Eyeline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eyeline Observatory clear sky chart from 2005-11-06 to 2024-08-30.

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% 5% 4 4 6% 7% 32%	95779
	Night	17% 12% 4 9% 5% 5% 4 4 5% 6% 29%	93274
Transparency	Day	49% 8% 14% 19% 9% 1	95505
	Night	53% 8% 12% 17% 9% 1	93101
Seeing	Day	20% 13% 26% 24% 12% 4	14248
	Night	22% 13% 27% 26% 11% 1	23612

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	16% 19% 7% 12% 7% 4 4 3 4 4 20%	2517
	11th to 20th	Day	13% 24% 8% 17% 8% 6% 5% 3 4 4 8%	2741
	21st to end	Day	22% 20% 8% 13% 8% 7% 4 3 3 3 9%	3124
February	1st to 10th	Day	18% 17% 6% 13% 7% 6% 4 3 5% 5% 15%	1710
	11th to 20th	Day	12% 17% 7% 16% 9% 7% 5% 3 4 5% 15%	1663
	21st to end	Day	19% 15% 3 12% 8% 6% 5% 3 5% 5% 19%	1474
March	1st to 10th	Day	19% 12% 4 10% 6% 5% 4 3 4 5% 29%	3432
	11th to 20th	Day	11% 16% 5% 9% 6% 6% 4 3 5% 5% 30%	3480
	21st to end	Day	16% 9% 4 8% 4 4 4 3 6% 7% 34%	3815
April	1st to 10th	Day	18% 11% 4 8% 5% 5% 4 3 4 6% 32%	2266
	11th to 20th	Day	19% 10% 3 6% 5% 3 4 4 4 7% 36%	2276
	21st to end	Day	19% 9% 4 7% 4 4 4 3 5% 6% 34%	2470
May	1st to 10th	Day	18% 9% 2 8% 4 4 4 4 5% 7% 34%	4108
	11th to 20th	Day	14% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	4398
	21st to end	Day	9% 6% 2 7% 4 4 4 4 6% 8% 46%	4837
June	1st to 10th	Day	14% 7% 2 5% 4 5% 4 4 6% 9% 40%	2660
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 40%	2660
	21st to end	Day	13% 5% 2 5% 3 4 5% 4 7% 8% 43%	2660
July	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 7% 11% 45%	4440
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 10% 44%	4440
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 9% 42%	4533
August	1st to 10th	Day	10% 5% 2 5% 4 5% 4 4 7% 10% 44%	2457
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 5% 8% 9% 40%	2433
	21st to end	Day	10% 6% 2 6% 4 4 4 3 7% 10% 45%	2557
September	1st to 10th	Day	14% 7% 3 5% 4 3 4 4 5% 9% 42%	2160
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 45%	2089
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	1922
October	1st to 10th	Day	19% 9% 2 8% 5% 5% 4 4 5% 7% 34%	1800
	11th to 20th	Day	21% 12% 3 9% 5% 4 4 4 5% 7% 27%	1800
	21st to end	Day	25% 14% 4 9% 5% 4 4 3 4 5% 23%	1980
November	1st to 10th	Day	12% 13% 4 10% 6% 5% 4 4 5% 7% 30%	1639
	11th to 20th	Day	14% 15% 4 10% 5% 4 5% 4 6% 6% 27%	1520
	21st to end	Day	20% 18% 4 10% 6% 4 4 3 5% 6% 19%	1520
December	1st to 10th	Day	18% 22% 7% 11% 7% 5% 3 3 5% 4 13%	1520
	11th to 20th	Day	21% 17% 7% 14% 6% 5% 4 3 5% 5% 13%	1906
	21st to end	Day	28% 22% 7% 12% 5% 5% 3 3 3 3 11%	2772
January	1st to 10th	Night	18% 19% 6% 13% 6% 5% 4 3 4 5% 15%	4429
	11th to 20th	Night	14% 21% 8% 16% 8% 5% 4 3 4 4 13%	4216
	21st to end	Night	24% 19% 6% 15% 8% 5% 4 3 3 3 10%	4532
February	1st to 10th	Night	16% 19% 5% 14% 7% 5% 5% 3 6% 5% 16%	2470
	11th to 20th	Night	15% 16% 6% 13% 8% 6% 5% 4 5% 5% 18%	2403
	21st to end	Night	17% 15% 5% 13% 7% 4 4 4 5% 6% 20%	1975
March	1st to 10th	Night	21% 12% 3 10% 6% 4 4 3 4 5% 29%	3408
	11th to 20th	Night	17% 16% 4 8% 5% 4 4 3 5% 6% 28%	3479
	21st to end	Night	16% 8% 3 8% 4 4 4 3 5% 6% 39%	3817
April	1st to 10th	Night	20% 13% 4 8% 5% 4 4 3 4 4 32%	1914
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 4 5% 5% 36%	1870
	21st to end	Night	18% 8% 4 7% 4 4 5% 4 5% 7% 34%	1710
May	1st to 10th	Night	21% 10% 3 7% 5% 4 4 3 5% 6% 33%	2833
	11th to 20th	Night	13% 8% 3 8% 6% 6% 4 5% 6% 8% 32%	2495
	21st to end	Night	10% 6% 2 6% 4 4 4 4 7% 8% 44%	2738
June	1st to 10th	Night	13% 5% 3 6% 6% 6% 6% 5% 7% 8% 35%	1520
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 8% 42%	1520
	21st to end	Night	13% 6% 2 6% 4 5% 5% 4 7% 9% 40%	1520
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 4 4 7% 8% 44%	2520
	11th to 20th	Night	11% 5% 2 6% 4 4 4 5% 5% 9% 46%	2520
	21st to end	Night	8% 5% 2 6% 4 4 4 5% 6% 8% 48%	3016
August	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 7% 9% 45%	1701
	11th to 20th	Night	9% 6% 3 6% 4 5% 5% 4 7% 8% 44%	1688
	21st to end	Night	10% 5% 3 6% 4 5% 4 4 7% 8% 46%	1990
September	1st to 10th	Night	12% 7% 2 6% 4 3 5% 4 6% 8% 43%	1800
	11th to 20th	Night	13% 5% 2 6% 3 3 4 4 5% 6% 49%	1867
	21st to end	Night	17% 7% 3 7% 4 4 5% 4 6% 7% 37%	2038
October	1st to 10th	Night	17% 8% 3 8% 4 5% 5% 3 6% 6% 36%	2160
	11th to 20th	Night	23% 9% 4 8% 5% 5% 4 3 5% 6% 28%	2160
	21st to end	Night	27% 11% 3 7% 6% 4 5% 3 5% 5% 24%	2376
November	1st to 10th	Night	16% 10% 4 7% 5% 4 3 4 4 7% 35%	2437
	11th to 20th	Night	16% 13% 3 9% 5% 6% 5% 4 5% 6% 27%	2660
	21st to end	Night	20% 16% 4 10% 6% 5% 5% 4 5% 5% 20%	2660
December	1st to 10th	Night	21% 21% 6% 13% 6% 5% 4 4 5% 4 12%	2660
	11th to 20th	Night	17% 21% 6% 13% 7% 5% 5% 3 4 5% 15%	3288
	21st to end	Night	30% 18% 5% 10% 6% 5% 5% 3 5% 4 9%	4884

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% 10% 10% 11% 3	2518
	11th to 20th	Day	78% 8% 8% 4 1	2741
	21st to end	Day	78% 7% 10% 3 1	3124
February	1st to 10th	Day	71% 7% 10% 10% 2	1710
	11th to 20th	Day	70% 7% 9% 10% 3	1672
	21st to end	Day	66% 7% 12% 10% 5%	1474
March	1st to 10th	Day	55% 7% 13% 19% 5%	3435
	11th to 20th	Day	53% 8% 17% 15% 7% 1	3480
	21st to end	Day	46% 7% 15% 23% 9% 1	3815
April	1st to 10th	Day	53% 5% 13% 18% 10% 1	2266
	11th to 20th	Day	48% 6% 11% 19% 14% 2	2276
	21st to end	Day	50% 5% 13% 18% 13% 2	2470
May	1st to 10th	Day	47% 6% 13% 19% 12% 2	4120
	11th to 20th	Day	47% 11% 14% 19% 7% 1	4402
	21st to end	Day	34% 7% 15% 24% 17% 3	4837
June	1st to 10th	Day	40% 8% 14% 22% 15% 2	2660
	11th to 20th	Day	38% 8% 15% 24% 15% 1	2660
	21st to end	Day	36% 8% 16% 24% 15% 1	2660
July	1st to 10th	Day	32% 8% 13% 28% 17% 2	4370
	11th to 20th	Day	33% 8% 15% 32% 12% 1	4365
	21st to end	Day	34% 10% 16% 24% 15% 1	4388
August	1st to 10th	Day	34% 8% 19% 26% 12% 1	2444
	11th to 20th	Day	38% 9% 17% 25% 11% 1	2433
	21st to end	Day	35% 6% 16% 29% 12% 1	2557
September	1st to 10th	Day	39% 7% 16% 26% 11% 1	2160
	11th to 20th	Day	36% 7% 15% 26% 15% 1	2089
	21st to end	Day	44% 7% 17% 22% 9% 1	1922
October	1st to 10th	Day	49% 7% 13% 18% 12% 1	1800
	11th to 20th	Day	55% 8% 15% 16% 6% 1	1800
	21st to end	Day	64% 6% 12% 13% 5% 1	1980
November	1st to 10th	Day	52% 9% 13% 17% 9% 1	1639
	11th to 20th	Day	54% 9% 17% 13% 7% 1	1520
	21st to end	Day	64% 8% 14% 11% 3	1520
December	1st to 10th	Day	73% 8% 11% 7% 1	1520
	11th to 20th	Day	70% 11% 10% 7% 2	1906
	21st to end	Day	78% 6% 9% 6% 1	2772
January	1st to 10th	Night	66% 12% 12% 8% 2	4429
	11th to 20th	Night	73% 9% 10% 7% 1	4216
	21st to end	Night	76% 9% 7% 5% 2	4532
February	1st to 10th	Night	68% 9% 9% 12% 3	2470
	11th to 20th	Night	65% 7% 11% 12% 4	2405
	21st to end	Night	63% 6% 12% 13% 6%	1975
March	1st to 10th	Night	56% 6% 11% 21% 6%	3418
	11th to 20th	Night	53% 9% 14% 16% 7% 1	3480
	21st to end	Night	42% 7% 10% 26% 14% 1	3817
April	1st to 10th	Night	55% 6% 9% 20% 10% 1	1914
	11th to 20th	Night	49% 6% 9% 20% 14% 2	1870
	21st to end	Night	48% 7% 10% 19% 14% 3	1710
May	1st to 10th	Night	51% 7% 11% 18% 13% 2	2840
	11th to 20th	Night	44% 11% 15% 20% 8% 2	2496
	21st to end	Night	33% 9% 12% 24% 20% 2	2739
June	1st to 10th	Night	43% 7% 11% 20% 17% 1	1520
	11th to 20th	Night	37% 8% 13% 22% 18% 2	1520
	21st to end	Night	40% 7% 15% 19% 17% 1	1520
July	1st to 10th	Night	37% 6% 12% 23% 20% 2	2480
	11th to 20th	Night	33% 7% 14% 31% 14% 1	2472
	21st to end	Night	32% 8% 17% 24% 18% 2	2920
August	1st to 10th	Night	33% 8% 17% 27% 14% 1	1690
	11th to 20th	Night	35% 8% 16% 29% 13%	1688
	21st to end	Night	33% 8% 14% 30% 13% 2	1990
September	1st to 10th	Night	38% 8% 16% 24% 14% 2	1800
	11th to 20th	Night	34% 7% 12% 29% 17% 1	1867
	21st to end	Night	43% 8% 12% 23% 13% 1	2038
October	1st to 10th	Night	46% 8% 11% 20% 14% 1	2160
	11th to 20th	Night	54% 7% 13% 18% 7% 1	2160
	21st to end	Night	61% 7% 11% 13% 8% 1	2376
November	1st to 10th	Night	48% 8% 11% 21% 11% 2	2437
	11th to 20th	Night	55% 8% 13% 16% 8%	2660
	21st to end	Night	61% 9% 14% 13% 3	2660
December	1st to 10th	Night	71% 10% 10% 6% 2	2660
	11th to 20th	Night	69% 10% 10% 9% 2	3288
	21st to end	Night	76% 9% 10% 4 1	4884

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	25% 26% 28% 16% 5% 1	180
	11th to 20th	Day	26% 29% 29% 12% 2 2	242
	21st to end	Day	24% 24% 30% 17% 4 1	297
February	1st to 10th	Day	33% 23% 30% 12% 2	270
	11th to 20th	Day	24% 25% 38% 11% 1	270
	21st to end	Day	35% 19% 34% 10% 2	277
March	1st to 10th	Day	23% 19% 33% 18% 5% 1	460
	11th to 20th	Day	25% 17% 32% 22% 3 1	460
	21st to end	Day	28% 17% 29% 19% 6% 2	506
April	1st to 10th	Day	23% 17% 31% 22% 6% 1	460
	11th to 20th	Day	27% 18% 23% 19% 9% 3	452
	21st to end	Day	26% 10% 35% 20% 7% 2	460
May	1st to 10th	Day	23% 10% 28% 23% 12% 4	463
	11th to 20th	Day	17% 9% 25% 28% 18% 4	487
	21st to end	Day	12% 9% 25% 32% 16% 6%	529
June	1st to 10th	Day	16% 11% 26% 33% 12% 3	490
	11th to 20th	Day	14% 10% 23% 33% 18% 2	490
	21st to end	Day	18% 7% 28% 31% 13% 3	490
July	1st to 10th	Day	9% 5% 21% 34% 23% 8%	485
	11th to 20th	Day	14% 8% 20% 31% 19% 7%	484
	21st to end	Day	11% 6% 27% 32% 19% 6%	532
August	1st to 10th	Day	13% 5% 22% 29% 20% 10%	486
	11th to 20th	Day	11% 6% 20% 31% 22% 9%	479
	21st to end	Day	11% 7% 22% 31% 20% 9%	532
September	1st to 10th	Day	14% 8% 22% 28% 19% 9%	450
	11th to 20th	Day	12% 6% 20% 25% 24% 13%	450
	21st to end	Day	16% 11% 28% 22% 16% 8%	450
October	1st to 10th	Day	23% 15% 21% 18% 16% 7%	450
	11th to 20th	Day	24% 22% 28% 18% 7% 2	450
	21st to end	Day	37% 14% 24% 18% 7%	495
November	1st to 10th	Day	23% 18% 28% 20% 8% 2	304
	11th to 20th	Day	28% 24% 25% 18% 3 2	180
	21st to end	Day	29% 24% 30% 12% 4	180
December	1st to 10th	Day	29% 21% 33% 11% 6% 1	180
	11th to 20th	Day	31% 27% 28% 12% 2	180
	21st to end	Day	29% 29% 30% 9% 3	198
January	1st to 10th	Night	28% 24% 29% 13% 5%	760
	11th to 20th	Night	31% 22% 28% 16% 3	760
	21st to end	Night	24% 18% 33% 21% 4	836
February	1st to 10th	Night	26% 23% 30% 18% 3	760
	11th to 20th	Night	23% 21% 34% 17% 4	743
	21st to end	Night	27% 19% 30% 21% 2	628
March	1st to 10th	Night	23% 16% 33% 22% 6%	760
	11th to 20th	Night	24% 19% 28% 22% 6% 1	760
	21st to end	Night	22% 12% 31% 25% 9% 1	836
April	1st to 10th	Night	25% 16% 30% 25% 4	760
	11th to 20th	Night	22% 13% 30% 25% 9% 1	746
	21st to end	Night	21% 8% 28% 32% 10%	570
May	1st to 10th	Night	20% 6% 25% 31% 15% 2	570
	11th to 20th	Night	18% 3 27% 41% 10% 1	374
	21st to end	Night	13% 7% 23% 37% 18% 2	411
June	1st to 10th	Night	16% 4 29% 36% 15% 1	380
	11th to 20th	Night	12% 5% 28% 38% 16% 1	380
	21st to end	Night	14% 5% 26% 41% 13% 1	380
July	1st to 10th	Night	11% 4 21% 42% 21% 1	370
	11th to 20th	Night	10% 4 25% 39% 20% 2	368
	21st to end	Night	10% 4 20% 42% 23% 1	579
August	1st to 10th	Night	11% 5% 14% 39% 27% 4	567
	11th to 20th	Night	12% 4 18% 34% 29% 4	564
	21st to end	Night	12% 4 21% 35% 24% 4	742
September	1st to 10th	Night	15% 6% 18% 35% 23% 4	720
	11th to 20th	Night	16% 4 19% 28% 26% 6%	720
	21st to end	Night	19% 7% 22% 29% 18% 5%	720
October	1st to 10th	Night	20% 10% 23% 28% 16% 3	720
	11th to 20th	Night	28% 14% 26% 24% 8% 1	720
	21st to end	Night	34% 14% 23% 24% 5% 1	792
November	1st to 10th	Night	24% 13% 28% 24% 10% 1	740
	11th to 20th	Night	25% 19% 29% 17% 8% 1	760
	21st to end	Night	28% 18% 30% 17% 6%	760
December	1st to 10th	Night	34% 21% 24% 16% 4	760
	11th to 20th	Night	25% 23% 32% 16% 4	760
	21st to end	Night	33% 18% 32% 15% 2	836

Page generated at 2024-08-27 09:49:38. Version ac07

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