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 2026-05-17.

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	14% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	132198
	Night	16% 12% 4 11% 6% 5% 5% 3 5% 6% 27%	130621
Transparency	Day	51% 7% 13% 19% 10% 1	135790
	Night	54% 7% 11% 17% 10% 1	135115
Seeing	Day	21% 14% 27% 22% 11% 4	16381
	Night	22% 14% 27% 25% 11% 1	25875

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	15% 19% 8% 18% 8% 5% 4 2 4 3 13%	4661
	11th to 20th	Day	11% 24% 9% 20% 9% 7% 5% 4 4 3 6%	5063
	21st to end	Day	14% 17% 7% 15% 12% 10% 5% 3 3 4 9%	5780
February	1st to 10th	Day	18% 17% 6% 14% 8% 7% 5% 3 4 5% 15%	1890
	11th to 20th	Day	13% 17% 7% 16% 9% 6% 5% 3 4 5% 15%	1843
	21st to end	Day	19% 16% 3 12% 8% 6% 5% 3 5% 5% 18%	1625
March	1st to 10th	Day	17% 12% 5% 11% 6% 5% 4 3 4 5% 29%	5042
	11th to 20th	Day	10% 14% 5% 9% 6% 5% 4 3 5% 6% 32%	5090
	21st to end	Day	18% 10% 4 10% 5% 4 4 3 5% 6% 31%	5586
April	1st to 10th	Day	18% 11% 4 8% 5% 5% 4 3 5% 6% 32%	2504
	11th to 20th	Day	18% 11% 3 7% 5% 4 4 4 4 6% 35%	2518
	21st to end	Day	19% 9% 3 7% 4 5% 4 3 5% 6% 34%	2730
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 5% 6% 31%	6018
	11th to 20th	Day	14% 11% 4 8% 5% 5% 4 3 5% 7% 35%	6402
	21st to end	Day	17% 7% 3 7% 4 4 4 4 6% 7% 38%	6949
June	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 6% 9% 40%	2800
	11th to 20th	Day	10% 5% 3 7% 5% 5% 5% 5% 7% 10% 39%	2800
	21st to end	Day	13% 5% 2 5% 3 4 5% 4 7% 8% 43%	2800
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 44%	6360
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 4 7% 10% 43%	6360
	21st to end	Day	8% 4 2 6% 4 4 5% 4 7% 9% 48%	6519
August	1st to 10th	Day	15% 7% 2 6% 4 4 4 4 6% 8% 40%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 4 7% 9% 37%	4174
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 7% 9% 46%	4426
September	1st to 10th	Day	13% 6% 3 5% 4 3 4 4 5% 9% 43%	2400
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	2321
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 7% 9% 34%	2134
October	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 5% 7% 37%	2000
	11th to 20th	Day	20% 11% 3 9% 5% 4 4 4 5% 8% 28%	2000
	21st to end	Day	24% 13% 3 9% 5% 5% 4 3 4 5% 24%	2200
November	1st to 10th	Day	13% 13% 4 10% 6% 5% 4 4 5% 7% 30%	1815
	11th to 20th	Day	14% 15% 4 10% 5% 5% 5% 4 5% 6% 27%	1680
	21st to end	Day	20% 18% 4 10% 6% 5% 4 4 5% 6% 19%	1680
December	1st to 10th	Day	19% 20% 6% 12% 7% 6% 5% 4 5% 4 11%	2680
	11th to 20th	Day	22% 17% 7% 12% 5% 4 4 3 5% 4 17%	3066
	21st to end	Day	24% 19% 7% 13% 7% 5% 4 3 4 4 11%	4048
January	1st to 10th	Night	16% 18% 7% 21% 7% 5% 5% 3 3 4 11%	8234
	11th to 20th	Night	11% 20% 9% 19% 8% 6% 5% 3 4 4 11%	7819
	21st to end	Night	16% 15% 6% 16% 10% 9% 5% 3 4 4 13%	8401
February	1st to 10th	Night	16% 18% 5% 14% 7% 5% 5% 3 6% 5% 16%	2730
	11th to 20th	Night	15% 16% 6% 13% 7% 6% 5% 4 5% 5% 18%	2663
	21st to end	Night	16% 16% 5% 13% 7% 4 4 4 5% 6% 19%	2175
March	1st to 10th	Night	18% 12% 5% 11% 8% 5% 4 3 4 5% 26%	5018
	11th to 20th	Night	14% 13% 3 8% 4 4 5% 3 6% 7% 34%	5089
	21st to end	Night	19% 10% 4 9% 4 4 4 3 5% 5% 33%	5588
April	1st to 10th	Night	20% 12% 4 9% 5% 4 4 3 4 4 31%	2116
	11th to 20th	Night	18% 9% 3 7% 5% 5% 4 4 5% 6% 36%	2069
	21st to end	Night	17% 8% 3 7% 5% 4 5% 4 5% 7% 34%	1890
May	1st to 10th	Night	23% 12% 3 8% 5% 4 4 3 4 6% 30%	4143
	11th to 20th	Night	12% 7% 3 8% 6% 5% 5% 5% 6% 8% 35%	3623
	21st to end	Night	18% 7% 2 7% 4 3 4 3 6% 7% 37%	3926
June	1st to 10th	Night	12% 5% 3 6% 6% 6% 6% 5% 7% 8% 35%	1600
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 8% 42%	1600
	21st to end	Night	12% 6% 2 6% 4 5% 5% 4 7% 9% 41%	1600
July	1st to 10th	Night	9% 5% 3 7% 4 5% 5% 4 7% 9% 43%	3600
	11th to 20th	Night	10% 5% 2 6% 4 4 5% 4 5% 9% 45%	3600
	21st to end	Night	9% 5% 2 6% 3 4 4 4 6% 7% 50%	4329
August	1st to 10th	Night	10% 6% 3 6% 5% 4 4 4 6% 10% 43%	2915
	11th to 20th	Night	13% 7% 3 7% 4 5% 4 4 6% 7% 40%	2875
	21st to end	Night	9% 5% 3 6% 4 5% 5% 4 7% 9% 45%	3439
September	1st to 10th	Night	12% 6% 2 6% 4 3 5% 4 6% 8% 45%	2000
	11th to 20th	Night	12% 5% 1 6% 3 4 4 4 5% 6% 50%	2075
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 7% 36%	2265
October	1st to 10th	Night	16% 7% 3 7% 4 5% 5% 3 6% 6% 39%	2400
	11th to 20th	Night	22% 10% 4 8% 4 5% 4 3 5% 6% 30%	2400
	21st to end	Night	25% 11% 3 7% 5% 4 5% 4 5% 5% 27%	2640
November	1st to 10th	Night	16% 10% 4 8% 6% 4 4 4 5% 6% 34%	2701
	11th to 20th	Night	16% 14% 3 8% 6% 6% 5% 4 5% 6% 26%	2940
	21st to end	Night	19% 16% 4 10% 6% 5% 5% 4 5% 5% 19%	2940
December	1st to 10th	Night	16% 23% 7% 14% 7% 6% 5% 4 5% 5% 10%	4720
	11th to 20th	Night	18% 21% 6% 12% 6% 5% 4 3 4 4 17%	5348
	21st to end	Night	25% 18% 6% 11% 7% 6% 4 3 4 4 12%	7150

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	73% 8% 9% 8% 2	4662
	11th to 20th	Day	79% 6% 7% 5% 2 1	5097
	21st to end	Day	77% 7% 9% 5% 2	5780
February	1st to 10th	Day	72% 7% 10% 10% 2	1890
	11th to 20th	Day	71% 7% 9% 10% 3	1852
	21st to end	Day	67% 6% 11% 11% 5%	1625
March	1st to 10th	Day	56% 6% 14% 15% 9%	5045
	11th to 20th	Day	51% 6% 18% 15% 8% 2	5090
	21st to end	Day	52% 6% 11% 21% 9% 1	5586
April	1st to 10th	Day	54% 5% 12% 18% 10% 1	2504
	11th to 20th	Day	50% 6% 10% 19% 14% 2	2518
	21st to end	Day	50% 4 13% 18% 12% 2	2730
May	1st to 10th	Day	51% 7% 11% 16% 10% 5%	6027
	11th to 20th	Day	48% 9% 11% 18% 13% 1	6406
	21st to end	Day	44% 7% 13% 21% 14% 2	6949
June	1st to 10th	Day	40% 8% 14% 22% 15% 2	2800
	11th to 20th	Day	39% 8% 15% 24% 14% 1	2800
	21st to end	Day	36% 8% 16% 24% 15% 1	2800
July	1st to 10th	Day	33% 8% 13% 28% 17% 1	6290
	11th to 20th	Day	34% 8% 14% 30% 13% 1	6285
	21st to end	Day	32% 8% 14% 29% 16% 1	6332
August	1st to 10th	Day	40% 8% 16% 22% 13% 1	4221
	11th to 20th	Day	41% 8% 16% 25% 9%	4213
	21st to end	Day	34% 6% 14% 34% 12% 1	4426
September	1st to 10th	Day	38% 7% 15% 27% 12% 1	2400
	11th to 20th	Day	34% 7% 15% 29% 14% 1	2321
	21st to end	Day	45% 8% 16% 22% 9% 1	2134
October	1st to 10th	Day	37% 7% 16% 25% 14% 1	3260
	11th to 20th	Day	47% 6% 15% 20% 12% 1	3240
	21st to end	Day	51% 6% 16% 20% 7%	3538
November	1st to 10th	Day	52% 8% 12% 18% 8% 1	1815
	11th to 20th	Day	54% 8% 16% 13% 7% 1	1680
	21st to end	Day	65% 8% 13% 11% 3	1680
December	1st to 10th	Day	72% 9% 11% 6% 1	2680
	11th to 20th	Day	67% 9% 8% 9% 6% 1	3066
	21st to end	Day	76% 6% 10% 6% 2	4048
January	1st to 10th	Night	73% 9% 10% 7% 2	8234
	11th to 20th	Night	73% 7% 8% 7% 4	7860
	21st to end	Night	73% 7% 8% 6% 6%	8401
February	1st to 10th	Night	68% 8% 9% 12% 3	2730
	11th to 20th	Night	65% 7% 12% 12% 4	2665
	21st to end	Night	65% 6% 12% 12% 6%	2175
March	1st to 10th	Night	58% 6% 11% 17% 8%	5028
	11th to 20th	Night	47% 7% 16% 20% 9% 2	5090
	21st to end	Night	50% 7% 9% 21% 12% 1	5588
April	1st to 10th	Night	56% 6% 9% 19% 10% 1	2116
	11th to 20th	Night	49% 6% 9% 20% 13% 2	2069
	21st to end	Night	48% 7% 10% 20% 14% 2	1890
May	1st to 10th	Night	56% 6% 9% 14% 11% 5%	4150
	11th to 20th	Night	43% 10% 14% 20% 12% 1	3624
	21st to end	Night	43% 8% 12% 21% 15% 1	3927
June	1st to 10th	Night	43% 7% 11% 20% 17% 2	1600
	11th to 20th	Night	37% 8% 13% 22% 18% 2	1600
	21st to end	Night	39% 7% 15% 19% 18% 2	1600
July	1st to 10th	Night	37% 6% 13% 23% 20% 1	3560
	11th to 20th	Night	34% 7% 13% 29% 16% 2	3552
	21st to end	Night	32% 6% 13% 24% 23% 1	4201
August	1st to 10th	Night	36% 7% 15% 26% 14% 2	2904
	11th to 20th	Night	40% 7% 14% 27% 11%	2902
	21st to end	Night	32% 7% 14% 32% 13% 1	3439
September	1st to 10th	Night	36% 7% 15% 26% 15% 1	2000
	11th to 20th	Night	32% 7% 12% 31% 17% 1	2075
	21st to end	Night	45% 8% 11% 22% 12% 1	2265
October	1st to 10th	Night	38% 8% 10% 26% 18% 1	3920
	11th to 20th	Night	48% 5% 13% 22% 11%	3899
	21st to end	Night	44% 7% 15% 25% 8%	4252
November	1st to 10th	Night	49% 7% 11% 21% 10% 1	2701
	11th to 20th	Night	55% 8% 13% 16% 8% 1	2940
	21st to end	Night	62% 9% 14% 13% 3	2940
December	1st to 10th	Night	71% 11% 10% 6% 2	4720
	11th to 20th	Night	68% 8% 7% 10% 7% 1	5348
	21st to end	Night	74% 7% 9% 8% 2	7150

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% 29% 27% 13% 4 1	220
	11th to 20th	Day	30% 31% 26% 10% 2 1	296
	21st to end	Day	27% 26% 28% 15% 3 1	363
February	1st to 10th	Day	34% 22% 30% 12% 2	330
	11th to 20th	Day	28% 24% 37% 10% 1	330
	21st to end	Day	36% 20% 33% 10% 2	331
March	1st to 10th	Day	26% 20% 31% 17% 5% 1	540
	11th to 20th	Day	28% 18% 30% 20% 4 1	540
	21st to end	Day	29% 18% 28% 18% 5% 2	594
April	1st to 10th	Day	26% 19% 30% 20% 5% 1	540
	11th to 20th	Day	31% 18% 22% 18% 8% 3	532
	21st to end	Day	25% 12% 35% 19% 7% 2	540
May	1st to 10th	Day	23% 12% 30% 21% 10% 3	543
	11th to 20th	Day	18% 10% 26% 26% 16% 4	551
	21st to end	Day	15% 9% 25% 30% 15% 6%	573
June	1st to 10th	Day	16% 12% 27% 31% 11% 2	530
	11th to 20th	Day	14% 11% 25% 31% 17% 2	530
	21st to end	Day	17% 8% 28% 31% 13% 3	530
July	1st to 10th	Day	9% 6% 22% 34% 22% 8%	525
	11th to 20th	Day	14% 8% 21% 31% 19% 7%	524
	21st to end	Day	11% 5% 29% 31% 18% 6%	576
August	1st to 10th	Day	12% 5% 22% 32% 20% 10%	526
	11th to 20th	Day	12% 6% 21% 32% 22% 8%	519
	21st to end	Day	12% 8% 22% 31% 19% 8%	583
September	1st to 10th	Day	14% 9% 22% 26% 20% 9%	530
	11th to 20th	Day	11% 5% 20% 25% 25% 15%	530
	21st to end	Day	16% 10% 28% 24% 15% 7%	530
October	1st to 10th	Day	21% 14% 22% 20% 16% 6%	530
	11th to 20th	Day	23% 20% 29% 18% 7% 2	530
	21st to end	Day	35% 16% 25% 18% 6%	583
November	1st to 10th	Day	25% 19% 29% 18% 7% 2	360
	11th to 20th	Day	29% 24% 26% 15% 4 2	220
	21st to end	Day	31% 25% 30% 11% 3	220
December	1st to 10th	Day	32% 19% 33% 11% 5%	220
	11th to 20th	Day	32% 26% 29% 11% 2	220
	21st to end	Day	32% 28% 28% 9% 3 1	242
January	1st to 10th	Night	28% 25% 30% 12% 5%	840
	11th to 20th	Night	32% 23% 28% 15% 2	840
	21st to end	Night	24% 19% 33% 19% 4	924
February	1st to 10th	Night	26% 22% 30% 19% 3	840
	11th to 20th	Night	24% 22% 33% 17% 4	823
	21st to end	Night	27% 19% 31% 21% 2	692
March	1st to 10th	Night	24% 16% 33% 22% 5%	840
	11th to 20th	Night	25% 19% 28% 21% 5% 1	840
	21st to end	Night	23% 13% 31% 24% 8% 1	924
April	1st to 10th	Night	26% 16% 30% 24% 4	840
	11th to 20th	Night	23% 13% 29% 25% 8% 1	825
	21st to end	Night	20% 9% 29% 33% 9%	630
May	1st to 10th	Night	21% 7% 26% 30% 14% 2	630
	11th to 20th	Night	18% 4 28% 40% 10%	406
	21st to end	Night	14% 7% 24% 36% 17% 2	433
June	1st to 10th	Night	16% 4 29% 37% 14% 1	400
	11th to 20th	Night	12% 5% 29% 38% 16% 1	400
	21st to end	Night	14% 5% 27% 41% 13% 1	400
July	1st to 10th	Night	10% 4 22% 43% 20% 1	390
	11th to 20th	Night	10% 5% 24% 39% 20% 2	388
	21st to end	Night	10% 4 20% 42% 23% 1	610
August	1st to 10th	Night	11% 5% 14% 40% 27% 3	597
	11th to 20th	Night	12% 4 17% 35% 29% 4	594
	21st to end	Night	13% 4 21% 36% 23% 3	785
September	1st to 10th	Night	14% 7% 19% 35% 23% 4	800
	11th to 20th	Night	14% 4 18% 30% 28% 6%	800
	21st to end	Night	20% 7% 21% 30% 18% 4	800
October	1st to 10th	Night	19% 9% 24% 28% 17% 2	800
	11th to 20th	Night	28% 14% 26% 24% 8% 1	800
	21st to end	Night	32% 14% 24% 24% 5%	880
November	1st to 10th	Night	24% 14% 28% 23% 10% 1	820
	11th to 20th	Night	26% 19% 28% 18% 8% 1	840
	21st to end	Night	29% 19% 30% 17% 5%	840
December	1st to 10th	Night	33% 21% 25% 17% 4	840
	11th to 20th	Night	25% 24% 32% 15% 3	840
	21st to end	Night	33% 17% 32% 15% 2	924

Page generated at 2026-05-14 01:58:55. Version ac07

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