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 2026-06-16.

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 6% 4 4 4 3 5% 7% 46%	137786
	Night	15% 6% 2 5% 3 3 3 3 5% 6% 49%	134619
Transparency	Day	37% 5% 9% 21% 19% 10%	141578
	Night	36% 4 10% 19% 19% 12%	138894
Seeing	Day	15% 8% 23% 30% 19% 5%	16920
	Night	17% 7% 25% 35% 14% 1	26872

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% 8% 2 5% 3 3 3 2 3 5% 52%	5289
	11th to 20th	Day	14% 6% 2 4 2 2 3 3 4 5% 55%	5310
	21st to end	Day	24% 7% 2 5% 3 2 3 2 4 6% 42%	6325
February	1st to 10th	Day	19% 7% 3 6% 3 4 3 2 4 5% 45%	2100
	11th to 20th	Day	18% 7% 2 4 3 3 4 3 5% 6% 45%	2049
	21st to end	Day	18% 6% 2 4 3 2 3 2 4 5% 52%	1730
March	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 52%	4952
	11th to 20th	Day	15% 8% 2 5% 3 3 3 2 3 4 51%	5090
	21st to end	Day	16% 8% 2 5% 3 3 3 3 4 6% 46%	5583
April	1st to 10th	Day	15% 7% 2 7% 4 3 4 3 5% 6% 43%	2310
	11th to 20th	Day	21% 6% 2 6% 4 4 3 4 4 5% 41%	2311
	21st to end	Day	15% 8% 2 6% 5% 4 4 3 5% 6% 42%	2655
May	1st to 10th	Day	22% 8% 2 6% 3 4 4 3 5% 6% 36%	7617
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 8% 43%	7627
	21st to end	Day	14% 7% 3 7% 5% 5% 6% 5% 7% 9% 34%	8392
June	1st to 10th	Day	7% 4 2 5% 4 5% 7% 5% 6% 9% 46%	2730
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 43%	2666
	21st to end	Day	7% 7% 3 7% 5% 6% 5% 6% 7% 9% 38%	2600
July	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 5% 6% 7% 37%	5900
	11th to 20th	Day	5% 5% 2 5% 5% 6% 6% 5% 8% 11% 43%	5900
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6422
August	1st to 10th	Day	1 2 1 2 3 3 3 3 5% 9% 67%	4149
	11th to 20th	Day	4 2 1 4 3 3 3 4 7% 11% 57%	3846
	21st to end	Day	4 3 2 6% 5% 6% 6% 5% 8% 10% 46%	4157
September	1st to 10th	Day	10% 6% 3 7% 7% 5% 5% 4 4 6% 44%	2200
	11th to 20th	Day	10% 6% 2 7% 6% 5% 6% 5% 7% 9% 39%	2200
	21st to end	Day	8% 4 2 6% 4 5% 5% 3 6% 7% 50%	2200
October	1st to 10th	Day	6% 4 2 5% 4 4 4 4 5% 7% 56%	2200
	11th to 20th	Day	12% 5% 1 4 3 3 4 3 4 6% 53%	2088
	21st to end	Day	12% 4 2 4 3 3 4 3 4 5% 58%	2200
November	1st to 10th	Day	14% 7% 3 6% 4 4 3 3 5% 6% 46%	2058
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 4 5% 51%	1942
	21st to end	Day	17% 7% 3 5% 4 4 3 3 4 6% 45%	1890
December	1st to 10th	Day	26% 8% 3 7% 3 2 2 3 3 5% 39%	3030
	11th to 20th	Day	16% 8% 2 6% 4 3 3 3 4 5% 44%	3481
	21st to end	Day	15% 6% 2 4 2 3 2 2 3 4 58%	4587
January	1st to 10th	Night	16% 7% 2 4 2 3 2 2 4 5% 53%	7634
	11th to 20th	Night	10% 3 1 3 2 2 3 3 4 4 64%	7680
	21st to end	Night	28% 6% 2 4 3 2 3 2 4 5% 41%	7944
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 2 4 5% 49%	2520
	11th to 20th	Night	20% 7% 2 4 3 2 3 2 4 5% 49%	2458
	21st to end	Night	20% 6% 2 4 3 3 3 3 4 4 47%	2076
March	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 5% 6% 45%	5124
	11th to 20th	Night	15% 9% 2 5% 3 3 3 2 3 4 51%	5090
	21st to end	Night	20% 9% 2 5% 3 2 3 3 4 6% 44%	5588
April	1st to 10th	Night	16% 6% 2 6% 4 3 3 4 5% 7% 44%	2310
	11th to 20th	Night	20% 9% 2 5% 3 3 3 3 4 5% 44%	2252
	21st to end	Night	22% 7% 2 5% 4 3 3 2 4 4 45%	1964
May	1st to 10th	Night	25% 10% 2 6% 3 3 3 3 4 5% 34%	5266
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 7% 44%	5265
	21st to end	Night	15% 8% 3 6% 4 4 5% 4 7% 9% 36%	5793
June	1st to 10th	Night	7% 4 1 5% 2 3 3 3 5% 9% 57%	1890
	11th to 20th	Night	6% 4 1 5% 4 3 4 3 5% 8% 56%	1845
	21st to end	Night	8% 5% 2 5% 3 3 4 4 7% 9% 51%	1800
July	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 9% 41%	4078
	11th to 20th	Night	5% 4 1 5% 4 5% 5% 5% 8% 10% 48%	4077
	21st to end	Night	8% 7% 2 5% 4 4 5% 4 7% 9% 45%	4449
August	1st to 10th	Night	1 1 1 2 2 2 3 2 5% 8% 74%	3005
	11th to 20th	Night	3 3 1 3 2 3 4 3 7% 11% 59%	3213
	21st to end	Night	5% 4 1 3 3 4 4 3 6% 10% 54%	3747
September	1st to 10th	Night	9% 6% 2 5% 5% 3 4 4 5% 8% 49%	2200
	11th to 20th	Night	9% 6% 3 6% 4 4 5% 4 5% 8% 46%	2200
	21st to end	Night	8% 5% 2 5% 3 4 5% 4 6% 8% 52%	2200
October	1st to 10th	Night	6% 4 1 5% 4 3 3 3 4 6% 61%	2200
	11th to 20th	Night	13% 5% 1 4 3 3 3 2 5% 6% 55%	2290
	21st to end	Night	15% 5% 1 4 2 2 2 2 3 4 60%	2640
November	1st to 10th	Night	13% 7% 2 5% 3 2 3 2 3 5% 54%	2460
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 6% 56%	2672
	21st to end	Night	19% 5% 1 5% 3 3 3 3 4 6% 47%	2730
December	1st to 10th	Night	24% 8% 2 5% 4 3 3 2 4 5% 39%	4380
	11th to 20th	Night	22% 9% 2 5% 3 2 2 2 3 3 47%	4946
	21st to end	Night	18% 6% 2 4 2 2 2 2 3 4 56%	6633

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	36% 2 5% 21% 25% 11%	5290
	11th to 20th	Day	34% 1 4 14% 25% 22%	5310
	21st to end	Day	45% 2 5% 12% 24% 11%	6325
February	1st to 10th	Day	43% 3 4 12% 20% 17%	2100
	11th to 20th	Day	41% 3 7% 18% 18% 12%	2059
	21st to end	Day	38% 2 6% 17% 22% 16%	1730
March	1st to 10th	Day	35% 3 5% 18% 17% 22%	4952
	11th to 20th	Day	38% 4 4 9% 16% 30%	5090
	21st to end	Day	39% 4 8% 18% 20% 11%	5585
April	1st to 10th	Day	42% 5% 7% 16% 17% 12%	2310
	11th to 20th	Day	46% 5% 5% 15% 18% 11%	2311
	21st to end	Day	44% 5% 6% 19% 16% 10%	2655
May	1st to 10th	Day	48% 4 9% 20% 15% 3	7655
	11th to 20th	Day	39% 5% 10% 18% 21% 7%	7631
	21st to end	Day	44% 6% 11% 26% 12% 1	8405
June	1st to 10th	Day	32% 8% 12% 28% 18% 3	2730
	11th to 20th	Day	35% 5% 12% 25% 19% 4	2666
	21st to end	Day	39% 7% 12% 23% 15% 3	2600
July	1st to 10th	Day	44% 7% 16% 28% 5% 1	5836
	11th to 20th	Day	32% 7% 17% 30% 13% 1	5831
	21st to end	Day	34% 8% 15% 26% 16% 2	6240
August	1st to 10th	Day	15% 5% 11% 41% 28% 1	4135
	11th to 20th	Day	20% 5% 23% 38% 12% 2	3882
	21st to end	Day	30% 9% 16% 25% 17% 2	4156
September	1st to 10th	Day	41% 5% 9% 20% 17% 9%	2200
	11th to 20th	Day	38% 7% 12% 20% 18% 5%	2200
	21st to end	Day	32% 6% 10% 21% 18% 13%	2200
October	1st to 10th	Day	18% 3 7% 12% 25% 35%	3590
	11th to 20th	Day	24% 3 8% 19% 25% 21%	3380
	21st to end	Day	28% 3 5% 17% 33% 14%	3538
November	1st to 10th	Day	39% 4 7% 18% 20% 12%	2058
	11th to 20th	Day	35% 3 7% 17% 22% 16%	1942
	21st to end	Day	42% 3 5% 17% 19% 14%	1890
December	1st to 10th	Day	50% 3 4 18% 15% 10%	3030
	11th to 20th	Day	41% 5% 7% 11% 22% 14%	3479
	21st to end	Day	32% 3 4 16% 28% 18%	4587
January	1st to 10th	Night	35% 2 6% 18% 24% 15%	7635
	11th to 20th	Night	26% 1 4 17% 30% 23%	7680
	21st to end	Night	48% 2 4 11% 22% 14%	7944
February	1st to 10th	Night	40% 2 6% 13% 21% 18%	2520
	11th to 20th	Night	40% 3 6% 17% 19% 15%	2458
	21st to end	Night	40% 3 5% 15% 19% 17%	2076
March	1st to 10th	Night	40% 3 6% 17% 17% 16%	5124
	11th to 20th	Night	38% 4 6% 11% 15% 27%	5090
	21st to end	Night	43% 3 8% 17% 19% 11%	5588
April	1st to 10th	Night	41% 4 8% 17% 15% 15%	2310
	11th to 20th	Night	44% 4 8% 13% 17% 14%	2252
	21st to end	Night	44% 4 9% 18% 16% 10%	1964
May	1st to 10th	Night	53% 3 9% 16% 14% 5%	5293
	11th to 20th	Night	37% 6% 10% 18% 20% 8%	5266
	21st to end	Night	44% 6% 15% 25% 8% 2	5805
June	1st to 10th	Night	25% 5% 16% 32% 19% 3	1890
	11th to 20th	Night	27% 5% 15% 29% 20% 5%	1845
	21st to end	Night	29% 7% 18% 27% 16% 3	1800
July	1st to 10th	Night	38% 9% 23% 26% 4	4035
	11th to 20th	Night	28% 9% 22% 30% 11% 1	4026
	21st to end	Night	34% 8% 20% 24% 12% 3	4320
August	1st to 10th	Night	11% 4 15% 47% 22% 1	2995
	11th to 20th	Night	19% 6% 31% 33% 9% 2	3243
	21st to end	Night	25% 11% 21% 26% 16% 2	3747
September	1st to 10th	Night	33% 7% 15% 21% 17% 8%	2200
	11th to 20th	Night	36% 8% 14% 23% 14% 5%	2200
	21st to end	Night	29% 5% 14% 21% 17% 13%	2200
October	1st to 10th	Night	17% 2 8% 15% 21% 37%	3590
	11th to 20th	Night	22% 2 9% 17% 30% 20%	3724
	21st to end	Night	27% 2 6% 17% 33% 15%	4252
November	1st to 10th	Night	35% 3 8% 18% 23% 13%	2460
	11th to 20th	Night	31% 4 8% 19% 20% 17%	2672
	21st to end	Night	39% 3 8% 17% 19% 14%	2730
December	1st to 10th	Night	49% 2 6% 16% 16% 12%	4380
	11th to 20th	Night	44% 2 5% 10% 21% 18%	4947
	21st to end	Night	34% 2 3 14% 29% 17%	6633

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% 16% 35% 18% 7%	330
	11th to 20th	Day	25% 17% 28% 22% 8% 1	330
	21st to end	Day	22% 20% 33% 20% 5%	363
February	1st to 10th	Day	31% 11% 33% 20% 5%	330
	11th to 20th	Day	25% 21% 36% 14% 3	330
	21st to end	Day	23% 16% 44% 15% 1	273
March	1st to 10th	Day	24% 17% 41% 16% 2	330
	11th to 20th	Day	17% 20% 39% 18% 5%	330
	21st to end	Day	20% 17% 36% 21% 5% 2	363
April	1st to 10th	Day	22% 15% 38% 22% 2	330
	11th to 20th	Day	23% 19% 34% 20% 4	349
	21st to end	Day	21% 14% 38% 23% 4	675
May	1st to 10th	Day	19% 14% 37% 25% 6%	753
	11th to 20th	Day	18% 11% 31% 31% 8% 2	775
	21st to end	Day	17% 7% 29% 34% 12% 1	844
June	1st to 10th	Day	9% 4 19% 39% 24% 4	780
	11th to 20th	Day	10% 2 19% 45% 22% 2	756
	21st to end	Day	9% 3 13% 38% 30% 7%	730
July	1st to 10th	Day	12% 2 10% 37% 30% 9%	721
	11th to 20th	Day	7% 2 8% 37% 39% 7%	718
	21st to end	Day	6% 2 5% 35% 43% 9%	794
August	1st to 10th	Day	2 2 5% 38% 40% 13%	680
	11th to 20th	Day	6% 2 4 40% 38% 10%	517
	21st to end	Day	4 1 5% 31% 39% 19%	493
September	1st to 10th	Day	10% 4 8% 29% 37% 12%	330
	11th to 20th	Day	5% 1 5% 32% 43% 13%	330
	21st to end	Day	7% 2 9% 35% 34% 12%	330
October	1st to 10th	Day	5% 2 16% 35% 32% 10%	330
	11th to 20th	Day	11% 2 26% 37% 19% 5%	330
	21st to end	Day	19% 10% 28% 25% 15% 3	363
November	1st to 10th	Day	20% 6% 26% 31% 13% 4	330
	11th to 20th	Day	19% 12% 27% 29% 10% 3	330
	21st to end	Day	26% 15% 32% 16% 9% 2	330
December	1st to 10th	Day	21% 9% 35% 25% 7% 2	330
	11th to 20th	Day	23% 11% 34% 24% 8% 1	330
	21st to end	Day	24% 10% 35% 24% 7% 1	363
January	1st to 10th	Night	20% 11% 30% 31% 7% 1	1050
	11th to 20th	Night	21% 13% 31% 28% 6%	1050
	21st to end	Night	25% 10% 28% 30% 6%	992
February	1st to 10th	Night	25% 11% 30% 28% 6%	840
	11th to 20th	Night	29% 11% 31% 26% 3	824
	21st to end	Night	26% 11% 39% 22% 2	692
March	1st to 10th	Night	24% 13% 36% 23% 4	677
	11th to 20th	Night	22% 11% 35% 28% 4	630
	21st to end	Night	22% 10% 37% 26% 4	693
April	1st to 10th	Night	18% 12% 40% 27% 3	630
	11th to 20th	Night	24% 11% 36% 25% 4	623
	21st to end	Night	26% 8% 37% 22% 7% 1	630
May	1st to 10th	Night	21% 9% 38% 28% 4	630
	11th to 20th	Night	16% 7% 36% 33% 7%	621
	21st to end	Night	16% 6% 30% 36% 11%	683
June	1st to 10th	Night	9% 3 25% 41% 20% 1	630
	11th to 20th	Night	8% 2 31% 44% 13%	615
	21st to end	Night	8% 2 18% 47% 22% 4	600
July	1st to 10th	Night	13% 2 12% 43% 27% 3	585
	11th to 20th	Night	4 2 11% 53% 28% 2	582
	21st to end	Night	7% 1 10% 49% 29% 3	657
August	1st to 10th	Night	2 1 9% 50% 34% 4	597
	11th to 20th	Night	6% 2 7% 50% 30% 5%	594
	21st to end	Night	8% 1 5% 42% 38% 6%	660
September	1st to 10th	Night	10% 2 10% 40% 35% 5%	600
	11th to 20th	Night	12% 2 8% 39% 37% 3	600
	21st to end	Night	11% 2 13% 36% 34% 4	600
October	1st to 10th	Night	8% 3 11% 45% 29% 5%	600
	11th to 20th	Night	14% 3 21% 39% 21% 2	690
	21st to end	Night	17% 8% 22% 32% 20% 2	880
November	1st to 10th	Night	15% 6% 20% 40% 17% 1	820
	11th to 20th	Night	16% 7% 25% 38% 14% 1	992
	21st to end	Night	22% 10% 27% 30% 10% 1	1050
December	1st to 10th	Night	21% 8% 28% 31% 9% 1	1050
	11th to 20th	Night	25% 9% 25% 32% 8%	1050
	21st to end	Night	22% 9% 28% 34% 7%	1155

Page generated at 2026-06-13 01:31:39. Version ac07

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