Horse Heaven Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Horse Heaven Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	6% 6% 2 6% 4 4 4 3 5% 7% 51%	135862
	Night	7% 7% 3 7% 4 4 4 4 5% 7% 47%	131475
Transparency	Day	33% 5% 10% 23% 20% 9%	130568
	Night	37% 6% 11% 22% 17% 7%	127772
Seeing	Day	9% 9% 21% 27% 23% 11%	15668
	Night	10% 7% 22% 35% 22% 3	24674

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% 16% 7% 15% 8% 6% 5% 4 5% 5% 15%	3639
	11th to 20th	Day	13% 12% 4 8% 5% 5% 5% 4 5% 7% 33%	3640
	21st to end	Day	14% 14% 4 12% 6% 5% 4 4 4 6% 28%	4004
February	1st to 10th	Day	12% 15% 5% 9% 6% 5% 6% 4 5% 7% 27%	1766
	11th to 20th	Day	11% 12% 4 9% 6% 5% 5% 3 5% 6% 34%	1800
	21st to end	Day	8% 8% 3 8% 6% 5% 7% 4 7% 9% 34%	1650
March	1st to 10th	Day	5% 7% 3 7% 6% 6% 7% 5% 8% 11% 35%	4518
	11th to 20th	Day	4 8% 3 8% 4 5% 5% 3 6% 7% 47%	4814
	21st to end	Day	7% 9% 4 10% 8% 6% 7% 5% 7% 8% 29%	5881
April	1st to 10th	Day	7% 8% 3 7% 7% 5% 6% 5% 6% 9% 37%	2400
	11th to 20th	Day	5% 6% 3 7% 5% 4 5% 5% 6% 8% 46%	2369
	21st to end	Day	4 6% 2 6% 5% 5% 5% 4 7% 8% 47%	2561
May	1st to 10th	Day	8% 5% 1 3 3 3 3 3 5% 7% 58%	6320
	11th to 20th	Day	4 5% 2 6% 4 4 4 3 6% 7% 54%	6277
	21st to end	Day	5% 5% 2 5% 4 4 5% 3 6% 8% 52%	6902
June	1st to 10th	Day	8% 6% 2 6% 4 3 5% 4 6% 7% 50%	2800
	11th to 20th	Day	5% 5% 2 4 4 4 4 3 7% 8% 54%	2800
	21st to end	Day	3 3 1 4 3 3 3 3 5% 6% 66%	2800
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 3 80%	6319
	11th to 20th	Day	1 1 1 2 1 1 1 1 2 3 84%	6320
	21st to end	Day	1 1 1 2 1 2 2 2 4 6% 78%	6880
August	1st to 10th	Day	3 3 1 4 2 3 3 2 4 6% 69%	6210
	11th to 20th	Day	2 1 1 2 2 3 3 3 4 6% 74%	5821
	21st to end	Day	3 4 1 5% 3 3 3 3 4 6% 64%	5984
September	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 7% 70%	2400
	11th to 20th	Day	3 4 2 5% 4 3 4 4 5% 7% 59%	2375
	21st to end	Day	5% 5% 2 5% 3 3 4 3 5% 6% 59%	2196
October	1st to 10th	Day	6% 3 1 4 2 2 3 3 6% 9% 62%	3280
	11th to 20th	Day	4 5% 2 5% 5% 5% 6% 5% 8% 9% 48%	3261
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 6% 8% 42%	3206
November	1st to 10th	Day	9% 9% 3 9% 6% 5% 6% 4 7% 7% 35%	1782
	11th to 20th	Day	10% 14% 4 10% 7% 5% 5% 3 6% 6% 29%	1635
	21st to end	Day	9% 12% 4 12% 7% 6% 6% 5% 6% 7% 24%	1600
December	1st to 10th	Day	8% 8% 5% 12% 8% 5% 6% 5% 8% 7% 27%	2620
	11th to 20th	Day	15% 16% 7% 15% 9% 6% 5% 4 5% 4 15%	3028
	21st to end	Day	14% 13% 6% 15% 9% 7% 6% 4 5% 4 17%	4004
January	1st to 10th	Night	15% 16% 6% 15% 7% 6% 5% 4 6% 5% 15%	6306
	11th to 20th	Night	12% 11% 3 8% 4 4 3 3 5% 8% 40%	6320
	21st to end	Night	12% 13% 5% 13% 6% 4 4 2 3 5% 32%	6952
February	1st to 10th	Night	11% 12% 3 8% 6% 5% 5% 4 6% 7% 32%	2630
	11th to 20th	Night	9% 10% 4 8% 6% 5% 5% 4 6% 7% 35%	2487
	21st to end	Night	8% 10% 4 9% 5% 5% 5% 5% 7% 8% 36%	1980
March	1st to 10th	Night	6% 6% 3 7% 5% 5% 6% 5% 7% 9% 41%	5376
	11th to 20th	Night	5% 7% 3 8% 4 5% 5% 4 6% 7% 46%	5185
	21st to end	Night	5% 9% 3 9% 7% 6% 7% 6% 8% 9% 32%	5081
April	1st to 10th	Night	8% 9% 3 7% 6% 5% 5% 5% 7% 8% 38%	2000
	11th to 20th	Night	5% 8% 3 7% 5% 4 5% 3 6% 6% 49%	1979
	21st to end	Night	4 6% 3 5% 5% 5% 5% 4 7% 7% 50%	1835
May	1st to 10th	Night	4 4 2 5% 3 4 4 4 6% 6% 59%	3640
	11th to 20th	Night	5% 4 1 5% 4 3 4 4 5% 6% 58%	3616
	21st to end	Night	4 4 2 4 2 3 3 4 6% 9% 59%	3972
June	1st to 10th	Night	7% 5% 1 4 4 4 4 4 5% 9% 52%	1600
	11th to 20th	Night	7% 5% 2 4 4 3 4 3 5% 6% 57%	1600
	21st to end	Night	4 3 2 5% 3 3 4 4 5% 5% 64%	1600
July	1st to 10th	Night	1 2 1 2 1 2 2 2 3 4 79%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 85%	3640
	21st to end	Night	2 2 1 2 1 1 2 2 3 6% 77%	3970
August	1st to 10th	Night	2 3 1 3 2 3 3 3 5% 7% 67%	3582
	11th to 20th	Night	2 2 1 3 2 2 3 3 4 6% 72%	4023
	21st to end	Night	3 3 1 4 3 4 4 3 5% 6% 64%	5015
September	1st to 10th	Night	2 2 1 3 2 2 3 3 4 6% 73%	2000
	11th to 20th	Night	3 3 1 4 3 4 4 4 5% 7% 63%	2025
	21st to end	Night	4 4 1 4 3 3 4 3 5% 8% 61%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 7% 65%	3920
	11th to 20th	Night	6% 4 2 5% 3 5% 5% 5% 8% 9% 51%	3897
	21st to end	Night	7% 5% 2 7% 5% 4 5% 4 6% 9% 46%	4592
November	1st to 10th	Night	11% 9% 3 9% 5% 5% 4 4 5% 6% 38%	2561
	11th to 20th	Night	10% 10% 4 8% 7% 6% 5% 5% 6% 6% 34%	2763
	21st to end	Night	10% 11% 3 9% 5% 5% 6% 5% 6% 6% 34%	2800
December	1st to 10th	Night	7% 8% 3 9% 5% 4 5% 5% 6% 10% 39%	4560
	11th to 20th	Night	17% 16% 6% 13% 8% 7% 6% 4 5% 6% 13%	5176
	21st to end	Night	15% 14% 5% 9% 6% 6% 6% 5% 6% 6% 22%	6952

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	69% 9% 8% 10% 4	3639
	11th to 20th	Day	50% 7% 18% 21% 4	3640
	21st to end	Day	58% 6% 9% 19% 8%	4004
February	1st to 10th	Day	57% 9% 15% 13% 5% 1	1766
	11th to 20th	Day	51% 7% 13% 20% 8% 1	1809
	21st to end	Day	44% 9% 17% 19% 10% 1	1650
March	1st to 10th	Day	38% 7% 11% 26% 18% 1	4518
	11th to 20th	Day	36% 5% 11% 32% 16%	4814
	21st to end	Day	48% 9% 15% 20% 7% 1	5881
April	1st to 10th	Day	40% 8% 15% 22% 15% 1	2400
	11th to 20th	Day	35% 5% 13% 24% 18% 4	2372
	21st to end	Day	31% 7% 13% 28% 18% 3	2561
May	1st to 10th	Day	25% 3 7% 23% 34% 7%	6320
	11th to 20th	Day	28% 4 10% 27% 24% 7%	6280
	21st to end	Day	27% 7% 14% 31% 19% 2	6904
June	1st to 10th	Day	32% 5% 9% 28% 20% 5%	2800
	11th to 20th	Day	26% 6% 10% 28% 25% 5%	2800
	21st to end	Day	20% 3 8% 29% 29% 11%	2800
July	1st to 10th	Day	11% 2 3 16% 31% 37%	6251
	11th to 20th	Day	9% 2 2 10% 32% 46%	6245
	21st to end	Day	10% 2 5% 24% 35% 24%	6681
August	1st to 10th	Day	15% 2 4 13% 36% 29%	4524
	11th to 20th	Day	12% 2 5% 34% 35% 12%	4211
	21st to end	Day	20% 3 7% 26% 33% 11%	4311
September	1st to 10th	Day	13% 4 9% 27% 31% 16%	2400
	11th to 20th	Day	23% 6% 10% 26% 26% 9%	2375
	21st to end	Day	26% 5% 13% 32% 19% 6%	2196
October	1st to 10th	Day	21% 4 11% 36% 26% 3	3280
	11th to 20th	Day	31% 7% 18% 27% 16% 1	3261
	21st to end	Day	39% 6% 13% 28% 12% 1	3206
November	1st to 10th	Day	47% 8% 16% 21% 7% 1	1782
	11th to 20th	Day	55% 8% 14% 17% 5% 1	1635
	21st to end	Day	58% 10% 15% 11% 6% 1	1600
December	1st to 10th	Day	53% 10% 15% 17% 4	2620
	11th to 20th	Day	72% 6% 9% 9% 2	3028
	21st to end	Day	68% 8% 9% 12% 2	4004
January	1st to 10th	Night	68% 9% 12% 8% 3	6306
	11th to 20th	Night	43% 6% 18% 25% 7%	6320
	21st to end	Night	56% 5% 10% 18% 10%	6952
February	1st to 10th	Night	49% 9% 18% 18% 5% 1	2630
	11th to 20th	Night	46% 8% 14% 21% 9% 1	2489
	21st to end	Night	44% 10% 13% 22% 9% 2	1980
March	1st to 10th	Night	36% 6% 10% 28% 19% 1	5376
	11th to 20th	Night	36% 5% 10% 31% 18%	5185
	21st to end	Night	43% 11% 15% 25% 6%	5081
April	1st to 10th	Night	42% 7% 14% 24% 12% 2	2000
	11th to 20th	Night	34% 6% 11% 27% 18% 5%	1979
	21st to end	Night	31% 8% 13% 25% 18% 5%	1835
May	1st to 10th	Night	24% 5% 6% 21% 37% 7%	3637
	11th to 20th	Night	25% 4 10% 28% 29% 4	3616
	21st to end	Night	21% 8% 16% 35% 18% 2	3972
June	1st to 10th	Night	29% 6% 9% 28% 22% 7%	1600
	11th to 20th	Night	29% 5% 7% 31% 23% 6%	1600
	21st to end	Night	22% 5% 7% 24% 28% 14%	1600
July	1st to 10th	Night	11% 2 3 18% 26% 40%	3600
	11th to 20th	Night	7% 2 3 9% 33% 47%	3592
	21st to end	Night	10% 2 5% 19% 34% 30%	3857
August	1st to 10th	Night	16% 2 3 10% 34% 34%	2601
	11th to 20th	Night	14% 3 5% 32% 31% 16%	2910
	21st to end	Night	15% 4 7% 26% 30% 18%	3608
September	1st to 10th	Night	14% 3 7% 25% 29% 21%	2000
	11th to 20th	Night	20% 6% 9% 26% 25% 15%	2025
	21st to end	Night	22% 5% 11% 30% 24% 10%	2200
October	1st to 10th	Night	20% 3 9% 32% 33% 2	3920
	11th to 20th	Night	28% 6% 14% 28% 22% 2	3897
	21st to end	Night	34% 5% 11% 32% 16% 1	4592
November	1st to 10th	Night	46% 7% 14% 23% 10% 1	2561
	11th to 20th	Night	49% 9% 15% 19% 7% 1	2763
	21st to end	Night	48% 9% 17% 18% 7% 1	2800
December	1st to 10th	Night	41% 8% 20% 25% 6%	4560
	11th to 20th	Night	71% 8% 12% 7% 2	5176
	21st to end	Night	60% 9% 12% 12% 6%	6952

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% 12% 27% 24% 12% 3	200
	11th to 20th	Day	20% 8% 19% 34% 15% 5%	200
	21st to end	Day	16% 7% 20% 31% 23% 3	220
February	1st to 10th	Day	20% 10% 24% 27% 15% 3	366
	11th to 20th	Day	16% 13% 26% 27% 16% 3	418
	21st to end	Day	11% 17% 25% 29% 14% 4	414
March	1st to 10th	Day	13% 13% 32% 25% 15% 3	500
	11th to 20th	Day	12% 12% 23% 28% 18% 7%	500
	21st to end	Day	12% 15% 27% 28% 14% 3	550
April	1st to 10th	Day	13% 16% 24% 25% 16% 6%	500
	11th to 20th	Day	10% 15% 29% 26% 13% 7%	492
	21st to end	Day	6% 13% 26% 27% 20% 7%	500
May	1st to 10th	Day	9% 9% 26% 27% 23% 7%	503
	11th to 20th	Day	12% 9% 23% 25% 19% 12%	528
	21st to end	Day	10% 5% 19% 28% 24% 14%	573
June	1st to 10th	Day	9% 8% 20% 25% 24% 14%	530
	11th to 20th	Day	6% 10% 29% 32% 16% 8%	530
	21st to end	Day	3 6% 18% 28% 30% 15%	530
July	1st to 10th	Day	1 2 14% 29% 41% 11%	526
	11th to 20th	Day	2 2 15% 28% 37% 15%	524
	21st to end	Day	1 3 11% 26% 36% 23%	575
August	1st to 10th	Day	3 4 13% 24% 32% 24%	527
	11th to 20th	Day	2 3 10% 24% 33% 27%	517
	21st to end	Day	5% 5% 16% 26% 27% 21%	583
September	1st to 10th	Day	3 2 12% 29% 35% 19%	530
	11th to 20th	Day	5% 4 16% 28% 32% 15%	530
	21st to end	Day	7% 9% 15% 30% 30% 10%	530
October	1st to 10th	Day	8% 7% 24% 25% 25% 11%	530
	11th to 20th	Day	9% 11% 19% 28% 23% 10%	530
	21st to end	Day	14% 9% 25% 31% 14% 7%	369
November	1st to 10th	Day	11% 14% 24% 33% 16% 3	306
	11th to 20th	Day	18% 12% 29% 21% 14% 6%	217
	21st to end	Day	11% 7% 24% 30% 23% 7%	200
December	1st to 10th	Day	10% 9% 23% 29% 17% 14%	200
	11th to 20th	Day	22% 6% 25% 31% 14% 3	200
	21st to end	Day	21% 7% 25% 31% 13% 4	220
January	1st to 10th	Night	22% 9% 22% 32% 13% 2	800
	11th to 20th	Night	16% 7% 23% 32% 20% 3	800
	21st to end	Night	16% 5% 19% 38% 20% 3	880
February	1st to 10th	Night	16% 10% 25% 33% 15% 2	800
	11th to 20th	Night	13% 10% 24% 33% 20% 2	784
	21st to end	Night	13% 12% 25% 34% 15% 1	660
March	1st to 10th	Night	11% 9% 25% 35% 18% 2	800
	11th to 20th	Night	8% 11% 24% 35% 20% 2	800
	21st to end	Night	10% 10% 33% 33% 13% 1	880
April	1st to 10th	Night	12% 11% 26% 32% 18% 2	800
	11th to 20th	Night	8% 13% 30% 29% 17% 3	791
	21st to end	Night	6% 11% 30% 31% 19% 1	635
May	1st to 10th	Night	4 9% 29% 35% 20% 4	400
	11th to 20th	Night	9% 8% 21% 33% 24% 5%	394
	21st to end	Night	5% 5% 21% 40% 24% 4	433
June	1st to 10th	Night	7% 6% 25% 35% 26% 2	400
	11th to 20th	Night	9% 8% 32% 36% 14% 1	400
	21st to end	Night	5% 5% 23% 37% 29% 2	400
July	1st to 10th	Night	3 2 15% 48% 28% 3	390
	11th to 20th	Night	4 3 21% 45% 26% 2	388
	21st to end	Night	1 2 13% 47% 33% 3	438
August	1st to 10th	Night	3 2 14% 40% 37% 4	398
	11th to 20th	Night	3 1 13% 36% 37% 11%	575
	21st to end	Night	2 3 17% 37% 34% 6%	880
September	1st to 10th	Night	3 2 16% 37% 36% 5%	800
	11th to 20th	Night	5% 1 16% 32% 38% 8%	800
	21st to end	Night	6% 5% 20% 33% 32% 6%	800
October	1st to 10th	Night	10% 5% 19% 35% 28% 4	800
	11th to 20th	Night	9% 6% 23% 33% 23% 6%	800
	21st to end	Night	10% 7% 26% 33% 19% 4	880
November	1st to 10th	Night	16% 7% 23% 37% 16% 1	788
	11th to 20th	Night	16% 9% 25% 32% 16% 3	800
	21st to end	Night	13% 6% 21% 37% 20% 3	800
December	1st to 10th	Night	14% 4 21% 37% 21% 3	800
	11th to 20th	Night	20% 7% 21% 36% 16% 1	800
	21st to end	Night	19% 9% 26% 34% 12% 1	880

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