Salem-Avilla Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Salem-Avilla Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 2 6% 4 4 4 3 5% 7% 45%	130770
	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	131609
Transparency	Day	37% 5% 9% 23% 19% 5%	134375
	Night	35% 4 9% 23% 22% 7%	136081
Seeing	Day	16% 10% 24% 26% 18% 5%	17202
	Night	15% 9% 26% 30% 17% 2	27120

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	17% 7% 3 7% 3 3 3 3 3 5% 45%	5289
	11th to 20th	Day	10% 7% 2 5% 4 3 3 3 3 4 54%	5310
	21st to end	Day	21% 6% 3 6% 3 2 2 2 3 4 48%	5812
February	1st to 10th	Day	22% 8% 2 6% 3 3 4 3 4 4 41%	1890
	11th to 20th	Day	18% 9% 2 6% 4 4 3 3 4 6% 41%	1981
	21st to end	Day	22% 7% 3 6% 4 4 4 3 4 5% 40%	1797
March	1st to 10th	Day	22% 9% 3 7% 4 3 3 3 4 4 38%	5046
	11th to 20th	Day	11% 5% 2 5% 4 4 4 3 4 6% 52%	5090
	21st to end	Day	15% 8% 3 7% 4 4 3 3 5% 5% 44%	5583
April	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 6% 7% 45%	2510
	11th to 20th	Day	16% 6% 2 6% 4 3 3 3 5% 6% 45%	2446
	21st to end	Day	16% 7% 2 6% 4 5% 4 3 5% 7% 41%	2400
May	1st to 10th	Day	19% 8% 2 7% 4 4 4 3 5% 6% 36%	5405
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	5806
	21st to end	Day	15% 7% 3 8% 5% 5% 5% 5% 6% 7% 34%	6434
June	1st to 10th	Day	10% 4 2 6% 5% 5% 5% 5% 8% 8% 43%	2644
	11th to 20th	Day	6% 4 1 4 5% 6% 6% 5% 7% 10% 45%	2711
	21st to end	Day	4 3 2 5% 5% 5% 6% 4 8% 10% 48%	2600
July	1st to 10th	Day	8% 6% 2 6% 5% 6% 6% 5% 7% 11% 37%	5900
	11th to 20th	Day	5% 3 1 4 3 4 4 3 7% 10% 55%	5900
	21st to end	Day	9% 6% 3 6% 5% 5% 5% 4 7% 9% 40%	6422
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 5% 9% 60%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 4 6% 9% 43%	3864
	21st to end	Day	6% 3 2 5% 4 5% 5% 4 6% 9% 51%	4312
September	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 6% 7% 46%	2400
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 3 6% 7% 45%	2398
	21st to end	Day	11% 4 2 5% 4 4 4 3 4 6% 53%	2085
October	1st to 10th	Day	7% 4 2 5% 3 3 4 3 4 5% 60%	2000
	11th to 20th	Day	13% 4 2 4 2 3 3 2 3 6% 58%	2000
	21st to end	Day	19% 5% 2 4 3 2 3 2 3 5% 53%	2200
November	1st to 10th	Day	15% 5% 2 4 3 2 2 2 4 6% 55%	2058
	11th to 20th	Day	11% 6% 1 4 4 3 3 3 4 6% 55%	2058
	21st to end	Day	19% 8% 2 6% 3 3 3 2 4 5% 45%	1844
December	1st to 10th	Day	18% 8% 2 6% 3 3 4 2 3 5% 46%	2690
	11th to 20th	Day	20% 8% 3 5% 4 3 3 3 3 4 44%	3088
	21st to end	Day	20% 8% 3 6% 3 4 4 3 4 4 41%	4563
January	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	7634
	11th to 20th	Night	12% 4 1 4 3 2 2 2 2 4 65%	7680
	21st to end	Night	23% 7% 2 5% 3 3 3 2 3 4 44%	8402
February	1st to 10th	Night	22% 8% 2 5% 4 3 3 2 3 6% 43%	2730
	11th to 20th	Night	21% 8% 2 5% 3 3 3 3 3 5% 45%	2525
	21st to end	Night	19% 6% 2 5% 3 3 3 3 4 5% 47%	2027
March	1st to 10th	Night	19% 7% 3 6% 3 3 3 3 4 4 44%	5029
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 4 5% 52%	5090
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 7% 45%	5588
April	1st to 10th	Night	13% 5% 2 5% 3 4 3 3 6% 6% 49%	2108
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 5% 7% 51%	2035
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 7% 49%	2000
May	1st to 10th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 36%	4490
	11th to 20th	Night	19% 9% 3 6% 4 4 4 3 5% 6% 36%	4103
	21st to end	Night	17% 7% 3 7% 5% 4 4 3 5% 6% 40%	4443
June	1st to 10th	Night	5% 4 2 4 3 4 5% 4 7% 10% 52%	1732
	11th to 20th	Night	5% 4 2 4 5% 3 3 3 5% 9% 57%	1664
	21st to end	Night	3 3 1 4 3 3 4 5% 6% 9% 60%	1800
July	1st to 10th	Night	7% 4 3 6% 5% 6% 6% 4 7% 10% 42%	4078
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 13% 56%	4077
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 9% 48%	4449
August	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 9% 69%	2926
	11th to 20th	Night	5% 2 1 4 3 4 4 5% 7% 10% 55%	3182
	21st to end	Night	4 3 1 5% 4 4 5% 4 6% 8% 57%	3586
September	1st to 10th	Night	6% 4 2 5% 3 4 4 4 6% 6% 56%	2000
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 5% 6% 52%	2000
	21st to end	Night	9% 4 2 4 2 4 3 3 5% 6% 59%	2292
October	1st to 10th	Night	7% 4 2 4 3 2 3 3 5% 6% 63%	2400
	11th to 20th	Night	9% 4 2 5% 3 3 3 2 3 6% 61%	2400
	21st to end	Night	16% 6% 2 5% 3 3 2 2 2 3 55%	2640
November	1st to 10th	Night	13% 5% 2 4 2 3 3 3 4 5% 57%	2460
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 4 5% 57%	2557
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 52%	2772
December	1st to 10th	Night	16% 6% 2 6% 3 3 3 3 4 4 51%	4720
	11th to 20th	Night	19% 9% 3 7% 4 3 3 2 3 4 45%	5339
	21st to end	Night	19% 7% 2 6% 4 4 3 3 4 5% 44%	6651

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	43% 2 9% 27% 17% 2	5290
	11th to 20th	Day	35% 2 7% 20% 30% 6%	5310
	21st to end	Day	42% 2 4 17% 25% 10%	5812
February	1st to 10th	Day	47% 4 8% 21% 17% 4	1890
	11th to 20th	Day	46% 4 8% 19% 18% 5%	1990
	21st to end	Day	48% 3 6% 14% 19% 9%	1797
March	1st to 10th	Day	50% 4 5% 14% 15% 12%	5046
	11th to 20th	Day	33% 4 6% 19% 24% 13%	5090
	21st to end	Day	43% 4 8% 23% 20% 2	5585
April	1st to 10th	Day	39% 5% 9% 23% 19% 5%	2510
	11th to 20th	Day	39% 6% 9% 21% 18% 7%	2446
	21st to end	Day	43% 6% 10% 19% 20% 2	2400
May	1st to 10th	Day	47% 6% 9% 23% 13% 1	5440
	11th to 20th	Day	44% 7% 11% 22% 14% 2	5810
	21st to end	Day	46% 8% 12% 18% 14% 3	6447
June	1st to 10th	Day	35% 8% 13% 25% 17% 2	2644
	11th to 20th	Day	32% 8% 13% 27% 18% 2	2711
	21st to end	Day	28% 9% 14% 28% 19% 3	2600
July	1st to 10th	Day	36% 9% 16% 28% 10% 1	5836
	11th to 20th	Day	24% 6% 16% 36% 17% 2	5831
	21st to end	Day	37% 8% 14% 27% 13% 1	6240
August	1st to 10th	Day	21% 6% 15% 29% 24% 5%	4220
	11th to 20th	Day	36% 7% 13% 32% 10% 1	3900
	21st to end	Day	29% 7% 13% 28% 20% 3	4311
September	1st to 10th	Day	37% 7% 10% 21% 18% 8%	2400
	11th to 20th	Day	37% 6% 11% 18% 22% 6%	2398
	21st to end	Day	32% 5% 8% 20% 26% 8%	2085
October	1st to 10th	Day	16% 3 6% 13% 33% 29%	3260
	11th to 20th	Day	19% 2 4 20% 36% 19%	3240
	21st to end	Day	32% 3 4 28% 27% 6%	3538
November	1st to 10th	Day	32% 4 8% 24% 24% 7%	2058
	11th to 20th	Day	31% 4 8% 21% 26% 10%	2058
	21st to end	Day	43% 3 6% 18% 24% 6%	1844
December	1st to 10th	Day	43% 3 6% 26% 19% 4	2690
	11th to 20th	Day	46% 3 4 22% 20% 5%	3085
	21st to end	Day	47% 3 6% 25% 15% 4	4563
January	1st to 10th	Night	41% 2 7% 28% 20% 2	7635
	11th to 20th	Night	27% 2 5% 19% 35% 11%	7680
	21st to end	Night	46% 3 6% 17% 21% 8%	8402
February	1st to 10th	Night	45% 3 7% 21% 18% 6%	2730
	11th to 20th	Night	44% 4 6% 20% 20% 6%	2526
	21st to end	Night	41% 3 7% 16% 22% 10%	2027
March	1st to 10th	Night	43% 4 7% 13% 20% 13%	5029
	11th to 20th	Night	35% 3 7% 22% 18% 15%	5090
	21st to end	Night	39% 4 8% 24% 22% 3	5588
April	1st to 10th	Night	35% 4 10% 23% 20% 8%	2108
	11th to 20th	Night	33% 5% 9% 23% 21% 9%	2035
	21st to end	Night	36% 5% 11% 23% 22% 4	2000
May	1st to 10th	Night	48% 6% 11% 21% 13% 2	4520
	11th to 20th	Night	47% 6% 11% 23% 12% 1	4104
	21st to end	Night	44% 6% 12% 20% 13% 4	4452
June	1st to 10th	Night	25% 9% 15% 29% 21% 2	1732
	11th to 20th	Night	25% 5% 12% 29% 24% 4	1664
	21st to end	Night	20% 7% 13% 32% 24% 5%	1800
July	1st to 10th	Night	36% 8% 16% 27% 13% 1	4035
	11th to 20th	Night	20% 6% 17% 37% 17% 2	4026
	21st to end	Night	31% 8% 15% 29% 17% 1	4320
August	1st to 10th	Night	14% 5% 16% 32% 27% 6%	2916
	11th to 20th	Night	23% 7% 19% 35% 15% 2	3212
	21st to end	Night	24% 7% 11% 28% 27% 3	3586
September	1st to 10th	Night	26% 7% 12% 24% 23% 9%	2000
	11th to 20th	Night	32% 6% 10% 22% 24% 6%	2000
	21st to end	Night	26% 5% 10% 21% 28% 10%	2292
October	1st to 10th	Night	16% 3 6% 14% 28% 33%	3920
	11th to 20th	Night	17% 2 4 16% 37% 24%	3899
	21st to end	Night	29% 3 4 22% 29% 13%	4252
November	1st to 10th	Night	31% 3 7% 21% 27% 10%	2460
	11th to 20th	Night	30% 4 7% 23% 25% 11%	2557
	21st to end	Night	36% 4 7% 19% 28% 6%	2772
December	1st to 10th	Night	38% 3 6% 31% 19% 4	4720
	11th to 20th	Night	46% 2 4 19% 21% 7%	5341
	21st to end	Night	44% 3 9% 23% 18% 3	6651

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	26% 17% 36% 16% 4 1	330
	11th to 20th	Day	26% 21% 38% 14% 1	330
	21st to end	Day	22% 25% 35% 14% 5%	363
February	1st to 10th	Day	28% 17% 35% 15% 4 1	330
	11th to 20th	Day	31% 28% 27% 11% 3	330
	21st to end	Day	34% 22% 29% 12% 3	273
March	1st to 10th	Day	28% 19% 35% 13% 4	330
	11th to 20th	Day	27% 24% 36% 12% 1	330
	21st to end	Day	23% 26% 34% 13% 3	363
April	1st to 10th	Day	22% 18% 35% 17% 7%	530
	11th to 20th	Day	25% 17% 33% 18% 6%	516
	21st to end	Day	21% 17% 31% 21% 8% 2	500
May	1st to 10th	Day	19% 16% 33% 22% 8% 1	503
	11th to 20th	Day	16% 9% 31% 27% 15% 2	695
	21st to end	Day	12% 7% 26% 33% 17% 4	789
June	1st to 10th	Day	9% 5% 21% 34% 25% 6%	730
	11th to 20th	Day	6% 4 20% 37% 25% 7%	730
	21st to end	Day	5% 2 22% 34% 29% 8%	730
July	1st to 10th	Day	5% 2 14% 34% 34% 10%	721
	11th to 20th	Day	6% 2 12% 35% 34% 10%	718
	21st to end	Day	8% 1 10% 34% 35% 12%	794
August	1st to 10th	Day	5% 2 12% 38% 34% 8%	726
	11th to 20th	Day	11% 2 11% 34% 31% 12%	534
	21st to end	Day	13% 1 7% 26% 37% 16%	583
September	1st to 10th	Day	11% 2 11% 31% 36% 8%	530
	11th to 20th	Day	13% 2 15% 25% 34% 11%	528
	21st to end	Day	12% 6% 19% 40% 16% 6%	330
October	1st to 10th	Day	5% 5% 29% 39% 19% 3	330
	11th to 20th	Day	12% 11% 32% 28% 13% 3	330
	21st to end	Day	25% 13% 32% 20% 9% 2	363
November	1st to 10th	Day	22% 13% 28% 22% 12% 2	330
	11th to 20th	Day	14% 15% 40% 23% 7% 2	330
	21st to end	Day	26% 16% 27% 23% 7%	330
December	1st to 10th	Day	23% 21% 33% 16% 7% 1	330
	11th to 20th	Day	25% 19% 31% 17% 7% 1	330
	21st to end	Day	27% 17% 37% 16% 3	363
January	1st to 10th	Night	23% 14% 32% 27% 4	1050
	11th to 20th	Night	22% 16% 36% 22% 4	1050
	21st to end	Night	21% 15% 36% 24% 3	1155
February	1st to 10th	Night	26% 15% 32% 24% 4	1050
	11th to 20th	Night	27% 16% 36% 20% 1	890
	21st to end	Night	25% 19% 32% 22% 2	643
March	1st to 10th	Night	22% 11% 36% 27% 5%	630
	11th to 20th	Night	21% 16% 33% 25% 4	630
	21st to end	Night	16% 14% 36% 26% 7%	693
April	1st to 10th	Night	17% 15% 35% 26% 7%	630
	11th to 20th	Night	15% 14% 37% 24% 10%	611
	21st to end	Night	15% 12% 36% 27% 10%	600
May	1st to 10th	Night	15% 6% 28% 41% 10% 1	600
	11th to 20th	Night	17% 8% 27% 30% 16% 2	591
	21st to end	Night	10% 6% 26% 32% 23% 3	650
June	1st to 10th	Night	6% 2 15% 40% 34% 4	600
	11th to 20th	Night	5% 2 16% 41% 34% 3	600
	21st to end	Night	4 2 12% 41% 37% 5%	600
July	1st to 10th	Night	4 1 11% 32% 43% 7%	585
	11th to 20th	Night	3 1 8% 36% 45% 8%	582
	21st to end	Night	5% 1 8% 37% 41% 9%	657
August	1st to 10th	Night	4 1 9% 42% 38% 6%	597
	11th to 20th	Night	6% 1 9% 39% 40% 7%	594
	21st to end	Night	8% 2 8% 33% 42% 6%	660
September	1st to 10th	Night	7% 2 11% 36% 41% 5%	600
	11th to 20th	Night	12% 3 11% 33% 37% 4	600
	21st to end	Night	9% 3 16% 33% 35% 4	695
October	1st to 10th	Night	9% 3 18% 36% 30% 4	800
	11th to 20th	Night	11% 6% 28% 36% 18% 1	800
	21st to end	Night	18% 10% 27% 29% 15% 1	880
November	1st to 10th	Night	14% 9% 25% 35% 16% 1	820
	11th to 20th	Night	12% 10% 31% 35% 11%	840
	21st to end	Night	18% 13% 30% 27% 12%	882
December	1st to 10th	Night	21% 11% 34% 29% 6%	1050
	11th to 20th	Night	23% 12% 30% 29% 6%	1050
	21st to end	Night	24% 15% 30% 26% 5%	1155

Page generated at 2026-04-14 01:44:25. Version ac07

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