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 2025-01-22.

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% 5% 4 5% 7% 32%	105194
	Night	16% 12% 4 9% 5% 5% 4 4 5% 6% 28%	104180
Transparency	Day	49% 8% 14% 19% 10% 1	108797
	Night	53% 8% 12% 18% 9% 1	108665
Seeing	Day	20% 14% 27% 23% 12% 4	14679
	Night	22% 13% 27% 26% 11% 1	24189

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% 20% 7% 12% 7% 4 4 3 4 4 19%	2597
	11th to 20th	Day	12% 24% 8% 17% 8% 6% 5% 3 4 4 7%	2828
	21st to end	Day	22% 20% 8% 13% 8% 7% 4 3 3 3 9%	3133
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	16% 7% 2 6% 4 4 4 4 6% 8% 40%	4104
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 4 7% 10% 36%	4044
	21st to end	Day	10% 5% 2 6% 4 4 4 4 7% 9% 46%	4290
September	1st to 10th	Day	13% 7% 3 5% 4 3 4 4 5% 9% 43%	2280
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	2205
	21st to end	Day	14% 8% 3 7% 5% 5% 5% 4 7% 8% 34%	2028
October	1st to 10th	Day	18% 8% 2 8% 4 4 4 4 5% 7% 35%	1900
	11th to 20th	Day	20% 12% 3 9% 5% 4 4 3 5% 7% 28%	1900
	21st to end	Day	24% 13% 3 9% 5% 4 4 3 4 5% 24%	2090
November	1st to 10th	Day	12% 13% 4 10% 6% 5% 4 4 5% 7% 30%	1727
	11th to 20th	Day	14% 15% 4 10% 5% 5% 5% 4 6% 6% 27%	1600
	21st to end	Day	20% 18% 4 10% 6% 5% 4 4 5% 6% 19%	1600
December	1st to 10th	Day	20% 20% 6% 12% 7% 6% 5% 4 5% 4 10%	2600
	11th to 20th	Day	22% 17% 7% 12% 6% 4 3 3 5% 4 18%	2986
	21st to end	Day	24% 19% 7% 13% 7% 5% 4 3 4 4 11%	3960
January	1st to 10th	Night	18% 19% 7% 14% 6% 5% 4 3 4 5% 15%	4569
	11th to 20th	Night	14% 21% 9% 16% 8% 6% 4 3 4 4 13%	4349
	21st to end	Night	24% 19% 6% 15% 8% 5% 4 3 3 3 11%	4546
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	10% 6% 3 6% 5% 4 4 4 6% 10% 42%	2825
	11th to 20th	Night	13% 7% 3 7% 4 5% 4 4 6% 7% 40%	2785
	21st to end	Night	9% 5% 3 6% 4 4 5% 4 7% 9% 45%	3334
September	1st to 10th	Night	12% 7% 2 6% 4 3 5% 4 5% 8% 44%	1900
	11th to 20th	Night	12% 5% 2 6% 3 4 4 4 5% 6% 49%	1971
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 7% 36%	2152
October	1st to 10th	Night	17% 7% 3 8% 4 5% 5% 3 6% 6% 36%	2280
	11th to 20th	Night	22% 10% 4 8% 4 5% 4 3 5% 5% 29%	2280
	21st to end	Night	26% 11% 3 7% 5% 4 5% 4 5% 5% 26%	2508
November	1st to 10th	Night	16% 10% 4 8% 5% 4 3 4 4 7% 35%	2569
	11th to 20th	Night	16% 13% 3 9% 5% 6% 5% 4 5% 6% 27%	2800
	21st to end	Night	20% 16% 4 10% 6% 5% 5% 4 5% 5% 20%	2800
December	1st to 10th	Night	16% 23% 7% 14% 7% 6% 5% 4 5% 5% 10%	4580
	11th to 20th	Night	18% 21% 6% 11% 6% 5% 4 3 4 4 17%	5208
	21st to end	Night	25% 18% 6% 11% 7% 6% 4 3 4 4 12%	6996

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% 10% 3	2598
	11th to 20th	Day	78% 8% 8% 4 1	2828
	21st to end	Day	78% 7% 10% 3 1	3133
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	41% 8% 16% 22% 12% 1	4091
	11th to 20th	Day	41% 8% 16% 25% 9%	4083
	21st to end	Day	34% 6% 14% 34% 12% 1	4290
September	1st to 10th	Day	38% 7% 15% 26% 12% 1	2280
	11th to 20th	Day	35% 7% 15% 28% 15% 1	2205
	21st to end	Day	45% 8% 16% 22% 9% 1	2028
October	1st to 10th	Day	38% 7% 17% 25% 13% 1	3160
	11th to 20th	Day	47% 6% 15% 20% 12% 1	3140
	21st to end	Day	50% 6% 16% 21% 6%	3428
November	1st to 10th	Day	52% 8% 13% 18% 8% 1	1727
	11th to 20th	Day	54% 8% 16% 13% 7% 1	1600
	21st to end	Day	65% 8% 13% 11% 4	1600
December	1st to 10th	Day	73% 9% 12% 6% 1	2600
	11th to 20th	Day	67% 9% 8% 9% 6% 1	2986
	21st to end	Day	76% 6% 9% 6% 2	3960
January	1st to 10th	Night	67% 12% 12% 8% 2	4569
	11th to 20th	Night	73% 9% 10% 7% 1	4349
	21st to end	Night	76% 9% 7% 5% 2	4546
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	36% 7% 14% 27% 13% 2	2814
	11th to 20th	Night	41% 7% 14% 27% 10%	2812
	21st to end	Night	32% 7% 14% 32% 14% 1	3334
September	1st to 10th	Night	37% 7% 15% 24% 15% 1	1900
	11th to 20th	Night	33% 7% 12% 30% 18% 1	1971
	21st to end	Night	45% 8% 12% 23% 12% 1	2152
October	1st to 10th	Night	39% 8% 10% 25% 18% 1	3800
	11th to 20th	Night	48% 5% 14% 22% 11%	3779
	21st to end	Night	44% 7% 15% 25% 8%	4120
November	1st to 10th	Night	48% 7% 11% 21% 11% 2	2569
	11th to 20th	Night	55% 8% 13% 16% 8%	2800
	21st to end	Night	62% 9% 14% 13% 3	2800
December	1st to 10th	Night	71% 11% 11% 6% 2	4580
	11th to 20th	Night	68% 7% 7% 10% 7% 1	5208
	21st to end	Night	73% 7% 9% 8% 2	6996

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% 28% 27% 15% 5% 1	200
	11th to 20th	Day	27% 31% 28% 11% 2 1	269
	21st to end	Day	24% 24% 30% 17% 4 1	300
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	12% 7% 22% 31% 20% 9%	539
September	1st to 10th	Day	14% 9% 22% 26% 19% 9%	490
	11th to 20th	Day	11% 6% 19% 24% 25% 15%	490
	21st to end	Day	17% 11% 28% 23% 15% 7%	490
October	1st to 10th	Day	22% 15% 22% 19% 15% 6%	490
	11th to 20th	Day	23% 21% 28% 19% 7% 2	490
	21st to end	Day	35% 15% 24% 19% 6%	539
November	1st to 10th	Day	24% 18% 29% 19% 8% 2	332
	11th to 20th	Day	28% 24% 27% 17% 3 2	200
	21st to end	Day	30% 26% 30% 12% 4	200
December	1st to 10th	Day	30% 20% 33% 12% 5% 1	200
	11th to 20th	Day	32% 26% 29% 12% 2	200
	21st to end	Day	29% 28% 29% 10% 3 1	220
January	1st to 10th	Night	28% 25% 30% 12% 5%	800
	11th to 20th	Night	31% 22% 29% 16% 3	800
	21st to end	Night	24% 18% 33% 21% 4	841
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	13% 4 20% 35% 24% 4	746
September	1st to 10th	Night	15% 6% 18% 35% 22% 4	760
	11th to 20th	Night	15% 4 19% 29% 27% 6%	760
	21st to end	Night	20% 8% 22% 29% 18% 4	760
October	1st to 10th	Night	20% 10% 24% 28% 16% 3	760
	11th to 20th	Night	28% 13% 25% 25% 8% 1	760
	21st to end	Night	33% 14% 23% 24% 5%	836
November	1st to 10th	Night	24% 13% 28% 23% 10% 1	780
	11th to 20th	Night	26% 18% 29% 18% 8% 1	800
	21st to end	Night	28% 19% 30% 17% 6%	800
December	1st to 10th	Night	34% 21% 25% 16% 4	800
	11th to 20th	Night	26% 24% 32% 16% 4	800
	21st to end	Night	33% 18% 32% 15% 3	880

Page generated at 2025-01-19 03:06:47. Version ac07

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