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 2025-03-27.

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% 10% 4 9% 5% 5% 5% 4 6% 7% 33%	109073
	Night	12% 11% 4 10% 5% 5% 4 4 5% 7% 34%	110477
Transparency	Day	49% 4 11% 23% 12% 1	112570
	Night	49% 4 11% 23% 12% 1	115120
Seeing	Day	19% 14% 32% 22% 10% 3	16467
	Night	18% 11% 33% 27% 10% 1	26544

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% 11% 5% 16% 9% 7% 5% 4 5% 6% 23%	3617
	11th to 20th	Day	9% 13% 5% 12% 8% 7% 8% 6% 7% 7% 20%	3620
	21st to end	Day	9% 14% 8% 14% 4 4 5% 4 5% 7% 26%	3982
February	1st to 10th	Day	8% 12% 5% 10% 6% 5% 7% 5% 6% 7% 30%	1788
	11th to 20th	Day	8% 13% 4 9% 5% 6% 5% 5% 7% 7% 31%	1839
	21st to end	Day	11% 14% 6% 10% 6% 5% 5% 4 5% 7% 27%	1650
March	1st to 10th	Day	12% 12% 3 8% 4 4 3 3 4 6% 42%	3220
	11th to 20th	Day	8% 10% 4 12% 8% 5% 6% 4 6% 7% 30%	3425
	21st to end	Day	18% 13% 4 13% 5% 4 4 3 4 5% 25%	4203
April	1st to 10th	Day	15% 14% 4 9% 6% 4 5% 4 5% 6% 29%	2280
	11th to 20th	Day	21% 11% 4 8% 5% 4 4 4 4 5% 29%	2254
	21st to end	Day	17% 12% 4 7% 6% 6% 5% 4 5% 6% 30%	2408
May	1st to 10th	Day	16% 8% 2 7% 5% 4 5% 4 5% 6% 37%	4353
	11th to 20th	Day	14% 8% 2 6% 4 4 4 4 5% 7% 40%	4399
	21st to end	Day	14% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	4824
June	1st to 10th	Day	11% 7% 3 7% 6% 4 5% 4 6% 8% 40%	2660
	11th to 20th	Day	14% 8% 2 7% 6% 5% 5% 4 6% 8% 37%	2660
	21st to end	Day	11% 7% 2 7% 4 5% 5% 5% 8% 10% 36%	2660
July	1st to 10th	Day	13% 4 2 6% 5% 5% 5% 4 7% 11% 37%	4440
	11th to 20th	Day	5% 4 2 5% 4 4 5% 5% 7% 10% 48%	4440
	21st to end	Day	8% 6% 2 7% 5% 4 5% 5% 6% 9% 44%	4813
August	1st to 10th	Day	10% 6% 2 8% 6% 5% 5% 5% 6% 7% 38%	4423
	11th to 20th	Day	13% 6% 2 6% 4 3 4 3 6% 8% 46%	3991
	21st to end	Day	13% 9% 3 8% 5% 4 4 3 5% 7% 38%	4180
September	1st to 10th	Day	12% 7% 3 6% 4 4 4 3 5% 8% 44%	2280
	11th to 20th	Day	12% 6% 3 6% 4 5% 5% 4 6% 8% 42%	2218
	21st to end	Day	13% 9% 2 7% 5% 5% 5% 4 6% 7% 36%	2090
October	1st to 10th	Day	15% 8% 3 6% 4 4 5% 4 6% 8% 38%	1933
	11th to 20th	Day	18% 14% 3 9% 6% 5% 4 4 4 7% 27%	1881
	21st to end	Day	20% 14% 4 9% 4 3 3 3 4 7% 29%	1881
November	1st to 10th	Day	15% 10% 3 8% 4 4 5% 4 7% 7% 34%	1762
	11th to 20th	Day	13% 15% 6% 15% 6% 4 4 4 6% 5% 24%	1699
	21st to end	Day	13% 13% 6% 10% 7% 5% 5% 4 5% 8% 24%	1600
December	1st to 10th	Day	8% 12% 5% 22% 8% 7% 7% 5% 4 5% 16%	2610
	11th to 20th	Day	10% 19% 7% 12% 8% 5% 5% 3 4 5% 22%	3008
	21st to end	Day	19% 24% 6% 11% 4 4 3 2 4 5% 18%	3982
January	1st to 10th	Night	9% 13% 5% 14% 10% 6% 5% 3 5% 6% 25%	6348
	11th to 20th	Night	8% 13% 6% 14% 7% 7% 6% 5% 6% 7% 21%	6360
	21st to end	Night	7% 15% 7% 16% 6% 4 4 3 4 6% 26%	6996
February	1st to 10th	Night	8% 12% 4 11% 7% 6% 5% 4 6% 7% 30%	2610
	11th to 20th	Night	7% 11% 4 10% 6% 6% 6% 5% 6% 7% 33%	2447
	21st to end	Night	13% 12% 4 10% 6% 5% 5% 4 5% 5% 32%	1980
March	1st to 10th	Night	10% 8% 3 7% 4 5% 4 3 6% 8% 42%	3853
	11th to 20th	Night	8% 9% 4 12% 6% 5% 4 4 5% 7% 35%	3770
	21st to end	Night	18% 11% 5% 11% 5% 4 4 3 4 6% 29%	3565
April	1st to 10th	Night	20% 10% 4 8% 4 3 4 4 5% 6% 32%	1900
	11th to 20th	Night	21% 9% 3 7% 4 3 3 3 5% 5% 36%	1875
	21st to end	Night	16% 9% 3 9% 5% 5% 5% 4 5% 6% 33%	1771
May	1st to 10th	Night	16% 10% 2 7% 5% 4 4 4 5% 7% 36%	2549
	11th to 20th	Night	12% 7% 3 6% 4 5% 5% 4 5% 6% 44%	2506
	21st to end	Night	15% 7% 3 7% 5% 5% 4 3 6% 6% 38%	2743
June	1st to 10th	Night	9% 7% 3 6% 4 4 4 5% 5% 8% 45%	1520
	11th to 20th	Night	12% 7% 3 9% 4 5% 5% 5% 5% 7% 38%	1520
	21st to end	Night	11% 5% 2 5% 4 5% 5% 5% 6% 9% 44%	1520
July	1st to 10th	Night	14% 5% 2 5% 3 4 4 4 5% 8% 45%	2530
	11th to 20th	Night	5% 3 2 4 3 4 4 4 6% 10% 55%	2530
	21st to end	Night	8% 7% 2 6% 3 3 4 4 5% 6% 52%	2750
August	1st to 10th	Night	11% 5% 2 7% 5% 4 5% 4 6% 8% 42%	2517
	11th to 20th	Night	11% 6% 3 5% 3 4 4 3 5% 7% 49%	2838
	21st to end	Night	12% 9% 3 7% 3 4 5% 3 4 6% 45%	3476
September	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 5% 7% 51%	1900
	11th to 20th	Night	11% 6% 1 5% 4 4 4 4 5% 7% 48%	1943
	21st to end	Night	13% 8% 3 7% 4 4 4 3 6% 6% 43%	2090
October	1st to 10th	Night	14% 6% 2 6% 3 5% 5% 3 6% 7% 44%	2243
	11th to 20th	Night	14% 12% 4 7% 4 4 5% 3 5% 7% 34%	2299
	21st to end	Night	17% 12% 3 9% 5% 5% 4 3 5% 7% 29%	2717
November	1st to 10th	Night	15% 10% 4 7% 5% 4 4 3 5% 7% 37%	2536
	11th to 20th	Night	13% 15% 5% 10% 5% 4 4 3 4 7% 29%	2700
	21st to end	Night	13% 13% 5% 13% 6% 5% 5% 3 5% 7% 26%	2800
December	1st to 10th	Night	11% 11% 5% 12% 9% 7% 5% 4 7% 8% 22%	4580
	11th to 20th	Night	10% 17% 8% 14% 6% 5% 4 3 4 5% 23%	5199
	21st to end	Night	21% 19% 5% 10% 5% 4 4 3 4 5% 21%	6996

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	60% 4 7% 19% 10% 1	3618
	11th to 20th	Day	61% 3 8% 15% 10% 2	3620
	21st to end	Day	59% 2 11% 18% 9%	3982
February	1st to 10th	Day	53% 4 10% 19% 14% 1	1788
	11th to 20th	Day	50% 5% 10% 23% 10% 2	1848
	21st to end	Day	56% 4 9% 19% 10% 1	1650
March	1st to 10th	Day	44% 3 11% 20% 20% 2	3220
	11th to 20th	Day	49% 7% 13% 19% 10% 1	3425
	21st to end	Day	61% 3 8% 18% 9% 1	4203
April	1st to 10th	Day	56% 4 10% 17% 12% 1	2280
	11th to 20th	Day	56% 4 7% 21% 11% 1	2254
	21st to end	Day	55% 4 7% 19% 13% 1	2408
May	1st to 10th	Day	46% 5% 9% 26% 14% 1	4394
	11th to 20th	Day	42% 4 12% 28% 13% 1	4403
	21st to end	Day	48% 4 10% 22% 15% 1	4837
June	1st to 10th	Day	41% 4 14% 29% 11% 1	2660
	11th to 20th	Day	44% 4 15% 23% 14% 1	2660
	21st to end	Day	40% 6% 17% 26% 11% 1	2660
July	1st to 10th	Day	40% 5% 16% 28% 11% 1	4371
	11th to 20th	Day	28% 5% 15% 37% 13% 3	4365
	21st to end	Day	35% 5% 12% 31% 17% 1	4662
August	1st to 10th	Day	42% 4 14% 25% 13% 1	4408
	11th to 20th	Day	35% 5% 14% 35% 11%	4033
	21st to end	Day	44% 4 14% 25% 12% 1	4179
September	1st to 10th	Day	40% 3 13% 29% 14% 1	2280
	11th to 20th	Day	40% 4 17% 30% 8% 1	2218
	21st to end	Day	46% 4 12% 27% 10%	2090
October	1st to 10th	Day	33% 3 11% 32% 20%	3206
	11th to 20th	Day	50% 4 10% 20% 15% 1	3109
	21st to end	Day	51% 4 16% 23% 6%	3081
November	1st to 10th	Day	48% 4 11% 24% 12% 1	1762
	11th to 20th	Day	61% 4 9% 18% 8% 1	1699
	21st to end	Day	58% 5% 13% 19% 4 1	1600
December	1st to 10th	Day	69% 5% 7% 15% 5%	2610
	11th to 20th	Day	65% 3 8% 13% 11%	3005
	21st to end	Day	70% 3 5% 13% 10%	3982
January	1st to 10th	Night	57% 6% 8% 18% 11% 1	6348
	11th to 20th	Night	60% 4 9% 15% 12% 1	6360
	21st to end	Night	60% 3 8% 19% 9% 1	6996
February	1st to 10th	Night	53% 3 8% 22% 13% 2	2610
	11th to 20th	Night	48% 6% 9% 23% 12% 3	2448
	21st to end	Night	53% 4 12% 19% 10% 2	1980
March	1st to 10th	Night	39% 4 15% 26% 15% 2	3853
	11th to 20th	Night	47% 5% 11% 24% 11% 1	3770
	21st to end	Night	58% 3 8% 20% 10% 1	3565
April	1st to 10th	Night	52% 4 8% 18% 17% 1	1900
	11th to 20th	Night	50% 3 9% 22% 13% 3	1875
	21st to end	Night	50% 4 9% 20% 15% 1	1771
May	1st to 10th	Night	48% 3 14% 21% 12% 1	2573
	11th to 20th	Night	38% 5% 12% 27% 15% 2	2507
	21st to end	Night	46% 4 15% 18% 16% 1	2751
June	1st to 10th	Night	38% 4 14% 29% 14% 1	1520
	11th to 20th	Night	44% 4 14% 21% 16% 1	1520
	21st to end	Night	35% 6% 16% 31% 11% 1	1520
July	1st to 10th	Night	38% 5% 18% 23% 13% 3	2490
	11th to 20th	Night	24% 5% 17% 38% 14% 2	2482
	21st to end	Night	32% 4 12% 32% 19% 2	2662
August	1st to 10th	Night	38% 5% 15% 22% 17% 2	2508
	11th to 20th	Night	35% 5% 15% 28% 14% 3	2862
	21st to end	Night	41% 4 13% 26% 15% 2	3476
September	1st to 10th	Night	34% 4 12% 34% 16% 1	1900
	11th to 20th	Night	35% 4 15% 36% 10%	1943
	21st to end	Night	41% 4 13% 29% 12% 1	2090
October	1st to 10th	Night	31% 3 8% 36% 19% 2	3750
	11th to 20th	Night	39% 4 10% 31% 15%	3810
	21st to end	Night	53% 4 10% 23% 11%	4467
November	1st to 10th	Night	47% 3 11% 25% 13% 1	2536
	11th to 20th	Night	56% 3 11% 17% 12% 1	2700
	21st to end	Night	58% 4 11% 20% 5% 1	2800
December	1st to 10th	Night	58% 6% 15% 17% 3	4580
	11th to 20th	Night	65% 3 7% 15% 10%	5201
	21st to end	Night	67% 3 5% 15% 10%	6996

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% 18% 32% 23% 6% 1	300
	11th to 20th	Day	22% 18% 36% 20% 4 1	300
	21st to end	Day	23% 16% 39% 19% 2	330
February	1st to 10th	Day	17% 25% 40% 13% 4 1	300
	11th to 20th	Day	20% 29% 35% 12% 4	300
	21st to end	Day	24% 24% 39% 12% 1	249
March	1st to 10th	Day	22% 23% 40% 12% 3	300
	11th to 20th	Day	25% 20% 33% 14% 6% 2	300
	21st to end	Day	25% 17% 32% 18% 6% 1	509
April	1st to 10th	Day	25% 17% 35% 18% 4 1	460
	11th to 20th	Day	32% 15% 28% 16% 6% 2	452
	21st to end	Day	27% 13% 26% 22% 9% 2	460
May	1st to 10th	Day	23% 18% 32% 20% 6% 2	620
	11th to 20th	Day	21% 15% 32% 21% 8% 3	675
	21st to end	Day	20% 12% 29% 25% 10% 3	734
June	1st to 10th	Day	13% 13% 35% 25% 11% 4	680
	11th to 20th	Day	21% 11% 32% 24% 10% 3	680
	21st to end	Day	15% 10% 32% 25% 14% 5%	680
July	1st to 10th	Day	11% 6% 29% 30% 18% 7%	671
	11th to 20th	Day	10% 9% 33% 27% 15% 7%	668
	21st to end	Day	9% 5% 32% 31% 18% 6%	739
August	1st to 10th	Day	11% 7% 28% 30% 19% 6%	676
	11th to 20th	Day	16% 4 23% 30% 20% 7%	582
	21st to end	Day	19% 8% 26% 24% 17% 6%	539
September	1st to 10th	Day	16% 9% 26% 27% 16% 5%	490
	11th to 20th	Day	16% 6% 30% 31% 13% 3	490
	21st to end	Day	20% 11% 29% 23% 14% 3	490
October	1st to 10th	Day	22% 14% 27% 23% 12% 2	333
	11th to 20th	Day	23% 24% 36% 15% 3	300
	21st to end	Day	31% 21% 35% 12% 1	330
November	1st to 10th	Day	23% 16% 39% 17% 3 1	300
	11th to 20th	Day	23% 24% 37% 11% 3 1	300
	21st to end	Day	21% 25% 38% 12% 3	300
December	1st to 10th	Day	19% 19% 44% 13% 4 1	300
	11th to 20th	Day	23% 24% 38% 11% 3	300
	21st to end	Day	29% 20% 35% 11% 3 1	330
January	1st to 10th	Night	17% 16% 40% 22% 5%	1000
	11th to 20th	Night	20% 14% 38% 21% 6% 1	1000
	21st to end	Night	19% 12% 38% 25% 6%	1100
February	1st to 10th	Night	15% 16% 39% 23% 7% 1	1000
	11th to 20th	Night	15% 19% 37% 23% 5%	893
	21st to end	Night	20% 15% 37% 23% 4	660
March	1st to 10th	Night	19% 14% 35% 23% 8% 1	800
	11th to 20th	Night	19% 15% 37% 22% 6% 1	715
	21st to end	Night	23% 9% 32% 25% 9% 1	647
April	1st to 10th	Night	25% 12% 33% 25% 5% 1	570
	11th to 20th	Night	26% 9% 32% 27% 6%	563
	21st to end	Night	22% 9% 32% 30% 6% 1	570
May	1st to 10th	Night	24% 9% 35% 24% 7% 1	570
	11th to 20th	Night	21% 7% 33% 30% 8% 1	561
	21st to end	Night	17% 7% 26% 38% 11% 1	617
June	1st to 10th	Night	13% 6% 31% 38% 12% 1	570
	11th to 20th	Night	18% 8% 29% 35% 10% 1	570
	21st to end	Night	13% 6% 25% 38% 17% 1	570
July	1st to 10th	Night	10% 4 21% 41% 22% 2	555
	11th to 20th	Night	10% 4 30% 34% 21% 1	552
	21st to end	Night	8% 2 25% 41% 21% 2	624
August	1st to 10th	Night	11% 2 22% 41% 23% 2	567
	11th to 20th	Night	13% 3 20% 34% 26% 4	564
	21st to end	Night	14% 5% 26% 32% 21% 2	627
September	1st to 10th	Night	15% 5% 26% 34% 18% 3	570
	11th to 20th	Night	14% 4 30% 35% 16% 1	613
	21st to end	Night	17% 8% 30% 30% 14% 1	760
October	1st to 10th	Night	19% 9% 30% 27% 13% 1	760
	11th to 20th	Night	22% 15% 31% 26% 6%	760
	21st to end	Night	25% 11% 32% 23% 9% 1	836
November	1st to 10th	Night	21% 12% 39% 23% 6% 1	780
	11th to 20th	Night	22% 18% 35% 20% 5%	900
	21st to end	Night	21% 18% 35% 21% 5%	1000
December	1st to 10th	Night	20% 16% 36% 21% 6% 1	1000
	11th to 20th	Night	19% 15% 36% 22% 6% 1	1000
	21st to end	Night	28% 15% 32% 21% 4	1100

Page generated at 2025-03-24 08:04:29. Version ac07

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