Summit Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summit Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	14% 6% 2 6% 4 4 4 4 6% 8% 42%	127379
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	124300
Transparency	Day	38% 6% 12% 24% 17% 3	130938
	Night	39% 5% 11% 23% 18% 4	128810
Seeing	Day	17% 12% 26% 26% 15% 5%	16490
	Night	18% 9% 30% 31% 12% 1	25924

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	13% 4 2 5% 4 4 4 3 5% 5% 51%	3677
	11th to 20th	Day	13% 8% 3 8% 5% 4 4 3 3 5% 44%	3897
	21st to end	Day	17% 6% 2 5% 4 4 4 3 4 6% 44%	4466
February	1st to 10th	Day	15% 7% 3 5% 4 4 5% 4 5% 6% 41%	1800
	11th to 20th	Day	17% 8% 2 5% 4 4 4 5% 5% 6% 40%	1885
	21st to end	Day	17% 10% 3 6% 4 4 3 3 6% 5% 40%	1650
March	1st to 10th	Day	20% 4 2 4 3 3 4 3 4 6% 48%	4571
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 5% 6% 49%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 43%	5734
April	1st to 10th	Day	16% 7% 2 5% 5% 4 4 4 5% 7% 42%	2400
	11th to 20th	Day	13% 6% 3 6% 4 4 5% 3 5% 7% 44%	2374
	21st to end	Day	16% 6% 2 6% 3 4 4 4 7% 8% 41%	2400
May	1st to 10th	Day	24% 7% 2 5% 4 4 4 3 6% 9% 32%	5710
	11th to 20th	Day	24% 6% 2 6% 4 4 4 4 5% 9% 32%	6318
	21st to end	Day	17% 5% 2 6% 3 4 4 4 6% 9% 39%	6934
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	2800
	11th to 20th	Day	12% 6% 2 6% 4 5% 5% 4 6% 10% 40%	2800
	21st to end	Day	9% 5% 2 5% 5% 4 5% 5% 7% 11% 44%	2800
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 5% 7% 11% 42%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 9% 12% 33%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	6638
August	1st to 10th	Day	16% 6% 2 5% 5% 5% 5% 4 8% 10% 34%	4234
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 7% 8% 43%	3867
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 8% 49%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 8% 49%	2390
	21st to end	Day	19% 7% 3 7% 4 4 5% 2 6% 6% 37%	2165
October	1st to 10th	Day	16% 6% 1 6% 3 3 4 3 4 6% 47%	2000
	11th to 20th	Day	16% 5% 2 4 3 3 4 3 5% 6% 49%	2000
	21st to end	Day	15% 5% 2 5% 3 3 4 3 5% 5% 49%	2175
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 53%	1851
	11th to 20th	Day	13% 6% 2 4 4 3 3 4 6% 6% 48%	1890
	21st to end	Day	15% 6% 2 5% 4 4 5% 3 4 6% 47%	1743
December	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 5% 47%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 4 3 5% 6% 41%	3070
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	4048
January	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 5% 6% 48%	6489
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 50%	6191
	21st to end	Night	17% 8% 2 6% 4 3 3 3 5% 5% 45%	6490
February	1st to 10th	Night	16% 8% 3 5% 5% 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	19% 8% 2 5% 4 3 3 3 4 5% 43%	2399
	21st to end	Night	17% 7% 3 4 4 4 4 4 4 6% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 4 3 2 4 6% 47%	5264
	11th to 20th	Night	11% 8% 2 6% 3 3 3 3 4 6% 51%	4980
	21st to end	Night	15% 7% 3 6% 3 4 4 4 5% 5% 44%	5197
April	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 5% 6% 41%	2000
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 43%	1975
	21st to end	Night	19% 6% 2 5% 4 3 3 3 5% 7% 44%	2000
May	1st to 10th	Night	21% 8% 3 8% 4 5% 5% 4 6% 7% 28%	4226
	11th to 20th	Night	23% 8% 3 6% 4 4 4 4 6% 7% 31%	3575
	21st to end	Night	20% 7% 2 5% 4 4 4 3 5% 8% 37%	3920
June	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	1600
	11th to 20th	Night	11% 7% 4 6% 5% 6% 5% 4 5% 10% 39%	1600
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 8% 10% 42%	1600
July	1st to 10th	Night	8% 6% 3 8% 6% 5% 6% 5% 6% 10% 38%	3598
	11th to 20th	Night	11% 8% 3 9% 5% 5% 6% 5% 8% 11% 30%	3597
	21st to end	Night	10% 5% 3 6% 5% 5% 5% 5% 6% 9% 42%	4195
August	1st to 10th	Night	18% 6% 2 7% 5% 4 5% 5% 6% 10% 34%	2915
	11th to 20th	Night	13% 7% 2 7% 4 5% 5% 5% 5% 8% 40%	3135
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3586
September	1st to 10th	Night	13% 6% 3 7% 4 3 4 2 5% 7% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 7% 52%	2010
	21st to end	Night	19% 7% 2 7% 4 4 4 3 5% 7% 39%	2230
October	1st to 10th	Night	17% 7% 3 6% 5% 3 3 3 5% 6% 44%	2400
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	15% 5% 2 5% 3 3 4 3 4 6% 50%	2664
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2667
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 48%	2730
	21st to end	Night	13% 5% 2 5% 5% 4 5% 3 6% 6% 47%	2872
December	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 4 7% 48%	4720
	11th to 20th	Night	21% 6% 2 5% 3 3 4 3 5% 7% 41%	5345
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 7% 39%	7150

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	34% 4 8% 27% 24% 4	3678
	11th to 20th	Day	43% 4 7% 23% 17% 6%	3897
	21st to end	Day	41% 5% 12% 26% 14% 3	4466
February	1st to 10th	Day	42% 6% 14% 25% 12% 2	1800
	11th to 20th	Day	43% 4 9% 25% 16% 3	1894
	21st to end	Day	45% 4 10% 22% 14% 4	1650
March	1st to 10th	Day	38% 4 8% 25% 19% 7%	4571
	11th to 20th	Day	35% 4 9% 28% 18% 6%	4980
	21st to end	Day	41% 5% 9% 18% 21% 6%	5734
April	1st to 10th	Day	41% 6% 12% 20% 16% 6%	2400
	11th to 20th	Day	38% 5% 11% 23% 18% 5%	2374
	21st to end	Day	39% 6% 11% 19% 20% 4	2400
May	1st to 10th	Day	48% 7% 14% 18% 11% 2	5742
	11th to 20th	Day	49% 6% 11% 22% 10% 1	6322
	21st to end	Day	40% 7% 12% 23% 16% 1	6949
June	1st to 10th	Day	37% 9% 14% 24% 16% 2	2800
	11th to 20th	Day	38% 8% 13% 25% 13% 3	2800
	21st to end	Day	32% 8% 16% 24% 19% 2	2800
July	1st to 10th	Day	34% 11% 17% 20% 15% 3	6290
	11th to 20th	Day	40% 10% 19% 22% 8% 1	6285
	21st to end	Day	36% 8% 11% 23% 21% 1	6448
August	1st to 10th	Day	41% 9% 16% 24% 9% 1	4221
	11th to 20th	Day	36% 7% 14% 29% 13% 1	3903
	21st to end	Day	32% 6% 12% 29% 19% 2	4312
September	1st to 10th	Day	38% 7% 11% 20% 21% 3	2400
	11th to 20th	Day	32% 6% 9% 25% 24% 4	2390
	21st to end	Day	47% 5% 9% 18% 19% 2	2165
October	1st to 10th	Day	34% 5% 10% 18% 29% 5%	3260
	11th to 20th	Day	26% 4 9% 25% 28% 9%	3240
	21st to end	Day	24% 3 9% 30% 28% 5%	3489
November	1st to 10th	Day	32% 3 13% 28% 20% 4	1851
	11th to 20th	Day	35% 5% 10% 24% 23% 3	1890
	21st to end	Day	38% 4 10% 31% 14% 2	1743
December	1st to 10th	Day	37% 5% 16% 27% 13% 3	2680
	11th to 20th	Day	44% 4 12% 28% 10% 2	3066
	21st to end	Day	39% 6% 15% 21% 17% 1	4048
January	1st to 10th	Night	37% 4 8% 23% 22% 5%	6489
	11th to 20th	Night	37% 4 8% 25% 19% 7%	6191
	21st to end	Night	42% 3 9% 27% 18% 2	6490
February	1st to 10th	Night	44% 5% 13% 24% 14% 1	2600
	11th to 20th	Night	44% 4 9% 21% 18% 4	2401
	21st to end	Night	43% 4 11% 19% 17% 6%	1980
March	1st to 10th	Night	39% 5% 8% 26% 13% 10%	5264
	11th to 20th	Night	34% 6% 13% 25% 17% 6%	4980
	21st to end	Night	41% 5% 8% 17% 24% 4	5197
April	1st to 10th	Night	43% 5% 10% 18% 19% 5%	2000
	11th to 20th	Night	40% 6% 11% 19% 19% 5%	1975
	21st to end	Night	39% 6% 10% 20% 20% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 10% 3	4256
	11th to 20th	Night	50% 7% 11% 19% 11% 1	3576
	21st to end	Night	44% 7% 13% 21% 13% 3	3927
June	1st to 10th	Night	38% 7% 10% 23% 19% 2	1600
	11th to 20th	Night	40% 8% 14% 24% 14% 2	1600
	21st to end	Night	34% 8% 15% 24% 17% 3	1600
July	1st to 10th	Night	41% 8% 17% 18% 14% 2	3560
	11th to 20th	Night	44% 11% 20% 18% 6% 1	3552
	21st to end	Night	37% 7% 12% 23% 19%	4072
August	1st to 10th	Night	45% 9% 18% 19% 9%	2904
	11th to 20th	Night	41% 7% 11% 25% 14% 1	3164
	21st to end	Night	33% 7% 11% 28% 21% 2	3586
September	1st to 10th	Night	38% 7% 10% 21% 21% 4	2000
	11th to 20th	Night	31% 5% 9% 25% 25% 4	2010
	21st to end	Night	45% 6% 10% 18% 17% 3	2230
October	1st to 10th	Night	41% 5% 9% 21% 21% 4	3920
	11th to 20th	Night	24% 4 11% 24% 30% 6%	3899
	21st to end	Night	24% 3 11% 31% 24% 6%	4300
November	1st to 10th	Night	32% 4 12% 25% 21% 6%	2667
	11th to 20th	Night	39% 4 9% 23% 20% 6%	2730
	21st to end	Night	36% 4 12% 26% 19% 2	2872
December	1st to 10th	Night	35% 4 13% 29% 15% 2	4720
	11th to 20th	Night	43% 4 12% 20% 16% 4	5348
	21st to end	Night	43% 6% 12% 20% 18% 1	7150

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	17% 22% 38% 17% 5% 1	220
	11th to 20th	Day	20% 36% 26% 16% 3	266
	21st to end	Day	20% 17% 41% 15% 7% 1	330
February	1st to 10th	Day	19% 22% 37% 17% 4	300
	11th to 20th	Day	24% 23% 37% 14% 1	433
	21st to end	Day	22% 21% 32% 20% 4	414
March	1st to 10th	Day	16% 24% 32% 22% 6%	500
	11th to 20th	Day	20% 21% 31% 21% 6% 2	500
	21st to end	Day	21% 15% 35% 22% 6% 1	550
April	1st to 10th	Day	21% 18% 35% 19% 5% 2	500
	11th to 20th	Day	17% 17% 30% 24% 8% 4	492
	21st to end	Day	20% 15% 32% 22% 10% 1	500
May	1st to 10th	Day	27% 14% 27% 21% 10% 2	503
	11th to 20th	Day	21% 9% 28% 28% 11% 3	527
	21st to end	Day	18% 5% 21% 27% 21% 7%	573
June	1st to 10th	Day	12% 7% 25% 28% 19% 8%	530
	11th to 20th	Day	15% 7% 22% 30% 20% 6%	530
	21st to end	Day	10% 5% 21% 33% 21% 9%	530
July	1st to 10th	Day	10% 3 16% 32% 29% 11%	525
	11th to 20th	Day	6% 2 14% 35% 31% 12%	524
	21st to end	Day	9% 2 18% 35% 27% 9%	576
August	1st to 10th	Day	12% 2 17% 33% 26% 11%	526
	11th to 20th	Day	13% 4 12% 29% 31% 11%	519
	21st to end	Day	10% 3 19% 31% 23% 13%	583
September	1st to 10th	Day	16% 6% 19% 25% 22% 12%	530
	11th to 20th	Day	14% 2 18% 29% 24% 13%	530
	21st to end	Day	23% 8% 21% 28% 15% 4	530
October	1st to 10th	Day	21% 5% 20% 27% 16% 10%	530
	11th to 20th	Day	16% 14% 24% 26% 17% 4	530
	21st to end	Day	18% 10% 31% 26% 12% 3	569
November	1st to 10th	Day	14% 10% 32% 28% 13% 3	425
	11th to 20th	Day	16% 18% 34% 22% 10%	430
	21st to end	Day	17% 19% 30% 25% 10%	283
December	1st to 10th	Day	20% 18% 34% 21% 6% 1	220
	11th to 20th	Day	25% 20% 29% 21% 6%	220
	21st to end	Day	22% 17% 30% 23% 7% 1	242
January	1st to 10th	Night	19% 16% 40% 22% 3	840
	11th to 20th	Night	21% 16% 34% 25% 4	825
	21st to end	Night	18% 13% 41% 26% 2	880
February	1st to 10th	Night	20% 12% 40% 26% 2	800
	11th to 20th	Night	23% 18% 40% 18% 1	783
	21st to end	Night	21% 16% 38% 20% 4	660
March	1st to 10th	Night	18% 15% 38% 24% 4	800
	11th to 20th	Night	20% 14% 38% 22% 6% 1	800
	21st to end	Night	20% 9% 41% 24% 6%	880
April	1st to 10th	Night	21% 13% 38% 22% 6%	800
	11th to 20th	Night	17% 12% 34% 31% 7%	791
	21st to end	Night	22% 10% 29% 31% 8%	800
May	1st to 10th	Night	20% 7% 32% 34% 6%	692
	11th to 20th	Night	20% 7% 29% 35% 9%	394
	21st to end	Night	17% 4 21% 37% 19% 2	433
June	1st to 10th	Night	14% 4 22% 42% 18% 1	400
	11th to 20th	Night	13% 1 25% 43% 18%	400
	21st to end	Night	10% 3 21% 48% 19%	400
July	1st to 10th	Night	11% 2 18% 42% 25% 2	390
	11th to 20th	Night	8% 1 12% 45% 33% 2	388
	21st to end	Night	9% 2 13% 46% 27% 2	555
August	1st to 10th	Night	11% 2 13% 44% 29% 2	597
	11th to 20th	Night	13% 2 14% 38% 30% 3	752
	21st to end	Night	13% 3 16% 38% 27% 4	880
September	1st to 10th	Night	16% 2 17% 38% 24% 3	800
	11th to 20th	Night	15% 3 15% 37% 26% 5%	800
	21st to end	Night	22% 4 20% 34% 20% 1	800
October	1st to 10th	Night	21% 5% 20% 30% 20% 5%	800
	11th to 20th	Night	18% 6% 29% 33% 13% 1	800
	21st to end	Night	18% 7% 34% 29% 11% 1	880
November	1st to 10th	Night	13% 8% 35% 32% 11% 1	820
	11th to 20th	Night	19% 12% 33% 28% 7%	840
	21st to end	Night	15% 14% 34% 29% 8% 1	840
December	1st to 10th	Night	21% 13% 35% 24% 7% 1	840
	11th to 20th	Night	18% 13% 38% 25% 5%	840
	21st to end	Night	21% 10% 36% 30% 2	924

Page generated at 2026-01-14 01:14:24. Version ac07

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