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 2025-01-07.

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 6% 4 4 5% 4 6% 7% 41%	105115
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	103875
Transparency	Day	40% 5% 10% 23% 18% 4	108862
	Night	39% 4 10% 22% 20% 5%	108267
Seeing	Day	17% 8% 23% 24% 19% 9%	15303
	Night	17% 8% 24% 29% 19% 3	25055

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% 14% 4 7% 4 4 4 3 4 6% 36%	2575
	11th to 20th	Day	20% 9% 2 7% 4 4 4 3 4 5% 39%	2697
	21st to end	Day	24% 10% 3 5% 3 3 3 3 4 6% 36%	3476
February	1st to 10th	Day	21% 10% 3 5% 4 3 3 2 4 6% 38%	1900
	11th to 20th	Day	21% 10% 3 7% 5% 3 3 3 4 4 37%	1848
	21st to end	Day	22% 9% 3 7% 4 3 4 4 5% 5% 36%	1570
March	1st to 10th	Day	22% 12% 3 8% 5% 4 4 3 5% 5% 30%	3120
	11th to 20th	Day	14% 7% 3 6% 4 3 4 3 4 7% 46%	3275
	21st to end	Day	13% 7% 3 8% 6% 4 4 4 6% 7% 39%	4123
April	1st to 10th	Day	14% 6% 2 6% 5% 4 4 3 5% 5% 45%	2280
	11th to 20th	Day	16% 7% 2 7% 3 3 4 3 5% 6% 44%	2254
	21st to end	Day	12% 6% 2 6% 5% 3 5% 4 6% 8% 43%	2280
May	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 3 6% 8% 37%	4036
	11th to 20th	Day	17% 8% 2 6% 4 5% 4 4 6% 8% 37%	4081
	21st to end	Day	13% 6% 2 7% 5% 4 4 4 7% 9% 39%	4476
June	1st to 10th	Day	9% 7% 3 6% 5% 5% 5% 5% 6% 9% 40%	2477
	11th to 20th	Day	5% 4 2 5% 5% 4 6% 5% 8% 10% 45%	2660
	21st to end	Day	5% 4 2 5% 6% 6% 6% 5% 8% 10% 42%	2660
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 9% 11% 34%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 6% 6% 6% 8% 12% 33%	4358
	21st to end	Day	16% 10% 3 8% 6% 5% 6% 5% 7% 8% 25%	4407
August	1st to 10th	Day	3 2 1 4 4 5% 5% 5% 7% 10% 53%	3785
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 44%	3729
	21st to end	Day	7% 4 2 5% 4 4 5% 4 5% 8% 53%	4180
September	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 3 6% 9% 42%	2280
	11th to 20th	Day	13% 5% 2 7% 4 5% 4 4 6% 7% 42%	2090
	21st to end	Day	11% 5% 1 5% 4 4 5% 4 5% 6% 50%	2090
October	1st to 10th	Day	10% 4 1 5% 4 4 3 3 4 6% 56%	2090
	11th to 20th	Day	10% 6% 2 5% 3 2 4 4 5% 7% 53%	2014
	21st to end	Day	16% 6% 1 4 3 4 4 2 5% 5% 50%	1953
November	1st to 10th	Day	12% 6% 2 6% 3 3 3 3 4 5% 54%	1762
	11th to 20th	Day	15% 6% 2 4 3 3 4 3 4 6% 49%	1800
	21st to end	Day	15% 8% 3 4 4 4 4 3 6% 7% 43%	1800
December	1st to 10th	Day	16% 8% 2 6% 4 4 4 3 3 4 46%	2930
	11th to 20th	Day	22% 10% 4 6% 4 3 3 2 4 4 37%	3373
	21st to end	Day	18% 10% 3 6% 4 4 5% 4 6% 7% 33%	4246
January	1st to 10th	Night	19% 13% 2 5% 3 3 3 3 5% 5% 40%	4513
	11th to 20th	Night	19% 10% 4 9% 3 3 3 2 4 4 39%	4245
	21st to end	Night	22% 12% 4 6% 3 3 3 2 4 4 38%	4180
February	1st to 10th	Night	19% 10% 3 7% 4 4 4 3 3 5% 37%	2280
	11th to 20th	Night	21% 10% 3 7% 4 4 4 3 3 3 38%	2218
	21st to end	Night	24% 9% 3 7% 3 3 4 4 4 5% 35%	1884
March	1st to 10th	Night	24% 9% 3 6% 4 3 3 3 4 6% 35%	3733
	11th to 20th	Night	13% 8% 2 4 4 3 4 3 5% 7% 47%	3683
	21st to end	Night	15% 7% 3 7% 5% 5% 5% 3 5% 6% 39%	3535
April	1st to 10th	Night	12% 7% 2 6% 4 3 3 3 5% 9% 46%	1900
	11th to 20th	Night	15% 6% 1 5% 4 3 4 3 4 8% 47%	1875
	21st to end	Night	12% 6% 2 4 4 4 4 4 6% 8% 45%	1900
May	1st to 10th	Night	16% 7% 3 6% 4 3 3 3 6% 8% 40%	2859
	11th to 20th	Night	14% 8% 3 6% 3 3 4 3 5% 8% 41%	2812
	21st to end	Night	9% 6% 2 7% 4 4 5% 5% 7% 10% 43%	3079
June	1st to 10th	Night	9% 6% 1 6% 5% 4 4 4 6% 11% 43%	1696
	11th to 20th	Night	4 3 1 4 2 4 4 6% 8% 10% 54%	1520
	21st to end	Night	4 4 2 4 5% 4 4 4 6% 10% 52%	1520
July	1st to 10th	Night	7% 4 3 7% 6% 5% 6% 5% 8% 11% 38%	2530
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 5% 8% 12% 42%	2592
	21st to end	Night	10% 10% 5% 10% 6% 6% 5% 4 7% 9% 29%	3141
August	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 9% 62%	3144
	11th to 20th	Night	4 4 1 4 3 4 5% 4 8% 12% 51%	3100
	21st to end	Night	5% 3 1 4 3 4 4 3 6% 9% 58%	3476
September	1st to 10th	Night	10% 5% 2 6% 4 3 4 3 6% 7% 49%	1900
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 8% 48%	2071
	21st to end	Night	10% 4 2 5% 4 4 3 3 5% 6% 54%	2090
October	1st to 10th	Night	9% 4 1 4 2 2 3 3 4 6% 61%	2090
	11th to 20th	Night	11% 6% 3 5% 3 4 3 2 4 6% 53%	2161
	21st to end	Night	15% 7% 2 4 3 2 3 2 4 6% 51%	2643
November	1st to 10th	Night	12% 5% 2 6% 3 4 3 3 4 5% 52%	2536
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 7% 50%	2600
	21st to end	Night	16% 8% 2 4 4 3 3 3 4 5% 47%	2600
December	1st to 10th	Night	19% 9% 1 6% 3 3 3 3 4 5% 44%	4250
	11th to 20th	Night	25% 8% 2 5% 3 2 2 2 4 4 42%	4822
	21st to end	Night	21% 10% 5% 7% 4 4 4 3 5% 6% 31%	6697

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	51% 2 6% 19% 20% 2	2576
	11th to 20th	Day	48% 4 8% 17% 22% 1	2697
	21st to end	Day	50% 3 7% 21% 18% 1	3476
February	1st to 10th	Day	49% 2 8% 19% 19% 4	1900
	11th to 20th	Day	52% 3 5% 20% 16% 5%	1858
	21st to end	Day	51% 3 8% 15% 17% 6%	1570
March	1st to 10th	Day	56% 4 6% 12% 13% 9%	3120
	11th to 20th	Day	40% 4 7% 26% 17% 7%	3275
	21st to end	Day	44% 4 11% 23% 16% 2	4123
April	1st to 10th	Day	41% 4 9% 23% 18% 5%	2280
	11th to 20th	Day	42% 3 7% 20% 21% 6%	2254
	21st to end	Day	39% 4 11% 27% 16% 3	2280
May	1st to 10th	Day	45% 5% 11% 22% 15% 2	4074
	11th to 20th	Day	44% 5% 14% 24% 12%	4085
	21st to end	Day	40% 6% 11% 25% 18% 1	4489
June	1st to 10th	Day	39% 6% 12% 24% 17% 2	2477
	11th to 20th	Day	31% 7% 15% 26% 18% 2	2660
	21st to end	Day	32% 9% 17% 22% 16% 4	2660
July	1st to 10th	Day	38% 10% 21% 23% 7% 2	4371
	11th to 20th	Day	38% 9% 21% 24% 7% 2	4283
	21st to end	Day	54% 8% 12% 19% 6%	4266
August	1st to 10th	Day	22% 7% 16% 34% 19% 2	3773
	11th to 20th	Day	33% 8% 14% 27% 15% 3	3765
	21st to end	Day	29% 6% 11% 26% 25% 3	4179
September	1st to 10th	Day	38% 7% 12% 21% 18% 5%	2280
	11th to 20th	Day	40% 5% 11% 21% 21% 3	2090
	21st to end	Day	33% 5% 10% 24% 21% 7%	2090
October	1st to 10th	Day	20% 3 6% 17% 39% 15%	3480
	11th to 20th	Day	21% 3 5% 23% 31% 18%	3332
	21st to end	Day	27% 3 6% 23% 32% 9%	3191
November	1st to 10th	Day	33% 3 7% 22% 28% 7%	1762
	11th to 20th	Day	37% 4 6% 21% 26% 8%	1800
	21st to end	Day	41% 4 7% 21% 23% 5%	1800
December	1st to 10th	Day	43% 3 5% 25% 21% 3	2930
	11th to 20th	Day	52% 3 5% 20% 16% 4	3370
	21st to end	Day	49% 5% 10% 23% 11% 2	4246
January	1st to 10th	Night	48% 2 6% 20% 22% 2	4513
	11th to 20th	Night	50% 3 5% 20% 17% 6%	4245
	21st to end	Night	51% 2 5% 21% 18% 3	4180
February	1st to 10th	Night	50% 3 8% 18% 18% 3	2280
	11th to 20th	Night	51% 2 7% 18% 15% 6%	2219
	21st to end	Night	52% 3 5% 18% 16% 6%	1884
March	1st to 10th	Night	51% 4 8% 13% 16% 8%	3733
	11th to 20th	Night	38% 4 8% 22% 18% 10%	3683
	21st to end	Night	46% 4 8% 20% 21% 1	3535
April	1st to 10th	Night	37% 4 12% 22% 21% 4	1900
	11th to 20th	Night	37% 3 8% 20% 23% 9%	1875
	21st to end	Night	36% 4 12% 27% 18% 3	1900
May	1st to 10th	Night	42% 5% 12% 23% 15% 2	2886
	11th to 20th	Night	40% 6% 15% 22% 16% 1	2813
	21st to end	Night	36% 7% 14% 26% 16% 2	3087
June	1st to 10th	Night	35% 8% 13% 24% 18% 2	1696
	11th to 20th	Night	22% 6% 18% 32% 19% 3	1520
	21st to end	Night	26% 8% 16% 29% 17% 4	1520
July	1st to 10th	Night	36% 9% 22% 24% 7% 2	2490
	11th to 20th	Night	31% 9% 23% 27% 7% 3	2544
	21st to end	Night	51% 9% 14% 20% 7%	3041
August	1st to 10th	Night	17% 7% 13% 38% 24% 1	3132
	11th to 20th	Night	24% 7% 24% 27% 17% 2	3130
	21st to end	Night	21% 6% 14% 30% 25% 3	3476
September	1st to 10th	Night	32% 6% 13% 26% 17% 6%	1900
	11th to 20th	Night	35% 5% 13% 19% 25% 3	2071
	21st to end	Night	30% 5% 9% 25% 24% 8%	2090
October	1st to 10th	Night	18% 4 6% 18% 38% 16%	3480
	11th to 20th	Night	21% 2 4 18% 32% 22%	3582
	21st to end	Night	25% 3 5% 27% 26% 14%	4355
November	1st to 10th	Night	35% 4 6% 22% 24% 10%	2536
	11th to 20th	Night	34% 3 6% 22% 26% 10%	2600
	21st to end	Night	40% 3 6% 21% 24% 7%	2600
December	1st to 10th	Night	43% 3 6% 20% 26% 2	4250
	11th to 20th	Night	47% 2 4 21% 22% 3	4824
	21st to end	Night	54% 4 8% 23% 10% 2	6697

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	27% 22% 31% 15% 5%	288
	11th to 20th	Day	34% 19% 31% 12% 2 2	270
	21st to end	Day	24% 17% 43% 12% 3	297
February	1st to 10th	Day	31% 17% 31% 17% 3	270
	11th to 20th	Day	37% 16% 34% 11% 2 1	270
	21st to end	Day	43% 18% 28% 9% 2	225
March	1st to 10th	Day	27% 15% 36% 18% 3 1	270
	11th to 20th	Day	25% 14% 36% 20% 4	333
	21st to end	Day	25% 15% 35% 17% 7%	506
April	1st to 10th	Day	23% 13% 32% 20% 11% 1	460
	11th to 20th	Day	22% 11% 31% 24% 11% 1	452
	21st to end	Day	19% 10% 32% 26% 12% 2	460
May	1st to 10th	Day	18% 10% 28% 27% 13% 5%	463
	11th to 20th	Day	14% 7% 26% 27% 15% 12%	487
	21st to end	Day	12% 5% 21% 28% 22% 12%	529
June	1st to 10th	Day	12% 4 20% 23% 25% 16%	497
	11th to 20th	Day	6% 2 17% 28% 30% 17%	680
	21st to end	Day	7% 2 17% 31% 28% 14%	680
July	1st to 10th	Day	9% 3 11% 30% 29% 18%	671
	11th to 20th	Day	10% 3 8% 25% 31% 22%	625
	21st to end	Day	8% 2 10% 27% 29% 24%	532
August	1st to 10th	Day	9% 2 8% 27% 38% 16%	486
	11th to 20th	Day	13% 3 10% 28% 35% 11%	479
	21st to end	Day	12% 3 8% 21% 35% 20%	539
September	1st to 10th	Day	11% 4 11% 30% 31% 13%	490
	11th to 20th	Day	15% 1 11% 26% 29% 17%	490
	21st to end	Day	14% 4 14% 34% 24% 9%	490
October	1st to 10th	Day	12% 2 19% 31% 26% 9%	490
	11th to 20th	Day	11% 11% 32% 30% 14% 3	414
	21st to end	Day	23% 9% 28% 30% 9% 2	330
November	1st to 10th	Day	18% 11% 30% 27% 12% 2	300
	11th to 20th	Day	16% 15% 38% 26% 5%	300
	21st to end	Day	22% 18% 28% 19% 11% 3	300
December	1st to 10th	Day	27% 18% 34% 15% 6%	300
	11th to 20th	Day	22% 19% 34% 17% 8%	300
	21st to end	Day	27% 16% 38% 17% 2	330
January	1st to 10th	Night	25% 14% 33% 24% 4	981
	11th to 20th	Night	26% 17% 30% 22% 5%	890
	21st to end	Night	20% 14% 43% 21% 2	836
February	1st to 10th	Night	24% 13% 37% 22% 4	760
	11th to 20th	Night	26% 15% 33% 23% 2	743
	21st to end	Night	29% 15% 35% 19% 2	628
March	1st to 10th	Night	21% 14% 31% 29% 5%	760
	11th to 20th	Night	21% 11% 38% 23% 6% 1	760
	21st to end	Night	17% 11% 35% 28% 9%	670
April	1st to 10th	Night	16% 10% 36% 26% 11% 1	570
	11th to 20th	Night	18% 8% 33% 27% 12% 1	563
	21st to end	Night	16% 7% 33% 28% 14% 1	570
May	1st to 10th	Night	15% 4 28% 31% 19% 2	570
	11th to 20th	Night	13% 6% 23% 35% 20% 4	561
	21st to end	Night	9% 2 15% 40% 29% 6%	617
June	1st to 10th	Night	10% 2 12% 38% 32% 6%	570
	11th to 20th	Night	5% 1 11% 38% 40% 7%	570
	21st to end	Night	5% 2 9% 38% 37% 8%	570
July	1st to 10th	Night	8% 3 7% 28% 43% 12%	555
	11th to 20th	Night	7% 1 7% 29% 41% 14%	552
	21st to end	Night	8% 3 7% 30% 41% 12%	624
August	1st to 10th	Night	6% 1 5% 27% 50% 11%	567
	11th to 20th	Night	7% 2 7% 38% 40% 6%	564
	21st to end	Night	11% 6% 29% 43% 11%	627
September	1st to 10th	Night	10% 2 9% 32% 37% 9%	570
	11th to 20th	Night	13% 3 14% 29% 35% 6%	741
	21st to end	Night	11% 3 15% 35% 32% 4	760
October	1st to 10th	Night	10% 3 19% 33% 29% 6%	760
	11th to 20th	Night	13% 6% 24% 36% 17% 3	760
	21st to end	Night	18% 6% 27% 31% 17% 2	836
November	1st to 10th	Night	15% 7% 24% 34% 18% 2	780
	11th to 20th	Night	16% 8% 34% 30% 12% 1	800
	21st to end	Night	21% 10% 29% 28% 12% 1	800
December	1st to 10th	Night	24% 13% 29% 28% 6%	800
	11th to 20th	Night	24% 11% 29% 28% 8%	800
	21st to end	Night	26% 12% 32% 26% 4	970

Page generated at 2025-01-04 02:23:58. Version ac07

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