N4LB Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the N4LB Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 8% 3 7% 4 4 5% 4 6% 7% 40%	120064
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	119142
Transparency	Day	42% 5% 10% 23% 17% 3	123771
	Night	41% 4 9% 23% 18% 4	123561
Seeing	Day	16% 8% 22% 24% 20% 10%	16191
	Night	17% 7% 24% 31% 19% 3	25912

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% 11% 9% 12% 4 4 4 2 4 5% 31%	3617
	11th to 20th	Day	14% 9% 3 12% 4 3 3 2 4 5% 41%	3639
	21st to end	Day	20% 9% 5% 5% 4 3 4 3 5% 6% 38%	4894
February	1st to 10th	Day	18% 12% 4 7% 4 4 4 3 4 6% 35%	2000
	11th to 20th	Day	22% 10% 4 8% 4 4 3 2 5% 5% 33%	1948
	21st to end	Day	17% 10% 4 6% 4 4 3 3 4 6% 38%	1650
March	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 4 4 36%	4480
	11th to 20th	Day	13% 9% 3 6% 4 3 4 3 4 5% 47%	4742
	21st to end	Day	11% 7% 3 7% 5% 5% 4 3 5% 7% 43%	5819
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	13% 8% 3 7% 3 4 4 4 5% 6% 43%	2374
	21st to end	Day	14% 8% 2 6% 6% 4 5% 4 5% 7% 40%	2440
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 7% 8% 27%	5862
	11th to 20th	Day	15% 9% 3 9% 5% 5% 5% 4 6% 8% 31%	5861
	21st to end	Day	14% 7% 3 8% 5% 4 5% 3 6% 7% 39%	6433
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 42%	2667
	11th to 20th	Day	5% 5% 2 5% 5% 5% 6% 5% 8% 9% 44%	2800
	21st to end	Day	4 4 2 4 4 5% 6% 6% 9% 13% 43%	2800
July	1st to 10th	Day	7% 5% 3 6% 6% 5% 6% 5% 8% 11% 38%	4580
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 6% 8% 10% 39%	4562
	21st to end	Day	16% 10% 4 10% 6% 6% 5% 5% 6% 8% 25%	4466
August	1st to 10th	Day	5% 3 2 5% 3 5% 5% 4 6% 9% 53%	3914
	11th to 20th	Day	7% 5% 3 6% 6% 6% 6% 5% 8% 10% 38%	3729
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 9% 54%	4180
September	1st to 10th	Day	12% 6% 2 5% 5% 5% 6% 4 6% 8% 41%	2261
	11th to 20th	Day	13% 6% 2 7% 4 4 4 3 5% 6% 46%	2090
	21st to end	Day	11% 6% 2 6% 4 4 4 4 5% 6% 48%	2090
October	1st to 10th	Day	10% 4 2 4 3 3 4 3 4 5% 57%	2090
	11th to 20th	Day	12% 6% 2 6% 2 3 3 2 4 6% 53%	2028
	21st to end	Day	18% 7% 2 5% 4 3 3 3 4 4 47%	1896
November	1st to 10th	Day	13% 5% 1 5% 2 3 2 3 4 7% 54%	1762
	11th to 20th	Day	14% 7% 2 4 4 3 3 4 4 7% 47%	1800
	21st to end	Day	14% 10% 3 6% 3 4 3 3 5% 5% 44%	1800
December	1st to 10th	Day	15% 9% 3 6% 4 4 5% 3 4 5% 44%	2930
	11th to 20th	Day	17% 11% 5% 9% 5% 4 5% 3 4 6% 32%	3373
	21st to end	Day	15% 12% 5% 9% 5% 6% 5% 3 6% 7% 27%	4087
January	1st to 10th	Night	15% 13% 4 10% 3 3 3 3 4 6% 35%	6348
	11th to 20th	Night	18% 10% 4 9% 4 4 4 2 3 4 40%	6334
	21st to end	Night	19% 10% 4 6% 4 3 3 2 4 4 40%	6058
February	1st to 10th	Night	21% 11% 4 8% 5% 4 3 3 4 6% 32%	2400
	11th to 20th	Night	22% 8% 4 9% 4 3 4 3 4 5% 35%	2338
	21st to end	Night	20% 10% 3 8% 4 4 3 3 4 5% 35%	1980
March	1st to 10th	Night	17% 10% 3 8% 4 4 4 3 5% 6% 35%	5373
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 6% 6% 49%	5216
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 4 7% 41%	5153
April	1st to 10th	Night	12% 6% 2 6% 5% 3 4 3 6% 7% 46%	2000
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 4 7% 47%	1975
	21st to end	Night	13% 7% 2 6% 5% 4 4 4 6% 9% 40%	1959
May	1st to 10th	Night	14% 10% 3 9% 5% 5% 4 4 6% 7% 32%	4033
	11th to 20th	Night	13% 9% 3 9% 5% 5% 5% 4 6% 8% 33%	4032
	21st to end	Night	12% 7% 3 7% 5% 4 5% 4 6% 8% 38%	4421
June	1st to 10th	Night	9% 6% 3 7% 4 3 4 3 7% 8% 46%	1727
	11th to 20th	Night	4 4 1 4 4 4 4 5% 8% 9% 53%	1600
	21st to end	Night	3 3 2 4 5% 4 5% 5% 9% 12% 49%	1600
July	1st to 10th	Night	5% 4 2 6% 5% 5% 5% 5% 8% 13% 42%	2610
	11th to 20th	Night	6% 5% 2 5% 4 4 4 4 6% 11% 49%	2610
	21st to end	Night	10% 9% 3 9% 6% 6% 6% 5% 7% 9% 30%	3087
August	1st to 10th	Night	4 3 1 4 3 3 4 3 4 8% 62%	3011
	11th to 20th	Night	4 3 1 5% 5% 5% 6% 4 8% 11% 48%	3100
	21st to end	Night	4 3 2 4 4 4 4 3 5% 8% 58%	3476
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 6% 50%	1900
	11th to 20th	Night	10% 5% 2 6% 4 3 4 3 5% 8% 50%	2088
	21st to end	Night	9% 5% 2 5% 4 3 4 4 5% 5% 53%	2090
October	1st to 10th	Night	9% 4 1 4 3 2 3 2 5% 6% 61%	2090
	11th to 20th	Night	16% 5% 2 4 3 3 3 2 5% 5% 52%	2147
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 6% 49%	2699
November	1st to 10th	Night	13% 5% 2 4 3 3 4 2 5% 5% 53%	2536
	11th to 20th	Night	12% 8% 2 5% 3 4 3 3 5% 7% 48%	2600
	21st to end	Night	16% 8% 3 5% 4 3 3 2 4 6% 46%	2600
December	1st to 10th	Night	17% 11% 3 8% 4 4 3 3 4 4 41%	4250
	11th to 20th	Night	18% 10% 4 7% 4 3 4 2 4 5% 39%	4822
	21st to end	Night	16% 13% 6% 10% 5% 5% 4 3 5% 5% 27%	6879

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	56% 4 9% 18% 10% 2	3618
	11th to 20th	Day	47% 3 5% 24% 20% 1	3639
	21st to end	Day	47% 3 6% 22% 17% 5%	4894
February	1st to 10th	Day	52% 3 9% 21% 14% 2	2000
	11th to 20th	Day	53% 3 7% 19% 14% 4	1958
	21st to end	Day	48% 3 8% 19% 15% 6%	1650
March	1st to 10th	Day	52% 4 4 15% 18% 7%	4480
	11th to 20th	Day	40% 4 8% 23% 19% 6%	4742
	21st to end	Day	42% 4 10% 28% 16% 1	5821
April	1st to 10th	Day	40% 4 10% 23% 20% 3	2400
	11th to 20th	Day	41% 4 8% 23% 20% 4	2374
	21st to end	Day	44% 5% 9% 26% 15% 2	2440
May	1st to 10th	Day	52% 6% 14% 19% 9% 1	5897
	11th to 20th	Day	50% 6% 13% 20% 10%	5865
	21st to end	Day	44% 6% 9% 22% 16% 2	6447
June	1st to 10th	Day	38% 7% 11% 25% 17% 3	2667
	11th to 20th	Day	33% 6% 16% 24% 18% 3	2800
	21st to end	Day	29% 9% 17% 25% 15% 5%	2800
July	1st to 10th	Day	36% 7% 19% 26% 9% 2	4511
	11th to 20th	Day	36% 7% 16% 30% 10% 2	4487
	21st to end	Day	55% 6% 13% 18% 7% 1	4324
August	1st to 10th	Day	25% 6% 14% 33% 21% 1	3901
	11th to 20th	Day	36% 7% 14% 26% 16% 1	3765
	21st to end	Day	26% 7% 10% 35% 21% 2	4179
September	1st to 10th	Day	39% 8% 10% 20% 19% 4	2261
	11th to 20th	Day	39% 5% 9% 24% 21% 2	2090
	21st to end	Day	36% 5% 9% 25% 21% 5%	2090
October	1st to 10th	Day	20% 4 6% 18% 44% 9%	3480
	11th to 20th	Day	24% 3 7% 25% 29% 13%	3346
	21st to end	Day	31% 3 6% 25% 28% 8%	3096
November	1st to 10th	Day	32% 4 8% 25% 26% 5%	1762
	11th to 20th	Day	37% 4 7% 23% 23% 6%	1800
	21st to end	Day	42% 3 7% 25% 19% 4	1800
December	1st to 10th	Day	43% 3 6% 27% 19% 1	2930
	11th to 20th	Day	52% 4 7% 21% 14% 2	3370
	21st to end	Day	55% 5% 11% 22% 7%	4087
January	1st to 10th	Night	51% 3 9% 22% 14% 1	6348
	11th to 20th	Night	51% 3 4 19% 19% 3	6334
	21st to end	Night	48% 3 6% 21% 16% 5%	6058
February	1st to 10th	Night	55% 3 9% 18% 14% 1	2400
	11th to 20th	Night	53% 4 5% 19% 15% 4	2339
	21st to end	Night	53% 3 6% 17% 17% 4	1980
March	1st to 10th	Night	49% 4 7% 16% 19% 6%	5373
	11th to 20th	Night	35% 4 9% 21% 19% 12%	5216
	21st to end	Night	44% 4 9% 26% 15% 2	5153
April	1st to 10th	Night	38% 4 10% 25% 20% 3	2000
	11th to 20th	Night	38% 4 8% 21% 22% 7%	1975
	21st to end	Night	41% 4 11% 26% 17% 2	1959
May	1st to 10th	Night	50% 6% 13% 20% 10% 1	4060
	11th to 20th	Night	49% 7% 15% 18% 11%	4033
	21st to end	Night	43% 6% 13% 23% 14% 1	4429
June	1st to 10th	Night	35% 5% 14% 26% 17% 4	1727
	11th to 20th	Night	24% 6% 18% 28% 22% 2	1600
	21st to end	Night	24% 8% 18% 29% 15% 6%	1600
July	1st to 10th	Night	32% 6% 18% 30% 11% 2	2570
	11th to 20th	Night	29% 5% 17% 36% 11% 3	2562
	21st to end	Night	49% 8% 14% 19% 8% 1	2988
August	1st to 10th	Night	20% 5% 11% 39% 23% 1	3000
	11th to 20th	Night	26% 7% 20% 30% 16% 1	3130
	21st to end	Night	22% 6% 11% 34% 25% 3	3476
September	1st to 10th	Night	34% 6% 10% 24% 20% 6%	1900
	11th to 20th	Night	33% 6% 9% 24% 25% 4	2088
	21st to end	Night	32% 4 8% 24% 25% 7%	2090
October	1st to 10th	Night	18% 4 6% 19% 42% 11%	3480
	11th to 20th	Night	21% 2 5% 24% 34% 14%	3555
	21st to end	Night	26% 3 7% 25% 27% 12%	4449
November	1st to 10th	Night	34% 4 7% 24% 24% 7%	2536
	11th to 20th	Night	35% 4 6% 23% 24% 7%	2600
	21st to end	Night	41% 3 7% 26% 19% 5%	2600
December	1st to 10th	Night	47% 3 7% 19% 22% 1	4250
	11th to 20th	Night	48% 3 6% 23% 17% 2	4824
	21st to end	Night	58% 5% 8% 19% 8% 1	6879

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	22% 16% 37% 19% 6% 1	300
	11th to 20th	Day	24% 20% 38% 14% 4 1	300
	21st to end	Day	23% 15% 44% 15% 4	330
February	1st to 10th	Day	25% 13% 39% 18% 4 1	300
	11th to 20th	Day	31% 19% 36% 11% 3	300
	21st to end	Day	27% 16% 41% 14% 3	249
March	1st to 10th	Day	27% 16% 32% 18% 5% 1	300
	11th to 20th	Day	23% 15% 38% 17% 6% 1	392
	21st to end	Day	19% 17% 36% 20% 7% 1	550
April	1st to 10th	Day	21% 14% 27% 26% 11% 1	500
	11th to 20th	Day	18% 14% 29% 24% 12% 2	492
	21st to end	Day	20% 9% 31% 23% 12% 4	500
May	1st to 10th	Day	22% 10% 26% 24% 13% 4	503
	11th to 20th	Day	18% 6% 22% 26% 19% 9%	527
	21st to end	Day	12% 5% 19% 28% 22% 15%	573
June	1st to 10th	Day	12% 4 18% 23% 27% 17%	597
	11th to 20th	Day	6% 4 17% 25% 31% 17%	730
	21st to end	Day	5% 2 15% 32% 27% 18%	730
July	1st to 10th	Day	9% 2 9% 26% 31% 23%	721
	11th to 20th	Day	8% 2 9% 25% 33% 22%	713
	21st to end	Day	10% 3 9% 26% 28% 24%	532
August	1st to 10th	Day	11% 2 7% 26% 31% 23%	486
	11th to 20th	Day	9% 3 11% 29% 33% 15%	479
	21st to end	Day	11% 2 6% 20% 36% 26%	539
September	1st to 10th	Day	11% 2 11% 24% 34% 18%	490
	11th to 20th	Day	14% 2 8% 24% 35% 17%	490
	21st to end	Day	15% 5% 17% 26% 28% 9%	490
October	1st to 10th	Day	12% 3 17% 33% 26% 11%	490
	11th to 20th	Day	13% 9% 29% 28% 16% 4	428
	21st to end	Day	23% 9% 25% 30% 11% 3	330
November	1st to 10th	Day	19% 7% 27% 26% 18% 2	300
	11th to 20th	Day	15% 15% 31% 30% 8%	300
	21st to end	Day	21% 10% 30% 24% 13% 2	300
December	1st to 10th	Day	21% 18% 32% 22% 7% 1	300
	11th to 20th	Day	20% 15% 31% 23% 10%	300
	21st to end	Day	22% 17% 34% 21% 5%	330
January	1st to 10th	Night	23% 15% 30% 27% 5%	1000
	11th to 20th	Night	28% 12% 33% 22% 5%	987
	21st to end	Night	23% 11% 39% 25% 3	880
February	1st to 10th	Night	26% 11% 34% 25% 4	800
	11th to 20th	Night	26% 14% 34% 23% 2	783
	21st to end	Night	26% 13% 36% 24% 1	660
March	1st to 10th	Night	23% 12% 32% 28% 5%	800
	11th to 20th	Night	21% 11% 34% 27% 6% 1	800
	21st to end	Night	18% 10% 33% 31% 7%	747
April	1st to 10th	Night	16% 9% 33% 33% 8% 1	600
	11th to 20th	Night	17% 8% 33% 31% 11% 1	593
	21st to end	Night	16% 7% 32% 33% 12% 2	600
May	1st to 10th	Night	16% 4 30% 32% 17% 2	600
	11th to 20th	Night	12% 5% 21% 38% 20% 4	591
	21st to end	Night	6% 2 16% 40% 30% 5%	650
June	1st to 10th	Night	12% 2 14% 37% 31% 4	600
	11th to 20th	Night	5% 2 10% 40% 38% 5%	600
	21st to end	Night	4 1 9% 40% 40% 7%	600
July	1st to 10th	Night	8% 2 5% 31% 43% 10%	585
	11th to 20th	Night	7% 3 7% 28% 48% 8%	582
	21st to end	Night	8% 2 6% 33% 41% 9%	624
August	1st to 10th	Night	8% 1 5% 33% 45% 8%	567
	11th to 20th	Night	6% 2 6% 42% 39% 5%	564
	21st to end	Night	8% 2 6% 30% 44% 10%	627
September	1st to 10th	Night	11% 3 7% 32% 41% 6%	570
	11th to 20th	Night	11% 4 10% 31% 38% 6%	758
	21st to end	Night	11% 2 16% 34% 34% 4	760
October	1st to 10th	Night	10% 3 17% 35% 28% 6%	760
	11th to 20th	Night	18% 6% 22% 34% 19% 1	760
	21st to end	Night	18% 5% 29% 31% 15% 2	836
November	1st to 10th	Night	15% 7% 22% 37% 17% 2	780
	11th to 20th	Night	16% 8% 34% 31% 11% 1	800
	21st to end	Night	19% 10% 28% 31% 11% 1	800
December	1st to 10th	Night	25% 10% 31% 27% 7% 1	800
	11th to 20th	Night	24% 7% 32% 30% 6% 1	800
	21st to end	Night	26% 10% 31% 28% 5%	1048

Page generated at 2025-07-18 00:09:59. Version ac07

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