John D. Nielsen Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John D. Nielsen 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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	132209
	Night	15% 12% 4 10% 5% 4 4 4 5% 6% 30%	130592
Transparency	Day	51% 7% 11% 19% 11% 1	135827
	Night	52% 7% 11% 18% 11% 1	135045
Seeing	Day	20% 15% 28% 21% 12% 4	16969
	Night	21% 12% 28% 27% 12% 1	26602

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	16% 20% 7% 21% 6% 4 4 3 4 5% 12%	5258
	11th to 20th	Day	12% 21% 8% 17% 9% 5% 5% 3 4 4 13%	5280
	21st to end	Day	19% 18% 8% 13% 8% 6% 5% 3 4 4 13%	5780
February	1st to 10th	Day	16% 18% 5% 12% 7% 4 4 3 4 5% 21%	1890
	11th to 20th	Day	14% 14% 6% 16% 8% 5% 5% 3 4 5% 20%	1942
	21st to end	Day	17% 14% 4 13% 5% 5% 4 3 5% 6% 23%	1730
March	1st to 10th	Day	18% 13% 6% 12% 5% 4 3 3 4 6% 27%	4857
	11th to 20th	Day	11% 12% 4 11% 5% 4 4 3 4 5% 36%	5090
	21st to end	Day	18% 13% 5% 10% 5% 5% 4 3 4 6% 27%	5586
April	1st to 10th	Day	15% 12% 4 9% 6% 4 4 3 6% 6% 30%	2335
	11th to 20th	Day	16% 11% 3 8% 5% 4 5% 3 5% 6% 33%	2702
	21st to end	Day	18% 11% 3 7% 4 4 4 4 4 7% 34%	2730
May	1st to 10th	Day	20% 10% 4 8% 5% 4 4 3 5% 6% 30%	5992
	11th to 20th	Day	16% 8% 3 8% 5% 4 5% 4 6% 7% 33%	5939
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 7% 35%	6433
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 5% 6% 8% 39%	2715
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 4 7% 8% 39%	2874
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 8% 10% 41%	2864
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 7% 10% 42%	6278
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 11% 40%	5900
	21st to end	Day	10% 6% 2 7% 5% 5% 4 4 6% 9% 41%	6424
August	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 41%	4234
	11th to 20th	Day	9% 6% 2 7% 4 5% 6% 4 8% 10% 38%	4174
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 7% 10% 41%	4647
September	1st to 10th	Day	13% 5% 2 5% 5% 4 5% 4 5% 7% 44%	2245
	11th to 20th	Day	10% 6% 3 6% 5% 4 4 4 5% 9% 45%	2200
	21st to end	Day	18% 8% 2 6% 4 4 4 4 6% 7% 38%	2200
October	1st to 10th	Day	12% 7% 3 6% 5% 5% 5% 3 6% 8% 41%	2095
	11th to 20th	Day	18% 8% 2 7% 4 4 5% 3 4 6% 38%	2000
	21st to end	Day	19% 11% 3 8% 4 4 4 4 5% 7% 29%	2154
November	1st to 10th	Day	13% 12% 3 8% 4 4 4 4 6% 7% 36%	1852
	11th to 20th	Day	17% 16% 4 7% 4 5% 4 3 4 6% 29%	1890
	21st to end	Day	17% 19% 5% 10% 6% 5% 5% 4 5% 5% 20%	1727
December	1st to 10th	Day	17% 16% 6% 12% 7% 5% 5% 4 4 6% 19%	2680
	11th to 20th	Day	20% 20% 7% 12% 6% 3 4 2 4 5% 17%	3070
	21st to end	Day	23% 22% 7% 13% 6% 5% 4 3 4 4 11%	4442
January	1st to 10th	Night	15% 19% 7% 21% 7% 5% 4 2 4 3 13%	7635
	11th to 20th	Night	9% 19% 9% 17% 8% 5% 6% 4 4 4 14%	7680
	21st to end	Night	17% 16% 6% 12% 5% 5% 5% 3 4 6% 20%	8401
February	1st to 10th	Night	19% 14% 5% 12% 6% 5% 3 3 5% 5% 22%	2730
	11th to 20th	Night	16% 16% 5% 11% 6% 5% 5% 4 5% 5% 22%	2561
	21st to end	Night	19% 14% 5% 13% 5% 5% 3 3 4 6% 24%	2076
March	1st to 10th	Night	19% 13% 4 8% 4 4 3 2 4 6% 31%	5216
	11th to 20th	Night	12% 9% 3 7% 5% 4 4 4 6% 7% 39%	5090
	21st to end	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 33%	5588
April	1st to 10th	Night	16% 13% 3 8% 5% 4 4 4 5% 6% 32%	2300
	11th to 20th	Night	16% 9% 3 8% 5% 5% 4 5% 7% 6% 33%	1870
	21st to end	Night	16% 8% 3 7% 5% 4 4 4 6% 7% 36%	1890
May	1st to 10th	Night	20% 11% 4 9% 5% 4 5% 4 5% 6% 26%	4123
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 4 5% 8% 37%	4086
	21st to end	Night	15% 7% 2 5% 4 5% 5% 4 6% 8% 39%	4421
June	1st to 10th	Night	11% 6% 3 7% 5% 5% 4 4 5% 7% 45%	1675
	11th to 20th	Night	9% 5% 2 7% 4 5% 6% 5% 7% 9% 40%	1516
	21st to end	Night	9% 5% 2 5% 5% 4 6% 4 7% 9% 43%	1527
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 5% 7% 10% 44%	3679
	11th to 20th	Night	9% 4 2 7% 5% 5% 5% 4 7% 10% 43%	4057
	21st to end	Night	7% 6% 3 6% 4 4 4 4 6% 9% 47%	4427
August	1st to 10th	Night	10% 6% 3 8% 4 4 4 4 6% 8% 42%	2915
	11th to 20th	Night	10% 7% 3 7% 5% 3 4 4 5% 7% 45%	2875
	21st to end	Night	7% 6% 2 7% 5% 5% 6% 5% 7% 8% 45%	3223
September	1st to 10th	Night	10% 6% 2 6% 5% 4 5% 4 5% 8% 47%	2143
	11th to 20th	Night	9% 4 2 4 4 3 4 4 5% 7% 53%	2200
	21st to end	Night	15% 7% 3 6% 5% 5% 5% 4 5% 6% 40%	2200
October	1st to 10th	Night	12% 7% 2 6% 4 5% 4 4 5% 6% 46%	2305
	11th to 20th	Night	18% 9% 2 7% 5% 4 4 3 5% 7% 37%	2400
	21st to end	Night	23% 10% 3 6% 4 4 4 3 5% 7% 30%	2684
November	1st to 10th	Night	14% 10% 3 7% 4 4 4 3 5% 7% 38%	2666
	11th to 20th	Night	16% 16% 3 9% 5% 4 4 3 4 6% 30%	2730
	21st to end	Night	22% 16% 4 9% 5% 5% 4 3 4 6% 22%	2888
December	1st to 10th	Night	15% 16% 6% 11% 5% 6% 4 4 5% 6% 21%	4720
	11th to 20th	Night	18% 19% 7% 12% 5% 4 4 3 4 5% 19%	5345
	21st to end	Night	23% 20% 6% 12% 5% 5% 4 3 5% 5% 13%	6750

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	75% 5% 10% 8% 2	5259
	11th to 20th	Day	75% 4 8% 6% 6%	5280
	21st to end	Day	76% 4 6% 10% 3	5780
February	1st to 10th	Day	65% 6% 11% 12% 6%	1890
	11th to 20th	Day	66% 6% 9% 13% 5% 1	1951
	21st to end	Day	60% 7% 14% 12% 6% 1	1730
March	1st to 10th	Day	58% 5% 10% 20% 6%	4857
	11th to 20th	Day	48% 7% 14% 22% 7% 1	5090
	21st to end	Day	57% 6% 9% 17% 10% 1	5586
April	1st to 10th	Day	53% 7% 12% 16% 12% 1	2335
	11th to 20th	Day	50% 7% 12% 17% 13% 2	2702
	21st to end	Day	49% 7% 12% 20% 10% 2	2730
May	1st to 10th	Day	53% 8% 9% 18% 11% 2	6027
	11th to 20th	Day	46% 9% 13% 20% 11% 1	5943
	21st to end	Day	43% 9% 12% 20% 12% 4	6447
June	1st to 10th	Day	41% 9% 11% 25% 13% 1	2715
	11th to 20th	Day	40% 10% 13% 21% 14% 2	2874
	21st to end	Day	36% 9% 14% 22% 16% 2	2864
July	1st to 10th	Day	36% 7% 11% 28% 17% 2	6210
	11th to 20th	Day	36% 8% 14% 25% 15% 1	5831
	21st to end	Day	38% 7% 13% 21% 20% 1	6240
August	1st to 10th	Day	40% 8% 15% 24% 13% 1	4221
	11th to 20th	Day	38% 8% 13% 30% 10% 1	4213
	21st to end	Day	36% 8% 15% 28% 11% 2	4647
September	1st to 10th	Day	36% 9% 15% 20% 16% 3	2245
	11th to 20th	Day	36% 8% 14% 25% 16% 2	2200
	21st to end	Day	45% 7% 11% 20% 18% 1	2200
October	1st to 10th	Day	31% 7% 14% 28% 19% 1	3407
	11th to 20th	Day	44% 6% 10% 20% 19% 1	3240
	21st to end	Day	42% 6% 12% 22% 17% 1	3456
November	1st to 10th	Day	47% 6% 11% 21% 13% 1	1852
	11th to 20th	Day	57% 6% 10% 17% 9% 2	1890
	21st to end	Day	64% 8% 10% 12% 5%	1727
December	1st to 10th	Day	67% 7% 13% 11% 2	2680
	11th to 20th	Day	70% 6% 8% 12% 4	3066
	21st to end	Day	78% 6% 6% 8% 2	4442
January	1st to 10th	Night	77% 6% 6% 9% 3	7636
	11th to 20th	Night	75% 5% 7% 8% 5%	7680
	21st to end	Night	66% 4 10% 13% 6%	8401
February	1st to 10th	Night	64% 6% 11% 14% 5% 1	2730
	11th to 20th	Night	63% 7% 9% 14% 6%	2563
	21st to end	Night	62% 6% 10% 13% 7% 1	2076
March	1st to 10th	Night	55% 4 13% 19% 8%	5216
	11th to 20th	Night	41% 9% 15% 25% 10% 1	5090
	21st to end	Night	50% 8% 9% 19% 13% 1	5588
April	1st to 10th	Night	52% 7% 12% 15% 12% 2	2300
	11th to 20th	Night	48% 8% 12% 16% 14% 3	1870
	21st to end	Night	46% 7% 11% 20% 14% 3	1890
May	1st to 10th	Night	55% 8% 10% 13% 10% 4	4150
	11th to 20th	Night	42% 9% 12% 22% 14% 1	4087
	21st to end	Night	40% 9% 13% 22% 13% 3	4429
June	1st to 10th	Night	39% 8% 10% 24% 15% 3	1675
	11th to 20th	Night	36% 9% 15% 20% 17% 2	1516
	21st to end	Night	34% 9% 14% 25% 16% 2	1527
July	1st to 10th	Night	34% 7% 14% 24% 17% 4	3640
	11th to 20th	Night	34% 9% 13% 26% 15% 3	4006
	21st to end	Night	33% 7% 13% 21% 25% 2	4297
August	1st to 10th	Night	37% 9% 13% 24% 15% 1	2904
	11th to 20th	Night	38% 7% 11% 29% 13% 1	2902
	21st to end	Night	35% 8% 13% 28% 14% 2	3223
September	1st to 10th	Night	35% 7% 13% 25% 16% 4	2143
	11th to 20th	Night	28% 8% 13% 28% 22% 2	2200
	21st to end	Night	43% 8% 11% 19% 18% 2	2200
October	1st to 10th	Night	30% 5% 9% 29% 25% 1	3773
	11th to 20th	Night	44% 6% 9% 20% 20% 1	3899
	21st to end	Night	41% 5% 10% 25% 17% 2	4332
November	1st to 10th	Night	44% 7% 12% 21% 13% 2	2666
	11th to 20th	Night	56% 6% 10% 17% 9% 2	2730
	21st to end	Night	62% 7% 11% 15% 5%	2888
December	1st to 10th	Night	64% 5% 13% 14% 4	4720
	11th to 20th	Night	67% 6% 10% 13% 3	5348
	21st to end	Night	72% 7% 10% 9% 1	6750

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	28% 23% 32% 12% 4 1	330
	11th to 20th	Day	29% 31% 30% 7% 3	330
	21st to end	Day	28% 28% 30% 12% 2 1	363
February	1st to 10th	Day	31% 22% 34% 11% 2	330
	11th to 20th	Day	29% 28% 31% 11% 2	330
	21st to end	Day	29% 31% 32% 8%	273
March	1st to 10th	Day	27% 21% 34% 14% 4	451
	11th to 20th	Day	28% 18% 32% 16% 5%	540
	21st to end	Day	30% 19% 30% 16% 4	594
April	1st to 10th	Day	25% 20% 32% 16% 6% 1	540
	11th to 20th	Day	25% 22% 25% 17% 7% 4	532
	21st to end	Day	26% 17% 32% 16% 7% 2	540
May	1st to 10th	Day	26% 16% 29% 21% 5% 3	543
	11th to 20th	Day	19% 11% 33% 23% 10% 3	551
	21st to end	Day	15% 11% 26% 28% 16% 4	573
June	1st to 10th	Day	14% 9% 32% 30% 14% 2	530
	11th to 20th	Day	14% 9% 25% 33% 16% 2	604
	21st to end	Day	11% 10% 30% 30% 15% 5%	730
July	1st to 10th	Day	9% 5% 25% 30% 22% 9%	692
	11th to 20th	Day	11% 7% 25% 28% 21% 8%	524
	21st to end	Day	9% 5% 26% 29% 22% 9%	576
August	1st to 10th	Day	11% 5% 19% 30% 25% 10%	526
	11th to 20th	Day	13% 5% 22% 29% 22% 10%	519
	21st to end	Day	10% 7% 22% 28% 22% 11%	583
September	1st to 10th	Day	14% 8% 23% 24% 19% 13%	530
	11th to 20th	Day	10% 4 18% 26% 29% 14%	530
	21st to end	Day	21% 12% 23% 22% 14% 8%	530
October	1st to 10th	Day	12% 12% 24% 27% 18% 7%	530
	11th to 20th	Day	20% 15% 33% 22% 7% 3	530
	21st to end	Day	25% 21% 31% 17% 5%	537
November	1st to 10th	Day	21% 22% 32% 17% 8% 1	330
	11th to 20th	Day	29% 23% 29% 14% 4 1	330
	21st to end	Day	32% 23% 33% 11%	245
December	1st to 10th	Day	24% 27% 31% 14% 4 1	220
	11th to 20th	Day	25% 29% 34% 10% 3	220
	21st to end	Day	30% 23% 33% 13% 1	333
January	1st to 10th	Night	27% 21% 32% 16% 4	840
	11th to 20th	Night	31% 20% 31% 15% 4	840
	21st to end	Night	26% 19% 32% 20% 3	924
February	1st to 10th	Night	28% 16% 32% 21% 3	840
	11th to 20th	Night	25% 19% 32% 21% 3	823
	21st to end	Night	28% 17% 36% 18% 2	692
March	1st to 10th	Night	24% 15% 33% 23% 5%	840
	11th to 20th	Night	22% 17% 33% 22% 6% 1	840
	21st to end	Night	25% 12% 31% 24% 8%	924
April	1st to 10th	Night	23% 16% 33% 24% 4	840
	11th to 20th	Night	21% 12% 33% 25% 8% 1	626
	21st to end	Night	19% 10% 35% 29% 7% 1	630
May	1st to 10th	Night	20% 9% 30% 31% 9% 1	630
	11th to 20th	Night	18% 8% 26% 33% 13% 1	609
	21st to end	Night	12% 5% 23% 36% 21% 3	650
June	1st to 10th	Night	12% 4 28% 41% 14% 1	475
	11th to 20th	Night	11% 5% 25% 43% 16% 1	400
	21st to end	Night	10% 4 25% 39% 21% 1	527
July	1st to 10th	Night	10% 3 15% 42% 29% 3	585
	11th to 20th	Night	9% 2 21% 40% 25% 3	582
	21st to end	Night	10% 2 16% 42% 28% 2	657
August	1st to 10th	Night	10% 2 14% 45% 26% 4	597
	11th to 20th	Night	11% 4 16% 38% 28% 4	594
	21st to end	Night	8% 4 17% 36% 31% 4	660
September	1st to 10th	Night	13% 5% 17% 35% 25% 4	793
	11th to 20th	Night	11% 4 13% 35% 32% 5%	800
	21st to end	Night	17% 8% 22% 30% 20% 3	800
October	1st to 10th	Night	16% 8% 22% 31% 20% 3	800
	11th to 20th	Night	24% 12% 28% 25% 9% 1	800
	21st to end	Night	29% 14% 28% 23% 6%	880
November	1st to 10th	Night	21% 13% 32% 24% 9% 1	820
	11th to 20th	Night	27% 15% 34% 18% 6% 1	840
	21st to end	Night	32% 14% 30% 18% 5%	840
December	1st to 10th	Night	30% 18% 30% 18% 5%	840
	11th to 20th	Night	27% 18% 34% 17% 3	840
	21st to end	Night	30% 16% 33% 18% 2	924

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

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