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 2024-01-22.

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% 10% 4 8% 6% 3 5% 5% 5% 7% 32%	75043
	Night	16% 11% 4 7% 6% 3 4 5% 5% 7% 32%	74871
Transparency	Day	51% 8% 12% 18% 11% 1	74761
	Night	51% 8% 12% 17% 11% 2	74673
Seeing	Day	20% 14% 28% 22% 12% 4	13899
	Night	21% 11% 27% 27% 12% 1	23583

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	18% 17% 7% 11% 8% 3 6% 4 4 5% 17%	1706
	11th to 20th	Day	19% 20% 8% 12% 8% 3 5% 3 4 5% 13%	1710
	21st to end	Day	16% 19% 8% 14% 10% 4 4 4 4 4 14%	1791
February	1st to 10th	Day	18% 17% 6% 11% 9% 3 4 4 4 4 20%	1620
	11th to 20th	Day	15% 13% 7% 14% 10% 5% 5% 4 4 5% 19%	1658
	21st to end	Day	19% 13% 5% 12% 6% 3 4 4 5% 6% 23%	1480
March	1st to 10th	Day	20% 12% 6% 8% 6% 3 3 4 4 6% 27%	1903
	11th to 20th	Day	16% 12% 6% 9% 5% 3 4 4 4 5% 30%	1980
	21st to end	Day	21% 11% 5% 9% 6% 3 4 4 4 6% 27%	2178
April	1st to 10th	Day	15% 11% 5% 8% 7% 3 4 4 5% 6% 32%	2000
	11th to 20th	Day	17% 10% 4 6% 5% 3 5% 4 4 6% 34%	2312
	21st to end	Day	20% 10% 3 6% 5% 2 4 4 4 7% 34%	2340
May	1st to 10th	Day	17% 9% 4 7% 6% 4 5% 5% 5% 6% 33%	2340
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 5% 6% 7% 34%	2301
	21st to end	Day	9% 6% 3 6% 6% 4 5% 6% 7% 9% 40%	2531
June	1st to 10th	Day	13% 6% 3 5% 5% 4 5% 6% 6% 8% 39%	2443
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 6% 8% 40%	2586
	21st to end	Day	11% 4 2 5% 5% 4 6% 5% 7% 10% 41%	2578
July	1st to 10th	Day	10% 5% 3 6% 6% 4 5% 4 6% 9% 44%	2494
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 5% 7% 11% 42%	2340
	21st to end	Day	9% 5% 2 5% 5% 3 5% 6% 7% 11% 41%	2547
August	1st to 10th	Day	8% 5% 2 5% 6% 4 5% 6% 6% 9% 43%	2327
	11th to 20th	Day	10% 6% 3 6% 6% 4 6% 6% 7% 9% 37%	2303
	21st to end	Day	11% 6% 2 5% 4 4 5% 5% 7% 10% 41%	2561
September	1st to 10th	Day	14% 5% 3 5% 5% 4 5% 5% 6% 7% 41%	2021
	11th to 20th	Day	11% 6% 3 5% 6% 3 4 5% 5% 9% 42%	1980
	21st to end	Day	17% 6% 2 5% 4 4 4 5% 6% 7% 40%	1980
October	1st to 10th	Day	13% 8% 3 6% 6% 4 5% 5% 5% 8% 38%	1886
	11th to 20th	Day	19% 8% 3 6% 5% 3 5% 4 4 6% 36%	1800
	21st to end	Day	21% 12% 4 7% 6% 3 5% 5% 5% 6% 28%	1940
November	1st to 10th	Day	13% 11% 3 7% 5% 3 4 4 5% 7% 37%	1672
	11th to 20th	Day	17% 16% 4 7% 4 4 5% 4 3 6% 30%	1710
	21st to end	Day	17% 17% 6% 9% 6% 4 5% 5% 5% 6% 21%	1563
December	1st to 10th	Day	17% 20% 7% 10% 8% 3 5% 4 4 6% 18%	1520
	11th to 20th	Day	17% 17% 7% 13% 7% 4 4 4 5% 6% 17%	1910
	21st to end	Day	23% 22% 7% 11% 7% 3 4 4 4 4 10%	3032
January	1st to 10th	Night	19% 15% 8% 10% 7% 4 4 3 4 4 21%	2459
	11th to 20th	Night	19% 19% 7% 13% 7% 4 4 4 4 4 16%	2470
	21st to end	Night	17% 17% 7% 11% 8% 3 4 4 4 5% 20%	2599
February	1st to 10th	Night	21% 12% 6% 11% 6% 4 4 4 4 5% 22%	2340
	11th to 20th	Night	18% 15% 6% 10% 7% 4 5% 5% 4 5% 22%	2186
	21st to end	Night	21% 12% 5% 11% 7% 4 3 3 4 6% 24%	1776
March	1st to 10th	Night	17% 11% 6% 8% 6% 3 4 4 4 6% 31%	2057
	11th to 20th	Night	16% 9% 5% 6% 6% 3 4 5% 6% 6% 34%	1980
	21st to end	Night	20% 12% 4 7% 5% 2 4 4 5% 6% 32%	2178
April	1st to 10th	Night	15% 11% 4 7% 6% 3 5% 5% 5% 7% 34%	1972
	11th to 20th	Night	16% 9% 4 7% 6% 3 5% 5% 6% 7% 34%	1600
	21st to end	Night	17% 8% 3 6% 6% 3 4 5% 6% 8% 36%	1620
May	1st to 10th	Night	17% 9% 4 7% 6% 4 5% 5% 6% 7% 29%	1620
	11th to 20th	Night	16% 8% 4 5% 5% 2 5% 5% 6% 8% 36%	1592
	21st to end	Night	9% 5% 2 4 5% 4 5% 6% 7% 10% 43%	1744
June	1st to 10th	Night	10% 5% 3 5% 5% 3 6% 5% 5% 7% 45%	1508
	11th to 20th	Night	9% 5% 2 6% 5% 3 6% 6% 6% 10% 41%	1365
	21st to end	Night	9% 5% 2 5% 5% 3 6% 6% 6% 9% 44%	1373
July	1st to 10th	Night	8% 6% 2 5% 5% 3 4 5% 7% 8% 48%	1462
	11th to 20th	Night	8% 3 2 4 5% 3 4 5% 8% 11% 47%	1620
	21st to end	Night	8% 4 2 5% 5% 3 5% 5% 6% 10% 46%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 2 4 6% 6% 9% 49%	1611
	11th to 20th	Night	9% 5% 3 5% 5% 2 5% 6% 7% 9% 45%	1598
	21st to end	Night	8% 5% 2 6% 4 4 5% 6% 6% 8% 46%	1782
September	1st to 10th	Night	11% 6% 2 5% 6% 3 5% 5% 5% 8% 45%	1927
	11th to 20th	Night	10% 4 3 4 5% 2 4 5% 5% 8% 51%	1980
	21st to end	Night	14% 6% 3 5% 6% 4 5% 5% 5% 7% 42%	1980
October	1st to 10th	Night	13% 7% 2 5% 5% 3 5% 5% 4 7% 43%	2074
	11th to 20th	Night	19% 9% 3 6% 6% 3 4 4 5% 7% 35%	2160
	21st to end	Night	25% 10% 4 6% 5% 3 4 4 5% 7% 28%	2415
November	1st to 10th	Night	15% 10% 3 6% 5% 3 5% 4 5% 7% 36%	2406
	11th to 20th	Night	16% 14% 4 8% 6% 3 4 4 4 6% 30%	2470
	21st to end	Night	22% 14% 5% 8% 6% 4 4 4 4 6% 23%	2612
December	1st to 10th	Night	19% 18% 7% 9% 6% 3 5% 4 5% 5% 20%	2660
	11th to 20th	Night	17% 16% 7% 10% 6% 3 4 5% 5% 6% 22%	3285
	21st to end	Night	25% 20% 6% 8% 6% 3 4 4 4 5% 13%	4619

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	70% 6% 10% 10% 3	1707
	11th to 20th	Day	74% 7% 10% 7% 2	1710
	21st to end	Day	74% 6% 8% 8% 4	1791
February	1st to 10th	Day	68% 5% 11% 11% 5%	1620
	11th to 20th	Day	67% 6% 9% 12% 5% 1	1667
	21st to end	Day	61% 6% 13% 11% 7% 1	1480
March	1st to 10th	Day	58% 6% 11% 18% 6%	1903
	11th to 20th	Day	57% 6% 11% 16% 10% 1	1980
	21st to end	Day	58% 6% 9% 18% 8% 1	2178
April	1st to 10th	Day	52% 7% 12% 17% 12% 1	2000
	11th to 20th	Day	51% 7% 12% 16% 13% 2	2312
	21st to end	Day	51% 6% 12% 20% 10% 2	2340
May	1st to 10th	Day	51% 7% 11% 19% 10% 2	2340
	11th to 20th	Day	48% 8% 12% 16% 14% 3	2305
	21st to end	Day	38% 9% 15% 23% 13% 2	2531
June	1st to 10th	Day	41% 9% 11% 25% 14% 1	2443
	11th to 20th	Day	40% 10% 14% 21% 13% 2	2586
	21st to end	Day	37% 9% 14% 22% 16% 2	2578
July	1st to 10th	Day	37% 8% 11% 23% 18% 3	2426
	11th to 20th	Day	34% 8% 16% 25% 15% 2	2271
	21st to end	Day	35% 10% 16% 23% 15% 2	2405
August	1st to 10th	Day	37% 10% 15% 25% 13% 1	2314
	11th to 20th	Day	40% 9% 16% 22% 11% 2	2303
	21st to end	Day	37% 9% 14% 22% 15% 3	2561
September	1st to 10th	Day	39% 9% 16% 18% 15% 3	2021
	11th to 20th	Day	38% 8% 14% 23% 15% 2	1980
	21st to end	Day	42% 8% 11% 21% 18% 1	1980
October	1st to 10th	Day	43% 9% 12% 21% 15% 1	1886
	11th to 20th	Day	48% 8% 12% 20% 11% 1	1800
	21st to end	Day	56% 8% 13% 14% 9% 1	1940
November	1st to 10th	Day	47% 7% 12% 20% 14% 1	1672
	11th to 20th	Day	56% 6% 10% 17% 9% 2	1710
	21st to end	Day	63% 8% 11% 12% 6%	1563
December	1st to 10th	Day	70% 7% 10% 11% 3	1520
	11th to 20th	Day	67% 8% 10% 12% 3	1906
	21st to end	Day	77% 7% 8% 6% 2	3032
January	1st to 10th	Night	68% 5% 9% 12% 6%	2459
	11th to 20th	Night	72% 6% 10% 9% 2	2470
	21st to end	Night	67% 6% 10% 11% 4 1	2599
February	1st to 10th	Night	66% 6% 11% 12% 4	2340
	11th to 20th	Night	64% 7% 9% 14% 7%	2188
	21st to end	Night	63% 6% 10% 13% 7% 2	1776
March	1st to 10th	Night	55% 5% 10% 20% 9% 1	2057
	11th to 20th	Night	49% 8% 11% 19% 12% 2	1980
	21st to end	Night	53% 8% 8% 20% 10% 2	2178
April	1st to 10th	Night	49% 7% 13% 16% 13% 2	1972
	11th to 20th	Night	47% 8% 12% 16% 15% 3	1600
	21st to end	Night	46% 7% 12% 19% 14% 3	1620
May	1st to 10th	Night	52% 8% 12% 15% 11% 3	1620
	11th to 20th	Night	45% 9% 11% 16% 16% 3	1593
	21st to end	Night	33% 10% 16% 24% 14% 2	1745
June	1st to 10th	Night	38% 9% 10% 25% 14% 3	1508
	11th to 20th	Night	36% 9% 16% 19% 18% 2	1365
	21st to end	Night	34% 9% 14% 25% 16% 2	1373
July	1st to 10th	Night	33% 7% 13% 22% 19% 4	1421
	11th to 20th	Night	30% 11% 15% 23% 19% 3	1566
	21st to end	Night	32% 10% 15% 22% 17% 4	1672
August	1st to 10th	Night	29% 10% 16% 26% 18% 1	1600
	11th to 20th	Night	34% 9% 16% 25% 13% 2	1598
	21st to end	Night	34% 9% 14% 23% 18% 3	1782
September	1st to 10th	Night	37% 8% 14% 23% 15% 4	1927
	11th to 20th	Night	30% 8% 13% 26% 21% 1	1980
	21st to end	Night	41% 8% 11% 20% 19% 1	1980
October	1st to 10th	Night	39% 8% 14% 20% 19% 1	2074
	11th to 20th	Night	49% 7% 11% 20% 13% 1	2160
	21st to end	Night	56% 6% 11% 17% 9% 1	2415
November	1st to 10th	Night	45% 8% 12% 20% 13% 2	2406
	11th to 20th	Night	56% 7% 10% 17% 9% 2	2470
	21st to end	Night	62% 7% 11% 15% 5%	2612
December	1st to 10th	Night	68% 6% 10% 11% 5%	2660
	11th to 20th	Night	63% 8% 13% 14% 3	3288
	21st to end	Night	73% 9% 10% 7% 2	4619

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	29% 23% 30% 12% 5% 1	270
	11th to 20th	Day	31% 27% 30% 7% 4	270
	21st to end	Day	28% 25% 31% 14% 1 1	267
February	1st to 10th	Day	35% 22% 33% 10%	240
	11th to 20th	Day	32% 27% 30% 10%	240
	21st to end	Day	31% 31% 30% 7% 1	198
March	1st to 10th	Day	28% 22% 31% 13% 5%	343
	11th to 20th	Day	27% 17% 33% 18% 5%	420
	21st to end	Day	32% 18% 27% 18% 5%	462
April	1st to 10th	Day	23% 17% 32% 19% 7% 1	420
	11th to 20th	Day	26% 19% 24% 19% 8% 4	412
	21st to end	Day	27% 16% 32% 16% 6% 3	420
May	1st to 10th	Day	25% 15% 29% 22% 6% 3	423
	11th to 20th	Day	21% 10% 33% 23% 10% 3	447
	21st to end	Day	13% 9% 26% 29% 18% 5%	485
June	1st to 10th	Day	12% 9% 31% 31% 14% 3	450
	11th to 20th	Day	15% 9% 25% 33% 15% 3	516
	21st to end	Day	11% 10% 31% 30% 15% 3	630
July	1st to 10th	Day	9% 5% 25% 29% 23% 10%	596
	11th to 20th	Day	13% 6% 24% 27% 21% 9%	444
	21st to end	Day	9% 6% 27% 30% 20% 8%	488
August	1st to 10th	Day	11% 5% 19% 31% 24% 11%	446
	11th to 20th	Day	14% 5% 21% 29% 21% 10%	439
	21st to end	Day	11% 7% 22% 27% 21% 11%	495
September	1st to 10th	Day	16% 7% 22% 25% 19% 11%	450
	11th to 20th	Day	12% 4 20% 26% 26% 11%	450
	21st to end	Day	19% 13% 24% 21% 14% 8%	450
October	1st to 10th	Day	13% 13% 25% 26% 18% 6%	450
	11th to 20th	Day	22% 16% 33% 22% 6% 1	450
	21st to end	Day	28% 22% 30% 16% 4	455
November	1st to 10th	Day	19% 23% 32% 17% 9% 1	270
	11th to 20th	Day	28% 23% 30% 14% 4 1	270
	21st to end	Day	29% 24% 35% 10%	201
December	1st to 10th	Day	23% 27% 31% 13% 4 2	180
	11th to 20th	Day	22% 30% 37% 9% 2	180
	21st to end	Day	28% 23% 36% 11% 1	272
January	1st to 10th	Night	28% 21% 32% 15% 4	760
	11th to 20th	Night	32% 19% 29% 15% 4	760
	21st to end	Night	26% 18% 32% 21% 3	800
February	1st to 10th	Night	30% 15% 32% 20% 2	720
	11th to 20th	Night	27% 18% 32% 21% 3	703
	21st to end	Night	29% 16% 35% 18% 2	592
March	1st to 10th	Night	23% 15% 33% 23% 5%	720
	11th to 20th	Night	21% 15% 33% 23% 7% 1	720
	21st to end	Night	25% 11% 29% 26% 9% 1	792
April	1st to 10th	Night	21% 15% 32% 26% 5%	720
	11th to 20th	Night	21% 12% 32% 25% 10% 1	536
	21st to end	Night	19% 9% 36% 28% 7% 1	540
May	1st to 10th	Night	20% 8% 29% 31% 10% 1	540
	11th to 20th	Night	19% 7% 25% 34% 13% 1	531
	21st to end	Night	11% 3 23% 36% 23% 3	584
June	1st to 10th	Night	11% 3 28% 43% 15% 1	428
	11th to 20th	Night	11% 4 24% 43% 17% 1	360
	21st to end	Night	10% 4 25% 40% 20% 1	473
July	1st to 10th	Night	10% 3 14% 40% 30% 3	525
	11th to 20th	Night	10% 2 21% 40% 24% 4	522
	21st to end	Night	10% 2 17% 42% 27% 2	591
August	1st to 10th	Night	9% 2 15% 44% 26% 4	537
	11th to 20th	Night	10% 4 16% 38% 28% 4	534
	21st to end	Night	9% 4 18% 34% 30% 4	594
September	1st to 10th	Night	13% 5% 17% 36% 25% 4	713
	11th to 20th	Night	12% 4 15% 35% 29% 5%	720
	21st to end	Night	16% 8% 23% 30% 20% 3	720
October	1st to 10th	Night	17% 8% 23% 31% 19% 2	720
	11th to 20th	Night	25% 13% 29% 24% 8% 1	720
	21st to end	Night	31% 15% 26% 23% 5%	792
November	1st to 10th	Night	22% 13% 31% 24% 10% 1	740
	11th to 20th	Night	27% 14% 34% 17% 7% 1	760
	21st to end	Night	31% 14% 30% 19% 6%	760
December	1st to 10th	Night	31% 18% 30% 17% 5%	760
	11th to 20th	Night	27% 18% 33% 17% 3	760
	21st to end	Night	30% 16% 33% 18% 2	836

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