Emerald Lane Observatory
Clear Sky Chart
History

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

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	12% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	127308
	Night	13% 7% 3 6% 4 4 4 3 5% 7% 43%	123408
Transparency	Day	40% 5% 11% 22% 17% 4	131046
	Night	39% 4 10% 21% 20% 6%	127729
Seeing	Day	15% 10% 23% 24% 18% 9%	16239
	Night	15% 9% 23% 29% 20% 4	25866

By Month

Percentage Cloud Cover SE (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% 10% 4 9% 4 3 3 3 4 5% 39%	4056
	11th to 20th	Day	14% 7% 6% 9% 3 2 2 3 3 4 46%	4175
	21st to end	Day	18% 12% 4 6% 4 3 3 2 4 6% 37%	4972
February	1st to 10th	Day	17% 11% 3 7% 4 3 4 3 5% 5% 38%	2000
	11th to 20th	Day	19% 9% 4 7% 6% 4 3 3 3 5% 37%	1948
	21st to end	Day	18% 9% 3 7% 5% 3 4 3 4 5% 38%	1650
March	1st to 10th	Day	18% 11% 3 7% 5% 5% 4 4 4 5% 34%	4480
	11th to 20th	Day	11% 8% 2 6% 4 3 5% 3 5% 6% 46%	4745
	21st to end	Day	12% 8% 2 7% 5% 5% 4 3 5% 6% 42%	5797
April	1st to 10th	Day	13% 6% 2 6% 4 3 4 4 6% 8% 45%	2400
	11th to 20th	Day	11% 7% 2 6% 4 4 5% 4 4 7% 46%	2374
	21st to end	Day	10% 6% 2 6% 5% 5% 4 4 7% 8% 43%	2400
May	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 4 6% 9% 33%	5814
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 7% 9% 29%	5861
	21st to end	Day	14% 7% 3 7% 5% 4 4 4 5% 8% 38%	6433
June	1st to 10th	Day	8% 6% 2 8% 5% 6% 7% 5% 6% 10% 37%	2600
	11th to 20th	Day	4 4 2 6% 5% 5% 7% 6% 8% 11% 44%	2600
	21st to end	Day	6% 4 2 7% 6% 6% 7% 5% 8% 11% 40%	2796
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 7% 6% 9% 11% 35%	6273
	11th to 20th	Day	6% 5% 2 6% 6% 7% 8% 6% 10% 14% 30%	5900
	21st to end	Day	10% 7% 3 8% 6% 7% 7% 6% 9% 11% 27%	6362
August	1st to 10th	Day	4 3 1 5% 4 5% 5% 5% 8% 11% 50%	3905
	11th to 20th	Day	5% 4 2 6% 4 6% 6% 5% 8% 10% 43%	3849
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 6% 7% 50%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 5% 7% 9% 42%	2368
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 6% 9% 43%	2200
	21st to end	Day	12% 4 2 5% 3 4 4 5% 5% 7% 49%	2200
October	1st to 10th	Day	8% 5% 2 6% 4 4 4 3 5% 5% 55%	2200
	11th to 20th	Day	9% 4 1 4 4 3 4 2 5% 7% 56%	2180
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 6% 51%	2039
November	1st to 10th	Day	11% 6% 2 5% 3 4 4 4 4 6% 53%	1852
	11th to 20th	Day	11% 7% 2 4 3 4 4 3 4 7% 51%	1890
	21st to end	Day	13% 8% 2 6% 4 4 4 3 5% 6% 44%	1890
December	1st to 10th	Day	14% 8% 3 7% 5% 4 3 3 4 4 44%	2948
	11th to 20th	Day	18% 10% 3 7% 4 5% 4 3 3 5% 37%	3373
	21st to end	Day	16% 10% 4 8% 4 4 3 3 5% 7% 35%	4466
January	1st to 10th	Night	15% 9% 4 8% 4 4 3 3 4 4 41%	5889
	11th to 20th	Night	16% 9% 4 6% 3 3 2 3 4 4 45%	5780
	21st to end	Night	18% 11% 2 6% 3 3 3 3 4 6% 41%	5984
February	1st to 10th	Night	17% 10% 2 7% 4 3 4 3 5% 5% 40%	2400
	11th to 20th	Night	21% 9% 2 7% 4 3 4 3 4 4 40%	2338
	21st to end	Night	19% 7% 3 7% 5% 4 4 3 4 5% 39%	1980
March	1st to 10th	Night	21% 9% 3 7% 5% 4 4 3 5% 6% 34%	5373
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 6% 8% 49%	5214
	21st to end	Night	15% 7% 3 8% 5% 4 4 3 4 5% 43%	5174
April	1st to 10th	Night	11% 5% 1 5% 5% 4 4 4 6% 8% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 6% 7% 49%	1975
	21st to end	Night	11% 6% 1 5% 4 4 4 3 5% 7% 50%	2000
May	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	4079
	11th to 20th	Night	15% 9% 4 10% 6% 6% 5% 4 5% 7% 31%	4032
	21st to end	Night	10% 7% 3 7% 5% 4 5% 4 7% 9% 39%	4421
June	1st to 10th	Night	5% 5% 2 7% 7% 5% 4 4 6% 10% 44%	1800
	11th to 20th	Night	3 3 1 4 5% 4 5% 5% 8% 11% 51%	1791
	21st to end	Night	5% 4 2 5% 4 4 5% 4 8% 11% 48%	1600
July	1st to 10th	Night	6% 6% 2 7% 6% 6% 6% 6% 7% 10% 38%	3679
	11th to 20th	Night	6% 5% 2 7% 5% 5% 6% 6% 9% 14% 35%	4057
	21st to end	Night	7% 7% 3 9% 6% 7% 6% 6% 9% 12% 29%	4472
August	1st to 10th	Night	2 2 1 4 3 4 5% 5% 8% 10% 56%	3244
	11th to 20th	Night	5% 4 1 4 4 4 5% 4 7% 10% 52%	3200
	21st to end	Night	5% 3 2 4 3 4 4 3 5% 7% 58%	3586
September	1st to 10th	Night	7% 5% 2 5% 5% 5% 4 4 6% 8% 49%	2010
	11th to 20th	Night	9% 7% 2 6% 4 5% 4 4 6% 9% 44%	2200
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 53%	2200
October	1st to 10th	Night	10% 4 2 4 4 3 3 2 4 6% 58%	2200
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 5% 55%	2215
	21st to end	Night	14% 6% 2 5% 3 3 3 3 4 5% 54%	2800
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 5% 54%	2666
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 53%	2730
	21st to end	Night	14% 7% 2 5% 4 3 4 3 4 6% 47%	2730
December	1st to 10th	Night	18% 9% 2 5% 3 3 3 3 4 5% 45%	4277
	11th to 20th	Night	20% 10% 3 6% 4 3 3 2 4 5% 41%	4822
	21st to end	Night	16% 10% 4 8% 5% 4 5% 4 5% 6% 35%	6490

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	48% 3 6% 21% 20% 2	4057
	11th to 20th	Day	42% 2 5% 23% 26% 2	4175
	21st to end	Day	49% 3 6% 19% 19% 4	4972
February	1st to 10th	Day	48% 3 7% 20% 19% 3	2000
	11th to 20th	Day	52% 3 5% 19% 16% 5%	1958
	21st to end	Day	49% 3 7% 15% 20% 6%	1650
March	1st to 10th	Day	52% 3 6% 14% 16% 8%	4480
	11th to 20th	Day	39% 3 7% 19% 20% 12%	4745
	21st to end	Day	44% 3 9% 25% 18% 2	5799
April	1st to 10th	Day	37% 3 11% 26% 17% 6%	2400
	11th to 20th	Day	38% 3 7% 23% 23% 6%	2374
	21st to end	Day	37% 5% 9% 28% 19% 2	2400
May	1st to 10th	Day	46% 5% 15% 21% 11% 2	5850
	11th to 20th	Day	49% 6% 14% 21% 9%	5865
	21st to end	Day	44% 5% 10% 22% 17% 2	6447
June	1st to 10th	Day	41% 6% 14% 21% 16% 1	2600
	11th to 20th	Day	31% 7% 15% 27% 17% 2	2600
	21st to end	Day	35% 8% 16% 21% 15% 4	2796
July	1st to 10th	Day	37% 10% 17% 25% 10% 1	6205
	11th to 20th	Day	36% 14% 25% 20% 4 1	5831
	21st to end	Day	46% 10% 23% 17% 4	6179
August	1st to 10th	Day	25% 6% 14% 37% 17% 1	3893
	11th to 20th	Day	32% 6% 17% 27% 14% 3	3885
	21st to end	Day	31% 7% 12% 21% 24% 4	4311
September	1st to 10th	Day	38% 5% 16% 19% 18% 5%	2368
	11th to 20th	Day	37% 5% 13% 20% 21% 3	2200
	21st to end	Day	33% 5% 10% 25% 20% 7%	2200
October	1st to 10th	Day	23% 3 5% 16% 34% 19%	3590
	11th to 20th	Day	20% 2 5% 19% 31% 23%	3536
	21st to end	Day	26% 4 6% 22% 33% 9%	3264
November	1st to 10th	Day	34% 3 8% 20% 27% 8%	1852
	11th to 20th	Day	35% 3 5% 22% 27% 7%	1890
	21st to end	Day	40% 3 6% 21% 24% 6%	1890
December	1st to 10th	Day	44% 3 5% 21% 23% 4	2948
	11th to 20th	Day	51% 3 6% 21% 14% 5%	3370
	21st to end	Day	49% 2 9% 24% 13% 2	4466
January	1st to 10th	Night	48% 2 6% 21% 20% 3	5889
	11th to 20th	Night	43% 3 5% 19% 25% 5%	5780
	21st to end	Night	47% 2 4 19% 22% 6%	5984
February	1st to 10th	Night	46% 3 7% 20% 20% 4	2400
	11th to 20th	Night	49% 3 6% 17% 17% 6%	2339
	21st to end	Night	49% 3 5% 17% 19% 7%	1980
March	1st to 10th	Night	53% 2 6% 15% 18% 6%	5373
	11th to 20th	Night	34% 3 8% 18% 23% 15%	5214
	21st to end	Night	45% 2 7% 22% 20% 4	5174
April	1st to 10th	Night	36% 4 11% 25% 20% 5%	2000
	11th to 20th	Night	35% 3 7% 21% 23% 10%	1975
	21st to end	Night	34% 4 11% 28% 19% 4	2000
May	1st to 10th	Night	44% 6% 14% 21% 14% 1	4106
	11th to 20th	Night	53% 6% 14% 19% 8% 1	4033
	21st to end	Night	41% 5% 13% 25% 15% 1	4429
June	1st to 10th	Night	36% 6% 14% 23% 19% 2	1800
	11th to 20th	Night	25% 6% 19% 27% 21% 3	1791
	21st to end	Night	28% 6% 18% 26% 17% 4	1600
July	1st to 10th	Night	37% 9% 21% 23% 9% 2	3640
	11th to 20th	Night	34% 12% 26% 22% 5% 1	4006
	21st to end	Night	43% 11% 24% 17% 5%	4341
August	1st to 10th	Night	20% 6% 14% 38% 21% 1	3232
	11th to 20th	Night	25% 7% 22% 29% 15% 2	3230
	21st to end	Night	25% 5% 15% 24% 27% 4	3586
September	1st to 10th	Night	32% 6% 17% 21% 19% 5%	2010
	11th to 20th	Night	35% 6% 14% 19% 22% 4	2200
	21st to end	Night	31% 4 10% 23% 24% 8%	2200
October	1st to 10th	Night	21% 3 6% 17% 34% 19%	3590
	11th to 20th	Night	21% 2 3 18% 32% 24%	3598
	21st to end	Night	24% 2 4 27% 28% 16%	4512
November	1st to 10th	Night	33% 4 8% 21% 24% 11%	2666
	11th to 20th	Night	34% 2 6% 19% 28% 10%	2730
	21st to end	Night	39% 3 5% 22% 24% 6%	2730
December	1st to 10th	Night	42% 3 5% 22% 25% 3	4277
	11th to 20th	Night	47% 3 5% 19% 22% 4	4824
	21st to end	Night	50% 3 7% 24% 14% 2	6490

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% 27% 34% 11% 3	300
	11th to 20th	Day	28% 22% 34% 10% 5% 1	300
	21st to end	Day	22% 25% 35% 15% 3	330
February	1st to 10th	Day	24% 24% 32% 17% 2 1	300
	11th to 20th	Day	36% 18% 34% 10% 1 1	300
	21st to end	Day	33% 20% 35% 10% 3	249
March	1st to 10th	Day	27% 23% 30% 16% 3 2	300
	11th to 20th	Day	23% 20% 33% 18% 5% 1	395
	21st to end	Day	23% 17% 36% 18% 6%	550
April	1st to 10th	Day	20% 19% 30% 21% 9% 2	500
	11th to 20th	Day	17% 16% 32% 24% 10% 1	492
	21st to end	Day	15% 14% 29% 30% 10% 2	500
May	1st to 10th	Day	17% 9% 32% 25% 12% 6%	503
	11th to 20th	Day	13% 8% 24% 27% 18% 9%	527
	21st to end	Day	12% 7% 22% 28% 19% 12%	573
June	1st to 10th	Day	13% 4 16% 31% 23% 14%	530
	11th to 20th	Day	5% 3 17% 29% 25% 21%	530
	21st to end	Day	8% 3 16% 28% 29% 16%	726
July	1st to 10th	Day	9% 2 9% 28% 33% 19%	687
	11th to 20th	Day	8% 2 6% 24% 30% 31%	524
	21st to end	Day	8% 1 7% 24% 35% 24%	576
August	1st to 10th	Day	9% 2 10% 26% 32% 22%	526
	11th to 20th	Day	9% 1 10% 29% 34% 17%	519
	21st to end	Day	14% 2 10% 22% 32% 21%	583
September	1st to 10th	Day	9% 3 11% 29% 33% 14%	530
	11th to 20th	Day	11% 2 12% 28% 30% 17%	530
	21st to end	Day	15% 4 16% 28% 28% 9%	530
October	1st to 10th	Day	11% 3 22% 31% 25% 8%	530
	11th to 20th	Day	12% 8% 28% 36% 14% 3	510
	21st to end	Day	22% 13% 23% 29% 11% 2	363
November	1st to 10th	Day	18% 12% 28% 26% 12% 2	330
	11th to 20th	Day	14% 16% 40% 23% 7% 1	330
	21st to end	Day	16% 21% 33% 21% 7% 2	330
December	1st to 10th	Day	26% 21% 29% 18% 5%	306
	11th to 20th	Day	21% 24% 31% 14% 10%	300
	21st to end	Day	22% 20% 39% 13% 6% 1	330
January	1st to 10th	Night	21% 19% 34% 21% 5%	800
	11th to 20th	Night	23% 19% 30% 22% 7%	800
	21st to end	Night	20% 17% 36% 25% 2	880
February	1st to 10th	Night	22% 15% 33% 25% 5%	800
	11th to 20th	Night	28% 16% 31% 23% 3	783
	21st to end	Night	25% 17% 33% 23% 2	660
March	1st to 10th	Night	21% 18% 29% 26% 6%	800
	11th to 20th	Night	19% 13% 37% 23% 8% 1	800
	21st to end	Night	17% 14% 34% 25% 10%	755
April	1st to 10th	Night	15% 13% 35% 26% 11%	600
	11th to 20th	Night	15% 10% 35% 28% 12% 1	593
	21st to end	Night	14% 9% 30% 30% 16% 1	600
May	1st to 10th	Night	11% 6% 30% 31% 20% 3	600
	11th to 20th	Night	13% 5% 23% 33% 22% 4	591
	21st to end	Night	6% 2 18% 37% 30% 6%	650
June	1st to 10th	Night	8% 2 16% 35% 34% 5%	600
	11th to 20th	Night	4 3 12% 33% 40% 9%	600
	21st to end	Night	7% 1 10% 30% 43% 9%	600
July	1st to 10th	Night	8% 2 8% 28% 43% 11%	585
	11th to 20th	Night	7% 2 5% 28% 42% 17%	582
	21st to end	Night	8% 2 7% 29% 42% 13%	657
August	1st to 10th	Night	4 1 3 29% 52% 11%	597
	11th to 20th	Night	9% 1 6% 32% 41% 11%	594
	21st to end	Night	10% 2 7% 30% 41% 10%	660
September	1st to 10th	Night	9% 2 10% 31% 41% 7%	610
	11th to 20th	Night	10% 3 13% 29% 38% 7%	800
	21st to end	Night	11% 4 13% 34% 36% 3	800
October	1st to 10th	Night	11% 5% 20% 31% 28% 7%	800
	11th to 20th	Night	14% 6% 24% 35% 19% 2	800
	21st to end	Night	17% 8% 22% 32% 20% 2	880
November	1st to 10th	Night	15% 9% 21% 34% 19% 3	820
	11th to 20th	Night	16% 10% 31% 30% 12% 2	840
	21st to end	Night	19% 12% 28% 29% 11% 1	840
December	1st to 10th	Night	24% 15% 26% 28% 7%	809
	11th to 20th	Night	23% 13% 30% 26% 9%	800
	21st to end	Night	24% 15% 29% 26% 6%	880

Page generated at 2025-11-30 00:00:53. Version ac07

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