Summit Crossing Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Summit Crossing Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	149062
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	142649
Transparency	Day	38% 7% 13% 22% 17% 4	152660
	Night	38% 6% 11% 22% 18% 4	147122
Seeing	Day	18% 11% 27% 25% 14% 5%	17606
	Night	17% 10% 29% 30% 13% 1	27160

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 6% 3 4 4 3 5% 5% 47%	5003
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 3 4 52%	5280
	21st to end	Day	13% 7% 3 6% 4 4 4 3 5% 5% 45%	5780
February	1st to 10th	Day	14% 8% 2 6% 5% 5% 4 4 4 7% 41%	1890
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 4 6% 41%	1904
	21st to end	Day	17% 8% 2 7% 4 4 4 3 5% 6% 39%	1730
March	1st to 10th	Day	18% 7% 2 6% 5% 4 4 4 5% 7% 38%	5034
	11th to 20th	Day	11% 5% 2 6% 4 3 4 3 5% 7% 51%	5090
	21st to end	Day	16% 6% 2 6% 4 4 4 3 6% 7% 41%	5632
April	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 8% 47%	2520
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2494
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 41%	2520
May	1st to 10th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 37%	7438
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 6% 9% 39%	7804
	21st to end	Day	21% 6% 2 6% 4 3 4 4 6% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 7% 9% 44%	2940
	11th to 20th	Day	10% 4 2 5% 5% 5% 5% 4 7% 11% 42%	2940
	21st to end	Day	8% 4 2 5% 5% 5% 5% 4 7% 11% 44%	2940
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 7% 11% 42%	8280
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 6% 9% 13% 29%	7749
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 38%	8340
August	1st to 10th	Day	13% 7% 2 7% 5% 5% 6% 6% 8% 11% 29%	6014
	11th to 20th	Day	8% 6% 3 7% 5% 6% 6% 5% 8% 11% 34%	5680
	21st to end	Day	9% 3 2 5% 4 4 5% 4 7% 10% 47%	6080
September	1st to 10th	Day	13% 5% 1 4 4 4 5% 3 5% 8% 48%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 5% 8% 47%	2520
	21st to end	Day	22% 7% 2 7% 4 4 4 3 5% 7% 35%	2199
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 4 5% 50%	2100
	11th to 20th	Day	17% 5% 1 4 3 3 3 3 4 5% 52%	2030
	21st to end	Day	13% 6% 2 5% 2 3 4 2 4 7% 51%	2200
November	1st to 10th	Day	12% 5% 2 4 4 4 4 3 4 6% 53%	2001
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 4 7% 51%	1887
	21st to end	Day	15% 5% 2 5% 4 3 3 3 5% 5% 51%	1680
December	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 6% 50%	2680
	11th to 20th	Day	17% 8% 2 6% 3 2 2 2 4 6% 47%	3070
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 7% 41%	4048
January	1st to 10th	Night	12% 6% 2 5% 4 3 4 4 6% 6% 48%	7873
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 3 5% 53%	7680
	21st to end	Night	14% 6% 2 6% 4 4 4 3 4 5% 49%	8401
February	1st to 10th	Night	14% 8% 2 5% 4 4 4 4 5% 6% 43%	2730
	11th to 20th	Night	21% 7% 2 5% 4 4 3 2 4 6% 43%	2600
	21st to end	Night	16% 7% 3 6% 3 3 4 3 5% 6% 45%	2076
March	1st to 10th	Night	16% 6% 2 5% 3 4 3 3 4 5% 48%	5035
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 6% 7% 50%	5090
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 7% 46%	5528
April	1st to 10th	Night	15% 6% 2 5% 5% 3 4 3 5% 6% 46%	2100
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 4 8% 49%	2075
	21st to end	Night	16% 5% 2 5% 4 4 3 4 7% 7% 44%	2100
May	1st to 10th	Night	15% 8% 3 8% 5% 4 5% 4 6% 8% 35%	5457
	11th to 20th	Night	18% 6% 2 5% 5% 4 4 4 6% 7% 38%	5055
	21st to end	Night	21% 8% 3 6% 4 4 4 4 4 7% 34%	5108
June	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 6% 9% 44%	1680
	11th to 20th	Night	12% 6% 1 6% 5% 4 5% 4 6% 9% 41%	1680
	21st to end	Night	8% 5% 2 5% 5% 4 5% 5% 6% 10% 44%	1680
July	1st to 10th	Night	10% 6% 2 8% 4 5% 5% 4 7% 9% 38%	4678
	11th to 20th	Night	11% 8% 3 9% 6% 6% 6% 5% 7% 10% 29%	5189
	21st to end	Night	9% 6% 3 7% 6% 6% 5% 5% 8% 9% 36%	5736
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 5% 4 7% 10% 30%	4135
	11th to 20th	Night	10% 7% 3 9% 6% 5% 6% 5% 7% 9% 32%	4305
	21st to end	Night	10% 6% 3 8% 4 4 4 3 6% 9% 44%	5045
September	1st to 10th	Night	14% 5% 2 5% 4 4 5% 4 5% 7% 46%	2100
	11th to 20th	Night	11% 7% 3 6% 4 3 4 4 5% 6% 47%	2100
	21st to end	Night	23% 7% 2 7% 4 4 3 4 4 6% 35%	2417
October	1st to 10th	Night	16% 6% 2 4 3 3 4 3 4 6% 47%	2520
	11th to 20th	Night	18% 5% 2 5% 3 2 3 3 5% 5% 50%	2437
	21st to end	Night	13% 5% 1 6% 4 3 4 3 5% 6% 50%	2640
November	1st to 10th	Night	12% 6% 3 5% 3 4 3 3 4 5% 54%	2514
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 5% 49%	2730
	21st to end	Night	12% 4 2 5% 3 4 4 3 5% 6% 51%	2940
December	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 6% 54%	4720
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 5% 41%	5345
	21st to end	Night	15% 7% 2 5% 4 4 4 3 6% 7% 42%	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	39% 4 8% 24% 19% 7%	5004
	11th to 20th	Day	36% 4 5% 18% 27% 10%	5280
	21st to end	Day	40% 4 8% 27% 17% 3	5780
February	1st to 10th	Day	42% 6% 11% 25% 14% 3	1890
	11th to 20th	Day	44% 4 8% 21% 18% 4	1913
	21st to end	Day	46% 4 12% 18% 16% 4	1730
March	1st to 10th	Day	43% 6% 9% 19% 16% 8%	5034
	11th to 20th	Day	31% 5% 9% 25% 21% 8%	5090
	21st to end	Day	42% 6% 11% 18% 17% 6%	5632
April	1st to 10th	Day	34% 6% 12% 21% 20% 7%	2520
	11th to 20th	Day	35% 6% 11% 20% 23% 6%	2494
	21st to end	Day	41% 6% 11% 25% 13% 5%	2520
May	1st to 10th	Day	43% 8% 12% 21% 15% 1	7471
	11th to 20th	Day	39% 7% 13% 26% 15%	7808
	21st to end	Day	44% 7% 12% 19% 16% 3	9061
June	1st to 10th	Day	34% 8% 15% 25% 16% 2	2940
	11th to 20th	Day	34% 9% 16% 23% 17% 1	2940
	21st to end	Day	34% 9% 15% 24% 17% 1	2940
July	1st to 10th	Day	33% 10% 20% 22% 11% 3	8210
	11th to 20th	Day	41% 12% 17% 23% 7% 1	7678
	21st to end	Day	36% 9% 14% 21% 19%	8156
August	1st to 10th	Day	44% 11% 20% 20% 5%	6001
	11th to 20th	Day	38% 10% 18% 21% 11%	5719
	21st to end	Day	30% 8% 19% 26% 17% 1	6080
September	1st to 10th	Day	34% 8% 14% 20% 20% 4	2520
	11th to 20th	Day	35% 6% 9% 26% 20% 4	2520
	21st to end	Day	48% 7% 9% 18% 14% 3	2199
October	1st to 10th	Day	35% 6% 7% 18% 29% 5%	3360
	11th to 20th	Day	27% 3 6% 22% 31% 11%	3270
	21st to end	Day	22% 3 10% 27% 26% 13%	3538
November	1st to 10th	Day	33% 4 9% 26% 23% 4	2001
	11th to 20th	Day	35% 5% 10% 22% 23% 6%	1887
	21st to end	Day	36% 3 10% 25% 22% 4	1680
December	1st to 10th	Day	34% 5% 10% 26% 18% 7%	2680
	11th to 20th	Day	41% 3 13% 25% 15% 4	3066
	21st to end	Day	40% 6% 13% 25% 16% 1	4048
January	1st to 10th	Night	35% 4 11% 22% 22% 5%	7874
	11th to 20th	Night	35% 3 5% 22% 25% 10%	7680
	21st to end	Night	39% 3 7% 28% 19% 4	8401
February	1st to 10th	Night	41% 5% 10% 24% 18% 2	2730
	11th to 20th	Night	44% 3 8% 20% 19% 6%	2602
	21st to end	Night	40% 5% 9% 22% 17% 7%	2076
March	1st to 10th	Night	37% 5% 8% 22% 20% 7%	5035
	11th to 20th	Night	32% 6% 9% 21% 25% 8%	5090
	21st to end	Night	37% 5% 9% 21% 20% 6%	5528
April	1st to 10th	Night	38% 6% 10% 19% 22% 5%	2100
	11th to 20th	Night	34% 6% 11% 23% 20% 6%	2075
	21st to end	Night	37% 6% 12% 26% 15% 5%	2100
May	1st to 10th	Night	44% 8% 14% 22% 9% 2	5486
	11th to 20th	Night	42% 7% 13% 23% 13% 1	5056
	21st to end	Night	48% 7% 11% 18% 14% 2	5115
June	1st to 10th	Night	35% 9% 12% 24% 17% 2	1680
	11th to 20th	Night	37% 9% 16% 22% 14% 1	1680
	21st to end	Night	33% 9% 15% 21% 19% 3	1680
July	1st to 10th	Night	41% 10% 16% 20% 12% 2	4640
	11th to 20th	Night	47% 11% 18% 17% 7% 1	5140
	21st to end	Night	41% 9% 14% 19% 16%	5606
August	1st to 10th	Night	47% 11% 17% 18% 6%	4124
	11th to 20th	Night	43% 10% 17% 19% 11%	4332
	21st to end	Night	36% 7% 14% 25% 17% 1	5045
September	1st to 10th	Night	36% 8% 13% 22% 15% 6%	2100
	11th to 20th	Night	35% 7% 10% 25% 21% 2	2100
	21st to end	Night	49% 6% 11% 16% 15% 3	2417
October	1st to 10th	Night	39% 6% 9% 19% 21% 6%	4040
	11th to 20th	Night	25% 4 9% 22% 31% 9%	3936
	21st to end	Night	22% 3 7% 30% 27% 11%	4252
November	1st to 10th	Night	33% 4 10% 24% 24% 5%	2514
	11th to 20th	Night	38% 4 8% 20% 23% 7%	2730
	21st to end	Night	33% 4 10% 26% 22% 5%	2940
December	1st to 10th	Night	30% 4 11% 28% 23% 4	4720
	11th to 20th	Night	45% 4 11% 19% 15% 6%	5348
	21st to end	Night	38% 6% 13% 26% 16% 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	26% 20% 31% 18% 4	283
	11th to 20th	Day	25% 28% 28% 16% 2	330
	21st to end	Day	23% 23% 37% 14% 3	363
February	1st to 10th	Day	23% 24% 35% 13% 4	330
	11th to 20th	Day	32% 22% 33% 11% 1	391
	21st to end	Day	28% 17% 31% 20% 3	446
March	1st to 10th	Day	23% 19% 33% 20% 5%	540
	11th to 20th	Day	23% 19% 34% 17% 6% 1	540
	21st to end	Day	24% 16% 35% 20% 5% 1	594
April	1st to 10th	Day	22% 16% 33% 21% 6% 1	540
	11th to 20th	Day	18% 16% 31% 23% 9% 3	532
	21st to end	Day	22% 12% 32% 24% 9% 1	540
May	1st to 10th	Day	22% 16% 31% 18% 10% 3	543
	11th to 20th	Day	20% 11% 28% 26% 13% 2	567
	21st to end	Day	21% 7% 21% 27% 19% 4	617
June	1st to 10th	Day	11% 6% 28% 30% 19% 6%	570
	11th to 20th	Day	13% 5% 26% 31% 18% 6%	570
	21st to end	Day	10% 5% 22% 35% 21% 6%	570
July	1st to 10th	Day	10% 4 16% 30% 28% 11%	565
	11th to 20th	Day	5% 3 14% 37% 30% 11%	564
	21st to end	Day	10% 5% 19% 33% 24% 10%	620
August	1st to 10th	Day	14% 3 21% 28% 25% 9%	566
	11th to 20th	Day	11% 4 18% 33% 24% 10%	559
	21st to end	Day	10% 3 20% 31% 22% 14%	627
September	1st to 10th	Day	15% 4 18% 31% 17% 15%	570
	11th to 20th	Day	14% 4 17% 33% 22% 10%	570
	21st to end	Day	28% 11% 21% 26% 9% 5%	570
October	1st to 10th	Day	18% 7% 23% 26% 18% 7%	570
	11th to 20th	Day	19% 13% 23% 27% 16% 2	542
	21st to end	Day	16% 14% 33% 25% 9% 3	583
November	1st to 10th	Day	16% 11% 33% 26% 12% 3	505
	11th to 20th	Day	19% 11% 35% 23% 11% 1	427
	21st to end	Day	21% 16% 33% 19% 9% 1	220
December	1st to 10th	Day	21% 15% 39% 19% 5% 1	220
	11th to 20th	Day	24% 13% 35% 20% 9%	220
	21st to end	Day	27% 16% 29% 18% 9% 1	242
January	1st to 10th	Night	20% 17% 40% 20% 3	840
	11th to 20th	Night	21% 16% 34% 25% 4	840
	21st to end	Night	20% 15% 38% 25% 2	924
February	1st to 10th	Night	20% 14% 38% 25% 3	840
	11th to 20th	Night	26% 18% 37% 17% 2	823
	21st to end	Night	22% 18% 36% 21% 3	692
March	1st to 10th	Night	20% 17% 35% 26% 3	840
	11th to 20th	Night	22% 14% 37% 21% 6%	840
	21st to end	Night	18% 17% 39% 22% 5%	924
April	1st to 10th	Night	18% 16% 38% 23% 4	840
	11th to 20th	Night	15% 12% 36% 28% 8%	831
	21st to end	Night	18% 9% 33% 32% 7%	840
May	1st to 10th	Night	19% 10% 34% 27% 10%	709
	11th to 20th	Night	16% 5% 31% 35% 12%	559
	21st to end	Night	15% 5% 20% 38% 21%	455
June	1st to 10th	Night	9% 5% 21% 42% 21% 1	420
	11th to 20th	Night	14% 3 21% 42% 18% 1	420
	21st to end	Night	9% 3 22% 46% 18% 2	420
July	1st to 10th	Night	10% 4 17% 38% 29% 3	410
	11th to 20th	Night	6% 3 12% 39% 34% 6%	578
	21st to end	Night	8% 2 14% 46% 27% 3	690
August	1st to 10th	Night	12% 1 13% 40% 31% 2	627
	11th to 20th	Night	11% 2 15% 41% 27% 3	717
	21st to end	Night	11% 3 15% 38% 28% 5%	924
September	1st to 10th	Night	15% 3 14% 37% 28% 4	840
	11th to 20th	Night	14% 4 14% 36% 28% 4	840
	21st to end	Night	25% 5% 18% 33% 18% 2	840
October	1st to 10th	Night	20% 5% 19% 32% 21% 4	840
	11th to 20th	Night	20% 8% 27% 29% 15% 1	813
	21st to end	Night	17% 8% 33% 30% 12% 1	880
November	1st to 10th	Night	15% 10% 30% 31% 12% 2	820
	11th to 20th	Night	18% 11% 34% 29% 8%	840
	21st to end	Night	15% 11% 36% 28% 9% 1	840
December	1st to 10th	Night	20% 10% 38% 23% 8%	840
	11th to 20th	Night	20% 11% 35% 29% 5%	840
	21st to end	Night	19% 13% 33% 31% 5%	924

Page generated at 2026-10-11 05:06:27. Version ac07

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