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 2024-01-17.

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% 3 5% 5% 3 5% 5% 6% 7% 41%	75046
	Night	13% 6% 3 5% 4 3 4 4 5% 7% 45%	74754
Transparency	Day	41% 5% 11% 22% 17% 4	74761
	Night	39% 5% 10% 22% 19% 5%	74559
Seeing	Day	17% 8% 22% 24% 19% 10%	13983
	Night	16% 8% 24% 29% 19% 4	23796

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	19% 9% 3 5% 5% 3 3 3 4 6% 41%	1517
	11th to 20th	Day	22% 10% 3 5% 4 2 4 4 4 5% 36%	1588
	21st to end	Day	17% 6% 3 5% 4 3 4 4 4 6% 45%	1980
February	1st to 10th	Day	22% 9% 4 5% 4 2 3 2 4 6% 38%	1800
	11th to 20th	Day	21% 10% 4 6% 6% 2 4 3 4 5% 36%	1748
	21st to end	Day	23% 8% 4 7% 4 2 3 4 4 5% 36%	1480
March	1st to 10th	Day	16% 8% 2 6% 6% 3 4 4 5% 6% 40%	1800
	11th to 20th	Day	20% 8% 3 7% 6% 3 4 5% 4 7% 34%	1859
	21st to end	Day	16% 7% 4 7% 6% 3 5% 4 5% 7% 36%	2348
April	1st to 10th	Day	13% 6% 2 5% 5% 3 4 4 5% 5% 46%	2160
	11th to 20th	Day	16% 7% 3 6% 4 2 4 3 4 6% 45%	2134
	21st to end	Day	12% 6% 2 6% 6% 3 5% 5% 5% 8% 42%	2160
May	1st to 10th	Day	16% 6% 3 5% 5% 3 5% 4 5% 6% 43%	2308
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 5% 5% 7% 39%	2301
	21st to end	Day	10% 6% 3 5% 6% 4 4 6% 8% 11% 38%	2531
June	1st to 10th	Day	9% 6% 3 5% 6% 4 5% 6% 6% 9% 41%	2346
	11th to 20th	Day	5% 4 2 5% 6% 3 6% 6% 8% 10% 45%	2520
	21st to end	Day	6% 4 2 4 6% 4 7% 7% 8% 11% 42%	2520
July	1st to 10th	Day	9% 5% 3 6% 7% 5% 7% 7% 8% 11% 32%	2520
	11th to 20th	Day	10% 5% 3 5% 6% 3 7% 6% 7% 11% 36%	2480
	21st to end	Day	6% 4 3 5% 6% 4 8% 7% 9% 11% 35%	2514
August	1st to 10th	Day	5% 4 2 6% 7% 5% 7% 8% 9% 10% 37%	2148
	11th to 20th	Day	11% 5% 3 6% 6% 4 6% 5% 8% 10% 36%	2125
	21st to end	Day	12% 5% 2 7% 6% 4 6% 6% 6% 9% 37%	2376
September	1st to 10th	Day	11% 5% 2 6% 5% 4 6% 5% 6% 9% 42%	2160
	11th to 20th	Day	13% 5% 2 6% 5% 3 4 5% 6% 7% 43%	1980
	21st to end	Day	10% 4 2 4 4 4 4 5% 5% 6% 52%	1980
October	1st to 10th	Day	11% 4 1 4 5% 3 3 4 4 6% 55%	1980
	11th to 20th	Day	11% 6% 2 4 4 2 3 5% 5% 7% 52%	1908
	21st to end	Day	16% 6% 1 4 3 3 4 3 5% 5% 49%	1851
November	1st to 10th	Day	11% 5% 2 5% 4 2 3 3 4 5% 57%	1672
	11th to 20th	Day	15% 5% 3 4 3 2 4 4 4 6% 50%	1710
	21st to end	Day	15% 7% 3 4 5% 2 4 4 5% 7% 42%	1710
December	1st to 10th	Day	23% 9% 3 5% 5% 3 4 4 4 4 35%	1710
	11th to 20th	Day	17% 7% 4 5% 6% 3 3 4 4 5% 42%	2153
	21st to end	Day	16% 10% 4 6% 5% 4 5% 5% 5% 6% 33%	2969
January	1st to 10th	Night	20% 9% 3 5% 4 2 3 3 4 5% 42%	2648
	11th to 20th	Night	21% 10% 5% 7% 5% 2 4 4 4 5% 33%	2506
	21st to end	Night	16% 7% 3 6% 4 3 3 4 4 5% 44%	2376
February	1st to 10th	Night	20% 10% 3 6% 5% 3 4 4 4 6% 37%	2160
	11th to 20th	Night	21% 8% 4 6% 5% 4 4 3 3 3 39%	2098
	21st to end	Night	24% 8% 3 6% 4 2 4 5% 4 6% 34%	1776
March	1st to 10th	Night	15% 6% 2 5% 4 3 4 4 5% 7% 44%	2160
	11th to 20th	Night	17% 8% 3 5% 6% 3 4 4 5% 8% 38%	2099
	21st to end	Night	15% 7% 3 5% 4 4 5% 5% 5% 7% 41%	2021
April	1st to 10th	Night	12% 6% 3 5% 4 2 4 3 5% 9% 47%	1800
	11th to 20th	Night	15% 5% 2 4 4 2 4 4 4 8% 48%	1775
	21st to end	Night	12% 5% 3 4 5% 2 5% 5% 6% 9% 45%	1800
May	1st to 10th	Night	11% 6% 2 4 4 2 3 4 6% 8% 49%	1652
	11th to 20th	Night	11% 5% 3 5% 4 2 4 5% 6% 9% 45%	1592
	21st to end	Night	8% 5% 2 5% 4 3 5% 6% 7% 11% 44%	1744
June	1st to 10th	Night	8% 5% 2 5% 5% 3 4 5% 6% 11% 44%	1607
	11th to 20th	Night	3 3 2 4 3 2 4 6% 8% 11% 54%	1440
	21st to end	Night	4 3 2 3 5% 3 4 5% 6% 11% 52%	1440
July	1st to 10th	Night	8% 4 2 6% 7% 4 6% 6% 7% 11% 40%	1440
	11th to 20th	Night	8% 4 2 5% 5% 3 5% 6% 7% 11% 44%	1476
	21st to end	Night	5% 5% 2 5% 5% 4 5% 7% 8% 12% 42%	1799
August	1st to 10th	Night	4 3 1 5% 5% 4 6% 5% 8% 12% 47%	1790
	11th to 20th	Night	7% 5% 2 4 4 4 5% 6% 8% 12% 42%	1776
	21st to end	Night	9% 4 2 6% 5% 3 5% 5% 6% 10% 44%	1980
September	1st to 10th	Night	10% 5% 3 5% 4 2 3 5% 6% 7% 50%	1800
	11th to 20th	Night	10% 5% 2 5% 5% 3 4 4 5% 9% 49%	1962
	21st to end	Night	9% 3 2 4 4 3 4 4 5% 6% 56%	1980
October	1st to 10th	Night	10% 5% 1 3 3 2 3 3 4 6% 60%	1980
	11th to 20th	Night	11% 6% 3 5% 5% 3 3 3 4 6% 51%	2047
	21st to end	Night	16% 7% 2 3 4 2 3 3 4 6% 50%	2503
November	1st to 10th	Night	11% 4 2 5% 5% 3 3 3 4 6% 54%	2406
	11th to 20th	Night	12% 5% 3 4 4 2 4 4 5% 7% 51%	2470
	21st to end	Night	17% 7% 2 4 4 2 4 3 5% 5% 47%	2470
December	1st to 10th	Night	20% 8% 2 6% 5% 3 4 4 4 6% 38%	2470
	11th to 20th	Night	17% 7% 2 5% 4 2 3 3 4 5% 48%	3042
	21st to end	Night	18% 10% 5% 7% 5% 4 4 5% 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	47% 2 7% 21% 19% 4	1518
	11th to 20th	Day	51% 3 8% 18% 18% 2	1588
	21st to end	Day	42% 3 8% 23% 22% 2	1980
February	1st to 10th	Day	49% 2 8% 19% 18% 3	1800
	11th to 20th	Day	53% 3 5% 21% 15% 4	1758
	21st to end	Day	52% 3 8% 15% 17% 7%	1480
March	1st to 10th	Day	46% 4 7% 17% 19% 7%	1800
	11th to 20th	Day	51% 4 9% 17% 16% 3	1859
	21st to end	Day	49% 4 10% 22% 13% 2	2348
April	1st to 10th	Day	40% 4 9% 24% 18% 5%	2160
	11th to 20th	Day	42% 3 7% 21% 22% 6%	2134
	21st to end	Day	40% 4 11% 26% 16% 3	2160
May	1st to 10th	Day	42% 4 11% 23% 16% 3	2308
	11th to 20th	Day	44% 6% 13% 22% 15% 1	2305
	21st to end	Day	39% 7% 15% 24% 13% 2	2531
June	1st to 10th	Day	39% 6% 12% 24% 17% 2	2346
	11th to 20th	Day	31% 7% 16% 26% 18% 2	2520
	21st to end	Day	33% 9% 17% 22% 16% 4	2520
July	1st to 10th	Day	41% 9% 19% 17% 11% 3	2451
	11th to 20th	Day	39% 8% 21% 19% 11% 3	2405
	21st to end	Day	36% 10% 18% 26% 8% 1	2373
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	46% 3 7% 22% 18% 4	2648
	11th to 20th	Night	55% 3 6% 20% 13% 3	2506
	21st to end	Night	44% 2 7% 24% 20% 3	2376
February	1st to 10th	Night	51% 3 9% 18% 17% 3	2160
	11th to 20th	Night	52% 2 7% 19% 15% 5%	2099
	21st to end	Night	53% 3 5% 18% 15% 6%	1776
March	1st to 10th	Night	40% 4 9% 17% 20% 10%	2160
	11th to 20th	Night	47% 5% 9% 20% 17% 3	2099
	21st to end	Night	42% 5% 11% 23% 17% 2	2021
April	1st to 10th	Night	36% 4 12% 23% 21% 4	1800
	11th to 20th	Night	36% 3 8% 21% 23% 9%	1775
	21st to end	Night	36% 4 12% 27% 18% 3	1800
May	1st to 10th	Night	35% 5% 11% 23% 23% 3	1652
	11th to 20th	Night	34% 7% 16% 24% 18% 2	1593
	21st to end	Night	33% 8% 14% 27% 15% 3	1745
June	1st to 10th	Night	34% 8% 14% 24% 19% 2	1607
	11th to 20th	Night	22% 6% 19% 32% 18% 3	1440
	21st to end	Night	25% 8% 16% 29% 18% 4	1440
July	1st to 10th	Night	36% 8% 21% 21% 12% 3	1400
	11th to 20th	Night	31% 8% 20% 24% 11% 5%	1428
	21st to end	Night	31% 10% 20% 27% 11% 1	1699
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% 17% 32% 12% 2 2	258
	21st to end	Day	23% 17% 45% 11% 3	264
February	1st to 10th	Day	31% 19% 32% 15% 3	240
	11th to 20th	Day	40% 15% 33% 11% 1	240
	21st to end	Day	42% 16% 29% 10% 3	198
March	1st to 10th	Day	25% 15% 39% 18% 3	240
	11th to 20th	Day	27% 14% 35% 19% 4	299
	21st to end	Day	26% 15% 33% 18% 8%	462
April	1st to 10th	Day	20% 13% 33% 21% 12% 1	420
	11th to 20th	Day	21% 11% 31% 25% 12% 1	412
	21st to end	Day	19% 10% 31% 26% 12% 2	420
May	1st to 10th	Day	19% 9% 27% 27% 14% 4	423
	11th to 20th	Day	13% 6% 26% 25% 16% 13%	447
	21st to end	Day	12% 4 19% 29% 23% 14%	485
June	1st to 10th	Day	12% 4 18% 23% 26% 18%	456
	11th to 20th	Day	6% 2 16% 28% 29% 18%	630
	21st to end	Day	7% 2 17% 32% 27% 15%	630
July	1st to 10th	Day	9% 3 10% 29% 29% 20%	621
	11th to 20th	Day	10% 3 8% 24% 32% 23%	578
	21st to end	Day	6% 1 9% 27% 30% 26%	488
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% 29% 22% 5%	878
	21st to end	Night	20% 13% 43% 21% 3	792
February	1st to 10th	Night	24% 13% 37% 22% 4	720
	11th to 20th	Night	25% 16% 34% 22% 2	703
	21st to end	Night	29% 14% 35% 19% 2	592
March	1st to 10th	Night	19% 15% 32% 29% 5%	720
	11th to 20th	Night	22% 11% 37% 23% 6% 1	720
	21st to end	Night	17% 11% 35% 28% 9%	635
April	1st to 10th	Night	15% 10% 35% 28% 12% 1	540
	11th to 20th	Night	18% 8% 33% 27% 13% 1	533
	21st to end	Night	16% 7% 33% 29% 14% 1	540
May	1st to 10th	Night	15% 4 29% 32% 18% 2	540
	11th to 20th	Night	12% 5% 23% 34% 21% 4	531
	21st to end	Night	9% 2 14% 39% 30% 6%	584
June	1st to 10th	Night	9% 2 11% 38% 33% 7%	540
	11th to 20th	Night	4 11% 38% 40% 7%	540
	21st to end	Night	5% 2 10% 39% 36% 9%	540
July	1st to 10th	Night	8% 2 6% 27% 44% 12%	525
	11th to 20th	Night	8% 1 8% 27% 41% 15%	522
	21st to end	Night	6% 2 6% 30% 42% 13%	591
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 2024-01-14 08:54:59. Version ac07

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