SkyWatcher Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyWatcher Observatory clear sky chart from 2005-11-06 to 2026-11-13.

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	6% 6% 2 6% 4 4 4 3 5% 6% 52%	162924
	Night	8% 7% 3 6% 4 4 4 4 6% 7% 47%	155484
Transparency	Day	31% 6% 11% 24% 19% 8%	157673
	Night	35% 7% 13% 24% 16% 5%	151737
Seeing	Day	11% 13% 25% 27% 18% 5%	17109
	Night	11% 8% 21% 38% 20% 2	26076

By Month

Percentage Cloud Cover SW (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% 15% 6% 14% 8% 7% 7% 5% 6% 6% 12%	4722
	11th to 20th	Day	11% 9% 2 6% 4 5% 5% 4 6% 7% 40%	4740
	21st to end	Day	10% 10% 4 10% 6% 5% 6% 5% 7% 7% 29%	5408
February	1st to 10th	Day	12% 14% 3 9% 7% 5% 5% 5% 5% 7% 27%	1890
	11th to 20th	Day	10% 11% 4 9% 6% 5% 7% 4 6% 7% 31%	1845
	21st to end	Day	9% 13% 4 8% 7% 5% 6% 5% 7% 7% 29%	1730
March	1st to 10th	Day	7% 10% 3 9% 5% 6% 6% 5% 8% 9% 32%	4618
	11th to 20th	Day	8% 14% 4 8% 5% 5% 5% 4 5% 6% 36%	5105
	21st to end	Day	9% 10% 4 11% 7% 7% 7% 5% 7% 8% 25%	5891
April	1st to 10th	Day	9% 11% 3 7% 6% 5% 6% 5% 7% 8% 32%	2520
	11th to 20th	Day	8% 9% 2 6% 6% 5% 5% 4 6% 7% 41%	2489
	21st to end	Day	7% 7% 2 6% 5% 5% 5% 4 7% 9% 45%	2647
May	1st to 10th	Day	7% 5% 2 4 3 3 4 3 5% 7% 57%	8195
	11th to 20th	Day	5% 5% 2 6% 4 4 4 4 5% 8% 54%	8160
	21st to end	Day	7% 5% 2 5% 4 4 5% 3 6% 8% 52%	8992
June	1st to 10th	Day	9% 7% 3 6% 5% 4 4 4 5% 7% 48%	2940
	11th to 20th	Day	6% 4 2 5% 5% 4 5% 3 5% 7% 54%	2940
	21st to end	Day	3 3 1 3 3 3 3 3 5% 6% 67%	2940
July	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 82%	8219
	11th to 20th	Day	2 2 1 2 1 1 1 1 2 3 82%	8220
	21st to end	Day	2 1 1 2 1 2 2 2 3 4 82%	8922
August	1st to 10th	Day	2 2 1 3 1 2 2 2 3 5% 78%	7928
	11th to 20th	Day	1 1 1 2 2 2 3 3 5% 6% 73%	7547
	21st to end	Day	4 3 1 3 2 2 3 2 3 6% 68%	7750
September	1st to 10th	Day	2 1 1 3 2 2 3 3 3 6% 73%	2520
	11th to 20th	Day	4 4 2 4 4 4 4 4 5% 6% 59%	2520
	21st to end	Day	8% 5% 1 4 4 3 3 3 4 6% 58%	2252
October	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 6% 65%	4660
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 4 8% 10% 47%	4639
	21st to end	Day	8% 8% 3 9% 5% 6% 7% 5% 7% 8% 36%	4760
November	1st to 10th	Day	11% 12% 3 8% 6% 6% 5% 4 7% 8% 28%	1939
	11th to 20th	Day	11% 13% 4 11% 8% 7% 6% 5% 7% 6% 21%	1696
	21st to end	Day	10% 13% 4 9% 5% 6% 5% 4 7% 7% 29%	1680
December	1st to 10th	Day	9% 9% 3 8% 5% 5% 5% 4 7% 6% 37%	2700
	11th to 20th	Day	12% 17% 6% 12% 7% 6% 6% 5% 7% 7% 15%	3108
	21st to end	Day	14% 15% 5% 13% 8% 7% 6% 4 5% 5% 18%	4092
January	1st to 10th	Night	15% 16% 6% 15% 8% 7% 6% 4 6% 5% 13%	8172
	11th to 20th	Night	12% 10% 2 5% 4 3 3 4 7% 7% 42%	8220
	21st to end	Night	11% 12% 4 10% 5% 5% 5% 4 6% 7% 32%	8769
February	1st to 10th	Night	12% 12% 4 8% 6% 5% 5% 4 6% 7% 30%	2730
	11th to 20th	Night	10% 11% 3 9% 5% 5% 5% 4 5% 8% 35%	2662
	21st to end	Night	9% 14% 3 8% 6% 5% 5% 4 6% 8% 31%	2076
March	1st to 10th	Night	6% 8% 3 8% 5% 6% 6% 5% 8% 9% 36%	5496
	11th to 20th	Night	8% 10% 3 7% 5% 5% 5% 4 6% 7% 40%	5114
	21st to end	Night	8% 10% 3 9% 7% 7% 7% 6% 8% 9% 25%	5325
April	1st to 10th	Night	10% 8% 3 6% 5% 4 4 5% 7% 8% 40%	2100
	11th to 20th	Night	7% 7% 3 5% 6% 4 6% 4 6% 7% 45%	2079
	21st to end	Night	8% 6% 2 5% 4 4 5% 4 7% 8% 48%	1968
May	1st to 10th	Night	6% 4 2 4 3 3 3 2 5% 7% 62%	4723
	11th to 20th	Night	4 4 2 5% 3 3 4 3 6% 7% 58%	4700
	21st to end	Night	7% 5% 1 5% 3 3 3 3 5% 8% 57%	5181
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 5% 7% 51%	1680
	11th to 20th	Night	6% 4 2 4 3 3 3 4 5% 6% 60%	1680
	21st to end	Night	4 3 1 4 2 3 4 3 4 4 69%	1680
July	1st to 10th	Night	1 1 1 1 2 2 2 3 4 82%	4740
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 78%	4740
	21st to end	Night	2 1 1 2 1 1 2 2 3 5% 81%	5153
August	1st to 10th	Night	1 2 1 3 2 2 3 2 4 5% 75%	4858
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 5% 73%	5308
	21st to end	Night	3 2 1 3 2 2 3 2 4 5% 72%	6499
September	1st to 10th	Night	2 2 1 3 2 2 3 3 4 5% 73%	2100
	11th to 20th	Night	4 3 1 3 3 4 4 3 5% 6% 64%	2100
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 6% 62%	2364
October	1st to 10th	Night	3 2 1 3 2 2 3 3 4 7% 70%	5560
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 8% 9% 49%	5537
	21st to end	Night	7% 6% 2 7% 4 5% 7% 5% 7% 8% 41%	6345
November	1st to 10th	Night	13% 9% 2 8% 7% 5% 5% 4 6% 8% 33%	2795
	11th to 20th	Night	13% 12% 5% 9% 6% 5% 5% 5% 7% 6% 28%	2968
	21st to end	Night	10% 9% 3 9% 5% 4 5% 5% 6% 8% 35%	2940
December	1st to 10th	Night	7% 8% 4 7% 5% 6% 5% 5% 8% 7% 39%	4700
	11th to 20th	Night	17% 16% 5% 12% 8% 7% 6% 5% 5% 5% 14%	5316
	21st to end	Night	16% 12% 5% 12% 7% 6% 6% 5% 6% 6% 20%	7106

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	67% 11% 11% 8% 3	4723
	11th to 20th	Day	39% 11% 10% 28% 10% 2	4740
	21st to end	Day	49% 10% 10% 21% 9% 2	5408
February	1st to 10th	Day	54% 8% 17% 16% 4	1890
	11th to 20th	Day	48% 11% 15% 17% 8%	1854
	21st to end	Day	50% 11% 16% 14% 9% 1	1730
March	1st to 10th	Day	42% 7% 13% 24% 13% 1	4618
	11th to 20th	Day	46% 7% 11% 26% 9%	5105
	21st to end	Day	50% 11% 16% 18% 5%	5891
April	1st to 10th	Day	46% 9% 17% 18% 9%	2520
	11th to 20th	Day	41% 9% 16% 18% 14% 3	2492
	21st to end	Day	35% 8% 15% 27% 12% 2	2647
May	1st to 10th	Day	25% 5% 9% 31% 26% 4	8195
	11th to 20th	Day	26% 7% 14% 29% 22% 2	8163
	21st to end	Day	27% 7% 19% 30% 16% 1	8994
June	1st to 10th	Day	35% 7% 13% 27% 15% 3	2940
	11th to 20th	Day	28% 6% 12% 32% 17% 4	2940
	21st to end	Day	18% 5% 12% 34% 23% 8%	2940
July	1st to 10th	Day	8% 2 5% 24% 33% 27%	8151
	11th to 20th	Day	10% 2 3 12% 31% 42%	8145
	21st to end	Day	9% 2 6% 29% 37% 17%	8723
August	1st to 10th	Day	10% 2 4 14% 37% 33%	6276
	11th to 20th	Day	12% 3 10% 38% 28% 9%	5946
	21st to end	Day	16% 3 9% 35% 29% 9%	6077
September	1st to 10th	Day	14% 4 9% 29% 29% 15%	2520
	11th to 20th	Day	24% 7% 12% 27% 23% 8%	2520
	21st to end	Day	27% 6% 14% 26% 21% 6%	2252
October	1st to 10th	Day	19% 5% 8% 30% 29% 9%	4659
	11th to 20th	Day	29% 7% 15% 27% 18% 3	4641
	21st to end	Day	41% 9% 14% 26% 10% 1	4758
November	1st to 10th	Day	50% 11% 15% 18% 6% 1	1939
	11th to 20th	Day	59% 12% 13% 12% 4	1696
	21st to end	Day	53% 8% 15% 16% 7% 1	1680
December	1st to 10th	Day	43% 10% 13% 24% 9%	2700
	11th to 20th	Day	63% 10% 15% 11% 2	3108
	21st to end	Day	63% 10% 13% 12% 2	4092
January	1st to 10th	Night	68% 9% 12% 9% 2	8172
	11th to 20th	Night	38% 11% 12% 28% 10% 1	8220
	21st to end	Night	48% 10% 11% 20% 10% 1	8769
February	1st to 10th	Night	51% 10% 17% 16% 5%	2730
	11th to 20th	Night	46% 10% 17% 19% 8% 1	2664
	21st to end	Night	48% 10% 14% 17% 9% 1	2076
March	1st to 10th	Night	39% 7% 14% 26% 14% 1	5496
	11th to 20th	Night	39% 9% 14% 27% 11%	5114
	21st to end	Night	48% 13% 20% 16% 3	5325
April	1st to 10th	Night	39% 10% 18% 25% 7% 1	2100
	11th to 20th	Night	36% 9% 19% 22% 11% 3	2079
	21st to end	Night	33% 10% 15% 27% 12% 3	1968
May	1st to 10th	Night	22% 4 9% 33% 29% 1	4721
	11th to 20th	Night	23% 5% 17% 34% 20% 1	4700
	21st to end	Night	23% 7% 25% 29% 14% 1	5182
June	1st to 10th	Night	34% 8% 13% 28% 15% 2	1680
	11th to 20th	Night	24% 7% 14% 37% 15% 3	1680
	21st to end	Night	19% 5% 12% 33% 23% 8%	1680
July	1st to 10th	Night	8% 2 6% 25% 33% 25%	4700
	11th to 20th	Night	12% 2 5% 12% 30% 38%	4692
	21st to end	Night	9% 1 5% 33% 36% 16%	5040
August	1st to 10th	Night	9% 2 4 13% 39% 33%	3840
	11th to 20th	Night	13% 3 11% 34% 31% 8%	4185
	21st to end	Night	12% 3 11% 34% 32% 8%	5092
September	1st to 10th	Night	13% 4 12% 28% 28% 15%	2100
	11th to 20th	Night	21% 6% 12% 28% 22% 11%	2100
	21st to end	Night	23% 5% 13% 29% 23% 8%	2364
October	1st to 10th	Night	16% 4 11% 31% 33% 6%	5560
	11th to 20th	Night	28% 6% 17% 24% 21% 4	5537
	21st to end	Night	36% 8% 17% 28% 10% 1	6346
November	1st to 10th	Night	47% 10% 14% 21% 8%	2795
	11th to 20th	Night	53% 11% 14% 18% 4	2968
	21st to end	Night	45% 10% 17% 20% 7%	2940
December	1st to 10th	Night	38% 12% 15% 29% 6%	4700
	11th to 20th	Night	66% 9% 12% 9% 3	5316
	21st to end	Night	58% 12% 14% 12% 4	7106

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	21% 17% 27% 22% 10% 3	220
	11th to 20th	Day	18% 9% 22% 30% 17% 4	220
	21st to end	Day	17% 9% 25% 29% 16% 4	321
February	1st to 10th	Day	20% 15% 24% 27% 13% 1	430
	11th to 20th	Day	17% 16% 29% 24% 13% 1	425
	21st to end	Day	18% 17% 27% 26% 10% 2	446
March	1st to 10th	Day	18% 18% 29% 25% 8% 2	540
	11th to 20th	Day	16% 22% 25% 25% 11% 2	540
	21st to end	Day	20% 17% 27% 25% 10% 1	594
April	1st to 10th	Day	20% 17% 26% 21% 10% 5%	540
	11th to 20th	Day	14% 18% 33% 21% 10% 4	532
	21st to end	Day	10% 18% 30% 24% 15% 3	540
May	1st to 10th	Day	11% 17% 27% 25% 16% 4	543
	11th to 20th	Day	17% 15% 24% 27% 13% 4	568
	21st to end	Day	11% 13% 24% 28% 18% 6%	617
June	1st to 10th	Day	12% 15% 25% 25% 16% 6%	570
	11th to 20th	Day	9% 19% 27% 26% 15% 4	570
	21st to end	Day	3 13% 25% 31% 21% 6%	570
July	1st to 10th	Day	2 8% 28% 32% 23% 7%	566
	11th to 20th	Day	2 12% 25% 31% 23% 7%	564
	21st to end	Day	3 6% 26% 32% 25% 9%	619
August	1st to 10th	Day	3 8% 22% 29% 24% 14%	567
	11th to 20th	Day	2 7% 19% 31% 28% 14%	557
	21st to end	Day	4 11% 22% 27% 27% 8%	627
September	1st to 10th	Day	2 8% 23% 31% 26% 10%	570
	11th to 20th	Day	6% 9% 24% 30% 24% 7%	570
	21st to end	Day	10% 11% 22% 28% 23% 7%	570
October	1st to 10th	Day	10% 10% 25% 30% 19% 6%	570
	11th to 20th	Day	12% 12% 23% 27% 22% 5%	570
	21st to end	Day	15% 18% 22% 26% 14% 4	490
November	1st to 10th	Day	17% 16% 26% 25% 14% 2	357
	11th to 20th	Day	16% 15% 27% 20% 18% 4	224
	21st to end	Day	18% 13% 20% 30% 18% 2	220
December	1st to 10th	Day	20% 8% 21% 27% 15% 8%	220
	11th to 20th	Day	21% 12% 31% 27% 9%	220
	21st to end	Day	18% 14% 29% 25% 13% 1	242
January	1st to 10th	Night	19% 12% 25% 29% 13% 1	840
	11th to 20th	Night	17% 9% 20% 31% 20% 2	840
	21st to end	Night	19% 7% 20% 36% 16% 2	924
February	1st to 10th	Night	17% 11% 23% 34% 13% 1	840
	11th to 20th	Night	16% 10% 26% 34% 14% 1	824
	21st to end	Night	17% 12% 26% 34% 10%	692
March	1st to 10th	Night	13% 12% 26% 34% 13% 1	840
	11th to 20th	Night	13% 11% 22% 38% 15% 1	840
	21st to end	Night	12% 12% 27% 35% 13% 1	924
April	1st to 10th	Night	15% 10% 22% 35% 17% 2	840
	11th to 20th	Night	11% 12% 27% 31% 16% 3	831
	21st to end	Night	10% 9% 26% 37% 17% 2	708
May	1st to 10th	Night	7% 7% 23% 39% 24% 1	420
	11th to 20th	Night	10% 6% 17% 39% 27% 2	414
	21st to end	Night	6% 6% 17% 40% 28% 3	455
June	1st to 10th	Night	10% 6% 17% 43% 22% 2	420
	11th to 20th	Night	6% 7% 23% 46% 17% 1	420
	21st to end	Night	7% 2 17% 45% 28%	420
July	1st to 10th	Night	2 1 10% 53% 33% 1	410
	11th to 20th	Night	1 14% 57% 25% 2	408
	21st to end	Night	2 1 9% 54% 32% 2	460
August	1st to 10th	Night	2 1 9% 47% 37% 5%	479
	11th to 20th	Night	2 1 7% 38% 44% 8%	627
	21st to end	Night	3 4 16% 46% 28% 4	924
September	1st to 10th	Night	2 3 15% 43% 32% 4	840
	11th to 20th	Night	5% 2 14% 41% 34% 5%	840
	21st to end	Night	7% 4 15% 40% 30% 4	840
October	1st to 10th	Night	9% 3 19% 43% 24% 3	840
	11th to 20th	Night	8% 8% 19% 39% 24% 3	840
	21st to end	Night	14% 8% 23% 35% 18% 3	924
November	1st to 10th	Night	14% 10% 21% 39% 15% 1	860
	11th to 20th	Night	17% 13% 24% 33% 11% 2	848
	21st to end	Night	12% 9% 22% 37% 17% 3	840
December	1st to 10th	Night	16% 9% 22% 32% 17% 3	840
	11th to 20th	Night	20% 11% 23% 32% 13% 1	840
	21st to end	Night	20% 12% 27% 30% 10% 1	924

Page generated at 2026-11-10 00:55:22. Version ac07

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