Heaven's Plateau
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heaven's Plateau clear sky chart from 2005-11-06 to 2026-08-30.

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 1 3 2 2 2 2 3 5% 67%	152465
	Night	10% 5% 1 3 2 2 3 2 4 5% 62%	147200
Transparency	Day	22% 3 6% 17% 27% 25%	147436
	Night	26% 3 6% 18% 24% 23%	143213
Seeing	Day	12% 12% 31% 29% 12% 4	17545
	Night	13% 4 17% 37% 24% 5%	26860

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% 7% 1 4 2 2 3 3 4 6% 47%	4722
	11th to 20th	Day	11% 5% 2 4 3 3 2 2 3 4 61%	5118
	21st to end	Day	14% 7% 2 6% 4 3 4 3 5% 7% 46%	5843
February	1st to 10th	Day	20% 9% 2 4 2 3 3 2 4 5% 47%	1890
	11th to 20th	Day	14% 11% 2 4 3 3 3 3 4 7% 45%	2040
	21st to end	Day	19% 11% 2 4 3 3 3 3 4 4 44%	1730
March	1st to 10th	Day	16% 15% 3 6% 4 4 4 3 5% 6% 34%	4653
	11th to 20th	Day	16% 10% 2 4 3 2 3 2 3 5% 49%	5110
	21st to end	Day	19% 11% 2 5% 4 4 4 3 5% 6% 38%	5907
April	1st to 10th	Day	14% 8% 1 4 3 3 4 3 4 7% 48%	2520
	11th to 20th	Day	12% 6% 2 5% 3 2 3 3 5% 5% 54%	2489
	21st to end	Day	12% 5% 1 4 3 2 3 3 4 6% 58%	2520
May	1st to 10th	Day	7% 5% 1 3 2 2 2 2 3 4 68%	7187
	11th to 20th	Day	3 2 1 2 1 2 2 2 2 4 79%	8043
	21st to end	Day	5% 2 1 2 1 2 2 2 3 4 76%	8992
June	1st to 10th	Day	5% 3 1 3 3 2 3 3 4 6% 67%	2940
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 4 81%	2940
	21st to end	Day	2 2 1 1 1 2 1 2 2 84%	2940
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	8219
	11th to 20th	Day	2 2 1 2 2 1 2 1 2 3 82%	8220
	21st to end	Day	1 1 1 1 1 1 1 1 2 3 87%	8440
August	1st to 10th	Day	2 1 2 1 1 1 1 2 3 86%	5864
	11th to 20th	Day	1 1 1 1 1 2 2 91%	5484
	21st to end	Day	2 1 1 2 1 2 2 2 2 4 82%	6093
September	1st to 10th	Day	2 1 1 2 1 1 2 3 87%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 4 76%	2390
	21st to end	Day	4 2 1 2 2 1 1 1 3 3 79%	2195
October	1st to 10th	Day	4 2 1 3 2 2 2 3 4 5% 74%	3280
	11th to 20th	Day	5% 2 1 2 1 2 2 2 3 4 76%	3261
	21st to end	Day	6% 3 1 4 2 3 3 3 5% 6% 66%	3558
November	1st to 10th	Day	11% 5% 1 4 3 3 3 3 4 7% 54%	1860
	11th to 20th	Day	13% 5% 1 4 3 3 4 3 4 7% 53%	1890
	21st to end	Day	12% 7% 1 4 3 3 2 3 4 6% 55%	1827
December	1st to 10th	Day	9% 4 1 3 3 3 4 2 5% 7% 60%	2700
	11th to 20th	Day	26% 8% 2 5% 3 3 3 3 5% 6% 37%	3108
	21st to end	Day	22% 8% 2 6% 4 4 5% 4 6% 7% 33%	4092
January	1st to 10th	Night	17% 8% 2 4 3 3 3 3 5% 7% 44%	8172
	11th to 20th	Night	12% 4 1 3 2 3 3 3 4 5% 59%	7846
	21st to end	Night	14% 7% 2 6% 4 4 3 3 4 6% 47%	8371
February	1st to 10th	Night	18% 8% 2 4 4 2 2 2 4 5% 48%	2730
	11th to 20th	Night	16% 9% 2 5% 3 3 3 3 5% 7% 45%	2467
	21st to end	Night	17% 8% 2 4 2 3 3 2 4 5% 50%	2076
March	1st to 10th	Night	18% 15% 2 4 3 3 3 3 4 6% 39%	5456
	11th to 20th	Night	18% 11% 2 3 2 2 2 2 3 5% 50%	5110
	21st to end	Night	21% 11% 1 4 3 3 4 3 4 7% 39%	5308
April	1st to 10th	Night	14% 6% 1 5% 4 3 3 3 4 6% 51%	2100
	11th to 20th	Night	12% 6% 1 4 3 3 4 3 4 6% 55%	2079
	21st to end	Night	11% 4 1 3 2 3 3 3 5% 6% 59%	2100
May	1st to 10th	Night	5% 5% 1 3 2 2 3 2 3 5% 69%	5766
	11th to 20th	Night	3 2 1 2 1 2 1 2 3 4 79%	4818
	21st to end	Night	4 2 1 2 1 2 2 2 2 4 79%	5181
June	1st to 10th	Night	5% 3 1 3 2 2 2 3 4 6% 69%	1680
	11th to 20th	Night	1 1 1 1 1 2 1 4 3 84%	1680
	21st to end	Night	1 1 1 2 2 1 1 1 2 3 85%	1680
July	1st to 10th	Night	1 1 1 1 1 2 93%	4740
	11th to 20th	Night	3 3 1 2 2 2 2 2 2 3 76%	4740
	21st to end	Night	2 1 1 2 1 2 2 2 2 3 83%	5644
August	1st to 10th	Night	2 1 1 1 1 1 2 2 2 2 84%	4168
	11th to 20th	Night	1 1 1 1 1 2 2 90%	4606
	21st to end	Night	3 1 1 2 1 1 2 2 2 3 82%	5105
September	1st to 10th	Night	2 1 1 1 1 1 2 3 88%	2000
	11th to 20th	Night	4 2 1 2 1 2 2 1 3 4 79%	2008
	21st to end	Night	3 2 1 2 2 3 2 2 2 3 81%	2200
October	1st to 10th	Night	3 2 1 3 2 2 2 2 3 3 78%	3920
	11th to 20th	Night	4 2 1 2 2 1 1 2 3 4 79%	3897
	21st to end	Night	8% 2 1 3 3 3 3 2 3 5% 67%	4253
November	1st to 10th	Night	12% 5% 1 3 2 2 2 2 4 6% 60%	2657
	11th to 20th	Night	12% 4 2 4 3 3 3 3 4 7% 55%	2730
	21st to end	Night	13% 5% 1 4 3 3 3 3 4 6% 57%	2790
December	1st to 10th	Night	10% 4 1 3 2 3 3 3 4 6% 62%	4700
	11th to 20th	Night	25% 8% 2 4 3 3 4 4 7% 8% 33%	5316
	21st to end	Night	21% 10% 3 7% 4 5% 4 4 5% 6% 32%	7106

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% 3 12% 22% 16% 7%	4723
	11th to 20th	Day	29% 3 5% 11% 26% 26%	5118
	21st to end	Day	39% 4 6% 16% 22% 13%	5843
February	1st to 10th	Day	41% 3 7% 21% 17% 12%	1890
	11th to 20th	Day	40% 4 8% 20% 23% 6%	2050
	21st to end	Day	44% 3 8% 22% 17% 7%	1730
March	1st to 10th	Day	49% 5% 10% 14% 17% 5%	4653
	11th to 20th	Day	39% 4 12% 28% 10% 7%	5110
	21st to end	Day	44% 7% 14% 22% 11% 2	5907
April	1st to 10th	Day	36% 5% 13% 23% 17% 6%	2520
	11th to 20th	Day	32% 4 13% 28% 17% 6%	2492
	21st to end	Day	27% 4 13% 30% 23% 3	2520
May	1st to 10th	Day	21% 3 7% 27% 32% 10%	7187
	11th to 20th	Day	12% 2 5% 25% 43% 13%	8046
	21st to end	Day	14% 2 6% 25% 45% 8%	8994
June	1st to 10th	Day	18% 4 9% 24% 31% 13%	2940
	11th to 20th	Day	9% 2 6% 21% 37% 25%	2940
	21st to end	Day	9% 1 2 11% 34% 42%	2940
July	1st to 10th	Day	5% 1 5% 23% 66%	8151
	11th to 20th	Day	11% 1 1 6% 13% 67%	8145
	21st to end	Day	6% 1 1 11% 28% 52%	8252
August	1st to 10th	Day	10% 2 1 6% 25% 55%	4304
	11th to 20th	Day	5% 1 1 7% 35% 51%	4000
	21st to end	Day	8% 1 2 8% 40% 41%	4420
September	1st to 10th	Day	7% 1 2 10% 32% 49%	2400
	11th to 20th	Day	13% 2 5% 21% 39% 21%	2390
	21st to end	Day	12% 2 4 18% 36% 27%	2195
October	1st to 10th	Day	15% 2 4 11% 26% 42%	3280
	11th to 20th	Day	15% 2 3 19% 40% 21%	3261
	21st to end	Day	21% 3 5% 19% 37% 16%	3558
November	1st to 10th	Day	30% 3 9% 21% 23% 15%	1860
	11th to 20th	Day	32% 5% 8% 20% 24% 12%	1890
	21st to end	Day	30% 4 6% 19% 27% 14%	1827
December	1st to 10th	Day	26% 3 6% 23% 26% 16%	2700
	11th to 20th	Day	50% 3 6% 14% 17% 9%	3108
	21st to end	Day	47% 7% 13% 19% 10% 5%	4092
January	1st to 10th	Night	39% 4 14% 19% 16% 8%	8172
	11th to 20th	Night	28% 3 5% 11% 25% 28%	7846
	21st to end	Night	38% 4 5% 15% 18% 19%	8371
February	1st to 10th	Night	39% 3 6% 18% 20% 14%	2730
	11th to 20th	Night	38% 4 7% 22% 19% 8%	2468
	21st to end	Night	38% 3 9% 24% 19% 8%	2076
March	1st to 10th	Night	47% 4 7% 18% 15% 8%	5456
	11th to 20th	Night	39% 4 9% 23% 17% 8%	5110
	21st to end	Night	45% 6% 13% 21% 12% 3	5308
April	1st to 10th	Night	34% 5% 14% 21% 20% 6%	2100
	11th to 20th	Night	31% 5% 12% 27% 16% 9%	2079
	21st to end	Night	26% 5% 11% 35% 18% 6%	2100
May	1st to 10th	Night	20% 3 9% 27% 26% 15%	5764
	11th to 20th	Night	11% 2 7% 26% 39% 16%	4818
	21st to end	Night	12% 2 5% 32% 42% 7%	5182
June	1st to 10th	Night	16% 4 12% 24% 30% 14%	1680
	11th to 20th	Night	6% 3 6% 23% 35% 27%	1680
	21st to end	Night	8% 1 2 12% 35% 41%	1680
July	1st to 10th	Night	3 1 1 6% 24% 66%	4700
	11th to 20th	Night	16% 1 2 6% 14% 62%	4692
	21st to end	Night	10% 1 1 11% 24% 53%	5520
August	1st to 10th	Night	12% 1 1 7% 24% 55%	3050
	11th to 20th	Night	5% 2 1 8% 32% 52%	3356
	21st to end	Night	9% 1 1 10% 37% 42%	3698
September	1st to 10th	Night	5% 2 2 9% 34% 48%	2000
	11th to 20th	Night	12% 2 3 20% 39% 24%	2008
	21st to end	Night	11% 2 2 20% 35% 29%	2200
October	1st to 10th	Night	13% 2 3 11% 26% 45%	3920
	11th to 20th	Night	12% 2 4 25% 36% 22%	3897
	21st to end	Night	21% 3 4 18% 35% 19%	4253
November	1st to 10th	Night	26% 4 8% 22% 22% 18%	2657
	11th to 20th	Night	29% 4 7% 21% 26% 12%	2730
	21st to end	Night	30% 4 6% 20% 25% 16%	2790
December	1st to 10th	Night	25% 2 6% 23% 26% 18%	4700
	11th to 20th	Night	48% 4 6% 14% 21% 8%	5316
	21st to end	Night	51% 7% 10% 18% 9% 6%	7106

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	32% 12% 24% 20% 9% 3	220
	11th to 20th	Day	20% 7% 24% 33% 13% 3	351
	21st to end	Day	20% 10% 28% 25% 15% 3	473
February	1st to 10th	Day	29% 11% 24% 22% 11% 2	430
	11th to 20th	Day	24% 16% 32% 19% 8% 1	530
	21st to end	Day	22% 16% 33% 20% 8%	446
March	1st to 10th	Day	27% 21% 27% 19% 5% 1	540
	11th to 20th	Day	27% 17% 26% 18% 10% 2	540
	21st to end	Day	23% 18% 32% 21% 6%	594
April	1st to 10th	Day	20% 16% 34% 20% 9% 1	540
	11th to 20th	Day	18% 23% 27% 23% 7% 2	532
	21st to end	Day	12% 18% 32% 24% 11% 3	540
May	1st to 10th	Day	12% 19% 33% 24% 9% 3	543
	11th to 20th	Day	14% 16% 31% 22% 12% 4	568
	21st to end	Day	11% 14% 26% 31% 13% 5%	617
June	1st to 10th	Day	6% 12% 35% 29% 11% 8%	570
	11th to 20th	Day	5% 16% 36% 29% 10% 5%	570
	21st to end	Day	5% 8% 38% 29% 12% 7%	570
July	1st to 10th	Day	2 7% 40% 33% 12% 6%	566
	11th to 20th	Day	2 3 35% 39% 13% 7%	564
	21st to end	Day	2 3 38% 36% 13% 7%	619
August	1st to 10th	Day	3 5% 37% 36% 14% 4	567
	11th to 20th	Day	2 6% 40% 34% 13% 5%	557
	21st to end	Day	2 5% 36% 38% 15% 4	620
September	1st to 10th	Day	3 8% 40% 34% 13% 2	530
	11th to 20th	Day	5% 12% 33% 35% 13% 1	530
	21st to end	Day	5% 10% 34% 38% 11% 2	530
October	1st to 10th	Day	7% 12% 32% 33% 14% 3	530
	11th to 20th	Day	8% 11% 28% 34% 16% 4	530
	21st to end	Day	11% 10% 27% 32% 15% 4	583
November	1st to 10th	Day	15% 10% 21% 30% 17% 7%	336
	11th to 20th	Day	18% 9% 20% 31% 15% 7%	330
	21st to end	Day	18% 9% 20% 30% 19% 4	297
December	1st to 10th	Day	16% 7% 24% 26% 17% 9%	220
	11th to 20th	Day	20% 9% 27% 26% 13% 5%	220
	21st to end	Day	29% 14% 22% 19% 12% 5%	242
January	1st to 10th	Night	24% 6% 24% 24% 16% 6%	840
	11th to 20th	Night	17% 6% 21% 39% 14% 4	840
	21st to end	Night	21% 5% 23% 30% 17% 4	924
February	1st to 10th	Night	21% 5% 23% 31% 17% 3	840
	11th to 20th	Night	21% 7% 20% 31% 16% 5%	824
	21st to end	Night	21% 8% 23% 33% 13% 2	692
March	1st to 10th	Night	25% 7% 21% 31% 14% 3	840
	11th to 20th	Night	21% 7% 18% 30% 19% 5%	840
	21st to end	Night	22% 8% 20% 31% 17% 2	924
April	1st to 10th	Night	18% 4 19% 40% 17% 2	840
	11th to 20th	Night	15% 7% 20% 37% 18% 3	831
	21st to end	Night	12% 3 20% 39% 21% 5%	840
May	1st to 10th	Night	10% 5% 21% 40% 22% 4	789
	11th to 20th	Night	7% 5% 14% 40% 31% 3	456
	21st to end	Night	7% 2 14% 39% 34% 5%	455
June	1st to 10th	Night	5% 2 11% 41% 36% 5%	420
	11th to 20th	Night	1 1 12% 48% 34% 5%	420
	21st to end	Night	2 11% 45% 37% 5%	420
July	1st to 10th	Night	1 5% 47% 42% 5%	410
	11th to 20th	Night	2 7% 41% 42% 6%	408
	21st to end	Night	3 1 6% 35% 43% 12%	630
August	1st to 10th	Night	3 1 6% 44% 38% 8%	645
	11th to 20th	Night	1 1 10% 46% 36% 8%	832
	21st to end	Night	1 1 7% 45% 43% 4	916
September	1st to 10th	Night	2 8% 47% 37% 6%	800
	11th to 20th	Night	4 2 10% 38% 37% 9%	800
	21st to end	Night	4 2 15% 41% 31% 9%	800
October	1st to 10th	Night	6% 4 15% 44% 25% 7%	800
	11th to 20th	Night	8% 3 15% 41% 29% 5%	800
	21st to end	Night	7% 4 19% 35% 23% 11%	880
November	1st to 10th	Night	14% 3 14% 37% 24% 8%	820
	11th to 20th	Night	14% 5% 21% 36% 19% 5%	840
	21st to end	Night	15% 5% 25% 32% 17% 6%	840
December	1st to 10th	Night	20% 5% 21% 35% 15% 5%	840
	11th to 20th	Night	27% 3 22% 30% 15% 3	840
	21st to end	Night	26% 8% 22% 27% 14% 3	924

Page generated at 2026-08-27 01:07:13. Version ac07

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