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 2024-12-24.

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%	100905
	Night	13% 6% 2 5% 3 3 3 3 5% 6% 50%	99231
Transparency	Day	37% 6% 10% 23% 19% 5%	104549
	Night	33% 5% 9% 22% 23% 7%	103727
Seeing	Day	15% 10% 24% 27% 19% 5%	15484
	Night	15% 9% 26% 30% 18% 2	25339

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% 8% 2 7% 3 3 3 2 3 5% 44%	2847
	11th to 20th	Day	15% 10% 2 5% 3 3 3 3 3 4 49%	2850
	21st to end	Day	25% 6% 2 4 3 2 2 2 2 4 46%	3135
February	1st to 10th	Day	21% 9% 2 6% 3 3 4 3 4 4 41%	1710
	11th to 20th	Day	18% 10% 2 5% 4 4 3 3 4 6% 41%	1787
	21st to end	Day	24% 7% 3 6% 4 3 4 3 4 5% 37%	1631
March	1st to 10th	Day	22% 9% 3 7% 4 3 3 3 4 4 38%	3436
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 5% 6% 44%	3480
	21st to end	Day	17% 8% 3 7% 4 4 3 2 4 5% 42%	3814
April	1st to 10th	Day	14% 6% 2 6% 4 4 3 3 6% 7% 45%	2270
	11th to 20th	Day	17% 6% 2 6% 5% 3 4 3 5% 6% 44%	2254
	21st to end	Day	16% 7% 2 6% 4 5% 4 3 4 6% 42%	2280
May	1st to 10th	Day	18% 7% 2 7% 4 4 4 3 5% 7% 37%	3765
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 6% 7% 43%	4040
	21st to end	Day	13% 7% 3 8% 6% 6% 6% 5% 6% 8% 33%	4477
June	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 5% 8% 8% 43%	2512
	11th to 20th	Day	6% 4 1 4 5% 6% 6% 5% 7% 10% 46%	2576
	21st to end	Day	5% 3 2 5% 5% 5% 6% 4 8% 10% 47%	2470
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	4120
	11th to 20th	Day	6% 4 1 4 4 4 4 3 6% 8% 54%	4120
	21st to end	Day	13% 8% 3 8% 6% 5% 5% 4 7% 8% 33%	4465
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 6% 9% 59%	4104
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 4 6% 9% 43%	3743
	21st to end	Day	6% 3 2 5% 4 5% 5% 4 6% 9% 51%	4180
September	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 46%	2280
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 3 6% 7% 45%	2278
	21st to end	Day	10% 4 2 5% 4 4 4 3 4 6% 54%	1981
October	1st to 10th	Day	7% 4 2 5% 3 3 4 3 4 5% 59%	1900
	11th to 20th	Day	14% 4 2 4 2 3 3 2 4 6% 57%	1900
	21st to end	Day	18% 4 2 4 3 2 3 2 3 5% 53%	2090
November	1st to 10th	Day	15% 5% 2 3 3 2 2 3 4 6% 54%	1958
	11th to 20th	Day	11% 6% 1 4 4 3 3 3 4 6% 55%	1960
	21st to end	Day	19% 8% 2 6% 3 3 3 2 3 5% 46%	1756
December	1st to 10th	Day	19% 8% 2 5% 4 3 4 2 4 5% 46%	1600
	11th to 20th	Day	16% 8% 3 5% 5% 3 4 4 4 4 44%	1998
	21st to end	Day	15% 8% 3 7% 4 5% 5% 4 5% 5% 38%	3138
January	1st to 10th	Night	17% 7% 3 6% 4 3 3 3 4 4 46%	4108
	11th to 20th	Night	19% 6% 1 4 3 2 2 2 3 4 53%	4120
	21st to end	Night	25% 7% 2 3 2 2 2 2 3 4 47%	4532
February	1st to 10th	Night	22% 8% 2 5% 4 3 3 2 3 6% 42%	2470
	11th to 20th	Night	20% 8% 2 5% 3 2 3 3 3 5% 46%	2279
	21st to end	Night	21% 7% 2 5% 3 3 3 3 4 5% 45%	1840
March	1st to 10th	Night	20% 7% 3 5% 3 3 3 4 5% 5% 41%	3419
	11th to 20th	Night	16% 7% 3 5% 3 3 3 2 4 5% 50%	3480
	21st to end	Night	12% 7% 2 7% 5% 5% 4 3 5% 7% 43%	3817
April	1st to 10th	Night	12% 5% 2 5% 3 4 3 3 6% 6% 50%	1908
	11th to 20th	Night	14% 4 2 5% 4 3 3 3 5% 7% 50%	1875
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 6% 49%	1900
May	1st to 10th	Night	18% 7% 2 5% 5% 4 4 4 5% 7% 38%	3130
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 5% 7% 45%	2860
	21st to end	Night	14% 6% 3 7% 5% 5% 4 3 5% 8% 42%	3090
June	1st to 10th	Night	6% 4 2 4 3 4 5% 4 7% 11% 52%	1645
	11th to 20th	Night	4 4 1 4 4 3 3 4 5% 9% 59%	1580
	21st to end	Night	4 3 1 4 3 3 4 4 6% 9% 60%	1710
July	1st to 10th	Night	7% 3 2 5% 5% 5% 4 4 7% 10% 49%	2850
	11th to 20th	Night	4 3 1 3 3 4 4 3 6% 12% 57%	2850
	21st to end	Night	10% 7% 3 7% 4 5% 5% 4 6% 8% 42%	3098
August	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 9% 69%	2836
	11th to 20th	Night	5% 2 1 4 3 4 4 5% 7% 10% 55%	3083
	21st to end	Night	4 3 1 5% 4 4 5% 4 6% 8% 57%	3476
September	1st to 10th	Night	6% 4 2 5% 3 4 4 4 6% 6% 56%	1900
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 5% 6% 52%	1900
	21st to end	Night	9% 4 2 4 2 4 3 3 5% 6% 59%	2177
October	1st to 10th	Night	8% 4 2 4 3 2 3 3 5% 6% 62%	2280
	11th to 20th	Night	10% 4 2 5% 3 3 3 2 3 5% 61%	2280
	21st to end	Night	16% 6% 2 5% 3 2 2 2 3 3 56%	2508
November	1st to 10th	Night	13% 5% 2 4 2 3 3 3 4 5% 56%	2340
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 4 5% 58%	2435
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 52%	2640
December	1st to 10th	Night	18% 6% 2 6% 3 3 3 3 4 5% 47%	2800
	11th to 20th	Night	17% 6% 2 6% 4 3 3 2 4 4 48%	3419
	21st to end	Night	15% 7% 2 6% 4 4 4 3 4 5% 45%	4596

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	45% 2 5% 23% 21% 4	2848
	11th to 20th	Day	41% 3 10% 22% 21% 3	2850
	21st to end	Day	44% 3 5% 18% 24% 7%	3135
February	1st to 10th	Day	47% 4 8% 20% 17% 4	1710
	11th to 20th	Day	45% 4 7% 19% 19% 6%	1796
	21st to end	Day	51% 3 6% 13% 17% 8%	1631
March	1st to 10th	Day	50% 5% 6% 12% 18% 10%	3436
	11th to 20th	Day	41% 5% 8% 23% 18% 5%	3480
	21st to end	Day	45% 5% 7% 21% 21% 1	3814
April	1st to 10th	Day	38% 5% 9% 22% 20% 5%	2270
	11th to 20th	Day	40% 5% 8% 21% 18% 7%	2254
	21st to end	Day	43% 6% 9% 19% 20% 2	2280
May	1st to 10th	Day	46% 7% 10% 21% 14% 2	3800
	11th to 20th	Day	39% 8% 11% 25% 16% 1	4044
	21st to end	Day	45% 9% 12% 21% 11% 2	4489
June	1st to 10th	Day	36% 8% 13% 25% 16% 2	2512
	11th to 20th	Day	31% 8% 13% 28% 18% 2	2576
	21st to end	Day	28% 9% 13% 27% 19% 4	2470
July	1st to 10th	Day	34% 8% 16% 29% 11% 1	4056
	11th to 20th	Day	26% 6% 10% 34% 22% 3	4051
	21st to end	Day	47% 8% 14% 22% 8%	4325
August	1st to 10th	Day	21% 6% 15% 28% 24% 5%	4090
	11th to 20th	Day	37% 7% 13% 32% 10% 1	3779
	21st to end	Day	29% 7% 13% 29% 20% 3	4179
September	1st to 10th	Day	37% 7% 10% 20% 18% 8%	2280
	11th to 20th	Day	37% 6% 11% 17% 23% 6%	2278
	21st to end	Day	32% 5% 8% 20% 26% 8%	1981
October	1st to 10th	Day	16% 3 6% 12% 33% 30%	3160
	11th to 20th	Day	19% 2 4 20% 36% 20%	3140
	21st to end	Day	31% 3 4 28% 27% 6%	3428
November	1st to 10th	Day	32% 4 9% 24% 23% 8%	1958
	11th to 20th	Day	31% 4 7% 20% 27% 11%	1960
	21st to end	Day	43% 3 6% 18% 24% 6%	1756
December	1st to 10th	Day	42% 4 7% 24% 17% 6%	1600
	11th to 20th	Day	44% 3 5% 20% 22% 5%	1995
	21st to end	Day	46% 4 9% 18% 17% 5%	3138
January	1st to 10th	Night	42% 3 6% 18% 27% 3	4108
	11th to 20th	Night	37% 4 8% 21% 25% 5%	4120
	21st to end	Night	43% 3 9% 18% 23% 4	4532
February	1st to 10th	Night	46% 3 8% 19% 18% 6%	2470
	11th to 20th	Night	43% 4 6% 20% 21% 7%	2280
	21st to end	Night	42% 4 7% 16% 21% 9%	1840
March	1st to 10th	Night	44% 5% 9% 12% 19% 11%	3419
	11th to 20th	Night	39% 4 7% 25% 18% 7%	3480
	21st to end	Night	40% 5% 9% 21% 23% 2	3817
April	1st to 10th	Night	34% 5% 10% 22% 20% 9%	1908
	11th to 20th	Night	34% 5% 8% 22% 21% 10%	1875
	21st to end	Night	36% 5% 10% 23% 22% 4	1900
May	1st to 10th	Night	45% 7% 10% 21% 13% 3	3160
	11th to 20th	Night	37% 6% 12% 28% 15% 1	2861
	21st to end	Night	40% 7% 14% 23% 14% 2	3099
June	1st to 10th	Night	25% 9% 15% 30% 20% 2	1645
	11th to 20th	Night	23% 5% 12% 30% 25% 4	1580
	21st to end	Night	20% 7% 13% 31% 24% 5%	1710
July	1st to 10th	Night	29% 8% 18% 28% 15% 1	2805
	11th to 20th	Night	20% 5% 13% 35% 23% 3	2796
	21st to end	Night	39% 8% 13% 26% 14%	2999
August	1st to 10th	Night	13% 5% 16% 32% 27% 6%	2826
	11th to 20th	Night	23% 7% 19% 35% 15% 2	3113
	21st to end	Night	24% 7% 11% 29% 27% 3	3476
September	1st to 10th	Night	26% 7% 12% 23% 22% 9%	1900
	11th to 20th	Night	32% 6% 10% 21% 25% 7%	1900
	21st to end	Night	25% 5% 9% 22% 28% 10%	2177
October	1st to 10th	Night	16% 3 6% 14% 28% 33%	3800
	11th to 20th	Night	17% 2 4 16% 37% 25%	3779
	21st to end	Night	28% 3 4 23% 29% 13%	4120
November	1st to 10th	Night	32% 3 7% 21% 27% 10%	2340
	11th to 20th	Night	29% 4 6% 22% 26% 12%	2435
	21st to end	Night	36% 4 7% 18% 28% 7%	2640
December	1st to 10th	Night	41% 4 8% 21% 20% 7%	2800
	11th to 20th	Night	41% 3 5% 19% 25% 7%	3421
	21st to end	Night	42% 4 8% 21% 21% 4	4596

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% 19% 34% 16% 5% 1	270
	11th to 20th	Day	30% 23% 31% 14% 1	270
	21st to end	Day	23% 25% 33% 14% 5%	297
February	1st to 10th	Day	28% 17% 36% 14% 4 1	270
	11th to 20th	Day	30% 27% 28% 11% 3	270
	21st to end	Day	39% 22% 28% 9% 1	225
March	1st to 10th	Day	28% 19% 34% 14% 4	270
	11th to 20th	Day	27% 24% 34% 14% 1	270
	21st to end	Day	25% 28% 32% 11% 3	297
April	1st to 10th	Day	21% 19% 36% 16% 7%	450
	11th to 20th	Day	25% 16% 33% 19% 7%	452
	21st to end	Day	22% 18% 30% 20% 8% 2	460
May	1st to 10th	Day	17% 17% 33% 22% 9% 2	463
	11th to 20th	Day	16% 7% 31% 29% 15% 2	646
	21st to end	Day	11% 7% 25% 33% 18% 5%	734
June	1st to 10th	Day	9% 5% 20% 33% 26% 7%	680
	11th to 20th	Day	6% 3 20% 38% 26% 7%	680
	21st to end	Day	5% 2 23% 34% 27% 8%	680
July	1st to 10th	Day	5% 3 15% 34% 33% 10%	671
	11th to 20th	Day	6% 2 12% 35% 34% 10%	668
	21st to end	Day	9% 1 10% 34% 34% 12%	739
August	1st to 10th	Day	6% 2 13% 38% 33% 9%	676
	11th to 20th	Day	12% 2 12% 35% 30% 10%	493
	21st to end	Day	12% 1 7% 24% 38% 17%	539
September	1st to 10th	Day	10% 2 12% 30% 37% 9%	490
	11th to 20th	Day	14% 2 15% 26% 32% 10%	488
	21st to end	Day	12% 7% 18% 42% 15% 6%	300
October	1st to 10th	Day	5% 5% 30% 37% 19% 3	300
	11th to 20th	Day	14% 11% 33% 28% 11% 2	300
	21st to end	Day	24% 13% 33% 20% 8% 2	330
November	1st to 10th	Day	23% 13% 28% 21% 13% 2	300
	11th to 20th	Day	14% 15% 40% 23% 7% 1	300
	21st to end	Day	26% 17% 26% 23% 8%	300
December	1st to 10th	Day	23% 21% 32% 17% 7% 1	300
	11th to 20th	Day	25% 21% 29% 17% 7% 1	300
	21st to end	Day	24% 17% 40% 14% 4 1	306
January	1st to 10th	Night	22% 15% 32% 26% 4	950
	11th to 20th	Night	24% 16% 33% 22% 4	950
	21st to end	Night	21% 16% 36% 25% 3	1045
February	1st to 10th	Night	26% 15% 31% 24% 4	950
	11th to 20th	Night	26% 15% 36% 22% 1	804
	21st to end	Night	27% 19% 32% 21% 3	584
March	1st to 10th	Night	21% 10% 35% 28% 5%	570
	11th to 20th	Night	22% 15% 33% 25% 5%	570
	21st to end	Night	17% 14% 36% 26% 7%	627
April	1st to 10th	Night	16% 15% 36% 25% 7% 1	570
	11th to 20th	Night	15% 13% 37% 24% 11%	563
	21st to end	Night	14% 13% 37% 26% 10%	570
May	1st to 10th	Night	15% 6% 28% 41% 10% 1	570
	11th to 20th	Night	16% 7% 27% 32% 17% 2	561
	21st to end	Night	9% 5% 26% 32% 24% 4	617
June	1st to 10th	Night	6% 2 14% 39% 34% 4	570
	11th to 20th	Night	4 1 15% 42% 35% 3	570
	21st to end	Night	4 2 12% 41% 36% 5%	570
July	1st to 10th	Night	4 1 12% 33% 43% 7%	555
	11th to 20th	Night	3 8% 36% 45% 8%	552
	21st to end	Night	5% 1 8% 36% 41% 9%	624
August	1st to 10th	Night	4 1 9% 40% 40% 7%	567
	11th to 20th	Night	6% 1 9% 39% 39% 6%	564
	21st to end	Night	7% 2 8% 33% 43% 7%	627
September	1st to 10th	Night	7% 2 10% 35% 42% 5%	570
	11th to 20th	Night	12% 3 12% 34% 36% 4	570
	21st to end	Night	9% 3 17% 32% 35% 4	660
October	1st to 10th	Night	9% 3 19% 34% 30% 4	760
	11th to 20th	Night	12% 6% 29% 36% 17% 1	760
	21st to end	Night	18% 10% 27% 29% 15% 1	836
November	1st to 10th	Night	15% 9% 25% 35% 16% 1	780
	11th to 20th	Night	13% 11% 30% 35% 11%	800
	21st to end	Night	18% 13% 30% 26% 12%	840
December	1st to 10th	Night	22% 11% 33% 28% 6%	1000
	11th to 20th	Night	23% 12% 29% 29% 7%	1000
	21st to end	Night	23% 14% 31% 26% 6%	1063

Page generated at 2024-12-21 03:13:49. Version ac07

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