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 2025-01-22.

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% 11% 4 9% 5% 5% 4 4 5% 7% 32%	105334
	Night	16% 12% 4 9% 5% 4 4 4 5% 6% 31%	103959
Transparency	Day	50% 8% 12% 19% 11% 1	108996
	Night	51% 7% 11% 18% 11% 1	108437
Seeing	Day	20% 14% 28% 22% 12% 4	15227
	Night	21% 11% 27% 27% 12% 1	24881

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% 23% 7% 15% 5% 4 4 2 4 4 13%	2926
	11th to 20th	Day	15% 25% 7% 13% 6% 5% 6% 3 4 5% 11%	2930
	21st to end	Day	27% 20% 11% 13% 8% 4 3 2 2 2 9%	3133
February	1st to 10th	Day	17% 18% 5% 12% 8% 4 4 3 4 4 21%	1710
	11th to 20th	Day	14% 14% 6% 16% 8% 5% 5% 3 4 5% 20%	1752
	21st to end	Day	18% 14% 4 13% 5% 4 4 3 5% 5% 24%	1570
March	1st to 10th	Day	22% 14% 6% 9% 4 4 3 3 4 6% 26%	3307
	11th to 20th	Day	13% 14% 5% 11% 5% 5% 4 3 4 5% 33%	3480
	21st to end	Day	20% 12% 6% 9% 5% 4 4 3 4 6% 28%	3815
April	1st to 10th	Day	14% 13% 4 9% 6% 4 4 3 6% 6% 31%	2112
	11th to 20th	Day	17% 11% 3 7% 5% 4 5% 3 4 6% 34%	2442
	21st to end	Day	19% 10% 3 7% 5% 4 3 4 4 7% 34%	2470
May	1st to 10th	Day	18% 10% 3 9% 5% 5% 4 3 5% 6% 31%	4082
	11th to 20th	Day	18% 8% 2 7% 5% 4 5% 4 6% 8% 35%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 8% 40%	4476
June	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 5% 6% 8% 39%	2579
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	2730
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 7% 10% 41%	2721
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 7% 10% 39%	4386
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 11% 45%	4120
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 6% 9% 40%	4466
August	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	4104
	11th to 20th	Day	9% 6% 2 7% 4 5% 6% 5% 8% 10% 38%	4044
	21st to end	Day	9% 6% 2 6% 4 5% 5% 4 7% 10% 41%	4505
September	1st to 10th	Day	14% 5% 2 5% 5% 4 5% 4 6% 7% 43%	2133
	11th to 20th	Day	11% 7% 3 6% 5% 4 4 3 5% 9% 44%	2090
	21st to end	Day	18% 8% 2 6% 4 4 4 3 6% 7% 38%	2090
October	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 3 6% 8% 40%	1990
	11th to 20th	Day	18% 8% 2 7% 4 5% 5% 3 4 6% 37%	1900
	21st to end	Day	19% 12% 3 8% 4 4 4 4 5% 7% 29%	2047
November	1st to 10th	Day	13% 12% 3 8% 4 5% 4 4 6% 7% 36%	1762
	11th to 20th	Day	17% 17% 4 7% 4 5% 5% 3 4 6% 30%	1800
	21st to end	Day	18% 19% 5% 10% 6% 5% 5% 4 5% 5% 20%	1645
December	1st to 10th	Day	17% 15% 6% 12% 7% 5% 5% 4 4 6% 19%	2600
	11th to 20th	Day	20% 20% 7% 12% 5% 3 4 2 4 5% 17%	2990
	21st to end	Day	23% 22% 7% 13% 6% 5% 4 3 4 4 11%	4346
January	1st to 10th	Night	16% 22% 8% 13% 5% 5% 4 2 4 4 16%	4239
	11th to 20th	Night	13% 21% 9% 17% 6% 4 7% 5% 4 3 10%	4250
	21st to end	Night	27% 19% 7% 12% 5% 4 3 2 3 4 14%	4546
February	1st to 10th	Night	20% 14% 5% 12% 6% 6% 3 3 5% 5% 22%	2470
	11th to 20th	Night	16% 17% 5% 11% 6% 5% 5% 4 4 5% 23%	2311
	21st to end	Night	20% 13% 5% 13% 5% 5% 3 3 4 5% 24%	1884
March	1st to 10th	Night	21% 14% 4 7% 4 4 3 2 4 5% 33%	3546
	11th to 20th	Night	14% 10% 3 6% 5% 4 4 4 6% 7% 37%	3480
	21st to end	Night	17% 12% 4 8% 5% 4 4 4 5% 6% 32%	3817
April	1st to 10th	Night	15% 13% 3 7% 5% 4 4 4 5% 6% 33%	2081
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 5% 7% 6% 33%	1690
	21st to end	Night	17% 9% 3 7% 5% 4 5% 4 6% 7% 36%	1710
May	1st to 10th	Night	19% 10% 4 8% 5% 5% 5% 4 5% 7% 28%	2813
	11th to 20th	Night	15% 8% 3 6% 5% 4 5% 4 6% 8% 36%	2812
	21st to end	Night	9% 6% 1 4 4 4 5% 4 6% 9% 47%	3079
June	1st to 10th	Night	11% 6% 3 6% 5% 5% 4 4 5% 7% 44%	1591
	11th to 20th	Night	9% 5% 2 7% 4 5% 6% 5% 7% 9% 41%	1440
	21st to end	Night	9% 6% 2 5% 5% 4 6% 4 7% 9% 43%	1450
July	1st to 10th	Night	10% 6% 2 6% 3 4 4 4 7% 9% 47%	2578
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 7% 10% 48%	2840
	21st to end	Night	6% 5% 3 7% 5% 4 5% 4 7% 10% 45%	3087
August	1st to 10th	Night	10% 6% 3 8% 5% 4 4 4 6% 8% 41%	2825
	11th to 20th	Night	10% 8% 3 7% 5% 3 4 4 5% 7% 45%	2785
	21st to end	Night	7% 6% 2 7% 5% 5% 6% 5% 7% 8% 45%	3124
September	1st to 10th	Night	10% 6% 2 6% 5% 4 5% 4 5% 8% 46%	2035
	11th to 20th	Night	10% 5% 2 4 4 3 4 4 5% 7% 52%	2090
	21st to end	Night	15% 7% 3 6% 5% 4 5% 4 5% 6% 40%	2090
October	1st to 10th	Night	12% 7% 2 6% 4 5% 4 4 5% 6% 44%	2190
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 2 5% 7% 36%	2280
	21st to end	Night	23% 11% 3 7% 4 4 4 3 5% 7% 29%	2550
November	1st to 10th	Night	14% 10% 3 7% 4 4 4 4 5% 7% 37%	2536
	11th to 20th	Night	16% 16% 3 9% 5% 4 4 3 4 6% 30%	2600
	21st to end	Night	21% 16% 4 9% 5% 5% 3 3 4 6% 23%	2750
December	1st to 10th	Night	15% 16% 6% 11% 5% 6% 4 4 5% 6% 21%	4580
	11th to 20th	Night	18% 19% 7% 12% 5% 4 4 3 4 5% 19%	5205
	21st to end	Night	23% 20% 6% 12% 5% 5% 4 3 5% 5% 13%	6605

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	74% 5% 10% 9% 2	2927
	11th to 20th	Day	75% 6% 10% 7% 3	2930
	21st to end	Day	83% 5% 5% 5% 2	3133
February	1st to 10th	Day	65% 6% 11% 12% 6%	1710
	11th to 20th	Day	65% 7% 9% 13% 5% 1	1761
	21st to end	Day	60% 7% 14% 11% 7% 1	1570
March	1st to 10th	Day	59% 6% 10% 20% 5%	3307
	11th to 20th	Day	51% 9% 13% 20% 6% 1	3480
	21st to end	Day	55% 8% 12% 15% 9% 1	3815
April	1st to 10th	Day	52% 7% 12% 17% 12% 1	2112
	11th to 20th	Day	50% 7% 12% 16% 13% 2	2442
	21st to end	Day	50% 7% 12% 20% 10% 2	2470
May	1st to 10th	Day	50% 9% 10% 21% 9% 1	4120
	11th to 20th	Day	44% 10% 14% 22% 9% 1	4085
	21st to end	Day	36% 11% 14% 19% 15% 5%	4489
June	1st to 10th	Day	41% 9% 11% 25% 13% 1	2579
	11th to 20th	Day	39% 10% 14% 22% 14% 2	2730
	21st to end	Day	37% 9% 14% 22% 16% 2	2721
July	1st to 10th	Day	39% 7% 11% 23% 17% 3	4318
	11th to 20th	Day	32% 8% 13% 28% 18% 1	4051
	21st to end	Day	38% 8% 15% 17% 19% 2	4324
August	1st to 10th	Day	40% 8% 15% 23% 13%	4091
	11th to 20th	Day	39% 8% 13% 30% 10% 1	4083
	21st to end	Day	36% 8% 15% 27% 12% 2	4505
September	1st to 10th	Day	37% 9% 16% 19% 16% 3	2133
	11th to 20th	Day	37% 8% 14% 23% 16% 2	2090
	21st to end	Day	45% 7% 11% 20% 17% 1	2090
October	1st to 10th	Day	32% 7% 15% 28% 19% 1	3302
	11th to 20th	Day	44% 7% 10% 20% 19% 1	3140
	21st to end	Day	42% 6% 12% 22% 17% 1	3349
November	1st to 10th	Day	46% 6% 12% 21% 14% 1	1762
	11th to 20th	Day	57% 6% 10% 16% 9% 1	1800
	21st to end	Day	64% 8% 11% 12% 5%	1645
December	1st to 10th	Day	67% 7% 13% 11% 2	2600
	11th to 20th	Day	70% 6% 8% 12% 4	2986
	21st to end	Day	78% 6% 6% 8% 2	4346
January	1st to 10th	Night	71% 8% 8% 10% 4	4239
	11th to 20th	Night	78% 7% 7% 7% 2	4250
	21st to end	Night	75% 5% 8% 8% 2	4546
February	1st to 10th	Night	65% 7% 11% 13% 5%	2470
	11th to 20th	Night	62% 7% 9% 15% 6%	2313
	21st to end	Night	63% 6% 10% 13% 7% 2	1884
March	1st to 10th	Night	54% 5% 13% 20% 8%	3546
	11th to 20th	Night	43% 11% 17% 18% 10% 1	3480
	21st to end	Night	47% 10% 10% 23% 9% 1	3817
April	1st to 10th	Night	50% 7% 13% 15% 12% 2	2081
	11th to 20th	Night	47% 8% 12% 16% 14% 3	1690
	21st to end	Night	45% 7% 12% 19% 14% 3	1710
May	1st to 10th	Night	52% 9% 11% 17% 9% 2	2840
	11th to 20th	Night	42% 10% 13% 20% 13% 2	2813
	21st to end	Night	29% 10% 15% 25% 17% 4	3087
June	1st to 10th	Night	39% 9% 10% 24% 14% 3	1591
	11th to 20th	Night	35% 9% 15% 20% 18% 2	1440
	21st to end	Night	34% 9% 14% 25% 16% 2	1450
July	1st to 10th	Night	33% 7% 13% 24% 17% 6%	2537
	11th to 20th	Night	31% 9% 13% 27% 18% 4	2786
	21st to end	Night	33% 8% 15% 21% 21% 2	2988
August	1st to 10th	Night	38% 9% 13% 24% 15% 1	2814
	11th to 20th	Night	38% 7% 11% 29% 13% 1	2812
	21st to end	Night	35% 8% 13% 27% 14% 3	3124
September	1st to 10th	Night	36% 8% 13% 23% 16% 4	2035
	11th to 20th	Night	29% 8% 13% 26% 21% 2	2090
	21st to end	Night	43% 8% 11% 20% 18% 1	2090
October	1st to 10th	Night	31% 5% 9% 29% 25% 1	3658
	11th to 20th	Night	45% 6% 9% 20% 19% 1	3779
	21st to end	Night	41% 5% 10% 25% 17% 2	4198
November	1st to 10th	Night	45% 7% 12% 21% 13% 2	2536
	11th to 20th	Night	56% 6% 10% 17% 9% 2	2600
	21st to end	Night	62% 7% 11% 15% 5% 1	2750
December	1st to 10th	Night	64% 5% 13% 14% 4	4580
	11th to 20th	Night	67% 6% 10% 13% 3	5208
	21st to end	Night	72% 8% 10% 9% 1	6605

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	300
	11th to 20th	Day	29% 29% 31% 7% 4	300
	21st to end	Day	29% 27% 29% 13% 1 1	300
February	1st to 10th	Day	33% 21% 33% 10% 2	270
	11th to 20th	Day	30% 27% 31% 11% 1	270
	21st to end	Day	32% 30% 29% 8%	225
March	1st to 10th	Day	27% 22% 32% 14% 5%	379
	11th to 20th	Day	27% 17% 33% 17% 5%	460
	21st to end	Day	32% 19% 28% 17% 5%	506
April	1st to 10th	Day	23% 18% 33% 18% 7% 1	460
	11th to 20th	Day	26% 21% 24% 18% 7% 4	452
	21st to end	Day	26% 17% 33% 15% 6% 2	460
May	1st to 10th	Day	25% 16% 29% 22% 6% 3	463
	11th to 20th	Day	20% 11% 33% 22% 10% 4	487
	21st to end	Day	13% 10% 26% 29% 18% 5%	529
June	1st to 10th	Day	13% 9% 31% 30% 14% 2	490
	11th to 20th	Day	14% 9% 25% 34% 16% 3	560
	21st to end	Day	11% 11% 31% 30% 14% 4	680
July	1st to 10th	Day	9% 5% 25% 30% 22% 9%	644
	11th to 20th	Day	12% 6% 25% 28% 20% 8%	484
	21st to end	Day	9% 6% 26% 30% 21% 8%	532
August	1st to 10th	Day	12% 6% 20% 30% 23% 10%	486
	11th to 20th	Day	13% 5% 23% 29% 21% 9%	479
	21st to end	Day	10% 7% 22% 27% 23% 11%	539
September	1st to 10th	Day	15% 7% 22% 24% 20% 11%	490
	11th to 20th	Day	11% 4 19% 26% 29% 12%	490
	21st to end	Day	22% 12% 24% 21% 13% 8%	490
October	1st to 10th	Day	12% 12% 26% 27% 18% 7%	490
	11th to 20th	Day	21% 16% 32% 21% 7% 3	490
	21st to end	Day	26% 21% 31% 17% 5%	496
November	1st to 10th	Day	19% 22% 32% 17% 8% 1	300
	11th to 20th	Day	29% 22% 30% 14% 4 1	300
	21st to end	Day	31% 24% 34% 10%	223
December	1st to 10th	Day	24% 27% 31% 14% 4 2	200
	11th to 20th	Day	24% 29% 34% 11% 3	200
	21st to end	Day	28% 23% 35% 13% 1	303
January	1st to 10th	Night	27% 21% 32% 15% 4	800
	11th to 20th	Night	31% 19% 31% 15% 4	800
	21st to end	Night	27% 17% 32% 21% 3	841
February	1st to 10th	Night	29% 16% 32% 21% 3	760
	11th to 20th	Night	26% 18% 32% 21% 3	743
	21st to end	Night	28% 17% 35% 18% 2	628
March	1st to 10th	Night	24% 15% 32% 24% 5%	760
	11th to 20th	Night	21% 17% 32% 23% 6% 1	760
	21st to end	Night	25% 11% 29% 25% 9%	836
April	1st to 10th	Night	22% 16% 32% 26% 5%	760
	11th to 20th	Night	21% 12% 33% 24% 9% 1	566
	21st to end	Night	19% 10% 36% 28% 7% 1	570
May	1st to 10th	Night	20% 8% 29% 32% 9% 1	570
	11th to 20th	Night	19% 7% 25% 34% 13% 1	561
	21st to end	Night	11% 4 23% 37% 22% 3	617
June	1st to 10th	Night	12% 4 27% 42% 15% 1	451
	11th to 20th	Night	11% 4 24% 43% 17% 1	380
	21st to end	Night	11% 4 26% 39% 20% 1	500
July	1st to 10th	Night	10% 3 15% 41% 28% 3	555
	11th to 20th	Night	10% 2 21% 39% 24% 3	552
	21st to end	Night	10% 2 17% 42% 28% 2	624
August	1st to 10th	Night	10% 2 14% 44% 25% 4	567
	11th to 20th	Night	10% 4 17% 38% 27% 4	564
	21st to end	Night	8% 4 17% 36% 30% 4	627
September	1st to 10th	Night	13% 5% 17% 36% 25% 4	753
	11th to 20th	Night	12% 4 14% 36% 30% 5%	760
	21st to end	Night	17% 8% 23% 29% 19% 3	760
October	1st to 10th	Night	17% 8% 23% 31% 19% 2	760
	11th to 20th	Night	25% 13% 28% 24% 9% 1	760
	21st to end	Night	30% 14% 27% 23% 6%	836
November	1st to 10th	Night	22% 13% 31% 24% 9% 1	780
	11th to 20th	Night	27% 14% 33% 18% 6% 1	800
	21st to end	Night	32% 15% 30% 19% 5%	800
December	1st to 10th	Night	30% 18% 30% 17% 5%	800
	11th to 20th	Night	28% 18% 33% 18% 3	800
	21st to end	Night	30% 16% 33% 19% 3	880

Page generated at 2025-01-19 03:06:43. Version ac07

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