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 2024-11-25.

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	15% 6% 2 6% 4 4 4 4 6% 8% 41%	101412
	Night	16% 7% 2 6% 4 4 4 4 5% 7% 42%	97983
Transparency	Day	38% 6% 12% 24% 16% 3	105016
	Night	40% 6% 11% 23% 17% 3	102518
Seeing	Day	17% 11% 26% 26% 15% 5%	15081
	Night	18% 9% 30% 31% 12% 1	24430

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	15% 5% 2 5% 4 3 3 4 5% 5% 48%	2517
	11th to 20th	Day	14% 10% 3 8% 5% 4 5% 3 4 6% 39%	2684
	21st to end	Day	20% 7% 3 5% 5% 4 5% 3 5% 6% 38%	3124
February	1st to 10th	Day	15% 7% 3 5% 4 4 5% 4 5% 6% 41%	1710
	11th to 20th	Day	16% 7% 2 4 4 4 4 5% 5% 7% 42%	1788
	21st to end	Day	17% 10% 3 6% 4 4 3 3 6% 5% 39%	1570
March	1st to 10th	Day	25% 5% 2 4 4 3 4 2 4 6% 41%	3183
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 6% 7% 50%	3480
	21st to end	Day	21% 6% 2 6% 4 3 4 3 4 5% 43%	4000
April	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 7% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 45%	2254
	21st to end	Day	16% 6% 2 6% 3 4 4 4 6% 8% 41%	2280
May	1st to 10th	Day	27% 8% 3 6% 4 4 4 3 5% 7% 29%	3972
	11th to 20th	Day	23% 7% 3 6% 5% 5% 4 4 6% 8% 29%	4398
	21st to end	Day	11% 5% 2 6% 3 4 4 4 6% 9% 46%	4823
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	2660
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 6% 10% 41%	2660
	21st to end	Day	9% 5% 2 5% 5% 4 5% 5% 7% 11% 44%	2660
July	1st to 10th	Day	6% 4 2 7% 5% 5% 6% 5% 8% 11% 41%	4440
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 12% 39%	4440
	21st to end	Day	9% 6% 3 8% 6% 6% 6% 5% 8% 10% 34%	4610
August	1st to 10th	Day	16% 6% 2 5% 5% 5% 5% 4 8% 10% 33%	4104
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 7% 8% 43%	3746
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 8% 48%	4180
September	1st to 10th	Day	13% 6% 2 6% 5% 4 3 4 5% 7% 44%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 49%	2271
	21st to end	Day	20% 6% 3 7% 4 4 5% 2 6% 6% 38%	2057
October	1st to 10th	Day	16% 7% 1 6% 3 3 4 3 4 6% 46%	1900
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 5% 6% 49%	1900
	21st to end	Day	16% 5% 2 5% 3 4 4 3 4 5% 49%	2066
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 5% 7% 52%	1761
	11th to 20th	Day	14% 6% 2 4 4 3 3 4 6% 6% 48%	1800
	21st to end	Day	14% 6% 2 5% 4 4 5% 3 4 6% 48%	1612
December	1st to 10th	Day	18% 6% 2 5% 5% 3 4 3 7% 5% 42%	1520
	11th to 20th	Day	19% 8% 2 4 4 3 4 4 5% 6% 42%	1910
	21st to end	Day	15% 7% 2 7% 5% 5% 4 4 6% 7% 38%	2772
January	1st to 10th	Night	17% 7% 2 6% 3 4 4 3 4 5% 44%	4429
	11th to 20th	Night	18% 8% 2 6% 3 3 3 3 4 4 44%	4272
	21st to end	Night	22% 8% 2 6% 5% 3 3 3 4 5% 38%	4532
February	1st to 10th	Night	16% 8% 3 5% 5% 4 4 3 5% 6% 41%	2470
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 4 5% 43%	2276
	21st to end	Night	18% 7% 3 4 4 4 4 4 4 6% 42%	1884
March	1st to 10th	Night	22% 7% 3 5% 4 4 4 3 4 6% 39%	3652
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 5% 6% 50%	3480
	21st to end	Night	17% 9% 3 6% 3 3 3 3 4 4 46%	3631
April	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 5% 7% 42%	1900
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	1875
	21st to end	Night	20% 6% 2 5% 4 3 3 3 5% 7% 43%	1900
May	1st to 10th	Night	22% 9% 3 8% 4 4 4 3 5% 7% 30%	2950
	11th to 20th	Night	23% 9% 3 7% 4 5% 4 4 5% 6% 29%	2495
	21st to end	Night	13% 6% 2 5% 4 4 4 4 6% 10% 43%	2732
June	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 44%	1520
	11th to 20th	Night	10% 7% 4 6% 5% 6% 5% 4 5% 10% 40%	1520
	21st to end	Night	9% 5% 2 6% 4 4 5% 5% 8% 10% 42%	1520
July	1st to 10th	Night	7% 5% 2 8% 6% 5% 6% 5% 6% 10% 39%	2520
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 5% 8% 11% 37%	2520
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 5% 6% 8% 36%	2925
August	1st to 10th	Night	18% 6% 2 7% 5% 4 5% 5% 6% 10% 34%	2825
	11th to 20th	Night	12% 7% 2 7% 5% 5% 5% 5% 5% 8% 40%	3037
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	13% 6% 3 6% 4 3 4 3 6% 7% 46%	1900
	11th to 20th	Night	11% 5% 2 5% 3 2 3 3 4 7% 53%	1909
	21st to end	Night	19% 7% 2 7% 4 4 3 3 5% 7% 40%	2118
October	1st to 10th	Night	18% 7% 3 6% 5% 3 3 3 5% 6% 43%	2280
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 5% 6% 51%	2280
	21st to end	Night	15% 5% 2 5% 3 3 4 3 4 6% 50%	2531
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2537
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 5% 5% 48%	2600
	21st to end	Night	13% 5% 2 4 5% 4 5% 3 6% 7% 47%	2658
December	1st to 10th	Night	18% 7% 2 6% 4 4 4 3 4 7% 40%	2660
	11th to 20th	Night	17% 5% 2 6% 4 4 4 3 6% 6% 43%	3285
	21st to end	Night	16% 7% 3 6% 4 4 5% 4 6% 8% 36%	4884

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% 4 11% 30% 14% 4	2518
	11th to 20th	Day	47% 5% 10% 21% 13% 4	2684
	21st to end	Day	46% 6% 13% 22% 11% 2	3124
February	1st to 10th	Day	42% 6% 14% 24% 12% 2	1710
	11th to 20th	Day	42% 4 9% 26% 15% 3	1797
	21st to end	Day	46% 5% 10% 21% 14% 4	1570
March	1st to 10th	Day	45% 4 9% 27% 12% 3	3183
	11th to 20th	Day	31% 5% 10% 31% 19% 4	3480
	21st to end	Day	44% 5% 8% 20% 19% 4	4000
April	1st to 10th	Day	40% 6% 12% 20% 17% 6%	2280
	11th to 20th	Day	37% 5% 11% 24% 18% 6%	2254
	21st to end	Day	40% 7% 11% 18% 19% 4	2280
May	1st to 10th	Day	52% 7% 10% 18% 11% 2	4007
	11th to 20th	Day	51% 7% 14% 18% 8% 1	4402
	21st to end	Day	32% 9% 13% 26% 18% 2	4837
June	1st to 10th	Day	36% 9% 14% 24% 15% 2	2660
	11th to 20th	Day	37% 8% 14% 25% 14% 3	2660
	21st to end	Day	32% 8% 16% 25% 19% 2	2660
July	1st to 10th	Day	34% 11% 17% 21% 13% 4	4370
	11th to 20th	Day	35% 8% 17% 26% 11% 2	4365
	21st to end	Day	42% 9% 13% 20% 15% 2	4463
August	1st to 10th	Day	42% 9% 16% 24% 8% 1	4091
	11th to 20th	Day	35% 7% 14% 29% 13% 1	3782
	21st to end	Day	32% 6% 12% 29% 18% 2	4180
September	1st to 10th	Day	39% 7% 11% 19% 21% 3	2280
	11th to 20th	Day	32% 6% 9% 25% 24% 4	2271
	21st to end	Day	47% 5% 8% 19% 19% 2	2057
October	1st to 10th	Day	34% 5% 10% 18% 29% 5%	3160
	11th to 20th	Day	25% 4 9% 25% 29% 9%	3140
	21st to end	Day	24% 3 9% 30% 28% 6%	3380
November	1st to 10th	Day	33% 3 12% 28% 19% 4	1761
	11th to 20th	Day	36% 5% 10% 24% 23% 3	1800
	21st to end	Day	38% 4 11% 31% 14% 3	1612
December	1st to 10th	Day	42% 6% 15% 24% 12% 2	1520
	11th to 20th	Day	42% 4 12% 29% 11% 2	1906
	21st to end	Day	42% 7% 16% 21% 13%	2772
January	1st to 10th	Night	42% 4 10% 26% 15% 4	4429
	11th to 20th	Night	43% 5% 9% 25% 15% 2	4272
	21st to end	Night	48% 4 8% 27% 11% 1	4532
February	1st to 10th	Night	43% 5% 13% 24% 14% 1	2470
	11th to 20th	Night	44% 4 9% 21% 18% 4	2278
	21st to end	Night	43% 4 11% 20% 17% 6%	1884
March	1st to 10th	Night	46% 5% 9% 25% 13% 2	3652
	11th to 20th	Night	33% 7% 12% 29% 17% 4	3480
	21st to end	Night	42% 5% 7% 18% 22% 5%	3631
April	1st to 10th	Night	42% 5% 10% 19% 20% 5%	1900
	11th to 20th	Night	39% 6% 12% 20% 19% 5%	1875
	21st to end	Night	40% 6% 10% 20% 19% 5%	1900
May	1st to 10th	Night	51% 7% 11% 19% 11% 2	2980
	11th to 20th	Night	53% 8% 12% 15% 10% 2	2496
	21st to end	Night	35% 8% 16% 21% 16% 4	2739
June	1st to 10th	Night	38% 8% 11% 24% 19% 2	1520
	11th to 20th	Night	38% 8% 14% 24% 14% 2	1520
	21st to end	Night	33% 8% 16% 24% 16% 3	1520
July	1st to 10th	Night	39% 9% 20% 17% 12% 3	2480
	11th to 20th	Night	37% 10% 19% 24% 8% 2	2472
	21st to end	Night	44% 8% 13% 20% 15% 1	2832
August	1st to 10th	Night	45% 9% 18% 19% 9%	2814
	11th to 20th	Night	41% 7% 11% 25% 14% 1	3066
	21st to end	Night	33% 7% 11% 28% 20% 2	3476
September	1st to 10th	Night	38% 7% 10% 20% 21% 4	1900
	11th to 20th	Night	31% 5% 9% 26% 26% 4	1909
	21st to end	Night	44% 6% 9% 19% 18% 3	2118
October	1st to 10th	Night	42% 5% 9% 21% 20% 4	3800
	11th to 20th	Night	24% 4 11% 24% 31% 6%	3779
	21st to end	Night	24% 4 12% 31% 23% 6%	4167
November	1st to 10th	Night	32% 5% 12% 25% 21% 6%	2537
	11th to 20th	Night	38% 4 9% 23% 20% 5%	2600
	21st to end	Night	36% 5% 12% 26% 18% 2	2658
December	1st to 10th	Night	44% 5% 12% 24% 14% 1	2660
	11th to 20th	Night	40% 5% 12% 21% 18% 4	3288
	21st to end	Night	43% 8% 14% 23% 11% 1	4884

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	18% 22% 36% 18% 5% 1	180
	11th to 20th	Day	20% 35% 25% 17% 3	227
	21st to end	Day	20% 18% 40% 15% 7% 1	297
February	1st to 10th	Day	20% 19% 38% 18% 5%	270
	11th to 20th	Day	23% 22% 38% 15% 1	396
	21st to end	Day	23% 21% 31% 19% 4 1	382
March	1st to 10th	Day	16% 23% 32% 22% 6%	460
	11th to 20th	Day	20% 21% 31% 20% 5% 2	460
	21st to end	Day	23% 15% 33% 22% 6% 1	506
April	1st to 10th	Day	19% 18% 36% 19% 6% 2	460
	11th to 20th	Day	17% 16% 31% 25% 9% 4	452
	21st to end	Day	21% 15% 32% 22% 8% 2	460
May	1st to 10th	Day	27% 14% 26% 21% 10% 2	463
	11th to 20th	Day	21% 9% 27% 29% 11% 3	487
	21st to end	Day	17% 4 20% 28% 23% 8%	529
June	1st to 10th	Day	12% 7% 26% 29% 19% 8%	490
	11th to 20th	Day	14% 7% 22% 30% 21% 7%	490
	21st to end	Day	10% 6% 22% 35% 20% 8%	490
July	1st to 10th	Day	9% 3 16% 32% 29% 10%	485
	11th to 20th	Day	6% 2 14% 35% 32% 11%	484
	21st to end	Day	9% 2 19% 35% 27% 8%	532
August	1st to 10th	Day	11% 2 17% 33% 27% 11%	486
	11th to 20th	Day	12% 3 13% 30% 30% 12%	479
	21st to end	Day	10% 3 19% 31% 23% 14%	539
September	1st to 10th	Day	16% 6% 18% 24% 22% 13%	490
	11th to 20th	Day	13% 3 19% 28% 23% 13%	490
	21st to end	Day	23% 9% 22% 27% 15% 4	490
October	1st to 10th	Day	22% 6% 21% 27% 15% 9%	490
	11th to 20th	Day	15% 13% 24% 27% 17% 4	490
	21st to end	Day	18% 8% 31% 27% 12% 3	526
November	1st to 10th	Day	14% 10% 31% 28% 14% 3	395
	11th to 20th	Day	15% 17% 34% 23% 11%	400
	21st to end	Day	16% 18% 29% 26% 10%	248
December	1st to 10th	Day	20% 19% 32% 21% 7% 1	180
	11th to 20th	Day	26% 21% 25% 22% 6%	180
	21st to end	Day	23% 16% 29% 24% 8% 1	198
January	1st to 10th	Night	20% 15% 40% 21% 4	760
	11th to 20th	Night	21% 16% 33% 26% 4	760
	21st to end	Night	19% 13% 40% 26% 2	836
February	1st to 10th	Night	20% 12% 40% 27% 2	760
	11th to 20th	Night	22% 18% 41% 18% 1	743
	21st to end	Night	22% 17% 37% 20% 4	628
March	1st to 10th	Night	18% 15% 38% 25% 4	760
	11th to 20th	Night	20% 14% 38% 22% 6% 1	760
	21st to end	Night	21% 9% 40% 24% 6%	836
April	1st to 10th	Night	20% 13% 38% 22% 6%	760
	11th to 20th	Night	16% 11% 34% 32% 7%	751
	21st to end	Night	23% 10% 29% 31% 8%	760
May	1st to 10th	Night	21% 6% 33% 33% 7%	658
	11th to 20th	Night	20% 7% 30% 34% 9%	374
	21st to end	Night	17% 4 19% 39% 20% 2	411
June	1st to 10th	Night	14% 4 22% 41% 19% 1	380
	11th to 20th	Night	11% 1 26% 43% 19%	380
	21st to end	Night	10% 2 22% 48% 18%	380
July	1st to 10th	Night	11% 2 18% 43% 24% 2	370
	11th to 20th	Night	8% 1 13% 46% 32% 1	368
	21st to end	Night	9% 2 13% 47% 25% 2	527
August	1st to 10th	Night	11% 2 13% 43% 29% 2	567
	11th to 20th	Night	13% 2 14% 39% 29% 3	714
	21st to end	Night	13% 3 15% 38% 27% 4	836
September	1st to 10th	Night	16% 2 17% 38% 24% 3	760
	11th to 20th	Night	14% 3 15% 36% 27% 5%	760
	21st to end	Night	22% 4 21% 33% 20% 1	760
October	1st to 10th	Night	21% 5% 21% 30% 19% 4	760
	11th to 20th	Night	17% 6% 29% 34% 13% 1	760
	21st to end	Night	18% 7% 33% 29% 11% 1	836
November	1st to 10th	Night	13% 7% 35% 32% 12% 1	780
	11th to 20th	Night	19% 12% 33% 29% 8% 1	800
	21st to end	Night	15% 13% 34% 29% 8% 1	779
December	1st to 10th	Night	22% 13% 34% 23% 8% 1	760
	11th to 20th	Night	18% 12% 39% 26% 6%	760
	21st to end	Night	21% 10% 37% 30% 2	836

Page generated at 2024-11-22 08:02:24. Version ac07

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