S&Y Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the S&Y Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	12% 6% 2 6% 4 4 4 4 6% 8% 45%	127380
	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	124598
Transparency	Day	36% 4 10% 22% 18% 9%	131110
	Night	34% 4 10% 21% 20% 11%	128931
Seeing	Day	12% 7% 20% 27% 25% 9%	16435
	Night	14% 7% 26% 35% 16% 2	26411

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	16% 10% 3 6% 3 3 3 3 4 6% 44%	4167
	11th to 20th	Day	15% 5% 2 4 2 3 3 3 5% 6% 53%	4134
	21st to end	Day	27% 9% 3 6% 3 2 2 1 3 5% 40%	4559
February	1st to 10th	Day	19% 8% 2 7% 4 3 3 3 4 5% 43%	2000
	11th to 20th	Day	20% 8% 2 7% 3 3 3 3 5% 5% 43%	1949
	21st to end	Day	18% 8% 2 5% 4 3 2 3 4 5% 46%	1694
March	1st to 10th	Day	13% 7% 3 6% 4 4 3 3 5% 7% 46%	4936
	11th to 20th	Day	14% 6% 2 6% 4 3 4 3 4 6% 49%	4980
	21st to end	Day	16% 9% 3 7% 4 3 3 3 5% 6% 42%	5462
April	1st to 10th	Day	14% 10% 3 6% 4 4 3 3 6% 6% 40%	2200
	11th to 20th	Day	13% 8% 4 6% 6% 4 3 4 5% 6% 42%	2215
	21st to end	Day	13% 7% 2 6% 4 4 4 4 5% 7% 44%	2477
May	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 4 6% 7% 34%	5862
	11th to 20th	Day	10% 5% 2 7% 4 4 5% 4 6% 8% 44%	5862
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 4 8% 10% 35%	6434
June	1st to 10th	Day	5% 4 2 5% 3 5% 5% 5% 7% 10% 49%	2600
	11th to 20th	Day	7% 4 2 5% 3 5% 5% 5% 7% 10% 48%	2600
	21st to end	Day	5% 6% 2 6% 6% 6% 6% 5% 7% 11% 38%	2600
July	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 5% 7% 11% 39%	5900
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 5% 8% 12% 44%	5900
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	6422
August	1st to 10th	Day	1 1 1 2 2 2 3 3 5% 9% 69%	4234
	11th to 20th	Day	3 2 1 3 3 4 4 4 8% 12% 56%	4087
	21st to end	Day	4 3 2 5% 5% 5% 6% 5% 9% 12% 45%	4018
September	1st to 10th	Day	5% 6% 3 8% 5% 5% 5% 4 6% 8% 46%	2200
	11th to 20th	Day	10% 5% 2 7% 4 5% 5% 4 7% 8% 44%	2200
	21st to end	Day	6% 3 2 5% 4 6% 6% 5% 6% 7% 51%	2200
October	1st to 10th	Day	5% 3 1 4 3 4 4 3 4 8% 60%	2189
	11th to 20th	Day	12% 4 2 5% 4 2 3 3 4 5% 57%	2000
	21st to end	Day	12% 4 2 4 3 3 4 3 4 5% 57%	2200
November	1st to 10th	Day	11% 6% 2 6% 3 3 4 4 5% 7% 48%	2058
	11th to 20th	Day	10% 6% 2 6% 4 3 3 2 5% 5% 53%	2053
	21st to end	Day	16% 8% 3 6% 4 3 4 3 4 6% 44%	1890
December	1st to 10th	Day	23% 7% 3 5% 4 3 3 2 4 5% 40%	3030
	11th to 20th	Day	19% 7% 2 6% 4 4 3 3 4 5% 43%	3481
	21st to end	Day	13% 7% 2 5% 3 3 3 2 3 4 53%	4587
January	1st to 10th	Night	18% 8% 2 4 2 2 2 2 4 5% 50%	6018
	11th to 20th	Night	12% 5% 1 3 2 2 3 3 4 5% 61%	5979
	21st to end	Night	28% 6% 2 5% 3 2 3 2 3 5% 40%	6462
February	1st to 10th	Night	18% 8% 3 6% 3 3 3 3 3 6% 46%	2400
	11th to 20th	Night	19% 7% 3 5% 3 3 3 2 4 4 48%	2338
	21st to end	Night	19% 8% 2 5% 3 3 3 3 4 5% 45%	1935
March	1st to 10th	Night	15% 7% 2 7% 3 4 4 3 5% 6% 44%	4919
	11th to 20th	Night	15% 7% 3 5% 3 3 4 3 4 5% 48%	4980
	21st to end	Night	16% 9% 3 6% 4 3 3 3 4 6% 43%	5467
April	1st to 10th	Night	12% 7% 2 5% 4 4 4 4 5% 6% 46%	2200
	11th to 20th	Night	15% 6% 2 6% 4 3 4 3 4 7% 46%	2130
	21st to end	Night	14% 7% 2 6% 3 3 3 3 5% 6% 48%	1917
May	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 5% 6% 34%	4053
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 7% 8% 46%	4053
	21st to end	Night	9% 7% 3 7% 5% 5% 5% 4 6% 10% 38%	4443
June	1st to 10th	Night	4 4 1 3 3 3 3 4 6% 10% 59%	1800
	11th to 20th	Night	4 3 1 3 3 3 4 4 6% 9% 59%	1800
	21st to end	Night	4 4 1 5% 3 4 5% 4 7% 11% 52%	1800
July	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 43%	4078
	11th to 20th	Night	3 4 1 4 3 4 5% 4 7% 12% 53%	4077
	21st to end	Night	6% 6% 2 6% 4 4 4 4 6% 10% 47%	4449
August	1st to 10th	Night	1 1 2 2 2 2 3 5% 8% 75%	2926
	11th to 20th	Night	2 2 1 4 2 3 4 4 7% 12% 58%	2953
	21st to end	Night	3 2 1 4 3 4 4 4 7% 12% 55%	3880
September	1st to 10th	Night	5% 5% 2 6% 5% 5% 4 4 6% 8% 52%	2200
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 9% 48%	2200
	21st to end	Night	6% 4 1 4 4 4 4 4 6% 8% 56%	2200
October	1st to 10th	Night	4 4 1 4 3 3 3 2 5% 6% 66%	2200
	11th to 20th	Night	10% 4 1 3 3 3 3 3 4 6% 59%	2390
	21st to end	Night	12% 4 1 4 3 3 2 2 3 4 61%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 4 3 4 6% 55%	2460
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 6% 54%	2562
	21st to end	Night	16% 7% 2 6% 3 4 3 2 4 7% 47%	2730
December	1st to 10th	Night	22% 9% 3 5% 3 3 4 3 3 5% 40%	4380
	11th to 20th	Night	17% 10% 2 5% 3 2 3 2 4 4 47%	4946
	21st to end	Night	15% 6% 2 4 3 3 2 2 3 4 56%	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	44% 2 3 23% 19% 8%	4168
	11th to 20th	Day	34% 2 4 20% 25% 15%	4134
	21st to end	Day	51% 1 2 9% 21% 15%	4559
February	1st to 10th	Day	46% 2 5% 12% 21% 14%	2000
	11th to 20th	Day	46% 2 7% 18% 18% 10%	1959
	21st to end	Day	43% 3 5% 19% 17% 14%	1694
March	1st to 10th	Day	40% 2 5% 20% 17% 17%	4936
	11th to 20th	Day	39% 2 6% 11% 17% 26%	4980
	21st to end	Day	45% 3 7% 20% 19% 7%	5464
April	1st to 10th	Day	45% 3 8% 17% 15% 13%	2200
	11th to 20th	Day	45% 3 7% 15% 19% 10%	2215
	21st to end	Day	41% 3 7% 21% 21% 7%	2477
May	1st to 10th	Day	48% 4 10% 21% 14% 3	5900
	11th to 20th	Day	37% 4 12% 19% 22% 5%	5866
	21st to end	Day	42% 4 10% 28% 13% 2	6447
June	1st to 10th	Day	29% 5% 15% 29% 19% 3	2600
	11th to 20th	Day	30% 7% 12% 28% 18% 5%	2600
	21st to end	Day	36% 8% 13% 26% 15% 3	2600
July	1st to 10th	Day	37% 8% 22% 28% 4 1	5836
	11th to 20th	Day	29% 9% 20% 29% 12% 1	5831
	21st to end	Day	35% 8% 17% 25% 13% 2	6240
August	1st to 10th	Day	13% 4 13% 41% 28% 1	4220
	11th to 20th	Day	20% 5% 26% 38% 10% 2	4126
	21st to end	Day	27% 11% 19% 24% 16% 2	4017
September	1st to 10th	Day	37% 6% 9% 21% 18% 8%	2200
	11th to 20th	Day	36% 6% 13% 21% 20% 5%	2200
	21st to end	Day	30% 6% 9% 23% 17% 15%	2200
October	1st to 10th	Day	15% 3 6% 14% 23% 39%	3566
	11th to 20th	Day	20% 2 6% 18% 34% 19%	3240
	21st to end	Day	25% 4 5% 20% 31% 15%	3538
November	1st to 10th	Day	36% 4 8% 18% 22% 12%	2058
	11th to 20th	Day	34% 3 8% 18% 23% 15%	2053
	21st to end	Day	42% 3 7% 17% 19% 12%	1890
December	1st to 10th	Day	48% 3 6% 17% 18% 8%	3030
	11th to 20th	Day	44% 3 7% 11% 23% 12%	3479
	21st to end	Day	37% 2 4 17% 26% 14%	4587
January	1st to 10th	Night	40% 2 4 20% 23% 12%	6018
	11th to 20th	Night	29% 2 3 19% 32% 16%	5979
	21st to end	Night	50% 1 2 12% 20% 14%	6462
February	1st to 10th	Night	42% 3 6% 12% 21% 17%	2400
	11th to 20th	Night	43% 3 6% 17% 19% 13%	2338
	21st to end	Night	44% 3 5% 16% 18% 15%	1935
March	1st to 10th	Night	43% 2 6% 16% 19% 14%	4919
	11th to 20th	Night	41% 2 7% 13% 13% 24%	4980
	21st to end	Night	45% 3 7% 17% 19% 10%	5467
April	1st to 10th	Night	39% 3 8% 19% 18% 13%	2200
	11th to 20th	Night	40% 3 10% 16% 17% 15%	2130
	21st to end	Night	39% 3 8% 22% 20% 8%	1917
May	1st to 10th	Night	51% 4 8% 22% 11% 4	4080
	11th to 20th	Night	36% 4 11% 22% 20% 7%	4054
	21st to end	Night	41% 4 13% 29% 12% 1	4452
June	1st to 10th	Night	22% 4 18% 32% 22% 3	1800
	11th to 20th	Night	21% 5% 14% 33% 20% 5%	1800
	21st to end	Night	26% 8% 17% 31% 17% 2	1800
July	1st to 10th	Night	35% 8% 27% 26% 4	4035
	11th to 20th	Night	23% 7% 24% 34% 11%	4026
	21st to end	Night	31% 8% 20% 26% 11% 3	4320
August	1st to 10th	Night	9% 4 15% 43% 28% 1	2916
	11th to 20th	Night	18% 6% 30% 33% 12% 1	2980
	21st to end	Night	21% 11% 23% 30% 14% 2	3880
September	1st to 10th	Night	29% 7% 16% 24% 17% 8%	2200
	11th to 20th	Night	34% 6% 14% 23% 17% 6%	2200
	21st to end	Night	26% 5% 15% 22% 18% 14%	2200
October	1st to 10th	Night	14% 3 7% 17% 20% 40%	3590
	11th to 20th	Night	17% 2 7% 19% 33% 22%	3889
	21st to end	Night	22% 3 4 23% 30% 17%	4252
November	1st to 10th	Night	33% 3 7% 20% 23% 14%	2460
	11th to 20th	Night	32% 3 8% 20% 22% 15%	2562
	21st to end	Night	39% 2 8% 19% 17% 15%	2730
December	1st to 10th	Night	49% 3 4 16% 19% 9%	4380
	11th to 20th	Night	42% 2 7% 10% 23% 16%	4947
	21st to end	Night	34% 2 4 16% 30% 14%	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	21% 10% 38% 21% 8% 1	330
	11th to 20th	Day	22% 13% 31% 25% 8% 1	318
	21st to end	Day	22% 13% 38% 22% 6%	330
February	1st to 10th	Day	27% 14% 26% 24% 9% 1	300
	11th to 20th	Day	29% 16% 32% 18% 5%	300
	21st to end	Day	26% 14% 39% 19% 3	249
March	1st to 10th	Day	19% 16% 35% 22% 7%	300
	11th to 20th	Day	16% 19% 37% 22% 5% 1	300
	21st to end	Day	19% 12% 38% 22% 7% 1	330
April	1st to 10th	Day	21% 14% 38% 20% 5% 1	300
	11th to 20th	Day	16% 14% 37% 25% 7% 1	333
	21st to end	Day	18% 12% 33% 26% 10% 2	577
May	1st to 10th	Day	17% 9% 34% 30% 11%	703
	11th to 20th	Day	14% 9% 28% 31% 16% 3	725
	21st to end	Day	12% 6% 25% 33% 21% 3	789
June	1st to 10th	Day	5% 2 13% 40% 30% 10%	730
	11th to 20th	Day	7% 2 12% 42% 30% 7%	730
	21st to end	Day	5% 3 12% 28% 36% 15%	730
July	1st to 10th	Day	9% 3 6% 30% 35% 17%	721
	11th to 20th	Day	4 2 3 28% 46% 18%	718
	21st to end	Day	5% 2 4 26% 46% 17%	794
August	1st to 10th	Day	3 3 23% 52% 19%	726
	11th to 20th	Day	6% 1 3 22% 50% 19%	670
	21st to end	Day	6% 1 3 19% 36% 34%	406
September	1st to 10th	Day	6% 2 8% 21% 34% 30%	330
	11th to 20th	Day	6% 1 6% 18% 43% 26%	330
	21st to end	Day	9% 3 6% 25% 35% 22%	330
October	1st to 10th	Day	4 2 8% 29% 40% 16%	330
	11th to 20th	Day	10% 3 19% 32% 26% 10%	330
	21st to end	Day	17% 9% 24% 23% 20% 7%	363
November	1st to 10th	Day	16% 8% 19% 29% 21% 7%	330
	11th to 20th	Day	15% 8% 24% 32% 16% 5%	330
	21st to end	Day	18% 18% 30% 19% 11% 3	330
December	1st to 10th	Day	23% 6% 31% 25% 11% 5%	330
	11th to 20th	Day	21% 15% 29% 22% 11% 2	330
	21st to end	Day	19% 11% 33% 25% 10% 2	363
January	1st to 10th	Night	19% 12% 32% 31% 7% 1	1050
	11th to 20th	Night	20% 11% 32% 30% 7%	1031
	21st to end	Night	22% 10% 32% 30% 6%	1085
February	1st to 10th	Night	23% 11% 34% 27% 4	800
	11th to 20th	Night	24% 12% 36% 24% 4	784
	21st to end	Night	24% 15% 39% 19% 2	615
March	1st to 10th	Night	19% 15% 38% 25% 3	600
	11th to 20th	Night	19% 11% 38% 28% 5%	600
	21st to end	Night	15% 11% 43% 26% 4	660
April	1st to 10th	Night	14% 11% 44% 27% 4	600
	11th to 20th	Night	18% 11% 40% 26% 5%	593
	21st to end	Night	15% 12% 41% 24% 8%	600
May	1st to 10th	Night	13% 11% 43% 27% 5% 1	600
	11th to 20th	Night	13% 7% 34% 34% 12% 1	591
	21st to end	Night	10% 4 32% 41% 13%	650
June	1st to 10th	Night	6% 2 19% 45% 26% 2	600
	11th to 20th	Night	4 2 25% 51% 17% 1	600
	21st to end	Night	4 1 12% 48% 31% 5%	600
July	1st to 10th	Night	7% 1 9% 46% 33% 3	585
	11th to 20th	Night	4 1 9% 49% 35% 3	582
	21st to end	Night	4 2 8% 48% 36% 2	657
August	1st to 10th	Night	1 1 7% 53% 35% 4	597
	11th to 20th	Night	5% 1 5% 52% 32% 5%	594
	21st to end	Night	4 1 5% 37% 46% 7%	660
September	1st to 10th	Night	8% 1 7% 40% 39% 6%	600
	11th to 20th	Night	9% 2 8% 36% 40% 4	600
	21st to end	Night	9% 2 11% 37% 37% 5%	600
October	1st to 10th	Night	5% 1 11% 48% 31% 5%	600
	11th to 20th	Night	11% 3 22% 41% 21% 2	790
	21st to end	Night	13% 8% 27% 33% 18% 1	880
November	1st to 10th	Night	13% 5% 23% 39% 19% 1	820
	11th to 20th	Night	12% 8% 27% 37% 15% 1	882
	21st to end	Night	20% 9% 28% 30% 12% 1	1050
December	1st to 10th	Night	20% 9% 27% 32% 10% 1	1050
	11th to 20th	Night	23% 12% 25% 32% 7% 1	1050
	21st to end	Night	20% 8% 29% 35% 7%	1155

Page generated at 2026-01-14 00:14:24. Version ac07

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