20th	Night	14% 4 21% 39% 20% 2	655
	21st to end	Night	17% 8% 22% 31% 20% 2	836
November	1st to 10th	Night	16% 6% 20% 39% 17% 1	780
	11th to 20th	Night	16% 7% 25% 37% 14% 1	945
	21st to end	Night	22% 10% 27% 30% 11% 1	1000
December	1st to 10th	Night	21% 9% 27% 32% 10% 1	1000
	11th to 20th	Night	26% 9% 25% 33% 7%	1000
	21st to end	Night	22% 8% 29% 34% 7%	1100

Page generated at 2025-02-18 02:26:33. Version ac07

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