Skywalker Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skywalker Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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% 7% 2 7% 4 4 5% 4 6% 7% 41%	131940
	Night	14% 7% 3 6% 4 4 4 3 5% 7% 43%	130600
Transparency	Day	40% 6% 11% 22% 17% 4	135647
	Night	39% 5% 10% 22% 19% 5%	134967
Seeing	Day	17% 9% 24% 24% 18% 9%	17041
	Night	17% 8% 25% 29% 18% 3	26793

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	18% 11% 4 9% 3 3 3 2 4 5% 38%	4691
	11th to 20th	Day	14% 6% 2 8% 3 3 3 2 3 5% 51%	5039
	21st to end	Day	21% 8% 3 7% 5% 3 4 3 4 6% 36%	6436
February	1st to 10th	Day	20% 10% 3 6% 4 3 3 2 4 6% 38%	2100
	11th to 20th	Day	21% 10% 3 7% 5% 3 3 2 4 4 36%	2048
	21st to end	Day	20% 8% 3 7% 4 3 3 3 5% 5% 38%	1730
March	1st to 10th	Day	17% 12% 3 8% 5% 4 4 3 5% 5% 34%	4580
	11th to 20th	Day	13% 8% 3 7% 4 3 4 3 4 6% 46%	4794
	21st to end	Day	12% 7% 3 7% 5% 4 4 3 5% 7% 43%	6042
April	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 6% 5% 44%	2520
	11th to 20th	Day	14% 7% 2 7% 3 3 4 3 4 6% 46%	2494
	21st to end	Day	12% 7% 2 6% 5% 3 5% 4 6% 7% 41%	2520
May	1st to 10th	Day	19% 8% 3 7% 5% 5% 5% 4 7% 8% 30%	5800
	11th to 20th	Day	16% 9% 3 9% 5% 6% 5% 4 6% 7% 30%	5861
	21st to end	Day	18% 8% 2 7% 4 4 4 3 6% 8% 37%	6433
June	1st to 10th	Day	9% 6% 3 6% 5% 6% 5% 5% 6% 9% 40%	2608
	11th to 20th	Day	6% 5% 2 6% 6% 4 6% 5% 8% 10% 44%	2800
	21st to end	Day	5% 4 2 5% 6% 6% 6% 6% 8% 11% 41%	2800
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 8% 11% 37%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 7% 6% 9% 13% 33%	6250
	21st to end	Day	11% 8% 3 7% 6% 5% 7% 5% 8% 11% 29%	6337
August	1st to 10th	Day	3 2 1 5% 4 5% 5% 5% 7% 10% 52%	3905
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	3849
	21st to end	Day	7% 4 2 5% 4 4 5% 4 5% 8% 53%	4312
September	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 3 6% 9% 43%	2400
	11th to 20th	Day	13% 5% 2 7% 4 5% 4 4 6% 7% 43%	2200
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 5% 7% 50%	2200
October	1st to 10th	Day	10% 4 1 5% 4 4 3 3 4 6% 56%	2200
	11th to 20th	Day	9% 5% 2 5% 3 2 4 3 5% 6% 55%	2120
	21st to end	Day	16% 6% 1 4 3 4 4 2 5% 5% 50%	2056
November	1st to 10th	Day	11% 6% 2 6% 3 3 3 3 4 5% 54%	1852
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 4 6% 50%	1890
	21st to end	Day	14% 8% 3 5% 4 4 4 3 6% 7% 43%	1890
December	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 3 4 45%	3020
	11th to 20th	Day	22% 10% 4 6% 4 3 3 2 3 4 38%	3463
	21st to end	Day	18% 10% 3 6% 4 4 5% 4 6% 7% 33%	4340
January	1st to 10th	Night	18% 10% 3 8% 3 3 4 3 4 4 40%	8232
	11th to 20th	Night	15% 8% 3 8% 3 3 3 2 3 4 49%	7903
	21st to end	Night	20% 10% 3 7% 4 3 3 2 4 4 38%	7745
February	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 4 6% 37%	2520
	11th to 20th	Night	21% 10% 3 7% 4 4 4 3 3 4 37%	2458
	21st to end	Night	22% 8% 3 7% 3 3 4 4 4 5% 37%	2076
March	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 34%	5493
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 7% 49%	5384
	21st to end	Night	16% 7% 3 7% 4 4 4 3 4 6% 42%	5171
April	1st to 10th	Night	12% 6% 2 5% 5% 3 3 3 5% 9% 45%	2100
	11th to 20th	Night	14% 6% 1 5% 4 3 4 3 4 9% 48%	2075
	21st to end	Night	12% 6% 3 5% 4 4 4 4 6% 8% 45%	2100
May	1st to 10th	Night	16% 9% 3 8% 5% 4 4 4 6% 7% 34%	4103
	11th to 20th	Night	17% 11% 3 9% 5% 4 4 3 5% 7% 32%	4032
	21st to end	Night	14% 9% 3 7% 4 4 4 4 6% 8% 37%	4421
June	1st to 10th	Night	9% 6% 1 6% 5% 4 4 4 6% 11% 44%	1785
	11th to 20th	Night	4 4 1 5% 2 4 4 6% 8% 10% 53%	1600
	21st to end	Night	4 4 2 4 5% 4 4 4 6% 11% 52%	1600
July	1st to 10th	Night	7% 4 2 6% 6% 5% 6% 5% 8% 11% 40%	3618
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 5% 8% 13% 41%	3705
	21st to end	Night	7% 7% 3 8% 5% 6% 5% 5% 9% 13% 31%	4509
August	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 9% 62%	3244
	11th to 20th	Night	4 4 1 4 3 4 5% 4 8% 12% 51%	3200
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 9% 58%	3586
September	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 6% 7% 50%	2000
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 9% 49%	2180
	21st to end	Night	10% 4 2 5% 4 4 3 4 5% 6% 54%	2200
October	1st to 10th	Night	9% 4 1 4 3 2 3 3 4 5% 61%	2200
	11th to 20th	Night	11% 6% 2 5% 3 3 3 2 4 6% 54%	2275
	21st to end	Night	15% 7% 2 4 3 3 3 2 4 5% 51%	2782
November	1st to 10th	Night	12% 5% 2 6% 3 4 3 3 4 5% 53%	2666
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 7% 51%	2730
	21st to end	Night	17% 8% 2 4 4 3 3 3 4 5% 46%	2730
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 6% 43%	4380
	11th to 20th	Night	25% 8% 2 5% 3 2 2 2 4 4 43%	4952
	21st to end	Night	22% 10% 5% 7% 4 4 4 3 5% 6% 31%	6845

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	50% 3 9% 23% 14% 1	4692
	11th to 20th	Day	37% 3 5% 21% 32% 2	5039
	21st to end	Day	49% 3 6% 20% 17% 4	6436
February	1st to 10th	Day	49% 2 8% 20% 18% 3	2100
	11th to 20th	Day	52% 3 5% 20% 15% 4	2058
	21st to end	Day	49% 3 7% 15% 19% 7%	1730
March	1st to 10th	Day	52% 4 5% 13% 16% 9%	4580
	11th to 20th	Day	40% 4 6% 22% 19% 10%	4794
	21st to end	Day	41% 4 10% 26% 18% 1	6044
April	1st to 10th	Day	40% 4 9% 24% 18% 4	2520
	11th to 20th	Day	40% 3 7% 22% 21% 6%	2494
	21st to end	Day	41% 5% 11% 25% 16% 2	2520
May	1st to 10th	Day	49% 8% 13% 19% 10% 1	5837
	11th to 20th	Day	51% 6% 15% 19% 9%	5865
	21st to end	Day	45% 5% 9% 23% 16% 2	6447
June	1st to 10th	Day	39% 6% 13% 23% 17% 2	2608
	11th to 20th	Day	33% 7% 15% 25% 18% 2	2800
	21st to end	Day	32% 10% 17% 22% 16% 3	2800
July	1st to 10th	Day	35% 10% 19% 25% 10% 1	6291
	11th to 20th	Day	35% 12% 24% 24% 5% 1	6175
	21st to end	Day	45% 11% 22% 18% 4	6154
August	1st to 10th	Day	23% 7% 16% 34% 18% 2	3893
	11th to 20th	Day	32% 8% 15% 27% 15% 3	3885
	21st to end	Day	29% 6% 11% 26% 25% 3	4311
September	1st to 10th	Day	37% 7% 13% 21% 18% 4	2400
	11th to 20th	Day	38% 5% 11% 22% 21% 3	2200
	21st to end	Day	33% 5% 10% 23% 21% 7%	2200
October	1st to 10th	Day	21% 3 6% 17% 38% 15%	3590
	11th to 20th	Day	21% 3 5% 23% 31% 18%	3438
	21st to end	Day	28% 3 6% 23% 31% 9%	3294
November	1st to 10th	Day	34% 3 7% 22% 28% 7%	1852
	11th to 20th	Day	36% 4 6% 22% 25% 8%	1890
	21st to end	Day	41% 4 6% 22% 23% 5%	1890
December	1st to 10th	Day	44% 3 5% 25% 21% 3	3020
	11th to 20th	Day	51% 3 6% 19% 17% 4	3460
	21st to end	Day	49% 5% 10% 23% 11% 2	4340
January	1st to 10th	Night	48% 3 12% 20% 16% 2	8233
	11th to 20th	Night	41% 2 5% 18% 28% 6%	7903
	21st to end	Night	50% 2 5% 19% 16% 7%	7745
February	1st to 10th	Night	49% 3 9% 19% 17% 3	2520
	11th to 20th	Night	53% 2 7% 18% 15% 5%	2459
	21st to end	Night	50% 3 5% 18% 18% 6%	2076
March	1st to 10th	Night	51% 4 7% 15% 17% 6%	5493
	11th to 20th	Night	35% 4 7% 19% 20% 15%	5384
	21st to end	Night	44% 3 8% 22% 20% 3	5171
April	1st to 10th	Night	37% 4 13% 22% 21% 4	2100
	11th to 20th	Night	35% 3 9% 22% 23% 8%	2075
	21st to end	Night	37% 4 13% 26% 18% 3	2100
May	1st to 10th	Night	48% 6% 13% 19% 12% 1	4130
	11th to 20th	Night	51% 6% 13% 18% 11% 1	4033
	21st to end	Night	44% 6% 11% 23% 14% 1	4429
June	1st to 10th	Night	34% 8% 13% 24% 19% 2	1785
	11th to 20th	Night	24% 6% 18% 31% 19% 3	1600
	21st to end	Night	25% 8% 16% 29% 17% 4	1600
July	1st to 10th	Night	34% 10% 21% 26% 8% 2	3580
	11th to 20th	Night	30% 11% 27% 26% 5% 2	3660
	21st to end	Night	40% 11% 26% 18% 5%	4378
August	1st to 10th	Night	17% 7% 13% 38% 23% 1	3232
	11th to 20th	Night	24% 7% 24% 27% 16% 2	3230
	21st to end	Night	22% 6% 14% 29% 26% 3	3586
September	1st to 10th	Night	32% 6% 13% 26% 18% 5%	2000
	11th to 20th	Night	34% 5% 13% 20% 26% 3	2180
	21st to end	Night	31% 5% 9% 24% 24% 7%	2200
October	1st to 10th	Night	18% 4 6% 18% 38% 16%	3590
	11th to 20th	Night	21% 2 4 19% 32% 22%	3696
	21st to end	Night	26% 3 5% 27% 26% 14%	4494
November	1st to 10th	Night	34% 4 6% 23% 24% 9%	2666
	11th to 20th	Night	33% 3 6% 22% 25% 9%	2730
	21st to end	Night	41% 3 6% 21% 23% 6%	2730
December	1st to 10th	Night	44% 3 6% 19% 25% 2	4380
	11th to 20th	Night	47% 2 5% 20% 23% 3	4954
	21st to end	Night	54% 4 8% 23% 10% 2	6845

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	28% 19% 33% 16% 4	330
	11th to 20th	Day	30% 21% 32% 12% 2 2	330
	21st to end	Day	24% 20% 43% 11% 2	363
February	1st to 10th	Day	28% 19% 34% 16% 3	330
	11th to 20th	Day	36% 18% 34% 10% 2 1	330
	21st to end	Day	37% 21% 30% 11% 2	273
March	1st to 10th	Day	27% 16% 37% 17% 2 1	330
	11th to 20th	Day	26% 16% 37% 18% 4	400
	21st to end	Day	25% 14% 38% 17% 6%	594
April	1st to 10th	Day	22% 14% 32% 21% 10% 1	540
	11th to 20th	Day	20% 13% 32% 23% 10% 1	532
	21st to end	Day	19% 11% 31% 26% 11% 2	540
May	1st to 10th	Day	20% 10% 29% 26% 12% 4	507
	11th to 20th	Day	15% 7% 28% 25% 14% 11%	527
	21st to end	Day	13% 7% 21% 27% 20% 12%	573
June	1st to 10th	Day	12% 4 20% 25% 24% 15%	538
	11th to 20th	Day	7% 3 17% 27% 30% 16%	730
	21st to end	Day	7% 3 16% 31% 28% 15%	730
July	1st to 10th	Day	9% 3 10% 31% 29% 18%	721
	11th to 20th	Day	9% 3 8% 25% 33% 22%	673
	21st to end	Day	8% 2 10% 26% 30% 24%	576
August	1st to 10th	Day	9% 3 8% 27% 37% 16%	526
	11th to 20th	Day	12% 3 10% 27% 35% 12%	519
	21st to end	Day	12% 3 9% 22% 34% 19%	583
September	1st to 10th	Day	11% 4 12% 31% 31% 12%	530
	11th to 20th	Day	14% 1 11% 27% 30% 16%	530
	21st to end	Day	13% 5% 15% 34% 24% 9%	530
October	1st to 10th	Day	12% 3 19% 32% 25% 8%	530
	11th to 20th	Day	10% 10% 30% 32% 14% 3	450
	21st to end	Day	24% 10% 30% 27% 8% 1	363
November	1st to 10th	Day	18% 12% 30% 26% 12% 2	330
	11th to 20th	Day	15% 16% 38% 26% 5%	330
	21st to end	Day	21% 19% 30% 18% 10% 2	330
December	1st to 10th	Day	25% 18% 36% 15% 5%	330
	11th to 20th	Day	22% 18% 35% 17% 7%	330
	21st to end	Day	25% 18% 39% 16% 2	363
January	1st to 10th	Night	25% 14% 33% 24% 4	1050
	11th to 20th	Night	25% 17% 32% 22% 5%	983
	21st to end	Night	20% 14% 43% 21% 2	924
February	1st to 10th	Night	23% 14% 37% 22% 3	840
	11th to 20th	Night	27% 16% 32% 22% 2	823
	21st to end	Night	28% 15% 34% 21% 2	692
March	1st to 10th	Night	20% 15% 31% 28% 5%	840
	11th to 20th	Night	21% 12% 38% 23% 5%	840
	21st to end	Night	17% 11% 36% 27% 8%	740
April	1st to 10th	Night	16% 11% 36% 27% 10%	630
	11th to 20th	Night	17% 8% 34% 28% 12% 1	623
	21st to end	Night	16% 8% 31% 30% 14% 1	630
May	1st to 10th	Night	16% 5% 29% 31% 18% 2	603
	11th to 20th	Night	14% 6% 24% 34% 19% 4	591
	21st to end	Night	10% 3 16% 39% 27% 5%	650
June	1st to 10th	Night	10% 2 13% 38% 32% 6%	600
	11th to 20th	Night	5% 1 11% 38% 39% 6%	600
	21st to end	Night	5% 3 9% 37% 38% 9%	600
July	1st to 10th	Night	8% 3 6% 28% 43% 11%	585
	11th to 20th	Night	7% 2 7% 28% 42% 14%	582
	21st to end	Night	7% 2 6% 30% 42% 12%	657
August	1st to 10th	Night	6% 1 5% 27% 49% 11%	597
	11th to 20th	Night	7% 2 7% 37% 41% 7%	594
	21st to end	Night	10% 1 7% 30% 42% 11%	660
September	1st to 10th	Night	10% 3 9% 33% 36% 9%	600
	11th to 20th	Night	12% 3 14% 29% 37% 6%	780
	21st to end	Night	11% 3 15% 35% 31% 4	800
October	1st to 10th	Night	11% 3 19% 33% 28% 6%	800
	11th to 20th	Night	13% 6% 23% 37% 18% 2	800
	21st to end	Night	18% 6% 28% 30% 16% 2	880
November	1st to 10th	Night	15% 8% 24% 34% 17% 2	820
	11th to 20th	Night	15% 8% 34% 30% 12% 1	840
	21st to end	Night	22% 10% 29% 27% 12% 1	840
December	1st to 10th	Night	25% 13% 28% 28% 6%	840
	11th to 20th	Night	23% 11% 30% 28% 8%	840
	21st to end	Night	27% 13% 31% 26% 4	1019

Page generated at 2026-04-29 01:21:45. Version ac07

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