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 2024-07-31.

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	14% 7% 2 6% 5% 4 5% 4 6% 7% 40%	90093
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	89670
Transparency	Day	42% 5% 11% 22% 17% 4	89859
	Night	41% 4 10% 22% 19% 5%	89509
Seeing	Day	17% 8% 23% 24% 19% 9%	14746
	Night	16% 8% 24% 29% 19% 3	24449

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	14% 14% 4 7% 4 4 4 3 4 6% 36%	2527
	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	9% 6% 3 7% 7% 6% 7% 6% 8% 11% 32%	2660
	11th to 20th	Day	10% 6% 2 6% 6% 6% 6% 5% 7% 11% 35%	2617
	21st to end	Day	8% 6% 2 6% 6% 6% 7% 6% 9% 11% 34%	2644
August	1st to 10th	Day	5% 4 2 7% 6% 7% 7% 6% 9% 10% 36%	2148
	11th to 20th	Day	11% 6% 3 6% 5% 6% 5% 4 8% 10% 35%	2125
	21st to end	Day	12% 6% 2 8% 5% 5% 6% 4 6% 9% 37%	2376
September	1st to 10th	Day	11% 5% 2 7% 4 5% 6% 3 6% 9% 42%	2160
	11th to 20th	Day	13% 6% 2 7% 4 5% 4 4 6% 7% 43%	1980
	21st to end	Day	10% 4 2 5% 4 4 4 4 5% 6% 52%	1980
October	1st to 10th	Day	11% 4 1 5% 4 4 3 3 4 6% 54%	1980
	11th to 20th	Day	10% 6% 2 5% 3 3 4 4 5% 7% 52%	1908
	21st to end	Day	16% 7% 2 4 3 4 4 2 5% 5% 49%	1851
November	1st to 10th	Day	11% 5% 2 6% 3 3 3 3 4 5% 57%	1672
	11th to 20th	Day	15% 6% 2 4 3 3 4 3 4 6% 50%	1710
	21st to end	Day	15% 8% 3 5% 4 4 4 3 6% 7% 42%	1710
December	1st to 10th	Day	23% 9% 3 6% 4 4 4 3 4 4 35%	1710
	11th to 20th	Day	17% 8% 3 6% 5% 4 3 3 4 5% 42%	2153
	21st to end	Day	16% 10% 4 7% 4 5% 5% 4 5% 6% 33%	2969
January	1st to 10th	Night	19% 13% 2 5% 3 3 3 3 5% 5% 40%	4428
	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 6% 7% 5% 6% 5% 7% 11% 39%	1520
	11th to 20th	Night	7% 4 2 5% 5% 4 5% 5% 8% 11% 44%	1558
	21st to end	Night	5% 6% 2 6% 4 5% 5% 5% 8% 11% 40%	1900
August	1st to 10th	Night	4 3 1 6% 4 5% 5% 4 8% 12% 47%	1790
	11th to 20th	Night	7% 6% 2 5% 3 5% 6% 5% 8% 12% 42%	1776
	21st to end	Night	9% 4 2 6% 5% 5% 5% 4 7% 10% 43%	1980
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 7% 7% 50%	1800
	11th to 20th	Night	10% 5% 1 5% 4 4 4 3 5% 9% 49%	1962
	21st to end	Night	9% 4 2 5% 4 4 3 3 5% 6% 56%	1980
October	1st to 10th	Night	10% 5% 1 4 2 2 3 3 4 6% 60%	1980
	11th to 20th	Night	11% 6% 3 6% 3 4 3 3 4 6% 50%	2047
	21st to end	Night	15% 7% 2 4 3 2 4 2 4 6% 50%	2503
November	1st to 10th	Night	11% 5% 2 6% 3 4 3 3 4 5% 54%	2406
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 5% 7% 50%	2470
	21st to end	Night	17% 8% 2 4 4 3 4 3 4 5% 47%	2470
December	1st to 10th	Night	20% 9% 2 7% 4 4 4 4 4 6% 38%	2470
	11th to 20th	Night	16% 7% 2 6% 3 3 3 2 4 5% 48%	3042
	21st to end	Night	18% 10% 5% 8% 4 5% 4 4 5% 6% 31%	4669

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 5% 19% 20% 2	2528
	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	41% 9% 19% 18% 10% 3	2591
	11th to 20th	Day	39% 8% 21% 19% 10% 3	2542
	21st to end	Day	38% 10% 18% 25% 8% 1	2503
August	1st to 10th	Day	35% 9% 21% 23% 10% 1	2136
	11th to 20th	Day	41% 8% 15% 20% 15% 2	2125
	21st to end	Day	42% 8% 13% 22% 12% 3	2376
September	1st to 10th	Day	38% 7% 13% 21% 17% 4	2160
	11th to 20th	Day	40% 5% 11% 20% 21% 4	1980
	21st to end	Day	32% 4 10% 25% 22% 7%	1980
October	1st to 10th	Day	32% 3 8% 26% 25% 6%	1980
	11th to 20th	Day	32% 4 8% 22% 26% 9%	1908
	21st to end	Day	38% 3 5% 21% 26% 8%	1851
November	1st to 10th	Day	32% 3 6% 22% 29% 8%	1672
	11th to 20th	Day	36% 3 6% 20% 26% 8%	1710
	21st to end	Day	41% 4 7% 21% 23% 5%	1710
December	1st to 10th	Day	53% 3 5% 18% 17% 4	1710
	11th to 20th	Day	46% 3 7% 21% 21% 3	2150
	21st to end	Day	50% 5% 10% 21% 12% 3	2969
January	1st to 10th	Night	48% 2 6% 20% 22% 2	4428
	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% 8% 21% 21% 11% 3	1480
	11th to 20th	Night	31% 8% 21% 24% 11% 4	1510
	21st to end	Night	34% 9% 19% 27% 11% 1	1800
August	1st to 10th	Night	27% 10% 19% 30% 14% 1	1778
	11th to 20th	Night	32% 9% 21% 20% 15% 3	1776
	21st to end	Night	34% 7% 16% 27% 12% 5%	1980
September	1st to 10th	Night	32% 6% 13% 27% 18% 5%	1800
	11th to 20th	Night	33% 5% 14% 19% 26% 3	1962
	21st to end	Night	29% 4 8% 26% 25% 8%	1980
October	1st to 10th	Night	26% 4 8% 24% 28% 9%	1980
	11th to 20th	Night	36% 3 7% 21% 22% 10%	2047
	21st to end	Night	37% 3 5% 22% 26% 7%	2503
November	1st to 10th	Night	33% 4 6% 23% 25% 10%	2406
	11th to 20th	Night	33% 3 6% 22% 25% 10%	2470
	21st to end	Night	41% 3 6% 20% 24% 6%	2470
December	1st to 10th	Night	49% 3 8% 19% 19% 3	2470
	11th to 20th	Night	40% 3 5% 22% 25% 5%	3044
	21st to end	Night	53% 4 8% 19% 12% 3	4669

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% 32% 15% 4	270
	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	9% 1 10% 27% 29% 24%	529
August	1st to 10th	Day	9% 2 7% 26% 39% 17%	446
	11th to 20th	Day	13% 3 8% 28% 38% 12%	439
	21st to end	Day	14% 3 8% 21% 34% 20%	495
September	1st to 10th	Day	10% 4 9% 30% 33% 14%	450
	11th to 20th	Day	14% 2 12% 25% 30% 18%	450
	21st to end	Day	13% 4 14% 34% 26% 10%	450
October	1st to 10th	Day	14% 2 19% 30% 26% 9%	450
	11th to 20th	Day	12% 11% 32% 29% 12% 3	378
	21st to end	Day	26% 9% 27% 28% 9% 2	297
November	1st to 10th	Day	18% 9% 30% 29% 12% 3	270
	11th to 20th	Day	14% 14% 39% 27% 5%	270
	21st to end	Day	22% 18% 27% 19% 11% 3	270
December	1st to 10th	Day	28% 16% 34% 16% 7%	270
	11th to 20th	Day	20% 19% 34% 19% 8%	270
	21st to end	Day	26% 16% 40% 16% 2	297
January	1st to 10th	Night	25% 14% 33% 24% 4	950
	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 6% 30% 41% 12%	624
August	1st to 10th	Night	6% 1 5% 26% 50% 12%	537
	11th to 20th	Night	7% 3 5% 38% 41% 6%	534
	21st to end	Night	11% 1 6% 29% 42% 12%	594
September	1st to 10th	Night	10% 2 8% 31% 39% 9%	540
	11th to 20th	Night	11% 3 14% 30% 36% 6%	702
	21st to end	Night	11% 3 15% 35% 32% 4	720
October	1st to 10th	Night	11% 3 19% 32% 29% 7%	720
	11th to 20th	Night	14% 6% 25% 36% 17% 3	720
	21st to end	Night	18% 6% 27% 29% 17% 2	792
November	1st to 10th	Night	14% 6% 24% 35% 19% 2	740
	11th to 20th	Night	15% 9% 35% 30% 12% 1	760
	21st to end	Night	21% 9% 29% 28% 12% 1	760
December	1st to 10th	Night	24% 12% 29% 28% 7%	760
	11th to 20th	Night	23% 11% 29% 29% 8%	760
	21st to end	Night	26% 12% 32% 26% 4	921

Page generated at 2024-07-28 08:52:13. Version ac07

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