ELAC-Stanleigh
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ELAC-Stanleigh clear sky chart from 2005-11-06 to 2025-03-12.

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	15% 11% 4 8% 5% 4 4 4 5% 7% 33%	108347
	Night	14% 12% 4 9% 5% 4 4 3 5% 6% 32%	110785
Transparency	Day	49% 6% 12% 20% 11% 1	111877
	Night	51% 6% 12% 20% 10% 1	115418
Seeing	Day	20% 15% 33% 22% 8% 2	15753
	Night	21% 11% 31% 27% 9% 1	25238

By Month

Percentage Cloud Cover NE (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	10% 18% 7% 9% 4 4 4 4 5% 7% 28%	3181
	11th to 20th	Day	9% 16% 7% 12% 8% 7% 6% 4 6% 6% 20%	3589
	21st to end	Day	12% 20% 9% 16% 6% 4 4 3 4 4 18%	4043
February	1st to 10th	Day	10% 14% 6% 13% 7% 6% 6% 5% 5% 5% 24%	1800
	11th to 20th	Day	11% 15% 5% 11% 7% 6% 4 4 5% 6% 26%	1758
	21st to end	Day	10% 16% 7% 11% 6% 6% 5% 4 4 6% 24%	1650
March	1st to 10th	Day	14% 14% 4 8% 4 4 4 4 5% 7% 31%	3306
	11th to 20th	Day	10% 15% 5% 9% 5% 4 5% 4 5% 6% 33%	3491
	21st to end	Day	13% 11% 9% 10% 5% 4 4 4 5% 6% 30%	3815
April	1st to 10th	Day	17% 13% 4 9% 5% 5% 5% 4 5% 6% 29%	2260
	11th to 20th	Day	20% 10% 3 7% 5% 4 4 3 4 7% 33%	2442
	21st to end	Day	19% 11% 3 6% 5% 3 4 3 5% 7% 35%	2470
May	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 38%	4108
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 41%	4234
	21st to end	Day	15% 7% 2 6% 4 3 3 3 6% 7% 44%	5153
June	1st to 10th	Day	16% 6% 2 7% 4 4 4 4 6% 8% 38%	2850
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 5% 8% 38%	2850
	21st to end	Day	18% 7% 2 5% 5% 5% 5% 4 5% 7% 36%	2850
July	1st to 10th	Day	16% 8% 3 6% 4 4 5% 4 6% 8% 35%	4760
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 6% 8% 44%	4673
	21st to end	Day	12% 5% 2 5% 4 3 4 4 5% 8% 49%	4792
August	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 4 7% 9% 40%	4104
	11th to 20th	Day	15% 5% 2 7% 4 5% 4 4 6% 7% 41%	4044
	21st to end	Day	15% 5% 2 6% 4 4 4 4 5% 7% 43%	4532
September	1st to 10th	Day	14% 7% 3 8% 4 4 4 4 5% 8% 40%	2252
	11th to 20th	Day	15% 5% 2 7% 4 4 5% 3 5% 7% 43%	2090
	21st to end	Day	20% 7% 3 7% 4 4 4 2 4 7% 37%	2090
October	1st to 10th	Day	21% 7% 3 6% 4 4 4 4 5% 7% 35%	1976
	11th to 20th	Day	22% 13% 3 8% 5% 5% 5% 4 5% 6% 24%	1900
	21st to end	Day	23% 14% 4 8% 5% 4 4 4 5% 7% 22%	1891
November	1st to 10th	Day	21% 11% 3 9% 5% 4 5% 3 5% 7% 26%	1742
	11th to 20th	Day	17% 17% 5% 11% 5% 4 4 3 5% 6% 24%	1600
	21st to end	Day	19% 24% 6% 9% 5% 3 3 4 4 4 19%	1600
December	1st to 10th	Day	12% 21% 6% 12% 5% 5% 5% 4 5% 5% 20%	2354
	11th to 20th	Day	14% 18% 8% 14% 6% 4 4 4 4 5% 18%	2621
	21st to end	Day	26% 22% 7% 10% 3 3 3 2 3 3 18%	3476
January	1st to 10th	Night	10% 12% 6% 11% 5% 4 5% 4 6% 7% 30%	6783
	11th to 20th	Night	10% 16% 7% 14% 7% 4 4 4 5% 6% 23%	6316
	21st to end	Night	10% 16% 9% 15% 8% 5% 4 3 4 5% 20%	6929
February	1st to 10th	Night	10% 15% 5% 12% 8% 6% 5% 4 5% 5% 25%	2600
	11th to 20th	Night	10% 14% 4 11% 7% 6% 4 4 6% 6% 27%	2524
	21st to end	Night	11% 16% 6% 10% 5% 6% 6% 3 4 7% 27%	1980
March	1st to 10th	Night	16% 10% 3 8% 4 4 4 3 4 7% 37%	3754
	11th to 20th	Night	9% 11% 5% 10% 5% 4 4 3 4 6% 38%	3497
	21st to end	Night	13% 10% 6% 9% 5% 4 4 3 4 6% 37%	3817
April	1st to 10th	Night	19% 11% 2 7% 4 4 5% 4 5% 7% 33%	1935
	11th to 20th	Night	20% 7% 2 6% 5% 4 4 3 4 5% 39%	1687
	21st to end	Night	19% 9% 3 7% 5% 5% 3 3 5% 5% 36%	1710
May	1st to 10th	Night	17% 7% 2 7% 4 5% 4 4 5% 6% 39%	2833
	11th to 20th	Night	11% 9% 3 7% 6% 5% 4 4 7% 8% 37%	2659
	21st to end	Night	14% 8% 3 5% 3 4 4 3 6% 7% 42%	2418
June	1st to 10th	Night	13% 7% 3 6% 4 4 4 4 5% 8% 41%	1330
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 5% 5% 8% 40%	1330
	21st to end	Night	14% 7% 3 7% 5% 4 5% 4 5% 6% 41%	1330
July	1st to 10th	Night	15% 7% 2 5% 4 4 4 4 6% 8% 42%	2210
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 4 7% 52%	2295
	21st to end	Night	9% 5% 2 5% 3 4 4 4 5% 7% 51%	2770
August	1st to 10th	Night	8% 8% 3 7% 4 5% 4 5% 7% 8% 42%	2825
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 6% 8% 45%	2785
	21st to end	Night	11% 5% 2 5% 4 4 3 3 4 6% 52%	3124
September	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 4 7% 44%	1906
	11th to 20th	Night	13% 4 1 4 3 4 3 3 6% 7% 51%	2090
	21st to end	Night	18% 7% 2 7% 4 4 4 3 4 7% 40%	2090
October	1st to 10th	Night	21% 7% 2 6% 4 5% 3 3 4 5% 38%	2204
	11th to 20th	Night	22% 10% 2 8% 5% 4 5% 3 5% 6% 29%	2280
	21st to end	Night	25% 11% 3 6% 5% 4 4 3 5% 6% 28%	2703
November	1st to 10th	Night	18% 11% 4 9% 4 4 3 4 6% 7% 32%	2554
	11th to 20th	Night	19% 13% 4 9% 6% 5% 5% 4 5% 5% 25%	2800
	21st to end	Night	19% 19% 5% 9% 5% 4 4 4 5% 5% 22%	2800
December	1st to 10th	Night	14% 18% 6% 10% 6% 5% 5% 4 5% 7% 21%	4829
	11th to 20th	Night	14% 18% 6% 12% 5% 4 4 3 4 5% 24%	5586
	21st to end	Night	23% 22% 7% 12% 4 2 3 2 3 4 18%	7502

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	56% 5% 11% 18% 7% 3	3182
	11th to 20th	Day	63% 6% 10% 10% 10% 1	3620
	21st to end	Day	70% 5% 4 10% 11% 1	4043
February	1st to 10th	Day	59% 7% 12% 15% 7% 1	1800
	11th to 20th	Day	56% 8% 10% 15% 10% 1	1767
	21st to end	Day	60% 7% 8% 15% 10% 1	1650
March	1st to 10th	Day	50% 7% 15% 11% 14% 2	3309
	11th to 20th	Day	50% 6% 11% 22% 9% 2	3491
	21st to end	Day	51% 9% 11% 14% 11% 4	3815
April	1st to 10th	Day	55% 6% 10% 16% 11% 2	2260
	11th to 20th	Day	50% 6% 10% 21% 11% 2	2442
	21st to end	Day	48% 6% 9% 18% 15% 4	2470
May	1st to 10th	Day	42% 8% 11% 23% 15% 2	4120
	11th to 20th	Day	40% 7% 13% 23% 15% 2	4238
	21st to end	Day	38% 7% 13% 18% 18% 6%	5153
June	1st to 10th	Day	42% 7% 12% 23% 14% 2	2850
	11th to 20th	Day	43% 6% 11% 22% 16% 1	2850
	21st to end	Day	46% 7% 11% 21% 13% 2	2850
July	1st to 10th	Day	45% 7% 15% 22% 11% 1	4686
	11th to 20th	Day	36% 6% 15% 32% 10% 1	4592
	21st to end	Day	33% 6% 11% 33% 17%	4640
August	1st to 10th	Day	39% 7% 13% 29% 11% 1	4091
	11th to 20th	Day	41% 6% 17% 25% 10%	4083
	21st to end	Day	40% 7% 14% 29% 10% 1	4531
September	1st to 10th	Day	42% 7% 14% 24% 12% 1	2252
	11th to 20th	Day	40% 6% 16% 27% 11%	2090
	21st to end	Day	48% 5% 15% 23% 9%	2090
October	1st to 10th	Day	38% 5% 13% 26% 18%	3288
	11th to 20th	Day	47% 7% 12% 27% 7%	3140
	21st to end	Day	61% 6% 14% 12% 7% 1	3091
November	1st to 10th	Day	57% 7% 13% 17% 6%	1742
	11th to 20th	Day	60% 7% 12% 14% 6%	1600
	21st to end	Day	69% 5% 11% 10% 5%	1600
December	1st to 10th	Day	65% 6% 12% 13% 4	2354
	11th to 20th	Day	67% 5% 8% 13% 7% 1	2621
	21st to end	Day	72% 3 6% 11% 7%	3476
January	1st to 10th	Night	52% 7% 12% 19% 8% 2	6783
	11th to 20th	Night	62% 5% 10% 14% 9%	6360
	21st to end	Night	67% 5% 6% 11% 10% 1	6929
February	1st to 10th	Night	60% 6% 10% 16% 8% 1	2600
	11th to 20th	Night	56% 7% 12% 17% 7% 1	2526
	21st to end	Night	56% 7% 11% 17% 9% 1	1980
March	1st to 10th	Night	46% 6% 12% 19% 15% 1	3764
	11th to 20th	Night	44% 8% 14% 22% 10% 2	3498
	21st to end	Night	47% 6% 13% 18% 14% 2	3817
April	1st to 10th	Night	50% 7% 10% 20% 13% 1	1935
	11th to 20th	Night	46% 5% 7% 25% 15% 2	1687
	21st to end	Night	50% 5% 9% 19% 14% 4	1710
May	1st to 10th	Night	44% 7% 9% 26% 13% 2	2840
	11th to 20th	Night	41% 7% 10% 26% 14% 1	2660
	21st to end	Night	39% 9% 12% 21% 13% 6%	2419
June	1st to 10th	Night	39% 7% 15% 23% 15% 2	1330
	11th to 20th	Night	39% 7% 13% 22% 17% 2	1330
	21st to end	Night	42% 8% 14% 22% 13% 2	1330
July	1st to 10th	Night	40% 6% 14% 25% 14% 2	2175
	11th to 20th	Night	32% 5% 15% 34% 14%	2253
	21st to end	Night	33% 6% 12% 30% 20%	2682
August	1st to 10th	Night	38% 6% 15% 28% 12% 1	2814
	11th to 20th	Night	36% 9% 19% 26% 10% 1	2812
	21st to end	Night	33% 5% 14% 32% 16% 1	3124
September	1st to 10th	Night	38% 7% 15% 25% 12% 2	1906
	11th to 20th	Night	32% 7% 19% 32% 10%	2090
	21st to end	Night	45% 7% 15% 25% 9%	2090
October	1st to 10th	Night	38% 5% 11% 31% 15%	3672
	11th to 20th	Night	42% 6% 15% 30% 8%	3779
	21st to end	Night	52% 6% 19% 16% 7%	4453
November	1st to 10th	Night	51% 7% 14% 20% 8%	2554
	11th to 20th	Night	60% 7% 12% 14% 7%	2800
	21st to end	Night	64% 7% 10% 12% 6% 1	2800
December	1st to 10th	Night	62% 5% 14% 15% 4	4829
	11th to 20th	Night	62% 5% 9% 13% 10% 1	5585
	21st to end	Night	72% 4 6% 12% 7%	7502

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	23% 19% 32% 20% 6%	205
	11th to 20th	Day	19% 15% 37% 23% 6% 1	300
	21st to end	Day	22% 16% 36% 19% 7% 1	330
February	1st to 10th	Day	21% 28% 34% 12% 5%	300
	11th to 20th	Day	20% 24% 40% 12% 4	300
	21st to end	Day	25% 27% 39% 9%	249
March	1st to 10th	Day	21% 23% 41% 12% 3 1	350
	11th to 20th	Day	20% 17% 38% 19% 5% 1	464
	21st to end	Day	25% 17% 34% 18% 5% 1	506
April	1st to 10th	Day	21% 16% 36% 21% 6% 1	460
	11th to 20th	Day	30% 19% 23% 17% 8% 2	452
	21st to end	Day	29% 14% 30% 19% 7% 2	460
May	1st to 10th	Day	21% 15% 35% 20% 6% 3	463
	11th to 20th	Day	17% 15% 32% 22% 10% 3	487
	21st to end	Day	17% 14% 36% 23% 8% 2	715
June	1st to 10th	Day	17% 15% 38% 23% 6% 1	680
	11th to 20th	Day	19% 14% 36% 22% 8% 1	680
	21st to end	Day	21% 14% 33% 22% 8% 3	680
July	1st to 10th	Day	11% 11% 38% 29% 8% 2	671
	11th to 20th	Day	13% 9% 34% 29% 10% 4	668
	21st to end	Day	15% 8% 33% 28% 11% 5%	729
August	1st to 10th	Day	14% 9% 27% 29% 16% 5%	486
	11th to 20th	Day	15% 6% 25% 28% 16% 10%	479
	21st to end	Day	17% 8% 28% 28% 13% 6%	539
September	1st to 10th	Day	17% 12% 27% 26% 12% 5%	490
	11th to 20th	Day	17% 8% 26% 32% 12% 5%	490
	21st to end	Day	23% 12% 27% 24% 12% 3	490
October	1st to 10th	Day	27% 15% 25% 20% 10% 2	490
	11th to 20th	Day	26% 17% 34% 18% 4 1	490
	21st to end	Day	29% 21% 34% 15% 1	340
November	1st to 10th	Day	29% 20% 33% 16% 3	290
	11th to 20th	Day	27% 21% 34% 15% 4	200
	21st to end	Day	29% 23% 31% 16% 3	200
December	1st to 10th	Day	23% 16% 33% 21% 8% 1	200
	11th to 20th	Day	29% 23% 33% 13% 3	200
	21st to end	Day	32% 17% 35% 11% 4	220
January	1st to 10th	Night	23% 12% 36% 23% 6%	1000
	11th to 20th	Night	23% 13% 32% 25% 7% 1	1000
	21st to end	Night	21% 10% 33% 26% 10%	1072
February	1st to 10th	Night	18% 14% 42% 22% 4	800
	11th to 20th	Night	19% 15% 36% 23% 7%	783
	21st to end	Night	18% 16% 37% 26% 4 1	660
March	1st to 10th	Night	17% 14% 39% 25% 4 1	800
	11th to 20th	Night	19% 13% 38% 23% 5% 1	767
	21st to end	Night	19% 12% 36% 26% 6% 1	836
April	1st to 10th	Night	23% 12% 33% 26% 5%	652
	11th to 20th	Night	25% 9% 31% 25% 9% 1	563
	21st to end	Night	21% 13% 29% 28% 8% 1	570
May	1st to 10th	Night	17% 9% 30% 34% 10% 1	570
	11th to 20th	Night	18% 8% 29% 35% 9% 1	473
	21st to end	Night	16% 8% 34% 32% 11%	411
June	1st to 10th	Night	13% 7% 42% 31% 7%	380
	11th to 20th	Night	15% 7% 32% 37% 9%	380
	21st to end	Night	15% 7% 26% 39% 12% 1	380
July	1st to 10th	Night	11% 7% 28% 45% 9%	370
	11th to 20th	Night	12% 5% 26% 40% 17% 1	414
	21st to end	Night	14% 4 25% 39% 17% 2	624
August	1st to 10th	Night	12% 2 27% 39% 19% 1	567
	11th to 20th	Night	14% 3 21% 33% 26% 2	564
	21st to end	Night	17% 6% 22% 37% 17% 1	627
September	1st to 10th	Night	18% 7% 26% 28% 19% 3	674
	11th to 20th	Night	15% 6% 27% 30% 19% 3	760
	21st to end	Night	21% 7% 25% 31% 14% 2	760
October	1st to 10th	Night	24% 8% 30% 24% 11% 2	760
	11th to 20th	Night	29% 12% 28% 24% 7%	760
	21st to end	Night	30% 14% 28% 23% 4 1	836
November	1st to 10th	Night	24% 13% 31% 25% 7% 1	780
	11th to 20th	Night	26% 12% 33% 24% 5%	800
	21st to end	Night	30% 15% 31% 19% 5% 1	800
December	1st to 10th	Night	24% 15% 33% 21% 6%	945
	11th to 20th	Night	27% 16% 33% 19% 5% 1	1000
	21st to end	Night	31% 14% 31% 18% 6%	1100

Page generated at 2025-03-09 08:08:45. Version ac07

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