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 2024-04-10.

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% 5% 3 5% 5% 5% 7% 40%	83417
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	85389
Transparency	Day	42% 5% 11% 23% 16% 3	83130
	Night	41% 5% 10% 23% 18% 4	85196
Seeing	Day	16% 8% 22% 24% 20% 11%	14387
	Night	17% 7% 24% 31% 19% 3	24291

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% 4 5% 4 4 5% 31%	2527
	11th to 20th	Day	19% 10% 3 9% 7% 3 4 3 4 4 34%	2536
	21st to end	Day	25% 11% 7% 5% 4 2 3 3 4 6% 31%	3412
February	1st to 10th	Day	18% 11% 4 6% 5% 3 5% 4 4 6% 36%	1900
	11th to 20th	Day	21% 9% 4 7% 4 3 3 3 5% 6% 35%	1848
	21st to end	Day	19% 9% 5% 6% 5% 3 4 4 4 6% 36%	1570
March	1st to 10th	Day	22% 13% 4 6% 6% 3 4 3 4 4 32%	3120
	11th to 20th	Day	14% 7% 3 5% 4 2 4 4 4 6% 47%	3312
	21st to end	Day	13% 7% 3 6% 6% 3 4 5% 5% 7% 42%	4059
April	1st to 10th	Day	12% 7% 2 5% 5% 3 5% 5% 5% 7% 44%	2269
	11th to 20th	Day	14% 7% 3 7% 4 3 4 4 5% 6% 43%	2134
	21st to end	Day	15% 7% 3 5% 6% 3 5% 5% 5% 6% 40%	2196
May	1st to 10th	Day	17% 5% 2 4 5% 3 5% 4 6% 8% 41%	2340
	11th to 20th	Day	14% 6% 3 6% 6% 3 5% 5% 5% 7% 39%	2301
	21st to end	Day	9% 5% 3 6% 7% 3 7% 6% 8% 9% 38%	2531
June	1st to 10th	Day	10% 5% 2 5% 6% 4 5% 6% 6% 9% 43%	2399
	11th to 20th	Day	5% 4 2 4 5% 4 6% 6% 8% 9% 45%	2520
	21st to end	Day	5% 3 2 4 5% 4 6% 8% 8% 12% 44%	2520
July	1st to 10th	Day	8% 6% 4 6% 7% 5% 7% 6% 7% 10% 34%	2520
	11th to 20th	Day	8% 5% 3 5% 6% 5% 6% 7% 8% 9% 38%	2516
	21st to end	Day	8% 5% 3 7% 6% 3 6% 7% 7% 12% 37%	2547
August	1st to 10th	Day	6% 5% 2 6% 5% 4 6% 7% 8% 10% 38%	2221
	11th to 20th	Day	9% 5% 3 6% 7% 4 6% 6% 7% 9% 38%	2125
	21st to end	Day	8% 6% 2 5% 6% 4 6% 6% 7% 10% 41%	2376
September	1st to 10th	Day	12% 6% 2 5% 6% 3 6% 6% 6% 8% 40%	2142
	11th to 20th	Day	13% 6% 2 6% 5% 3 5% 4 5% 6% 46%	1980
	21st to end	Day	10% 5% 2 5% 4 3 4 4 4 6% 51%	1980
October	1st to 10th	Day	10% 4 2 4 4 2 4 4 4 6% 55%	1980
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 4 6% 52%	1922
	21st to end	Day	18% 7% 2 4 4 2 4 4 4 4 46%	1797
November	1st to 10th	Day	12% 4 2 5% 3 2 3 3 4 7% 56%	1672
	11th to 20th	Day	14% 6% 2 4 4 3 3 4 4 7% 48%	1710
	21st to end	Day	15% 9% 3 5% 4 3 4 4 5% 5% 45%	1710
December	1st to 10th	Day	20% 10% 3 6% 4 2 5% 4 4 5% 37%	1710
	11th to 20th	Day	16% 10% 3 6% 5% 3 5% 4 5% 7% 36%	2153
	21st to end	Day	15% 12% 5% 6% 6% 4 5% 4 6% 7% 29%	2862
January	1st to 10th	Night	17% 13% 4 6% 4 2 3 4 4 7% 37%	4428
	11th to 20th	Night	19% 10% 4 9% 6% 3 5% 4 4 4 33%	4428
	21st to end	Night	23% 11% 5% 6% 4 2 3 3 4 5% 34%	4240
February	1st to 10th	Night	22% 10% 4 7% 5% 3 4 4 4 6% 33%	2280
	11th to 20th	Night	21% 7% 4 8% 5% 2 4 4 4 5% 37%	2218
	21st to end	Night	21% 10% 4 7% 6% 3 4 4 4 5% 33%	1884
March	1st to 10th	Night	22% 10% 3 7% 5% 3 4 4 5% 5% 33%	3733
	11th to 20th	Night	13% 7% 2 5% 4 3 4 4 4 6% 49%	3646
	21st to end	Night	14% 8% 3 5% 5% 4 4 4 4 7% 42%	3601
April	1st to 10th	Night	12% 5% 3 5% 5% 2 5% 4 5% 8% 47%	1900
	11th to 20th	Night	16% 6% 2 4 4 2 4 4 4 7% 46%	1775
	21st to end	Night	14% 6% 2 6% 5% 3 4 5% 5% 9% 41%	1764
May	1st to 10th	Night	12% 5% 3 5% 6% 3 5% 4 5% 7% 45%	1620
	11th to 20th	Night	10% 5% 3 6% 5% 4 5% 5% 7% 8% 44%	1592
	21st to end	Night	5% 3 2 6% 7% 4 5% 6% 7% 11% 44%	1744
June	1st to 10th	Night	9% 5% 3 6% 5% 3 4 4 6% 8% 47%	1555
	11th to 20th	Night	3 3 1 3 4 3 4 5% 8% 9% 55%	1440
	21st to end	Night	3 3 2 4 5% 3 6% 6% 9% 12% 49%	1440
July	1st to 10th	Night	7% 5% 3 5% 5% 3 6% 5% 6% 13% 43%	1440
	11th to 20th	Night	7% 4 3 5% 5% 3 5% 4 6% 11% 47%	1440
	21st to end	Night	6% 4 1 4 5% 3 6% 7% 7% 11% 47%	1771
August	1st to 10th	Night	7% 4 2 6% 4 4 4 6% 6% 9% 49%	1713
	11th to 20th	Night	6% 4 2 5% 5% 4 5% 5% 8% 10% 46%	1776
	21st to end	Night	7% 4 2 4 5% 4 5% 5% 6% 10% 47%	1980
September	1st to 10th	Night	11% 5% 2 5% 4 3 4 5% 6% 6% 49%	1800
	11th to 20th	Night	10% 4 2 4 5% 2 4 4 5% 8% 51%	1978
	21st to end	Night	8% 4 2 3 5% 3 4 5% 5% 6% 55%	1980
October	1st to 10th	Night	9% 4 1 4 3 2 3 3 4 6% 60%	1980
	11th to 20th	Night	17% 5% 2 4 4 2 3 3 5% 6% 50%	2034
	21st to end	Night	16% 7% 3 4 4 2 3 4 4 5% 48%	2556
November	1st to 10th	Night	12% 5% 2 4 4 2 4 3 4 5% 55%	2406
	11th to 20th	Night	13% 6% 2 4 4 3 3 4 5% 7% 50%	2470
	21st to end	Night	16% 7% 3 5% 5% 2 3 3 4 6% 46%	2470
December	1st to 10th	Night	20% 11% 3 6% 5% 3 4 4 4 5% 35%	2470
	11th to 20th	Night	17% 9% 3 5% 4 2 4 4 4 5% 43%	3042
	21st to end	Night	16% 12% 5% 8% 6% 4 5% 4 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	39% 5% 10% 23% 20% 3	2269
	11th to 20th	Day	42% 4 8% 22% 20% 4	2134
	21st to end	Day	44% 5% 9% 26% 15% 2	2196
May	1st to 10th	Day	41% 5% 13% 26% 13% 1	2340
	11th to 20th	Day	45% 6% 11% 23% 14% 1	2305
	21st to end	Day	39% 8% 15% 24% 12% 2	2531
June	1st to 10th	Day	37% 7% 11% 25% 18% 3	2399
	11th to 20th	Day	32% 6% 17% 25% 18% 3	2520
	21st to end	Day	29% 8% 17% 25% 15% 5%	2520
July	1st to 10th	Day	41% 8% 16% 19% 12% 4	2451
	11th to 20th	Day	38% 8% 18% 21% 12% 3	2441
	21st to end	Day	36% 7% 18% 25% 11% 3	2405
August	1st to 10th	Day	37% 9% 17% 25% 12% 1	2208
	11th to 20th	Day	40% 7% 13% 22% 16% 2	2125
	21st to end	Day	37% 7% 13% 24% 15% 3	2376
September	1st to 10th	Day	40% 8% 10% 21% 18% 3	2142
	11th to 20th	Day	39% 5% 8% 24% 21% 2	1980
	21st to end	Day	33% 4 9% 26% 23% 5%	1980
October	1st to 10th	Day	30% 5% 8% 24% 28% 5%	1980
	11th to 20th	Day	35% 4 6% 21% 25% 8%	1922
	21st to end	Day	42% 4 6% 22% 22% 5%	1797
November	1st to 10th	Day	31% 4 8% 26% 27% 6%	1672
	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	38% 4 8% 21% 22% 7%	1775
	21st to end	Night	41% 4 10% 27% 17% 2	1764
May	1st to 10th	Night	39% 5% 10% 24% 18% 3	1620
	11th to 20th	Night	37% 7% 17% 22% 16% 1	1593
	21st to end	Night	31% 9% 18% 24% 15% 2	1745
June	1st to 10th	Night	35% 5% 14% 26% 17% 4	1555
	11th to 20th	Night	22% 6% 19% 30% 21% 3	1440
	21st to end	Night	25% 8% 19% 30% 13% 6%	1440
July	1st to 10th	Night	34% 6% 19% 25% 13% 3	1400
	11th to 20th	Night	32% 5% 18% 23% 16% 5%	1392
	21st to end	Night	29% 9% 17% 30% 13% 2	1672
August	1st to 10th	Night	30% 7% 16% 29% 18% 1	1702
	11th to 20th	Night	30% 8% 20% 23% 17% 2	1776
	21st to end	Night	31% 7% 13% 28% 17% 5%	1980
September	1st to 10th	Night	34% 6% 11% 25% 19% 5%	1800
	11th to 20th	Night	32% 6% 9% 24% 25% 4	1978
	21st to end	Night	30% 4 8% 24% 26% 7%	1980
October	1st to 10th	Night	26% 4 8% 25% 29% 8%	1980
	11th to 20th	Night	37% 4 8% 21% 24% 7%	2034
	21st to end	Night	38% 4 6% 24% 24% 5%	2556
November	1st to 10th	Night	32% 4 7% 25% 25% 8%	2406
	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	19% 15% 26% 28% 12% 1	457
	11th to 20th	Day	19% 12% 27% 25% 14% 3	412
	21st to end	Day	22% 9% 29% 23% 13% 4	420
May	1st to 10th	Day	20% 10% 26% 26% 14% 4	423
	11th to 20th	Day	18% 5% 21% 27% 19% 10%	447
	21st to end	Day	11% 4 15% 28% 25% 17%	485
June	1st to 10th	Day	13% 4 15% 21% 29% 18%	509
	11th to 20th	Day	5% 4 16% 26% 31% 18%	630
	21st to end	Day	5% 3 17% 33% 26% 17%	630
July	1st to 10th	Day	10% 2 9% 24% 30% 25%	621
	11th to 20th	Day	8% 3 10% 24% 32% 24%	614
	21st to end	Day	8% 3 9% 25% 29% 26%	488
August	1st to 10th	Day	11% 2 8% 25% 32% 23%	446
	11th to 20th	Day	10% 3 8% 28% 35% 16%	439
	21st to end	Day	12% 2 7% 19% 36% 24%	495
September	1st to 10th	Day	11% 2 10% 23% 35% 19%	450
	11th to 20th	Day	14% 2 8% 23% 35% 18%	450
	21st to end	Day	14% 5% 16% 26% 30% 10%	450
October	1st to 10th	Day	12% 3 18% 33% 24% 11%	450
	11th to 20th	Day	14% 10% 29% 30% 14% 3	392
	21st to end	Day	25% 10% 24% 28% 10% 3	297
November	1st to 10th	Day	19% 5% 28% 27% 19% 3	270
	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	15% 9% 33% 34% 8% 1	570
	11th to 20th	Night	18% 8% 32% 30% 13% 1	533
	21st to end	Night	16% 6% 31% 33% 12% 1	540
May	1st to 10th	Night	15% 4 31% 31% 17% 2	540
	11th to 20th	Night	12% 5% 20% 38% 21% 5%	531
	21st to end	Night	5% 2 14% 40% 34% 5%	584
June	1st to 10th	Night	12% 1 14% 36% 32% 4	540
	11th to 20th	Night	4 2 10% 40% 38% 6%	540
	21st to end	Night	4 1 9% 40% 39% 6%	540
July	1st to 10th	Night	9% 2 5% 31% 43% 11%	525
	11th to 20th	Night	7% 3 7% 27% 48% 9%	522
	21st to end	Night	7% 2 6% 33% 43% 10%	591
August	1st to 10th	Night	9% 1 5% 33% 45% 8%	537
	11th to 20th	Night	6% 2 5% 41% 40% 5%	534
	21st to end	Night	8% 2 5% 30% 45% 9%	594
September	1st to 10th	Night	11% 3 6% 31% 42% 7%	540
	11th to 20th	Night	11% 3 10% 31% 38% 6%	718
	21st to end	Night	9% 2 17% 33% 34% 4	720
October	1st to 10th	Night	11% 4 17% 34% 28% 6%	720
	11th to 20th	Night	19% 6% 22% 34% 18% 1	720
	21st to end	Night	19% 5% 29% 30% 15% 2	792
November	1st to 10th	Night	13% 6% 22% 38% 18% 2	740
	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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