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 2024-02-10.

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	16% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	77863
	Night	16% 12% 5% 8% 6% 3 4 5% 5% 6% 32%	81135
Transparency	Day	51% 7% 12% 19% 10% 1	77560
	Night	52% 7% 12% 18% 9% 1	80973
Seeing	Day	20% 14% 32% 22% 8% 2	14325
	Night	21% 11% 31% 27% 9% 1	23812

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	14% 20% 9% 10% 5% 4 3 4 4 6% 21%	2217
	11th to 20th	Day	10% 14% 6% 12% 11% 5% 6% 6% 5% 6% 18%	2530
	21st to end	Day	14% 22% 11% 14% 7% 3 5% 4 4 3 13%	2818
February	1st to 10th	Day	10% 13% 6% 12% 8% 4 6% 6% 5% 5% 24%	1701
	11th to 20th	Day	12% 14% 5% 10% 7% 4 4 5% 5% 6% 27%	1577
	21st to end	Day	11% 15% 8% 10% 8% 5% 6% 5% 4 5% 23%	1480
March	1st to 10th	Day	13% 11% 5% 8% 6% 3 4 4 4 6% 36%	1841
	11th to 20th	Day	12% 11% 5% 8% 5% 2 4 4 4 5% 38%	1980
	21st to end	Day	16% 10% 3 6% 6% 3 5% 5% 5% 6% 35%	2178
April	1st to 10th	Day	18% 11% 5% 8% 6% 4 5% 4 5% 6% 28%	2140
	11th to 20th	Day	20% 9% 4 6% 5% 3 4 4 4 7% 34%	2312
	21st to end	Day	19% 11% 3 6% 5% 3 4 4 5% 7% 35%	2340
May	1st to 10th	Day	17% 8% 3 6% 5% 3 4 5% 6% 7% 35%	2340
	11th to 20th	Day	16% 7% 3 6% 5% 3 5% 5% 5% 7% 38%	2384
	21st to end	Day	14% 6% 2 6% 5% 3 4 5% 6% 9% 41%	2901
June	1st to 10th	Day	16% 5% 3 6% 5% 3 5% 5% 6% 8% 39%	2700
	11th to 20th	Day	15% 7% 2 6% 6% 3 5% 5% 5% 8% 38%	2700
	21st to end	Day	17% 7% 3 5% 5% 4 6% 5% 5% 7% 37%	2700
July	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 6% 6% 9% 44%	2700
	11th to 20th	Day	13% 5% 3 6% 6% 4 5% 4 6% 9% 41%	2655
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 5% 9% 42%	2734
August	1st to 10th	Day	13% 5% 3 5% 6% 4 6% 5% 6% 9% 39%	2327
	11th to 20th	Day	13% 4 2 6% 6% 5% 4 5% 6% 8% 40%	2303
	21st to end	Day	14% 6% 2 6% 6% 4 5% 5% 5% 8% 39%	2574
September	1st to 10th	Day	14% 7% 3 7% 5% 3 4 5% 5% 7% 40%	2134
	11th to 20th	Day	15% 5% 2 6% 5% 3 5% 5% 5% 7% 41%	1980
	21st to end	Day	20% 6% 4 6% 5% 3 4 3 4 7% 36%	1980
October	1st to 10th	Day	21% 7% 3 5% 5% 3 5% 5% 4 7% 34%	1872
	11th to 20th	Day	24% 12% 4 7% 6% 4 5% 4 5% 6% 23%	1800
	21st to end	Day	23% 13% 4 7% 6% 3 4 5% 4 7% 23%	1792
November	1st to 10th	Day	21% 11% 4 8% 6% 3 5% 4 5% 8% 26%	1653
	11th to 20th	Day	17% 16% 5% 10% 6% 3 4 4 5% 6% 24%	1520
	21st to end	Day	20% 23% 6% 8% 6% 2 4 4 4 4 19%	1520
December	1st to 10th	Day	12% 20% 6% 11% 5% 3 6% 5% 4 5% 23%	1378
	11th to 20th	Day	16% 19% 6% 10% 7% 4 4 4 4 5% 22%	1671
	21st to end	Day	24% 23% 5% 9% 4 3 4 3 3 3 18%	2431
January	1st to 10th	Night	15% 15% 7% 11% 7% 3 4 4 5% 6% 24%	4737
	11th to 20th	Night	10% 14% 7% 13% 10% 4 5% 6% 5% 6% 21%	4440
	21st to end	Night	15% 18% 9% 11% 8% 4 5% 3 4 4 19%	4845
February	1st to 10th	Night	11% 14% 5% 10% 9% 4 6% 4 5% 5% 26%	2464
	11th to 20th	Night	11% 13% 5% 10% 8% 3 5% 5% 5% 6% 28%	2265
	21st to end	Night	12% 15% 7% 9% 6% 3 6% 5% 4 6% 27%	1776
March	1st to 10th	Night	11% 10% 4 8% 6% 3 5% 5% 5% 6% 38%	2106
	11th to 20th	Night	14% 10% 4 6% 5% 3 4 4 4 7% 37%	1979
	21st to end	Night	13% 10% 4 5% 6% 4 4 5% 4 6% 39%	2178
April	1st to 10th	Night	19% 10% 3 6% 5% 3 4 5% 5% 7% 33%	1834
	11th to 20th	Night	19% 6% 2 6% 6% 3 5% 4 4 6% 40%	1597
	21st to end	Night	19% 8% 3 7% 6% 4 4 3 4 6% 36%	1620
May	1st to 10th	Night	14% 7% 3 7% 6% 4 6% 6% 6% 6% 35%	1620
	11th to 20th	Night	15% 7% 3 5% 6% 4 4 5% 6% 7% 38%	1509
	21st to end	Night	12% 6% 3 5% 4 3 4 5% 6% 6% 46%	1373
June	1st to 10th	Night	13% 6% 4 5% 4 3 4 5% 5% 7% 43%	1260
	11th to 20th	Night	12% 5% 2 5% 6% 4 5% 6% 5% 8% 41%	1260
	21st to end	Night	13% 6% 3 6% 5% 3 5% 5% 5% 6% 42%	1260
July	1st to 10th	Night	10% 3 2 5% 4 2 4 5% 6% 8% 51%	1260
	11th to 20th	Night	10% 4 2 5% 6% 3 5% 6% 5% 8% 47%	1303
	21st to end	Night	10% 5% 2 5% 4 3 5% 6% 5% 8% 47%	1583
August	1st to 10th	Night	10% 5% 2 4 4 3 4 5% 6% 8% 48%	1611
	11th to 20th	Night	12% 4 2 4 5% 2 4 5% 7% 7% 48%	1598
	21st to end	Night	14% 5% 3 5% 5% 3 4 4 5% 8% 45%	1782
September	1st to 10th	Night	14% 5% 2 4 5% 3 4 5% 4 8% 44%	1805
	11th to 20th	Night	13% 4 2 3 4 3 4 4 6% 8% 49%	1980
	21st to end	Night	18% 7% 3 6% 5% 3 4 4 4 7% 39%	1980
October	1st to 10th	Night	21% 7% 2 5% 5% 4 3 4 4 5% 38%	2088
	11th to 20th	Night	24% 9% 3 7% 6% 3 5% 4 5% 6% 29%	2160
	21st to end	Night	26% 10% 4 6% 5% 3 4 4 4 6% 28%	2560
November	1st to 10th	Night	17% 11% 4 8% 5% 3 4 4 5% 7% 32%	2423
	11th to 20th	Night	19% 12% 5% 8% 7% 4 5% 5% 5% 5% 26%	2660
	21st to end	Night	20% 18% 5% 8% 5% 3 4 4 5% 6% 23%	2660
December	1st to 10th	Night	15% 18% 6% 8% 6% 3 4 5% 5% 6% 24%	2797
	11th to 20th	Night	17% 16% 6% 8% 6% 3 4 4 4 5% 25%	3526
	21st to end	Night	23% 23% 6% 10% 4 2 3 3 3 5% 18%	5236

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	66% 6% 13% 11% 4	2218
	11th to 20th	Day	64% 8% 12% 10% 5% 1	2530
	21st to end	Day	75% 6% 5% 7% 6% 1	2818
February	1st to 10th	Day	59% 7% 12% 14% 7% 1	1701
	11th to 20th	Day	56% 8% 10% 15% 10% 1	1586
	21st to end	Day	61% 7% 7% 14% 10% 1	1480
March	1st to 10th	Day	48% 7% 13% 14% 15% 3	1844
	11th to 20th	Day	47% 5% 10% 24% 10% 4	1980
	21st to end	Day	48% 7% 10% 19% 11% 4	2178
April	1st to 10th	Day	56% 6% 10% 16% 11% 2	2140
	11th to 20th	Day	50% 5% 10% 22% 11% 2	2312
	21st to end	Day	49% 5% 9% 18% 15% 4	2340
May	1st to 10th	Day	46% 7% 11% 20% 13% 4	2340
	11th to 20th	Day	44% 6% 10% 20% 18% 2	2388
	21st to end	Day	39% 7% 15% 22% 13% 3	2901
June	1st to 10th	Day	41% 7% 12% 23% 14% 2	2700
	11th to 20th	Day	44% 6% 11% 22% 16% 1	2700
	21st to end	Day	45% 8% 11% 21% 12% 2	2700
July	1st to 10th	Day	33% 8% 15% 26% 16% 1	2626
	11th to 20th	Day	39% 8% 12% 26% 14% 1	2574
	21st to end	Day	38% 9% 14% 25% 13% 1	2582
August	1st to 10th	Day	41% 9% 17% 22% 11% 1	2314
	11th to 20th	Day	40% 8% 18% 23% 11% 1	2303
	21st to end	Day	42% 8% 13% 24% 11% 1	2574
September	1st to 10th	Day	43% 7% 14% 22% 12% 1	2134
	11th to 20th	Day	41% 6% 17% 26% 9%	1980
	21st to end	Day	48% 5% 16% 23% 7%	1980
October	1st to 10th	Day	50% 7% 12% 23% 8%	1872
	11th to 20th	Day	60% 9% 10% 16% 5%	1800
	21st to end	Day	61% 8% 12% 11% 7% 1	1792
November	1st to 10th	Day	56% 7% 13% 17% 7%	1653
	11th to 20th	Day	60% 8% 13% 14% 6%	1520
	21st to end	Day	69% 5% 12% 10% 5%	1520
December	1st to 10th	Day	62% 7% 10% 14% 6% 1	1378
	11th to 20th	Day	64% 6% 10% 13% 5% 1	1671
	21st to end	Day	72% 5% 7% 11% 6%	2431
January	1st to 10th	Night	60% 8% 12% 12% 7% 1	4737
	11th to 20th	Night	62% 7% 12% 13% 5%	4440
	21st to end	Night	69% 6% 7% 12% 6%	4845
February	1st to 10th	Night	59% 6% 10% 16% 8% 1	2464
	11th to 20th	Night	56% 7% 13% 16% 7% 1	2267
	21st to end	Night	57% 7% 10% 16% 9% 1	1776
March	1st to 10th	Night	45% 5% 12% 19% 16% 2	2116
	11th to 20th	Night	47% 5% 13% 21% 13% 2	1980
	21st to end	Night	45% 6% 12% 17% 16% 4	2178
April	1st to 10th	Night	50% 6% 9% 20% 14% 1	1834
	11th to 20th	Night	46% 5% 7% 26% 15% 2	1597
	21st to end	Night	50% 4 9% 19% 14% 4	1620
May	1st to 10th	Night	46% 7% 10% 20% 14% 3	1620
	11th to 20th	Night	44% 5% 8% 25% 16% 2	1510
	21st to end	Night	37% 9% 11% 24% 15% 4	1374
June	1st to 10th	Night	39% 6% 15% 24% 15% 2	1260
	11th to 20th	Night	39% 8% 13% 22% 16% 2	1260
	21st to end	Night	41% 8% 14% 22% 13% 2	1260
July	1st to 10th	Night	29% 7% 14% 29% 18% 2	1225
	11th to 20th	Night	34% 7% 12% 30% 16% 1	1261
	21st to end	Night	35% 8% 16% 26% 15%	1495
August	1st to 10th	Night	33% 8% 19% 28% 11% 2	1600
	11th to 20th	Night	33% 9% 18% 28% 10% 1	1598
	21st to end	Night	38% 7% 16% 24% 14% 1	1782
September	1st to 10th	Night	38% 8% 15% 25% 13% 2	1805
	11th to 20th	Night	33% 7% 19% 32% 8%	1980
	21st to end	Night	45% 7% 15% 26% 7%	1980
October	1st to 10th	Night	48% 6% 12% 23% 10%	2088
	11th to 20th	Night	55% 6% 14% 19% 5%	2160
	21st to end	Night	57% 6% 12% 16% 7% 1	2560
November	1st to 10th	Night	51% 7% 14% 20% 8%	2423
	11th to 20th	Night	60% 7% 12% 14% 7%	2660
	21st to end	Night	63% 7% 11% 12% 6% 1	2660
December	1st to 10th	Night	61% 6% 12% 17% 4	2797
	11th to 20th	Night	60% 7% 11% 12% 7% 2	3525
	21st to end	Night	71% 5% 8% 11% 5%	5236

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	24% 21% 31% 18% 6% 1	184
	11th to 20th	Day	19% 15% 36% 23% 6% 1	270
	21st to end	Day	22% 16% 35% 20% 7% 1	297
February	1st to 10th	Day	22% 25% 34% 13% 5%	267
	11th to 20th	Day	20% 24% 39% 12% 5%	240
	21st to end	Day	26% 27% 39% 8%	198
March	1st to 10th	Day	21% 24% 39% 13% 3 1	284
	11th to 20th	Day	20% 17% 37% 20% 5% 1	420
	21st to end	Day	26% 16% 34% 19% 5% 1	462
April	1st to 10th	Day	21% 14% 35% 22% 6% 1	420
	11th to 20th	Day	28% 19% 23% 18% 9% 3	412
	21st to end	Day	28% 15% 30% 19% 7% 2	420
May	1st to 10th	Day	21% 15% 33% 21% 7% 3	423
	11th to 20th	Day	18% 16% 32% 21% 10% 3	447
	21st to end	Day	17% 13% 37% 22% 9% 2	661
June	1st to 10th	Day	17% 14% 38% 24% 6% 1	630
	11th to 20th	Day	19% 14% 36% 22% 8% 2	630
	21st to end	Day	20% 14% 32% 23% 8% 3	630
July	1st to 10th	Day	10% 11% 38% 30% 8% 2	621
	11th to 20th	Day	13% 10% 34% 28% 10% 5%	618
	21st to end	Day	15% 8% 34% 27% 11% 5%	675
August	1st to 10th	Day	15% 9% 24% 30% 16% 5%	446
	11th to 20th	Day	15% 6% 24% 29% 16% 10%	439
	21st to end	Day	17% 8% 28% 28% 13% 6%	495
September	1st to 10th	Day	18% 11% 26% 27% 13% 5%	450
	11th to 20th	Day	18% 9% 27% 31% 11% 4	450
	21st to end	Day	23% 13% 27% 24% 11% 3	450
October	1st to 10th	Day	28% 14% 25% 20% 11% 2	450
	11th to 20th	Day	27% 17% 33% 18% 4 1	450
	21st to end	Day	28% 20% 34% 17% 1	307
November	1st to 10th	Day	28% 20% 33% 17% 3	261
	11th to 20th	Day	25% 22% 35% 15% 3	180
	21st to end	Day	31% 22% 29% 14% 3	180
December	1st to 10th	Day	22% 16% 33% 22% 7% 1	180
	11th to 20th	Day	29% 23% 32% 12% 3	180
	21st to end	Day	32% 19% 37% 9% 3 1	198
January	1st to 10th	Night	24% 13% 35% 23% 5%	950
	11th to 20th	Night	23% 13% 32% 24% 7% 1	950
	21st to end	Night	21% 9% 32% 27% 10%	1019
February	1st to 10th	Night	18% 14% 41% 23% 4	759
	11th to 20th	Night	19% 14% 36% 23% 8%	703
	21st to end	Night	19% 16% 36% 25% 4 1	592
March	1st to 10th	Night	15% 13% 39% 27% 5% 1	720
	11th to 20th	Night	20% 13% 37% 24% 6% 1	720
	21st to end	Night	19% 11% 36% 26% 7% 1	792
April	1st to 10th	Night	23% 11% 33% 26% 5%	618
	11th to 20th	Night	24% 9% 31% 26% 9% 2	533
	21st to end	Night	21% 12% 30% 29% 8% 1	540
May	1st to 10th	Night	16% 9% 30% 35% 10% 1	540
	11th to 20th	Night	19% 8% 29% 34% 9% 1	448
	21st to end	Night	14% 8% 34% 32% 12%	389
June	1st to 10th	Night	13% 8% 41% 31% 8%	360
	11th to 20th	Night	15% 7% 31% 37% 9%	360
	21st to end	Night	14% 7% 25% 40% 13% 1	360
July	1st to 10th	Night	10% 7% 28% 45% 9%	350
	11th to 20th	Night	13% 4 26% 39% 18% 1	391
	21st to end	Night	14% 4 25% 39% 16% 2	591
August	1st to 10th	Night	12% 2 26% 39% 19% 1	537
	11th to 20th	Night	15% 3 20% 34% 26% 2	534
	21st to end	Night	17% 6% 23% 36% 17% 2	594
September	1st to 10th	Night	18% 6% 27% 28% 19% 3	638
	11th to 20th	Night	16% 7% 28% 29% 17% 3	720
	21st to end	Night	21% 7% 26% 30% 14% 2	720
October	1st to 10th	Night	25% 8% 29% 25% 12% 2	720
	11th to 20th	Night	29% 12% 28% 24% 7%	720
	21st to end	Night	30% 14% 28% 23% 4 1	792
November	1st to 10th	Night	23% 13% 31% 25% 7% 1	740
	11th to 20th	Night	25% 13% 33% 24% 5%	760
	21st to end	Night	30% 14% 31% 19% 5% 1	760
December	1st to 10th	Night	25% 15% 34% 20% 6%	897
	11th to 20th	Night	27% 16% 32% 19% 4 1	950
	21st to end	Night	31% 14% 32% 18% 5%	1045

Page generated at 2024-02-07 06:11:54. Version ac07

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