SkyWatcher Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SkyWatcher Ranch clear sky chart from 2005-11-06 to 2026-04-02.

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	9% 6% 2 5% 4 4 4 3 5% 7% 51%	131788
	Night	9% 6% 2 5% 3 3 3 3 5% 7% 54%	130378
Transparency	Day	32% 5% 9% 21% 21% 12%	135619
	Night	30% 5% 9% 20% 21% 14%	134598
Seeing	Day	12% 9% 26% 31% 18% 4	16600
	Night	12% 9% 31% 35% 13% 1	26685

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	8% 5% 3 5% 2 3 3 3 4 6% 58%	5289
	11th to 20th	Day	11% 6% 2 4 2 2 2 2 5% 5% 58%	5407
	21st to end	Day	17% 6% 2 5% 4 3 3 3 4 6% 46%	6468
February	1st to 10th	Day	12% 6% 2 6% 4 3 3 3 4 6% 52%	2100
	11th to 20th	Day	11% 7% 2 5% 4 3 3 3 5% 6% 50%	2049
	21st to end	Day	11% 6% 2 4 3 3 3 3 4 6% 55%	1730
March	1st to 10th	Day	9% 6% 2 4 3 3 3 2 4 6% 56%	4780
	11th to 20th	Day	10% 6% 2 5% 3 3 3 2 3 5% 56%	5090
	21st to end	Day	8% 6% 2 6% 4 4 4 3 5% 6% 53%	5583
April	1st to 10th	Day	10% 7% 2 6% 3 3 5% 4 4 7% 48%	2211
	11th to 20th	Day	12% 6% 2 7% 4 4 4 3 4 6% 49%	2337
	21st to end	Day	10% 8% 2 6% 5% 3 4 3 4 6% 51%	2452
May	1st to 10th	Day	13% 7% 3 7% 4 5% 5% 4 6% 7% 41%	5862
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 47%	5862
	21st to end	Day	11% 7% 3 6% 4 5% 5% 5% 7% 8% 39%	6434
June	1st to 10th	Day	4 4 1 5% 5% 5% 4 4 6% 9% 53%	2600
	11th to 20th	Day	4 4 2 5% 5% 4 4 4 7% 9% 52%	2600
	21st to end	Day	5% 5% 1 6% 5% 5% 6% 5% 7% 10% 45%	2600
July	1st to 10th	Day	12% 7% 2 7% 4 5% 6% 4 6% 8% 38%	5900
	11th to 20th	Day	3 4 2 6% 5% 5% 6% 5% 6% 8% 50%	5900
	21st to end	Day	5% 5% 2 5% 4 4 5% 5% 7% 10% 49%	6422
August	1st to 10th	Day	1 1 1 3 3 3 3 3 4 8% 69%	3953
	11th to 20th	Day	4 2 1 4 3 3 4 3 5% 8% 62%	3846
	21st to end	Day	4 3 2 6% 5% 5% 5% 4 7% 10% 49%	4282
September	1st to 10th	Day	10% 5% 3 7% 5% 4 6% 4 5% 7% 45%	2200
	11th to 20th	Day	9% 5% 3 7% 5% 6% 5% 5% 6% 7% 43%	2200
	21st to end	Day	9% 4 2 6% 4 5% 4 5% 6% 6% 51%	2200
October	1st to 10th	Day	5% 4 2 5% 3 4 4 4 5% 7% 58%	2200
	11th to 20th	Day	10% 5% 1 5% 4 3 3 2 4 6% 57%	2157
	21st to end	Day	9% 5% 2 5% 2 3 3 2 4 5% 60%	2200
November	1st to 10th	Day	11% 6% 2 6% 3 3 3 3 5% 6% 52%	1996
	11th to 20th	Day	8% 5% 1 5% 3 4 5% 3 5% 6% 57%	1890
	21st to end	Day	12% 7% 2 7% 3 4 2 3 4 6% 50%	1890
December	1st to 10th	Day	20% 9% 3 6% 3 2 2 2 4 5% 43%	3030
	11th to 20th	Day	10% 9% 3 6% 4 4 4 3 5% 5% 47%	3481
	21st to end	Day	9% 6% 2 5% 2 2 2 2 3 4 63%	4587
January	1st to 10th	Night	9% 5% 2 6% 3 3 3 2 3 5% 60%	7634
	11th to 20th	Night	8% 4 1 3 2 2 2 2 4 6% 67%	7638
	21st to end	Night	23% 7% 2 5% 3 3 3 2 3 5% 44%	7746
February	1st to 10th	Night	10% 6% 2 6% 4 3 2 3 5% 6% 54%	2520
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 6% 52%	2458
	21st to end	Night	12% 5% 2 6% 3 3 3 4 5% 5% 53%	2076
March	1st to 10th	Night	7% 6% 2 6% 3 3 4 4 6% 8% 50%	5291
	11th to 20th	Night	10% 7% 2 5% 3 3 4 3 4 6% 54%	5090
	21st to end	Night	9% 8% 3 6% 3 3 3 3 5% 6% 52%	5588
April	1st to 10th	Night	7% 6% 2 6% 4 4 3 4 6% 5% 53%	2211
	11th to 20th	Night	10% 7% 3 6% 4 4 3 3 4 6% 50%	2007
	21st to end	Night	11% 6% 2 5% 4 3 3 4 5% 6% 51%	1945
May	1st to 10th	Night	16% 9% 3 6% 4 4 4 3 4 6% 42%	4053
	11th to 20th	Night	5% 6% 3 6% 4 4 4 3 6% 8% 52%	4053
	21st to end	Night	10% 7% 2 6% 3 4 4 4 7% 10% 42%	4443
June	1st to 10th	Night	2 3 1 3 2 3 4 3 7% 11% 60%	1800
	11th to 20th	Night	3 3 2 4 3 3 3 5% 6% 7% 61%	1800
	21st to end	Night	4 4 2 5% 4 3 5% 4 5% 10% 55%	1800
July	1st to 10th	Night	8% 7% 2 7% 4 4 4 4 6% 9% 44%	4078
	11th to 20th	Night	2 3 2 5% 3 4 4 3 6% 9% 59%	4077
	21st to end	Night	3 6% 3 6% 4 4 4 3 6% 9% 52%	4449
August	1st to 10th	Night	1 1 1 2 2 2 2 2 4 7% 75%	3188
	11th to 20th	Night	3 2 1 4 2 3 3 4 7% 10% 62%	3213
	21st to end	Night	3 4 2 5% 4 4 5% 4 6% 10% 54%	3624
September	1st to 10th	Night	7% 5% 3 5% 4 4 3 3 5% 8% 52%	2200
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 7% 8% 48%	2200
	21st to end	Night	7% 4 2 4 4 4 4 4 6% 8% 54%	2200
October	1st to 10th	Night	5% 3 1 4 3 3 3 3 4 7% 65%	2200
	11th to 20th	Night	9% 4 2 5% 3 3 3 3 4 6% 60%	2220
	21st to end	Night	11% 5% 2 4 3 2 3 2 3 4 63%	2640
November	1st to 10th	Night	10% 6% 2 5% 3 2 3 2 5% 5% 58%	2517
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 4 6% 59%	2730
	21st to end	Night	13% 5% 2 6% 3 3 3 3 4 6% 51%	2730
December	1st to 10th	Night	20% 6% 2 5% 3 4 3 3 4 5% 43%	4380
	11th to 20th	Night	13% 10% 2 6% 3 3 3 3 3 5% 49%	4946
	21st to end	Night	10% 7% 2 4 2 2 2 2 3 4 61%	6633

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	29% 3 5% 23% 26% 14%	5290
	11th to 20th	Day	30% 2 4 16% 27% 21%	5407
	21st to end	Day	41% 3 5% 13% 25% 13%	6468
February	1st to 10th	Day	34% 4 7% 13% 22% 20%	2100
	11th to 20th	Day	36% 4 7% 19% 19% 16%	2059
	21st to end	Day	31% 3 7% 16% 24% 18%	1730
March	1st to 10th	Day	30% 3 5% 16% 17% 28%	4780
	11th to 20th	Day	32% 4 5% 9% 16% 33%	5090
	21st to end	Day	32% 5% 7% 19% 21% 16%	5585
April	1st to 10th	Day	36% 5% 9% 17% 18% 15%	2211
	11th to 20th	Day	38% 5% 7% 16% 21% 14%	2337
	21st to end	Day	36% 5% 7% 20% 19% 12%	2452
May	1st to 10th	Day	41% 7% 8% 17% 19% 8%	5900
	11th to 20th	Day	34% 6% 10% 15% 22% 13%	5866
	21st to end	Day	39% 8% 10% 25% 14% 3	6447
June	1st to 10th	Day	28% 7% 12% 29% 21% 3	2600
	11th to 20th	Day	28% 8% 11% 26% 21% 6%	2600
	21st to end	Day	34% 9% 12% 23% 17% 5%	2600
July	1st to 10th	Day	43% 7% 15% 29% 6%	5836
	11th to 20th	Day	30% 7% 14% 33% 14% 2	5831
	21st to end	Day	28% 7% 18% 27% 19% 1	6240
August	1st to 10th	Day	14% 5% 10% 40% 30% 1	3939
	11th to 20th	Day	21% 5% 21% 39% 13% 2	3882
	21st to end	Day	28% 10% 16% 25% 19% 2	4281
September	1st to 10th	Day	39% 7% 9% 19% 18% 9%	2200
	11th to 20th	Day	38% 8% 11% 20% 18% 6%	2200
	21st to end	Day	33% 6% 10% 21% 19% 12%	2200
October	1st to 10th	Day	15% 5% 7% 13% 27% 33%	3590
	11th to 20th	Day	26% 3 6% 19% 22% 23%	3488
	21st to end	Day	24% 3 6% 17% 38% 13%	3538
November	1st to 10th	Day	34% 4 8% 18% 23% 13%	1996
	11th to 20th	Day	30% 5% 7% 18% 22% 19%	1890
	21st to end	Day	38% 3 5% 17% 23% 14%	1890
December	1st to 10th	Day	45% 3 5% 16% 20% 11%	3030
	11th to 20th	Day	38% 5% 8% 12% 23% 15%	3479
	21st to end	Day	28% 2 3 17% 28% 21%	4587
January	1st to 10th	Night	29% 2 6% 18% 29% 15%	7635
	11th to 20th	Night	22% 1 3 17% 30% 26%	7638
	21st to end	Night	45% 3 3 9% 23% 17%	7746
February	1st to 10th	Night	32% 4 7% 13% 22% 21%	2520
	11th to 20th	Night	34% 4 7% 18% 21% 17%	2458
	21st to end	Night	34% 4 6% 12% 24% 19%	2076
March	1st to 10th	Night	30% 6% 6% 18% 20% 19%	5291
	11th to 20th	Night	33% 5% 6% 11% 16% 30%	5090
	21st to end	Night	34% 4 7% 19% 19% 17%	5588
April	1st to 10th	Night	32% 6% 10% 19% 15% 19%	2211
	11th to 20th	Night	36% 6% 9% 15% 18% 17%	2007
	21st to end	Night	36% 5% 9% 20% 19% 12%	1945
May	1st to 10th	Night	43% 7% 7% 18% 13% 11%	4080
	11th to 20th	Night	30% 6% 11% 19% 19% 14%	4054
	21st to end	Night	36% 8% 14% 25% 12% 5%	4452
June	1st to 10th	Night	18% 8% 18% 34% 20% 2	1800
	11th to 20th	Night	22% 6% 16% 28% 23% 6%	1800
	21st to end	Night	26% 7% 16% 28% 18% 4	1800
July	1st to 10th	Night	37% 8% 21% 29% 5%	4035
	11th to 20th	Night	23% 7% 19% 35% 15% 1	4026
	21st to end	Night	29% 7% 19% 27% 15% 3	4320
August	1st to 10th	Night	12% 4 14% 43% 26% 1	3178
	11th to 20th	Night	17% 6% 26% 38% 11% 2	3243
	21st to end	Night	26% 10% 20% 25% 18% 1	3624
September	1st to 10th	Night	31% 6% 12% 23% 19% 8%	2200
	11th to 20th	Night	32% 8% 15% 23% 16% 6%	2200
	21st to end	Night	29% 7% 11% 22% 19% 13%	2200
October	1st to 10th	Night	14% 3 8% 15% 23% 36%	3590
	11th to 20th	Night	23% 3 8% 17% 29% 19%	3602
	21st to end	Night	25% 2 5% 19% 34% 16%	4252
November	1st to 10th	Night	30% 4 7% 19% 25% 16%	2517
	11th to 20th	Night	27% 5% 7% 22% 21% 19%	2730
	21st to end	Night	36% 3 7% 16% 21% 16%	2730
December	1st to 10th	Night	44% 3 7% 16% 17% 13%	4380
	11th to 20th	Night	39% 3 5% 12% 21% 19%	4947
	21st to end	Night	30% 2 4 16% 29% 18%	6633

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	18% 18% 42% 17% 5% 1	330
	11th to 20th	Day	21% 16% 35% 21% 6%	330
	21st to end	Day	17% 18% 42% 19% 5%	363
February	1st to 10th	Day	23% 13% 35% 22% 7%	330
	11th to 20th	Day	21% 17% 42% 18% 2 1	330
	21st to end	Day	20% 19% 41% 18% 2	273
March	1st to 10th	Day	22% 22% 39% 15% 2	330
	11th to 20th	Day	18% 23% 37% 17% 5%	330
	21st to end	Day	14% 17% 43% 21% 4 1	363
April	1st to 10th	Day	18% 21% 39% 19% 2	303
	11th to 20th	Day	16% 21% 42% 15% 5%	455
	21st to end	Day	15% 18% 44% 18% 5%	552
May	1st to 10th	Day	15% 13% 42% 25% 5%	703
	11th to 20th	Day	17% 10% 35% 29% 8% 1	725
	21st to end	Day	15% 9% 32% 34% 9% 1	789
June	1st to 10th	Day	6% 4 25% 42% 19% 3	730
	11th to 20th	Day	8% 3 24% 47% 17% 2	730
	21st to end	Day	6% 2 18% 40% 29% 5%	730
July	1st to 10th	Day	10% 3 12% 39% 30% 7%	721
	11th to 20th	Day	4 1 8% 43% 37% 6%	718
	21st to end	Day	4 1 8% 40% 36% 11%	794
August	1st to 10th	Day	2 1 4 42% 42% 9%	562
	11th to 20th	Day	8% 6% 35% 39% 12%	517
	21st to end	Day	5% 1 7% 34% 39% 14%	566
September	1st to 10th	Day	11% 1 9% 33% 33% 13%	330
	11th to 20th	Day	5% 7% 35% 42% 11%	330
	21st to end	Day	11% 1 8% 35% 35% 9%	330
October	1st to 10th	Day	5% 2 15% 38% 31% 8%	330
	11th to 20th	Day	8% 5% 25% 41% 18% 3	330
	21st to end	Day	15% 14% 28% 30% 12% 2	363
November	1st to 10th	Day	18% 7% 32% 29% 11% 2	330
	11th to 20th	Day	18% 10% 32% 25% 10% 5%	330
	21st to end	Day	21% 21% 30% 18% 7% 3	330
December	1st to 10th	Day	17% 12% 33% 28% 7% 2	330
	11th to 20th	Day	16% 17% 35% 23% 9% 1	330
	21st to end	Day	17% 14% 33% 28% 9%	363
January	1st to 10th	Night	15% 11% 36% 30% 7%	1050
	11th to 20th	Night	16% 12% 38% 30% 5%	1034
	21st to end	Night	19% 13% 34% 28% 6%	924
February	1st to 10th	Night	17% 14% 35% 28% 6%	840
	11th to 20th	Night	18% 15% 37% 26% 4	824
	21st to end	Night	17% 15% 46% 21% 1	692
March	1st to 10th	Night	17% 15% 40% 24% 3	754
	11th to 20th	Night	16% 12% 40% 29% 3	630
	21st to end	Night	13% 13% 48% 23% 3	693
April	1st to 10th	Night	11% 14% 50% 22% 3	603
	11th to 20th	Night	13% 20% 46% 19% 2	593
	21st to end	Night	16% 12% 46% 21% 4 1	600
May	1st to 10th	Night	13% 14% 45% 25% 3	600
	11th to 20th	Night	12% 9% 40% 33% 7%	591
	21st to end	Night	12% 8% 43% 29% 8% 1	650
June	1st to 10th	Night	3 3 31% 47% 15% 1	600
	11th to 20th	Night	5% 3 40% 43% 9%	600
	21st to end	Night	6% 3 22% 48% 19% 2	600
July	1st to 10th	Night	8% 2 15% 47% 26% 2	585
	11th to 20th	Night	3 1 13% 55% 26% 2	582
	21st to end	Night	4 1 11% 55% 26% 3	657
August	1st to 10th	Night	1 1 10% 56% 31% 2	597
	11th to 20th	Night	5% 1 10% 50% 32% 3	594
	21st to end	Night	5% 2 8% 43% 38% 4	660
September	1st to 10th	Night	9% 2 12% 42% 34% 3	600
	11th to 20th	Night	11% 1 9% 42% 35% 3	600
	21st to end	Night	9% 2 13% 45% 30% 2	600
October	1st to 10th	Night	6% 1 16% 48% 29% 2	600
	11th to 20th	Night	11% 5% 25% 41% 16% 2	620
	21st to end	Night	13% 10% 29% 30% 17% 2	880
November	1st to 10th	Night	13% 6% 27% 38% 14% 1	877
	11th to 20th	Night	11% 8% 32% 35% 12% 2	1050
	21st to end	Night	18% 10% 31% 30% 10% 1	1050
December	1st to 10th	Night	16% 8% 33% 34% 8% 1	1050
	11th to 20th	Night	17% 12% 33% 31% 7%	1050
	21st to end	Night	14% 12% 34% 33% 7%	1155

Page generated at 2026-03-30 01:18:39. Version ac07

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