Summit View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summit View Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	5% 4 2 4 3 3 3 3 5% 7% 61%	139500
	Night	7% 5% 2 5% 3 3 4 3 5% 7% 54%	138825
Transparency	Day	23% 6% 8% 18% 25% 21%	134445
	Night	26% 8% 9% 16% 22% 19%	134866
Seeing	Day	8% 10% 22% 31% 22% 7%	17067
	Night	9% 5% 15% 33% 32% 7%	26555

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	13% 7% 3 7% 3 3 4 4 5% 8% 43%	5100
	11th to 20th	Day	8% 5% 2 6% 4 4 3 3 4 6% 57%	5340
	21st to end	Day	10% 9% 3 7% 5% 4 4 4 6% 7% 40%	5843
February	1st to 10th	Day	17% 8% 2 5% 4 2 4 3 5% 5% 44%	1890
	11th to 20th	Day	12% 8% 3 6% 4 4 4 4 6% 7% 43%	1915
	21st to end	Day	13% 8% 3 7% 5% 3 4 3 5% 6% 44%	1650
March	1st to 10th	Day	11% 10% 4 10% 7% 6% 6% 4 5% 6% 31%	4859
	11th to 20th	Day	9% 7% 3 7% 4 4 4 4 6% 7% 45%	5000
	21st to end	Day	9% 8% 3 10% 6% 6% 7% 5% 7% 9% 31%	5573
April	1st to 10th	Day	12% 8% 2 6% 4 4 3 3 5% 7% 46%	2400
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 5% 8% 52%	2369
	21st to end	Day	5% 5% 2 5% 4 3 4 3 5% 8% 55%	2400
May	1st to 10th	Day	4 3 1 2 1 1 2 2 4 6% 73%	5523
	11th to 20th	Day	2 2 1 2 2 2 3 2 4 7% 71%	5941
	21st to end	Day	2 3 1 3 2 2 2 2 4 6% 73%	6902
June	1st to 10th	Day	4 4 1 4 3 2 3 3 6% 8% 62%	2800
	11th to 20th	Day	1 2 2 2 3 2 3 5% 7% 73%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 77%	2800
July	1st to 10th	Day	1 1 1 1 2 2 2 5% 86%	6288
	11th to 20th	Day	1 1 1 1 1 2 2 3 7% 80%	5880
	21st to end	Day	1 1 2 2 2 3 2 5% 9% 73%	6402
August	1st to 10th	Day	1 1 1 1 1 2 3 3 4 7% 77%	5777
	11th to 20th	Day	1 1 1 1 2 2 3 6% 83%	5455
	21st to end	Day	1 1 2 1 2 3 3 4 6% 78%	5984
September	1st to 10th	Day	1 2 1 2 2 1 2 2 3 6% 79%	2400
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 9% 63%	2400
	21st to end	Day	3 2 1 2 2 2 2 2 4 6% 74%	2129
October	1st to 10th	Day	2 3 1 4 2 2 2 3 4 6% 71%	3280
	11th to 20th	Day	3 2 1 3 3 2 3 3 4 7% 68%	3261
	21st to end	Day	4 3 2 4 3 3 4 4 5% 8% 61%	3558
November	1st to 10th	Day	8% 5% 2 4 4 3 4 4 5% 6% 55%	2027
	11th to 20th	Day	8% 7% 2 5% 4 4 5% 4 6% 7% 48%	1890
	21st to end	Day	11% 5% 2 3 3 3 4 3 5% 7% 54%	1764
December	1st to 10th	Day	7% 5% 2 6% 4 4 4 3 6% 9% 53%	2700
	11th to 20th	Day	18% 8% 3 8% 4 4 4 3 6% 7% 35%	3108
	21st to end	Day	14% 13% 5% 10% 5% 5% 4 4 5% 6% 29%	4092
January	1st to 10th	Night	13% 8% 3 8% 4 3 3 3 5% 7% 42%	7797
	11th to 20th	Night	8% 6% 2 6% 3 4 4 3 4 7% 54%	7650
	21st to end	Night	10% 9% 3 9% 4 4 4 3 5% 7% 41%	8371
February	1st to 10th	Night	13% 9% 2 6% 3 3 3 3 4 5% 47%	2730
	11th to 20th	Night	12% 7% 2 7% 4 3 5% 4 6% 7% 43%	2482
	21st to end	Night	11% 7% 2 6% 3 3 3 3 5% 7% 49%	1980
March	1st to 10th	Night	14% 12% 4 9% 6% 5% 4 4 5% 6% 32%	5031
	11th to 20th	Night	11% 8% 3 7% 4 3 3 3 5% 6% 47%	5000
	21st to end	Night	13% 11% 4 9% 4 4 4 4 5% 6% 37%	5388
April	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 5% 8% 47%	2000
	11th to 20th	Night	8% 5% 2 6% 4 4 3 3 6% 7% 51%	1979
	21st to end	Night	4 5% 1 4 3 3 4 4 7% 11% 55%	2000
May	1st to 10th	Night	2 3 1 4 3 3 4 3 5% 5% 68%	4468
	11th to 20th	Night	1 2 1 6% 4 4 5% 4 8% 11% 55%	3964
	21st to end	Night	1 2 1 3 3 3 3 2 5% 7% 69%	3972
June	1st to 10th	Night	2 3 1 3 3 2 4 4 7% 10% 61%	1600
	11th to 20th	Night	1 1 3 2 3 3 3 7% 9% 68%	1600
	21st to end	Night	1 2 1 3 2 2 2 3 5% 8% 71%	1600
July	1st to 10th	Night	1 1 1 3 2 5% 7% 79%	3655
	11th to 20th	Night	1 1 2 3 4 4 7% 9% 68%	4100
	21st to end	Night	2 1 5% 3 4 5% 4 7% 13% 57%	4472
August	1st to 10th	Night	1 2 1 3 2 2 4 4 7% 8% 68%	4035
	11th to 20th	Night	1 2 2 2 4 4 4 8% 73%	4408
	21st to end	Night	1 1 2 3 3 3 3 5% 7% 73%	5015
September	1st to 10th	Night	1 1 2 2 2 4 3 5% 8% 73%	2000
	11th to 20th	Night	3 2 1 5% 3 3 4 3 6% 8% 61%	2000
	21st to end	Night	2 3 1 4 2 2 3 2 4 7% 71%	2266
October	1st to 10th	Night	2 2 1 3 2 3 2 2 4 6% 72%	3920
	11th to 20th	Night	2 3 2 5% 2 3 4 3 5% 7% 64%	3897
	21st to end	Night	3 3 2 5% 3 3 4 3 5% 6% 63%	4253
November	1st to 10th	Night	6% 5% 2 5% 4 3 3 3 5% 7% 57%	2489
	11th to 20th	Night	9% 5% 2 6% 4 4 4 4 6% 8% 49%	2730
	21st to end	Night	8% 5% 2 5% 3 4 4 3 4 7% 54%	2851
December	1st to 10th	Night	8% 4 2 5% 3 4 4 4 5% 6% 56%	4700
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 8% 8% 31%	5316
	21st to end	Night	16% 11% 4 11% 6% 6% 4 4 5% 5% 28%	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	36% 7% 15% 21% 15% 6%	5101
	11th to 20th	Day	28% 4 6% 9% 21% 31%	5340
	21st to end	Day	38% 5% 7% 15% 20% 14%	5843
February	1st to 10th	Day	40% 4 6% 20% 18% 12%	1890
	11th to 20th	Day	38% 6% 8% 19% 20% 7%	1924
	21st to end	Day	40% 6% 7% 22% 18% 8%	1650
March	1st to 10th	Day	45% 11% 8% 14% 19% 3	4859
	11th to 20th	Day	34% 8% 12% 28% 10% 8%	5000
	21st to end	Day	40% 13% 14% 20% 12% 2	5573
April	1st to 10th	Day	36% 5% 13% 21% 20% 5%	2400
	11th to 20th	Day	28% 8% 12% 25% 20% 7%	2372
	21st to end	Day	27% 6% 12% 28% 23% 5%	2400
May	1st to 10th	Day	13% 3 8% 27% 35% 14%	5523
	11th to 20th	Day	11% 13% 7% 29% 34% 7%	5944
	21st to end	Day	13% 7% 6% 29% 38% 5%	6904
June	1st to 10th	Day	20% 7% 11% 27% 27% 8%	2800
	11th to 20th	Day	12% 7% 9% 21% 34% 18%	2800
	21st to end	Day	12% 5% 5% 13% 32% 33%	2800
July	1st to 10th	Day	5% 2 3 6% 22% 62%	6220
	11th to 20th	Day	7% 3 5% 5% 17% 64%	5811
	21st to end	Day	9% 5% 4 16% 31% 35%	6217
August	1st to 10th	Day	12% 5% 5% 9% 28% 41%	4206
	11th to 20th	Day	6% 4 3 8% 39% 40%	3948
	21st to end	Day	10% 6% 6% 9% 34% 36%	4311
September	1st to 10th	Day	10% 4 5% 9% 30% 42%	2400
	11th to 20th	Day	17% 7% 6% 22% 31% 17%	2400
	21st to end	Day	13% 4 5% 19% 35% 23%	2129
October	1st to 10th	Day	14% 3 6% 12% 27% 39%	3280
	11th to 20th	Day	15% 4 8% 23% 32% 18%	3261
	21st to end	Day	20% 5% 7% 21% 29% 19%	3558
November	1st to 10th	Day	27% 5% 10% 22% 25% 12%	2027
	11th to 20th	Day	32% 6% 10% 21% 22% 8%	1890
	21st to end	Day	29% 3 9% 18% 29% 12%	1764
December	1st to 10th	Day	27% 5% 7% 19% 32% 10%	2700
	11th to 20th	Day	46% 6% 8% 16% 15% 8%	3108
	21st to end	Day	50% 7% 11% 18% 10% 3	4092
January	1st to 10th	Night	35% 9% 14% 20% 18% 4	7797
	11th to 20th	Night	28% 5% 8% 9% 24% 26%	7650
	21st to end	Night	37% 7% 8% 13% 19% 16%	8371
February	1st to 10th	Night	38% 3 8% 17% 20% 14%	2730
	11th to 20th	Night	36% 6% 11% 18% 21% 8%	2484
	21st to end	Night	32% 4 11% 28% 18% 7%	1980
March	1st to 10th	Night	46% 9% 11% 13% 18% 4	5031
	11th to 20th	Night	34% 9% 9% 22% 17% 8%	5000
	21st to end	Night	40% 9% 14% 21% 13% 3	5388
April	1st to 10th	Night	33% 6% 13% 23% 20% 5%	2000
	11th to 20th	Night	29% 8% 12% 26% 17% 8%	1979
	21st to end	Night	22% 9% 12% 28% 22% 6%	2000
May	1st to 10th	Night	14% 6% 10% 23% 29% 18%	4465
	11th to 20th	Night	13% 27% 9% 19% 23% 8%	3965
	21st to end	Night	12% 13% 8% 19% 38% 10%	3972
June	1st to 10th	Night	17% 12% 12% 25% 24% 9%	1600
	11th to 20th	Night	14% 7% 12% 14% 29% 25%	1600
	21st to end	Night	17% 8% 5% 12% 25% 33%	1600
July	1st to 10th	Night	5% 4 5% 7% 20% 60%	3615
	11th to 20th	Night	8% 6% 8% 8% 17% 54%	4046
	21st to end	Night	17% 10% 8% 11% 24% 30%	4345
August	1st to 10th	Night	17% 6% 7% 8% 23% 39%	2929
	11th to 20th	Night	9% 6% 4 8% 25% 47%	3183
	21st to end	Night	12% 7% 8% 8% 27% 37%	3608
September	1st to 10th	Night	9% 6% 9% 10% 26% 41%	2000
	11th to 20th	Night	18% 9% 6% 20% 30% 17%	2000
	21st to end	Night	14% 6% 5% 16% 33% 26%	2266
October	1st to 10th	Night	14% 5% 6% 9% 18% 48%	3920
	11th to 20th	Night	17% 6% 8% 21% 28% 19%	3897
	21st to end	Night	20% 6% 6% 19% 26% 23%	4253
November	1st to 10th	Night	24% 5% 10% 23% 23% 15%	2489
	11th to 20th	Night	29% 6% 10% 21% 23% 9%	2730
	21st to end	Night	29% 4 8% 19% 24% 16%	2851
December	1st to 10th	Night	27% 5% 6% 20% 26% 17%	4700
	11th to 20th	Night	45% 7% 9% 13% 18% 8%	5316
	21st to end	Night	50% 10% 12% 15% 9% 4	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	28% 13% 22% 22% 13% 2	351
	11th to 20th	Day	14% 11% 21% 32% 17% 4	430
	21st to end	Day	14% 11% 24% 28% 19% 4	473
February	1st to 10th	Day	21% 16% 20% 24% 14% 5%	430
	11th to 20th	Day	20% 16% 26% 25% 11% 2	469
	21st to end	Day	12% 15% 30% 30% 12% 2	414
March	1st to 10th	Day	21% 17% 26% 24% 10% 1	500
	11th to 20th	Day	20% 16% 24% 22% 13% 4	500
	21st to end	Day	14% 17% 32% 25% 11% 1	550
April	1st to 10th	Day	17% 14% 26% 26% 14% 3	500
	11th to 20th	Day	12% 15% 31% 24% 14% 4	492
	21st to end	Day	6% 16% 27% 31% 14% 6%	500
May	1st to 10th	Day	8% 13% 32% 28% 14% 5%	503
	11th to 20th	Day	9% 12% 29% 28% 17% 6%	528
	21st to end	Day	6% 11% 23% 34% 19% 7%	573
June	1st to 10th	Day	5% 8% 25% 33% 21% 8%	530
	11th to 20th	Day	2 9% 29% 36% 20% 4	530
	21st to end	Day	2 4 19% 41% 25% 9%	530
July	1st to 10th	Day	2 1 18% 38% 32% 9%	526
	11th to 20th	Day	1 1 10% 43% 33% 12%	524
	21st to end	Day	1 10% 42% 34% 13%	575
August	1st to 10th	Day	2 1 19% 38% 32% 9%	527
	11th to 20th	Day	1 16% 42% 27% 13%	517
	21st to end	Day	1 3 12% 40% 32% 13%	583
September	1st to 10th	Day	1 4 13% 42% 28% 12%	530
	11th to 20th	Day	3 5% 25% 35% 22% 10%	530
	21st to end	Day	3 7% 18% 32% 32% 8%	530
October	1st to 10th	Day	2 9% 21% 29% 29% 10%	530
	11th to 20th	Day	5% 9% 20% 28% 28% 10%	530
	21st to end	Day	7% 8% 21% 31% 25% 8%	583
November	1st to 10th	Day	8% 12% 18% 24% 27% 10%	503
	11th to 20th	Day	9% 13% 20% 25% 25% 8%	330
	21st to end	Day	11% 7% 22% 30% 25% 5%	264
December	1st to 10th	Day	11% 12% 19% 29% 21% 9%	220
	11th to 20th	Day	15% 16% 18% 21% 20% 10%	220
	21st to end	Day	17% 22% 26% 17% 14% 5%	242
January	1st to 10th	Night	19% 8% 16% 30% 23% 5%	840
	11th to 20th	Night	13% 6% 15% 34% 25% 7%	840
	21st to end	Night	16% 5% 14% 35% 25% 5%	924
February	1st to 10th	Night	18% 9% 15% 31% 23% 5%	840
	11th to 20th	Night	16% 8% 19% 33% 19% 5%	804
	21st to end	Night	14% 7% 22% 35% 20% 3	660
March	1st to 10th	Night	18% 9% 19% 30% 21% 3	800
	11th to 20th	Night	16% 9% 18% 28% 25% 5%	800
	21st to end	Night	13% 8% 21% 35% 20% 3	880
April	1st to 10th	Night	13% 6% 21% 35% 23% 2	800
	11th to 20th	Night	10% 10% 19% 34% 22% 5%	791
	21st to end	Night	5% 7% 25% 32% 24% 7%	800
May	1st to 10th	Night	5% 5% 23% 36% 26% 5%	760
	11th to 20th	Night	4 3 14% 35% 37% 7%	547
	21st to end	Night	2 5% 21% 36% 32% 4	433
June	1st to 10th	Night	3 3 14% 42% 32% 6%	400
	11th to 20th	Night	1 3 14% 45% 35% 2	400
	21st to end	Night	2 1 9% 37% 45% 6%	400
July	1st to 10th	Night	5% 32% 57% 6%	392
	11th to 20th	Night	1 3 26% 53% 16%	582
	21st to end	Night	1 5% 24% 57% 14%	657
August	1st to 10th	Night	2 8% 31% 50% 8%	597
	11th to 20th	Night	1 1 9% 35% 45% 9%	744
	21st to end	Night	1 8% 29% 49% 12%	880
September	1st to 10th	Night	1 1 8% 31% 47% 12%	800
	11th to 20th	Night	4 2 9% 34% 42% 10%	800
	21st to end	Night	2 4 12% 31% 40% 11%	800
October	1st to 10th	Night	4 3 12% 34% 36% 12%	800
	11th to 20th	Night	6% 3 12% 29% 39% 11%	800
	21st to end	Night	7% 4 10% 33% 31% 15%	880
November	1st to 10th	Night	7% 6% 13% 29% 35% 10%	820
	11th to 20th	Night	11% 4 15% 32% 31% 6%	840
	21st to end	Night	10% 5% 14% 39% 26% 6%	840
December	1st to 10th	Night	15% 4 16% 33% 25% 6%	840
	11th to 20th	Night	18% 6% 15% 35% 22% 4	840
	21st to end	Night	19% 11% 21% 29% 17% 3	924

Page generated at 2026-02-12 22:42:14. Version ac07

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