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 2024-11-04.

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% 7% 3 6% 4 4 5% 4 6% 7% 40%	101078
	Night	13% 8% 3 6% 4 4 4 3 5% 7% 43%	97926
Transparency	Day	40% 5% 10% 24% 17% 3	104785
	Night	39% 5% 10% 24% 19% 4	102345
Seeing	Day	16% 8% 22% 24% 20% 10%	15243
	Night	17% 7% 24% 31% 19% 3	24969

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	14% 12% 8% 9% 5% 5% 5% 3 4 5% 31%	2527
	11th to 20th	Day	19% 11% 3 11% 5% 4 4 2 4 4 34%	2536
	21st to end	Day	24% 11% 7% 5% 4 3 3 2 4 5% 31%	3412
February	1st to 10th	Day	18% 12% 4 7% 4 4 5% 3 4 6% 35%	1900
	11th to 20th	Day	21% 10% 4 7% 4 4 3 2 5% 5% 35%	1848
	21st to end	Day	18% 10% 5% 7% 4 4 4 3 4 6% 36%	1570
March	1st to 10th	Day	22% 13% 4 7% 5% 3 4 3 4 4 32%	3120
	11th to 20th	Day	13% 7% 3 5% 4 3 4 3 4 6% 47%	3312
	21st to end	Day	12% 8% 3 7% 5% 4 4 3 5% 7% 41%	4059
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 44%	2280
	11th to 20th	Day	13% 8% 3 7% 3 4 4 4 5% 6% 43%	2254
	21st to end	Day	14% 7% 2 6% 6% 4 5% 4 5% 6% 41%	2318
May	1st to 10th	Day	19% 6% 2 6% 4 4 5% 4 7% 8% 34%	4082
	11th to 20th	Day	16% 7% 3 7% 4 4 5% 3 5% 8% 37%	4081
	21st to end	Day	11% 6% 3 7% 5% 4 5% 4 7% 8% 41%	4476
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 8% 42%	2533
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 8% 9% 46%	2660
	21st to end	Day	5% 4 2 4 4 5% 6% 6% 9% 12% 43%	2660
July	1st to 10th	Day	7% 5% 3 6% 6% 5% 6% 5% 7% 11% 38%	4440
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 6% 8% 10% 39%	4422
	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	12% 5% 1 5% 2 3 2 3 4 7% 56%	1699
	11th to 20th	Day	14% 7% 2 4 4 3 3 4 4 7% 48%	1710
	21st to end	Day	14% 9% 2 5% 3 4 3 3 5% 5% 45%	1710
December	1st to 10th	Day	20% 10% 3 7% 3 3 5% 3 4 5% 37%	1710
	11th to 20th	Day	16% 10% 3 7% 5% 5% 5% 3 5% 7% 36%	2153
	21st to end	Day	15% 13% 4 8% 5% 5% 5% 3 6% 7% 29%	2862
January	1st to 10th	Night	17% 13% 3 7% 3 3 3 3 4 6% 37%	4428
	11th to 20th	Night	18% 11% 4 10% 5% 4 4 3 4 4 33%	4428
	21st to end	Night	23% 12% 5% 6% 3 3 3 3 4 5% 34%	4240
February	1st to 10th	Night	21% 11% 4 8% 5% 4 3 3 4 6% 32%	2280
	11th to 20th	Night	20% 8% 4 9% 4 3 4 3 4 5% 36%	2218
	21st to end	Night	21% 10% 3 8% 5% 4 3 3 4 5% 33%	1884
March	1st to 10th	Night	22% 10% 3 8% 5% 4 3 3 5% 5% 33%	3733
	11th to 20th	Night	13% 7% 2 6% 4 4 3 3 4 5% 49%	3646
	21st to end	Night	14% 9% 3 6% 5% 5% 4 4 4 6% 42%	3601
April	1st to 10th	Night	11% 6% 2 6% 5% 3 4 3 6% 7% 47%	1900
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 7% 46%	1875
	21st to end	Night	13% 7% 2 6% 5% 4 3 4 5% 9% 41%	1861
May	1st to 10th	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 39%	2813
	11th to 20th	Night	11% 7% 3 7% 4 5% 4 4 6% 7% 43%	2812
	21st to end	Night	7% 5% 3 7% 5% 4 5% 4 7% 9% 44%	3079
June	1st to 10th	Night	10% 6% 3 7% 4 4 4 3 6% 8% 46%	1641
	11th to 20th	Night	3 3 1 4 4 4 3 5% 8% 10% 55%	1520
	21st to end	Night	3 3 2 4 5% 4 5% 5% 9% 12% 49%	1520
July	1st to 10th	Night	5% 5% 2 6% 5% 5% 5% 5% 7% 14% 42%	2530
	11th to 20th	Night	6% 5% 2 6% 4 3 4 4 6% 10% 50%	2530
	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	12% 6% 2 4 3 3 3 2 5% 5% 55%	2452
	11th to 20th	Night	12% 7% 2 5% 3 4 3 3 5% 7% 49%	2470
	21st to end	Night	16% 8% 3 5% 4 3 3 2 4 6% 46%	2470
December	1st to 10th	Night	20% 11% 3 6% 4 4 3 4 4 6% 35%	2470
	11th to 20th	Night	17% 9% 3 6% 3 3 4 3 4 5% 42%	3042
	21st to end	Night	16% 13% 5% 10% 5% 5% 5% 3 5% 6% 28%	4795

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% 5% 6% 22% 11% 2	2528
	11th to 20th	Day	53% 3 7% 22% 14% 1	2536
	21st to end	Day	56% 4 7% 23% 9% 1	3412
February	1st to 10th	Day	52% 3 8% 20% 14% 2	1900
	11th to 20th	Day	52% 3 7% 20% 14% 4	1858
	21st to end	Day	50% 4 8% 18% 14% 6%	1570
March	1st to 10th	Day	56% 4 5% 16% 13% 7%	3120
	11th to 20th	Day	38% 5% 9% 26% 16% 5%	3312
	21st to end	Day	42% 4 10% 26% 16% 1	4059
April	1st to 10th	Day	40% 4 10% 23% 20% 3	2280
	11th to 20th	Day	41% 4 8% 22% 20% 4	2254
	21st to end	Day	43% 5% 9% 26% 15% 2	2318
May	1st to 10th	Day	46% 6% 13% 22% 12% 1	4120
	11th to 20th	Day	46% 6% 10% 25% 12% 1	4085
	21st to end	Day	40% 6% 11% 22% 20% 1	4489
June	1st to 10th	Day	37% 7% 11% 25% 17% 3	2533
	11th to 20th	Day	32% 6% 17% 24% 19% 3	2660
	21st to end	Day	29% 8% 17% 25% 15% 5%	2660
July	1st to 10th	Day	37% 7% 19% 26% 9% 2	4371
	11th to 20th	Day	35% 7% 15% 30% 10% 2	4347
	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	30% 4 8% 25% 27% 5%	1699
	11th to 20th	Day	36% 4 7% 23% 23% 6%	1710
	21st to end	Day	41% 3 7% 25% 19% 4	1710
December	1st to 10th	Day	50% 4 8% 23% 14% 2	1710
	11th to 20th	Day	47% 5% 8% 22% 16% 2	2150
	21st to end	Day	53% 5% 12% 20% 10% 1	2862
January	1st to 10th	Night	49% 4 7% 25% 14% 1	4428
	11th to 20th	Night	55% 4 5% 18% 15% 3	4428
	21st to end	Night	54% 4 8% 22% 12% 1	4240
February	1st to 10th	Night	54% 3 8% 19% 14% 1	2280
	11th to 20th	Night	52% 4 5% 20% 15% 4	2219
	21st to end	Night	55% 3 7% 16% 15% 4	1884
March	1st to 10th	Night	52% 5% 9% 13% 16% 5%	3733
	11th to 20th	Night	37% 4 10% 24% 14% 11%	3646
	21st to end	Night	43% 4 9% 25% 18% 1	3601
April	1st to 10th	Night	36% 4 10% 26% 20% 3	1900
	11th to 20th	Night	39% 4 8% 20% 22% 7%	1875
	21st to end	Night	41% 4 10% 27% 17% 2	1861
May	1st to 10th	Night	44% 6% 13% 22% 13% 2	2840
	11th to 20th	Night	40% 6% 15% 23% 16% 1	2813
	21st to end	Night	34% 8% 15% 24% 18% 1	3087
June	1st to 10th	Night	36% 5% 14% 26% 17% 4	1641
	11th to 20th	Night	23% 5% 18% 29% 22% 2	1520
	21st to end	Night	25% 8% 19% 29% 14% 6%	1520
July	1st to 10th	Night	32% 6% 18% 31% 11% 2	2490
	11th to 20th	Night	29% 5% 16% 36% 12% 3	2482
	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	32% 4 7% 25% 25% 8%	2452
	11th to 20th	Night	35% 4 6% 23% 25% 8%	2470
	21st to end	Night	41% 3 7% 25% 20% 4	2470
December	1st to 10th	Night	51% 4 8% 20% 14% 2	2470
	11th to 20th	Night	44% 4 6% 23% 20% 3	3044
	21st to end	Night	55% 5% 8% 18% 11% 2	4795

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% 15% 37% 19% 6% 1	270
	11th to 20th	Day	26% 19% 38% 12% 4 1	270
	21st to end	Day	23% 14% 42% 15% 4	297
February	1st to 10th	Day	26% 13% 37% 18% 5% 1	270
	11th to 20th	Day	31% 18% 36% 12% 3	270
	21st to end	Day	29% 17% 38% 12% 3	225
March	1st to 10th	Day	28% 15% 33% 19% 4 1	270
	11th to 20th	Day	23% 14% 38% 17% 7% 1	357
	21st to end	Day	20% 16% 35% 21% 8% 1	506
April	1st to 10th	Day	20% 14% 27% 27% 12% 1	460
	11th to 20th	Day	19% 13% 27% 25% 13% 3	452
	21st to end	Day	21% 9% 31% 23% 13% 4	460
May	1st to 10th	Day	21% 10% 25% 25% 14% 5%	463
	11th to 20th	Day	18% 5% 21% 26% 20% 10%	487
	21st to end	Day	11% 4 18% 29% 23% 16%	529
June	1st to 10th	Day	13% 4 17% 22% 28% 17%	553
	11th to 20th	Day	5% 4 16% 27% 31% 18%	680
	21st to end	Day	5% 3 16% 33% 26% 16%	680
July	1st to 10th	Day	9% 3 9% 26% 30% 23%	671
	11th to 20th	Day	8% 2 10% 25% 32% 22%	663
	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	18% 5% 28% 28% 19% 3	279
	11th to 20th	Day	14% 14% 32% 31% 8%	270
	21st to end	Day	21% 10% 29% 24% 13% 2	270
December	1st to 10th	Day	22% 17% 30% 23% 7% 1	270
	11th to 20th	Day	19% 14% 32% 23% 11%	270
	21st to end	Day	22% 18% 34% 20% 6%	297
January	1st to 10th	Night	23% 15% 30% 27% 5%	950
	11th to 20th	Night	29% 12% 32% 22% 5%	938
	21st to end	Night	23% 11% 39% 25% 3	836
February	1st to 10th	Night	26% 12% 33% 25% 4	760
	11th to 20th	Night	25% 14% 35% 23% 2	743
	21st to end	Night	27% 13% 35% 23% 1	628
March	1st to 10th	Night	24% 11% 32% 28% 5%	760
	11th to 20th	Night	21% 11% 34% 27% 6% 1	760
	21st to end	Night	18% 10% 33% 31% 8%	710
April	1st to 10th	Night	14% 9% 33% 34% 8% 1	570
	11th to 20th	Night	18% 8% 33% 30% 12% 1	563
	21st to end	Night	16% 6% 32% 33% 12% 1	570
May	1st to 10th	Night	15% 4 30% 31% 18% 2	570
	11th to 20th	Night	12% 5% 20% 38% 21% 4	561
	21st to end	Night	5% 2 15% 40% 32% 5%	617
June	1st to 10th	Night	12% 2 14% 36% 31% 4	570
	11th to 20th	Night	5% 2 10% 40% 38% 6%	570
	21st to end	Night	5% 1 9% 40% 39% 6%	570
July	1st to 10th	Night	9% 2 6% 32% 42% 10%	555
	11th to 20th	Night	7% 3 7% 28% 47% 9%	552
	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	13% 7% 22% 38% 18% 2	755
	11th to 20th	Night	16% 8% 34% 31% 11% 1	760
	21st to end	Night	20% 9% 28% 32% 11% 1	760
December	1st to 10th	Night	26% 10% 30% 27% 7% 1	760
	11th to 20th	Night	23% 7% 31% 31% 7% 1	760
	21st to end	Night	26% 10% 32% 28% 5%	995

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