Summit Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summit Lake clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 9% 3 9% 5% 5% 5% 4 6% 7% 35%	138611
	Night	12% 10% 4 10% 5% 5% 4 4 5% 7% 34%	134729
Transparency	Day	47% 4 11% 24% 13% 1	142063
	Night	49% 4 11% 22% 13% 1	139353
Seeing	Day	19% 14% 32% 22% 10% 3	18456
	Night	19% 12% 32% 27% 10% 1	28286

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	8% 13% 5% 16% 9% 6% 5% 4 5% 7% 22%	4691
	11th to 20th	Day	7% 14% 5% 13% 7% 6% 7% 6% 7% 7% 20%	4710
	21st to end	Day	7% 11% 6% 12% 4 5% 6% 5% 6% 8% 29%	5156
February	1st to 10th	Day	8% 12% 5% 10% 6% 5% 7% 5% 5% 7% 30%	1878
	11th to 20th	Day	8% 13% 4 9% 5% 6% 5% 5% 7% 7% 31%	1933
	21st to end	Day	11% 14% 6% 11% 6% 5% 5% 4 5% 7% 27%	1730
March	1st to 10th	Day	13% 10% 2 6% 4 4 4 3 4 7% 43%	4580
	11th to 20th	Day	11% 9% 3 10% 7% 5% 6% 4 6% 8% 31%	4855
	21st to end	Day	21% 13% 4 11% 5% 4 4 3 4 5% 27%	6051
April	1st to 10th	Day	15% 14% 4 9% 6% 4 5% 4 5% 6% 28%	2520
	11th to 20th	Day	20% 10% 4 9% 6% 5% 4 4 4 5% 29%	2494
	21st to end	Day	17% 12% 4 7% 6% 6% 5% 4 5% 6% 29%	2662
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 5% 6% 42%	8110
	11th to 20th	Day	12% 7% 2 7% 4 5% 5% 4 5% 7% 42%	8222
	21st to end	Day	12% 7% 3 7% 4 4 5% 4 6% 8% 40%	9047
June	1st to 10th	Day	12% 7% 3 6% 5% 4 5% 4 6% 8% 40%	2940
	11th to 20th	Day	14% 8% 2 7% 5% 5% 5% 4 6% 8% 36%	2940
	21st to end	Day	12% 7% 2 7% 4 5% 6% 4 8% 10% 35%	2940
July	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 7% 10% 41%	6500
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 10% 49%	6500
	21st to end	Day	9% 6% 3 7% 5% 4 5% 4 6% 9% 43%	7065
August	1st to 10th	Day	10% 6% 3 8% 6% 5% 5% 5% 6% 7% 38%	4563
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 6% 8% 46%	4119
	21st to end	Day	13% 8% 3 8% 5% 4 4 4 5% 7% 39%	4312
September	1st to 10th	Day	12% 7% 3 7% 4 4 4 3 5% 8% 43%	2400
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 6% 8% 42%	2334
	21st to end	Day	12% 9% 2 7% 5% 5% 6% 4 6% 7% 37%	2200
October	1st to 10th	Day	14% 8% 3 6% 4 4 5% 4 6% 8% 38%	2035
	11th to 20th	Day	17% 14% 3 9% 6% 5% 4 3 4 7% 27%	1980
	21st to end	Day	19% 14% 4 9% 4 3 3 3 5% 7% 29%	1980
November	1st to 10th	Day	14% 10% 4 8% 5% 4 5% 4 7% 7% 34%	1852
	11th to 20th	Day	13% 14% 5% 14% 6% 4 4 4 6% 5% 25%	1784
	21st to end	Day	13% 13% 6% 10% 7% 5% 5% 4 5% 7% 24%	1680
December	1st to 10th	Day	8% 12% 6% 22% 8% 7% 7% 5% 4 5% 16%	2690
	11th to 20th	Day	10% 19% 7% 12% 8% 5% 5% 3 4 5% 22%	3088
	21st to end	Day	19% 23% 6% 11% 4 4 3 2 4 5% 18%	4070
January	1st to 10th	Night	10% 14% 5% 14% 9% 7% 5% 4 5% 6% 23%	8232
	11th to 20th	Night	7% 14% 6% 15% 9% 7% 6% 6% 6% 7% 18%	8280
	21st to end	Night	6% 12% 6% 14% 7% 6% 6% 4 5% 6% 28%	9058
February	1st to 10th	Night	9% 12% 5% 12% 6% 6% 5% 4 5% 7% 30%	2740
	11th to 20th	Night	8% 11% 4 10% 6% 6% 5% 5% 6% 7% 33%	2572
	21st to end	Night	12% 12% 4 10% 6% 5% 5% 4 5% 5% 31%	2076
March	1st to 10th	Night	10% 7% 3 8% 4 4 4 3 5% 8% 43%	5493
	11th to 20th	Night	10% 11% 4 11% 6% 5% 4 4 6% 8% 31%	5354
	21st to end	Night	20% 11% 4 10% 5% 4 4 3 4 6% 29%	5119
April	1st to 10th	Night	20% 11% 4 8% 4 3 4 4 5% 6% 32%	2100
	11th to 20th	Night	22% 9% 3 8% 4 4 3 3 5% 5% 35%	2075
	21st to end	Night	15% 9% 3 8% 5% 5% 4 4 5% 6% 35%	1957
May	1st to 10th	Night	10% 8% 3 6% 4 4 4 4 5% 8% 44%	4753
	11th to 20th	Night	12% 7% 3 7% 5% 4 4 4 6% 7% 42%	4670
	21st to end	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	5139
June	1st to 10th	Night	9% 7% 3 6% 4 5% 4 5% 5% 8% 44%	1680
	11th to 20th	Night	13% 7% 3 8% 4 5% 4 5% 5% 7% 37%	1680
	21st to end	Night	12% 5% 2 5% 4 5% 5% 4 6% 9% 43%	1680
July	1st to 10th	Night	12% 5% 2 5% 4 5% 4 4 6% 9% 44%	3698
	11th to 20th	Night	8% 4 2 4 3 3 3 3 6% 9% 54%	3697
	21st to end	Night	8% 7% 2 6% 4 4 4 4 5% 8% 49%	4027
August	1st to 10th	Night	10% 5% 2 7% 5% 4 5% 4 6% 8% 42%	2597
	11th to 20th	Night	11% 6% 3 5% 3 4 4 3 5% 7% 48%	2930
	21st to end	Night	12% 9% 3 7% 4 4 5% 3 4 6% 46%	3586
September	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 5% 7% 51%	2000
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 5% 7% 48%	2045
	21st to end	Night	12% 8% 3 7% 4 4 4 3 6% 6% 43%	2200
October	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 5% 7% 45%	2361
	11th to 20th	Night	14% 12% 4 7% 4 4 5% 3 4 7% 35%	2420
	21st to end	Night	17% 12% 3 9% 5% 5% 4 3 5% 7% 30%	2860
November	1st to 10th	Night	15% 10% 4 7% 5% 4 4 3 5% 7% 37%	2666
	11th to 20th	Night	13% 14% 5% 10% 5% 4 4 3 4 7% 30%	2835
	21st to end	Night	13% 13% 5% 13% 6% 5% 5% 3 4 7% 25%	2940
December	1st to 10th	Night	11% 11% 5% 12% 9% 7% 5% 4 7% 8% 22%	4720
	11th to 20th	Night	10% 17% 8% 14% 6% 5% 4 3 4 5% 22%	5339
	21st to end	Night	21% 18% 5% 10% 5% 4 4 3 4 5% 21%	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	61% 4 9% 17% 9%	4692
	11th to 20th	Day	60% 3 8% 16% 10% 1	4710
	21st to end	Day	53% 2 10% 22% 12%	5156
February	1st to 10th	Day	53% 4 10% 19% 14% 1	1878
	11th to 20th	Day	49% 5% 10% 24% 10% 2	1942
	21st to end	Day	56% 4 9% 19% 10% 1	1730
March	1st to 10th	Day	42% 3 13% 20% 18% 5%	4580
	11th to 20th	Day	48% 6% 14% 19% 12% 1	4855
	21st to end	Day	61% 3 8% 15% 13%	6053
April	1st to 10th	Day	56% 4 10% 18% 12% 1	2520
	11th to 20th	Day	56% 4 8% 20% 10% 1	2494
	21st to end	Day	55% 4 8% 19% 12% 1	2662
May	1st to 10th	Day	40% 4 9% 28% 17% 1	8148
	11th to 20th	Day	41% 5% 11% 27% 16% 1	8226
	21st to end	Day	42% 4 9% 24% 20% 1	9061
June	1st to 10th	Day	41% 4 13% 28% 12% 1	2940
	11th to 20th	Day	45% 4 15% 23% 13% 1	2940
	21st to end	Day	41% 6% 17% 26% 10% 1	2940
July	1st to 10th	Day	37% 5% 16% 33% 9%	6431
	11th to 20th	Day	30% 5% 16% 35% 12% 2	6425
	21st to end	Day	36% 5% 12% 29% 16%	6869
August	1st to 10th	Day	42% 5% 14% 24% 13% 1	4548
	11th to 20th	Day	35% 5% 14% 35% 11%	4161
	21st to end	Day	44% 5% 14% 26% 11% 1	4311
September	1st to 10th	Day	40% 4 14% 29% 13% 1	2400
	11th to 20th	Day	40% 4 17% 30% 8% 1	2334
	21st to end	Day	45% 4 12% 27% 11%	2200
October	1st to 10th	Day	32% 3 12% 33% 20%	3308
	11th to 20th	Day	50% 4 10% 20% 16% 1	3208
	21st to end	Day	50% 4 16% 23% 6%	3180
November	1st to 10th	Day	48% 4 11% 23% 12% 1	1852
	11th to 20th	Day	59% 4 9% 19% 8% 1	1784
	21st to end	Day	59% 5% 13% 19% 4 1	1680
December	1st to 10th	Day	68% 5% 7% 15% 5%	2690
	11th to 20th	Day	65% 3 8% 13% 11%	3085
	21st to end	Day	70% 3 5% 13% 10%	4070
January	1st to 10th	Night	60% 5% 9% 17% 9% 1	8233
	11th to 20th	Night	64% 4 9% 13% 10%	8280
	21st to end	Night	56% 3 8% 20% 13% 1	9058
February	1st to 10th	Night	53% 2 7% 22% 13% 2	2740
	11th to 20th	Night	48% 6% 8% 23% 12% 2	2573
	21st to end	Night	54% 4 12% 20% 9% 2	2076
March	1st to 10th	Night	40% 3 15% 23% 16% 3	5493
	11th to 20th	Night	50% 5% 15% 19% 9% 2	5354
	21st to end	Night	57% 3 10% 17% 12% 1	5119
April	1st to 10th	Night	53% 4 8% 18% 17% 1	2100
	11th to 20th	Night	51% 3 9% 22% 12% 3	2075
	21st to end	Night	49% 4 10% 21% 15% 1	1957
May	1st to 10th	Night	38% 4 12% 30% 14% 2	4777
	11th to 20th	Night	40% 6% 11% 23% 18% 1	4671
	21st to end	Night	42% 3 12% 21% 19% 3	5149
June	1st to 10th	Night	38% 5% 14% 28% 14% 1	1680
	11th to 20th	Night	45% 4 14% 21% 15% 1	1680
	21st to end	Night	36% 6% 15% 31% 11% 1	1680
July	1st to 10th	Night	36% 6% 19% 28% 9% 2	3660
	11th to 20th	Night	27% 4 17% 37% 13% 2	3652
	21st to end	Night	33% 5% 15% 28% 18% 1	3912
August	1st to 10th	Night	38% 5% 16% 22% 17% 2	2588
	11th to 20th	Night	36% 5% 15% 28% 13% 3	2954
	21st to end	Night	40% 4 13% 27% 14% 2	3586
September	1st to 10th	Night	35% 3 12% 34% 16% 1	2000
	11th to 20th	Night	36% 4 14% 35% 10%	2045
	21st to end	Night	40% 5% 13% 29% 13% 1	2200
October	1st to 10th	Night	31% 3 9% 36% 19% 2	3868
	11th to 20th	Night	39% 4 10% 31% 16%	3931
	21st to end	Night	53% 4 9% 23% 11%	4610
November	1st to 10th	Night	47% 3 10% 25% 13% 1	2666
	11th to 20th	Night	55% 3 11% 18% 11% 1	2835
	21st to end	Night	59% 4 11% 19% 6% 1	2940
December	1st to 10th	Night	59% 6% 15% 17% 3	4720
	11th to 20th	Night	65% 3 7% 15% 10%	5341
	21st to end	Night	67% 3 5% 15% 10%	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	20% 19% 32% 22% 6% 1	330
	11th to 20th	Day	20% 20% 36% 19% 4 1	330
	21st to end	Day	21% 18% 40% 19% 2	363
February	1st to 10th	Day	17% 25% 41% 13% 4 1	330
	11th to 20th	Day	19% 28% 36% 12% 4	330
	21st to end	Day	24% 25% 39% 11% 1	273
March	1st to 10th	Day	23% 24% 39% 12% 3	330
	11th to 20th	Day	26% 21% 33% 13% 5% 2	330
	21st to end	Day	27% 18% 32% 16% 6% 1	572
April	1st to 10th	Day	28% 18% 34% 16% 3 1	540
	11th to 20th	Day	32% 16% 28% 16% 6% 2	532
	21st to end	Day	28% 13% 27% 21% 8% 2	540
May	1st to 10th	Day	22% 18% 32% 21% 6% 2	716
	11th to 20th	Day	21% 16% 31% 21% 8% 3	775
	21st to end	Day	19% 13% 30% 24% 11% 3	844
June	1st to 10th	Day	14% 12% 35% 25% 11% 3	780
	11th to 20th	Day	22% 12% 33% 22% 9% 2	780
	21st to end	Day	16% 11% 32% 24% 13% 4	780
July	1st to 10th	Day	10% 6% 29% 30% 18% 6%	771
	11th to 20th	Day	10% 9% 34% 27% 15% 6%	768
	21st to end	Day	9% 6% 33% 30% 17% 5%	845
August	1st to 10th	Day	11% 7% 27% 30% 19% 6%	726
	11th to 20th	Day	17% 5% 23% 30% 19% 7%	627
	21st to end	Day	18% 8% 27% 24% 17% 6%	583
September	1st to 10th	Day	17% 9% 26% 26% 16% 6%	530
	11th to 20th	Day	16% 6% 29% 33% 13% 3	530
	21st to end	Day	19% 11% 28% 25% 14% 3	530
October	1st to 10th	Day	20% 15% 28% 23% 12% 2	365
	11th to 20th	Day	22% 23% 36% 15% 3	330
	21st to end	Day	28% 20% 36% 15% 2	363
November	1st to 10th	Day	24% 16% 39% 17% 3 1	330
	11th to 20th	Day	24% 23% 37% 12% 3 2	330
	21st to end	Day	22% 26% 37% 12% 3	330
December	1st to 10th	Day	18% 21% 45% 13% 3 1	330
	11th to 20th	Day	24% 24% 38% 11% 3	330
	21st to end	Day	29% 21% 35% 10% 3 1	363
January	1st to 10th	Night	17% 17% 40% 22% 5%	1050
	11th to 20th	Night	19% 15% 38% 21% 6% 1	1050
	21st to end	Night	18% 13% 39% 24% 6%	1155
February	1st to 10th	Night	15% 16% 39% 23% 7% 1	1050
	11th to 20th	Night	15% 19% 37% 23% 5%	938
	21st to end	Night	20% 15% 37% 23% 4	692
March	1st to 10th	Night	19% 14% 35% 23% 8% 1	840
	11th to 20th	Night	20% 16% 37% 21% 5% 1	751
	21st to end	Night	23% 11% 32% 24% 9% 1	693
April	1st to 10th	Night	26% 13% 32% 24% 4 1	630
	11th to 20th	Night	27% 10% 31% 26% 6%	623
	21st to end	Night	22% 9% 33% 29% 6% 1	630
May	1st to 10th	Night	23% 9% 35% 25% 8% 1	630
	11th to 20th	Night	20% 9% 33% 30% 7%	621
	21st to end	Night	17% 7% 27% 38% 10% 1	683
June	1st to 10th	Night	13% 6% 30% 38% 12%	630
	11th to 20th	Night	19% 8% 29% 34% 9% 1	630
	21st to end	Night	15% 5% 26% 38% 16% 1	630
July	1st to 10th	Night	10% 4 22% 41% 21% 2	615
	11th to 20th	Night	9% 5% 29% 35% 20% 1	612
	21st to end	Night	9% 3 26% 41% 19% 2	687
August	1st to 10th	Night	10% 2 22% 40% 23% 2	597
	11th to 20th	Night	13% 3 20% 36% 25% 3	594
	21st to end	Night	14% 5% 26% 33% 21% 2	660
September	1st to 10th	Night	15% 5% 25% 33% 18% 4	600
	11th to 20th	Night	15% 4 29% 35% 16% 1	645
	21st to end	Night	17% 8% 29% 31% 15% 1	800
October	1st to 10th	Night	19% 9% 31% 27% 14% 1	800
	11th to 20th	Night	23% 15% 31% 26% 6%	800
	21st to end	Night	24% 11% 31% 24% 10% 1	880
November	1st to 10th	Night	21% 12% 38% 23% 5% 1	820
	11th to 20th	Night	21% 19% 35% 20% 6%	945
	21st to end	Night	21% 18% 35% 21% 4	1050
December	1st to 10th	Night	20% 17% 36% 21% 6% 1	1050
	11th to 20th	Night	20% 16% 35% 22% 6% 1	1050
	21st to end	Night	28% 16% 32% 20% 4	1155

Page generated at 2026-07-28 02:25:25. Version ac07

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