Nightfall Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Nightfall Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 41%	120192
	Night	13% 7% 3 6% 4 4 4 3 5% 7% 44%	118953
Transparency	Day	39% 7% 10% 20% 18% 5%	124001
	Night	38% 6% 9% 20% 20% 7%	123283
Seeing	Day	17% 11% 24% 23% 17% 8%	15425
	Night	16% 9% 24% 29% 20% 4	25410

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	17% 10% 5% 6% 3 2 3 3 4 4 44%	4056
	11th to 20th	Day	12% 8% 6% 6% 3 2 2 2 3 5% 50%	4220
	21st to end	Day	21% 11% 3 5% 3 3 3 3 4 6% 39%	4972
February	1st to 10th	Day	19% 10% 3 6% 3 4 3 3 4 5% 39%	2000
	11th to 20th	Day	24% 8% 2 5% 4 3 3 3 4 4 39%	1948
	21st to end	Day	21% 8% 3 6% 4 4 4 3 4 5% 38%	1650
March	1st to 10th	Day	20% 9% 3 6% 4 5% 4 3 5% 5% 36%	4480
	11th to 20th	Day	13% 6% 2 7% 4 4 4 3 6% 6% 45%	4782
	21st to end	Day	14% 7% 3 7% 5% 4 4 4 5% 6% 42%	5751
April	1st to 10th	Day	11% 6% 3 6% 5% 4 4 3 6% 7% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 4 6% 47%	2374
	21st to end	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 46%	2400
May	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	5806
	11th to 20th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 29%	5861
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	6433
June	1st to 10th	Day	9% 6% 2 8% 6% 6% 6% 6% 8% 9% 34%	2600
	11th to 20th	Day	5% 4 2 6% 5% 5% 7% 6% 9% 10% 41%	2600
	21st to end	Day	7% 4 2 6% 6% 7% 6% 6% 7% 11% 39%	2600
July	1st to 10th	Day	6% 5% 2 8% 6% 6% 8% 7% 9% 13% 30%	4250
	11th to 20th	Day	8% 5% 2 6% 5% 6% 7% 6% 9% 13% 32%	4250
	21st to end	Day	11% 9% 4 10% 7% 7% 7% 6% 9% 9% 20%	4387
August	1st to 10th	Day	4 3 2 4 5% 4 5% 5% 8% 10% 51%	3785
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 10% 46%	3729
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 6% 8% 50%	4180
September	1st to 10th	Day	9% 5% 3 6% 5% 5% 6% 4 7% 8% 41%	2237
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 3 6% 7% 43%	2090
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 6% 51%	2090
October	1st to 10th	Day	9% 4 2 5% 4 4 4 3 5% 6% 55%	2090
	11th to 20th	Day	10% 5% 2 4 3 3 3 4 5% 7% 55%	2090
	21st to end	Day	13% 4 2 5% 3 4 4 3 4 5% 54%	1950
November	1st to 10th	Day	13% 6% 2 6% 3 3 3 2 5% 4 53%	1762
	11th to 20th	Day	13% 6% 2 4 4 3 3 3 5% 6% 52%	1800
	21st to end	Day	14% 6% 2 6% 3 3 4 4 6% 6% 45%	1800
December	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 5% 44%	2930
	11th to 20th	Day	17% 10% 3 7% 4 4 4 3 3 4 40%	3373
	21st to end	Day	20% 9% 3 6% 4 4 3 3 4 6% 38%	4466
January	1st to 10th	Night	16% 10% 4 7% 3 3 4 2 4 5% 43%	5889
	11th to 20th	Night	18% 8% 4 6% 3 2 3 2 4 5% 45%	5738
	21st to end	Night	19% 11% 2 5% 3 3 3 2 4 4 43%	5984
February	1st to 10th	Night	18% 9% 3 6% 4 4 3 3 4 6% 42%	2400
	11th to 20th	Night	21% 8% 3 6% 4 3 3 3 4 4 41%	2338
	21st to end	Night	18% 8% 2 6% 4 4 4 4 5% 6% 39%	1980
March	1st to 10th	Night	21% 9% 3 8% 4 4 4 3 4 6% 34%	5373
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 49%	5176
	21st to end	Night	15% 7% 3 8% 4 4 4 3 4 5% 44%	5206
April	1st to 10th	Night	12% 6% 1 5% 5% 3 4 4 6% 7% 48%	2000
	11th to 20th	Night	13% 6% 2 4 3 4 4 3 5% 6% 50%	1975
	21st to end	Night	10% 5% 3 5% 4 4 3 4 5% 7% 49%	2000
May	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	4089
	11th to 20th	Night	15% 9% 3 10% 6% 5% 5% 4 6% 7% 30%	4032
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 7% 10% 40%	4421
June	1st to 10th	Night	6% 5% 2 6% 5% 6% 5% 6% 8% 9% 41%	1800
	11th to 20th	Night	4 3 1 4 4 4 5% 4 6% 11% 52%	1800
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 4 7% 11% 45%	1800
July	1st to 10th	Night	7% 5% 2 8% 5% 6% 6% 5% 8% 13% 34%	2930
	11th to 20th	Night	7% 5% 2 6% 5% 6% 6% 5% 8% 13% 36%	2930
	21st to end	Night	8% 9% 4 10% 8% 8% 6% 6% 7% 9% 24%	3149
August	1st to 10th	Night	2 2 1 3 4 4 4 4 7% 10% 58%	3144
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 9% 54%	3100
	21st to end	Night	5% 4 1 4 4 3 5% 4 6% 8% 56%	3476
September	1st to 10th	Night	7% 6% 2 6% 5% 5% 6% 4 6% 8% 46%	1924
	11th to 20th	Night	11% 5% 2 7% 4 5% 4 4 5% 8% 44%	2090
	21st to end	Night	7% 4 2 5% 3 4 4 3 6% 7% 54%	2090
October	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 4 61%	2090
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 4 5% 55%	2090
	21st to end	Night	14% 6% 2 4 3 3 2 2 3 4 56%	2641
November	1st to 10th	Night	11% 4 2 5% 4 3 3 3 4 6% 55%	2536
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 4 6% 53%	2600
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 47%	2600
December	1st to 10th	Night	21% 6% 2 5% 3 3 3 2 4 5% 47%	4250
	11th to 20th	Night	18% 9% 3 6% 4 3 3 2 3 5% 43%	4822
	21st to end	Night	20% 8% 3 7% 3 4 4 3 5% 6% 37%	6490

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	44% 3 5% 23% 22% 3	4057
	11th to 20th	Day	38% 3 5% 25% 26% 3	4220
	21st to end	Day	48% 3 6% 16% 20% 6%	4972
February	1st to 10th	Day	47% 4 7% 19% 19% 4	2000
	11th to 20th	Day	48% 4 4 18% 18% 6%	1958
	21st to end	Day	48% 4 7% 13% 22% 6%	1650
March	1st to 10th	Day	50% 4 8% 13% 18% 8%	4480
	11th to 20th	Day	38% 4 6% 16% 21% 14%	4782
	21st to end	Day	43% 5% 8% 21% 20% 4	5753
April	1st to 10th	Day	37% 5% 11% 22% 19% 7%	2400
	11th to 20th	Day	38% 5% 7% 21% 23% 7%	2374
	21st to end	Day	34% 6% 11% 24% 22% 3	2400
May	1st to 10th	Day	44% 9% 13% 22% 12% 2	5842
	11th to 20th	Day	47% 9% 14% 20% 8% 1	5865
	21st to end	Day	41% 7% 11% 24% 16% 2	6447
June	1st to 10th	Day	42% 10% 14% 17% 15% 1	2600
	11th to 20th	Day	32% 11% 16% 24% 15% 2	2600
	21st to end	Day	35% 11% 17% 22% 11% 4	2600
July	1st to 10th	Day	37% 15% 23% 19% 5% 1	4185
	11th to 20th	Day	37% 14% 22% 21% 5% 1	4181
	21st to end	Day	54% 13% 13% 14% 5%	4247
August	1st to 10th	Day	23% 9% 14% 36% 17% 1	3773
	11th to 20th	Day	31% 9% 17% 25% 15% 3	3765
	21st to end	Day	30% 9% 15% 18% 23% 4	4179
September	1st to 10th	Day	37% 10% 14% 17% 17% 5%	2237
	11th to 20th	Day	40% 7% 12% 18% 20% 4	2090
	21st to end	Day	34% 6% 10% 22% 21% 7%	2090
October	1st to 10th	Day	21% 5% 5% 14% 33% 22%	3480
	11th to 20th	Day	19% 3 5% 17% 30% 26%	3458
	21st to end	Day	23% 5% 6% 22% 33% 12%	3188
November	1st to 10th	Day	34% 3 8% 18% 27% 10%	1762
	11th to 20th	Day	34% 4 7% 18% 27% 10%	1800
	21st to end	Day	38% 5% 7% 17% 25% 8%	1800
December	1st to 10th	Day	43% 3 5% 18% 26% 4	2930
	11th to 20th	Day	48% 3 7% 19% 17% 6%	3370
	21st to end	Day	47% 4 7% 23% 18% 1	4466
January	1st to 10th	Night	45% 3 5% 22% 23% 3	5889
	11th to 20th	Night	43% 3 5% 19% 24% 7%	5738
	21st to end	Night	45% 3 5% 16% 25% 6%	5984
February	1st to 10th	Night	44% 4 6% 19% 21% 7%	2400
	11th to 20th	Night	48% 3 6% 17% 18% 8%	2339
	21st to end	Night	45% 5% 7% 14% 20% 9%	1980
March	1st to 10th	Night	51% 4 6% 14% 18% 6%	5373
	11th to 20th	Night	34% 5% 7% 16% 22% 16%	5176
	21st to end	Night	44% 4 6% 22% 18% 6%	5206
April	1st to 10th	Night	35% 6% 11% 22% 19% 7%	2000
	11th to 20th	Night	35% 5% 6% 20% 22% 12%	1975
	21st to end	Night	33% 6% 10% 25% 21% 5%	2000
May	1st to 10th	Night	42% 10% 13% 18% 15% 2	4116
	11th to 20th	Night	50% 8% 14% 18% 8% 1	4033
	21st to end	Night	37% 9% 12% 25% 16% 2	4429
June	1st to 10th	Night	33% 11% 15% 20% 19% 2	1800
	11th to 20th	Night	25% 9% 19% 27% 18% 3	1800
	21st to end	Night	32% 9% 16% 22% 17% 4	1800
July	1st to 10th	Night	38% 11% 24% 19% 6% 2	2885
	11th to 20th	Night	33% 14% 24% 21% 6% 1	2876
	21st to end	Night	53% 12% 15% 14% 6% 1	3048
August	1st to 10th	Night	18% 9% 14% 40% 18% 1	3132
	11th to 20th	Night	25% 8% 18% 30% 16% 3	3130
	21st to end	Night	25% 8% 15% 20% 26% 6%	3476
September	1st to 10th	Night	35% 9% 17% 18% 15% 7%	1924
	11th to 20th	Night	38% 7% 10% 18% 21% 5%	2090
	21st to end	Night	28% 6% 11% 23% 24% 8%	2090
October	1st to 10th	Night	20% 4 7% 17% 30% 22%	3480
	11th to 20th	Night	19% 3 5% 16% 31% 27%	3461
	21st to end	Night	22% 3 5% 23% 32% 15%	4353
November	1st to 10th	Night	31% 5% 8% 18% 25% 14%	2536
	11th to 20th	Night	32% 4 6% 18% 28% 12%	2600
	21st to end	Night	37% 5% 6% 20% 24% 9%	2600
December	1st to 10th	Night	40% 4 5% 20% 27% 4	4250
	11th to 20th	Night	45% 4 6% 17% 23% 4	4824
	21st to end	Night	46% 4 7% 24% 17% 2	6490

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	30% 24% 31% 12% 3	300
	11th to 20th	Day	30% 22% 28% 15% 3 1	300
	21st to end	Day	25% 26% 34% 11% 4	330
February	1st to 10th	Day	26% 24% 33% 14% 2 1	300
	11th to 20th	Day	41% 20% 28% 9% 2	300
	21st to end	Day	32% 24% 35% 7% 2	249
March	1st to 10th	Day	28% 21% 36% 11% 4 1	300
	11th to 20th	Day	26% 21% 32% 16% 5% 1	406
	21st to end	Day	23% 18% 34% 18% 6%	550
April	1st to 10th	Day	22% 16% 31% 19% 11% 1	500
	11th to 20th	Day	18% 16% 32% 22% 11% 1	492
	21st to end	Day	14% 15% 32% 27% 12% 1	500
May	1st to 10th	Day	16% 10% 32% 24% 13% 5%	503
	11th to 20th	Day	9% 10% 31% 26% 18% 6%	527
	21st to end	Day	12% 6% 25% 29% 17% 11%	573
June	1st to 10th	Day	13% 6% 20% 29% 21% 12%	530
	11th to 20th	Day	6% 5% 18% 28% 26% 18%	530
	21st to end	Day	12% 2 14% 28% 28% 16%	530
July	1st to 10th	Day	6% 2 13% 28% 30% 21%	525
	11th to 20th	Day	7% 2 9% 23% 31% 27%	524
	21st to end	Day	10% 2 11% 20% 37% 19%	532
August	1st to 10th	Day	9% 2 11% 27% 28% 22%	486
	11th to 20th	Day	10% 1 11% 32% 32% 13%	479
	21st to end	Day	15% 2 9% 27% 31% 17%	539
September	1st to 10th	Day	10% 3 11% 34% 30% 11%	490
	11th to 20th	Day	12% 3 16% 25% 33% 10%	490
	21st to end	Day	14% 4 18% 29% 26% 9%	490
October	1st to 10th	Day	11% 5% 26% 33% 17% 8%	490
	11th to 20th	Day	15% 8% 27% 32% 17% 2	490
	21st to end	Day	16% 13% 24% 30% 16% 1	340
November	1st to 10th	Day	21% 10% 33% 23% 10% 3	300
	11th to 20th	Day	16% 19% 38% 19% 8%	300
	21st to end	Day	18% 21% 32% 20% 8% 2	300
December	1st to 10th	Day	30% 20% 30% 17% 3	300
	11th to 20th	Day	27% 23% 30% 14% 6% 1	300
	21st to end	Day	28% 21% 29% 18% 4 1	330
January	1st to 10th	Night	23% 17% 36% 20% 5%	800
	11th to 20th	Night	25% 18% 27% 23% 7%	800
	21st to end	Night	22% 17% 36% 23% 3	880
February	1st to 10th	Night	24% 13% 33% 27% 4	800
	11th to 20th	Night	27% 17% 31% 22% 3	783
	21st to end	Night	25% 16% 34% 24% 1	660
March	1st to 10th	Night	21% 17% 31% 27% 4	800
	11th to 20th	Night	17% 15% 35% 23% 8% 1	800
	21st to end	Night	20% 13% 33% 25% 9%	774
April	1st to 10th	Night	15% 13% 34% 26% 11% 1	600
	11th to 20th	Night	17% 10% 34% 25% 13% 1	593
	21st to end	Night	13% 9% 30% 31% 16% 1	600
May	1st to 10th	Night	12% 6% 29% 31% 20% 3	600
	11th to 20th	Night	10% 6% 25% 32% 22% 4	591
	21st to end	Night	6% 3 15% 39% 30% 6%	650
June	1st to 10th	Night	7% 3 15% 36% 33% 7%	600
	11th to 20th	Night	4 2 11% 31% 38% 14%	600
	21st to end	Night	7% 2 11% 26% 44% 10%	600
July	1st to 10th	Night	6% 2 7% 28% 41% 16%	585
	11th to 20th	Night	6% 1 6% 24% 44% 19%	582
	21st to end	Night	6% 2 10% 26% 41% 14%	624
August	1st to 10th	Night	4 1 4 29% 46% 16%	567
	11th to 20th	Night	7% 1 9% 31% 41% 10%	564
	21st to end	Night	11% 1 7% 32% 37% 10%	627
September	1st to 10th	Night	9% 3 11% 31% 36% 9%	594
	11th to 20th	Night	14% 3 12% 30% 37% 5%	760
	21st to end	Night	9% 2 15% 35% 34% 4	760
October	1st to 10th	Night	10% 3 21% 34% 25% 5%	760
	11th to 20th	Night	13% 7% 24% 34% 20% 2	760
	21st to end	Night	15% 8% 21% 36% 18% 2	836
November	1st to 10th	Night	14% 9% 21% 36% 18% 3	780
	11th to 20th	Night	16% 10% 30% 31% 12% 1	800
	21st to end	Night	18% 11% 30% 29% 11% 1	800
December	1st to 10th	Night	25% 13% 28% 26% 8% 1	800
	11th to 20th	Night	24% 14% 29% 25% 8%	800
	21st to end	Night	26% 13% 30% 25% 7%	880

Page generated at 2025-07-18 00:10:04. Version ac07

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