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 2024-11-04.

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	12% 7% 2 6% 4 4 5% 4 6% 8% 41%	100846
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	98139
Transparency	Day	38% 7% 11% 20% 18% 5%	104654
	Night	36% 6% 10% 20% 20% 7%	102469
Seeing	Day	17% 10% 24% 24% 18% 8%	14520
	Night	15% 9% 23% 29% 20% 4	24494

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% 12% 4 7% 4 3 3 3 5% 4 38%	2836
	11th to 20th	Day	16% 9% 3 7% 4 3 3 2 3 4 46%	2944
	21st to end	Day	23% 11% 3 4 2 2 3 3 3 6% 41%	3476
February	1st to 10th	Day	19% 11% 3 6% 3 3 3 3 4 6% 39%	1900
	11th to 20th	Day	23% 8% 2 5% 4 3 3 3 4 5% 40%	1848
	21st to end	Day	22% 9% 3 5% 4 4 4 3 4 5% 37%	1570
March	1st to 10th	Day	22% 10% 3 6% 4 4 4 3 5% 6% 33%	3120
	11th to 20th	Day	15% 7% 3 7% 4 3 4 3 5% 7% 42%	3338
	21st to end	Day	15% 7% 2 7% 5% 4 4 4 6% 7% 38%	4019
April	1st to 10th	Day	11% 6% 3 6% 5% 4 4 3 6% 7% 46%	2280
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 3 4 6% 46%	2254
	21st to end	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	2280
May	1st to 10th	Day	11% 5% 3 6% 4 5% 5% 3 7% 9% 43%	4040
	11th to 20th	Day	16% 7% 2 6% 4 5% 4 5% 7% 8% 37%	4081
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 4 6% 10% 41%	4476
June	1st to 10th	Day	9% 6% 2 8% 6% 6% 7% 6% 8% 9% 34%	2470
	11th to 20th	Day	5% 4 2 6% 4 5% 6% 6% 9% 10% 42%	2470
	21st to end	Day	7% 4 2 6% 6% 7% 6% 6% 7% 10% 40%	2470
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 8% 7% 9% 13% 30%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 6% 7% 6% 9% 13% 31%	4120
	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	12% 5% 2 5% 3 3 3 2 4 5% 54%	1699
	11th to 20th	Day	12% 6% 2 4 4 3 3 3 5% 5% 52%	1710
	21st to end	Day	15% 6% 2 6% 3 3 4 4 6% 6% 44%	1710
December	1st to 10th	Day	22% 9% 2 6% 3 3 4 3 5% 5% 38%	1710
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 3 4 4 43%	2153
	21st to end	Day	18% 10% 4 7% 4 4 4 3 5% 6% 34%	3124
January	1st to 10th	Night	18% 12% 2 6% 3 3 4 2 4 5% 40%	4109
	11th to 20th	Night	19% 9% 4 5% 3 3 3 3 4 5% 43%	4014
	21st to end	Night	21% 14% 3 5% 4 3 3 2 4 4 39%	4180
February	1st to 10th	Night	18% 9% 3 5% 4 4 3 3 4 6% 42%	2280
	11th to 20th	Night	21% 8% 3 6% 4 3 3 3 4 4 43%	2218
	21st to end	Night	19% 8% 2 6% 4 4 4 4 5% 6% 38%	1884
March	1st to 10th	Night	23% 9% 3 7% 4 3 3 2 4 6% 35%	3733
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 6% 7% 48%	3620
	21st to end	Night	16% 8% 3 8% 5% 5% 4 3 5% 5% 40%	3640
April	1st to 10th	Night	12% 6% 1 5% 5% 4 4 4 6% 7% 47%	1900
	11th to 20th	Night	14% 6% 2 4 3 4 4 3 5% 6% 49%	1875
	21st to end	Night	10% 5% 2 5% 4 3 3 4 5% 7% 50%	1900
May	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	2855
	11th to 20th	Night	15% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	2812
	21st to end	Night	5% 4 2 6% 4 5% 6% 5% 7% 11% 45%	3079
June	1st to 10th	Night	6% 5% 2 6% 5% 6% 5% 6% 9% 9% 40%	1710
	11th to 20th	Night	4 3 1 4 4 4 5% 4 6% 11% 54%	1710
	21st to end	Night	5% 5% 2 6% 5% 5% 5% 4 7% 11% 45%	1710
July	1st to 10th	Night	7% 5% 2 8% 6% 6% 6% 5% 8% 13% 33%	2840
	11th to 20th	Night	8% 5% 2 6% 5% 6% 6% 5% 9% 13% 36%	2840
	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 4 3 3 4 3 4 6% 56%	2452
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 4 6% 53%	2470
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 47%	2470
December	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 40%	2470
	11th to 20th	Night	18% 7% 3 5% 4 3 3 2 4 5% 47%	3042
	21st to end	Night	19% 9% 4 7% 4 5% 4 4 6% 7% 32%	4532

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	49% 4 6% 15% 22% 5%	2837
	11th to 20th	Day	43% 4 5% 24% 21% 3	2944
	21st to end	Day	47% 3 7% 18% 21% 5%	3476
February	1st to 10th	Day	48% 3 7% 19% 19% 4	1900
	11th to 20th	Day	48% 4 4 18% 18% 7%	1858
	21st to end	Day	49% 4 7% 12% 21% 6%	1570
March	1st to 10th	Day	51% 5% 9% 11% 15% 9%	3120
	11th to 20th	Day	42% 4 7% 18% 22% 7%	3338
	21st to end	Day	44% 6% 10% 17% 18% 4	4019
April	1st to 10th	Day	38% 5% 11% 21% 19% 8%	2280
	11th to 20th	Day	39% 5% 7% 20% 22% 7%	2254
	21st to end	Day	33% 6% 10% 25% 23% 3	2280
May	1st to 10th	Day	36% 8% 12% 25% 16% 2	4078
	11th to 20th	Day	42% 7% 12% 26% 12% 1	4085
	21st to end	Day	36% 8% 12% 27% 16% 1	4489
June	1st to 10th	Day	42% 10% 13% 17% 15% 1	2470
	11th to 20th	Day	30% 10% 17% 25% 16% 2	2470
	21st to end	Day	36% 11% 16% 22% 11% 4	2470
July	1st to 10th	Day	38% 15% 23% 18% 5% 1	4055
	11th to 20th	Day	37% 14% 22% 21% 6% 1	4051
	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	33% 3 8% 17% 28% 10%	1699
	11th to 20th	Day	34% 4 7% 19% 26% 10%	1710
	21st to end	Day	39% 5% 7% 17% 25% 7%	1710
December	1st to 10th	Day	48% 5% 7% 15% 20% 5%	1710
	11th to 20th	Day	47% 2 5% 18% 22% 5%	2150
	21st to end	Day	50% 4 9% 18% 17% 2	3124
January	1st to 10th	Night	48% 3 4 16% 25% 3	4109
	11th to 20th	Night	44% 4 5% 21% 18% 7%	4014
	21st to end	Night	50% 3 5% 15% 22% 4	4180
February	1st to 10th	Night	45% 4 6% 19% 21% 7%	2280
	11th to 20th	Night	47% 3 6% 17% 19% 9%	2219
	21st to end	Night	46% 5% 7% 13% 19% 9%	1884
March	1st to 10th	Night	51% 4 7% 15% 15% 7%	3733
	11th to 20th	Night	34% 6% 8% 19% 22% 10%	3620
	21st to end	Night	46% 5% 7% 21% 19% 4	3640
April	1st to 10th	Night	35% 6% 11% 22% 20% 6%	1900
	11th to 20th	Night	36% 5% 6% 20% 22% 12%	1875
	21st to end	Night	32% 5% 10% 26% 22% 5%	1900
May	1st to 10th	Night	36% 10% 12% 20% 20% 3	2882
	11th to 20th	Night	42% 8% 14% 23% 11% 2	2813
	21st to end	Night	28% 10% 13% 29% 17% 2	3087
June	1st to 10th	Night	34% 12% 15% 20% 18% 2	1710
	11th to 20th	Night	23% 9% 19% 28% 18% 3	1710
	21st to end	Night	32% 8% 16% 21% 18% 5%	1710
July	1st to 10th	Night	39% 11% 24% 19% 6% 2	2795
	11th to 20th	Night	34% 14% 24% 21% 7% 1	2786
	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	30% 5% 8% 18% 26% 14%	2452
	11th to 20th	Night	32% 4 7% 18% 28% 12%	2470
	21st to end	Night	38% 5% 6% 19% 24% 8%	2470
December	1st to 10th	Night	45% 6% 8% 17% 21% 4	2470
	11th to 20th	Night	41% 4 7% 16% 27% 5%	3044
	21st to end	Night	48% 4 8% 19% 18% 2	4532

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	270
	11th to 20th	Day	33% 21% 27% 15% 4 1	270
	21st to end	Day	26% 27% 34% 10% 3	297
February	1st to 10th	Day	28% 24% 33% 13% 1 1	270
	11th to 20th	Day	40% 20% 29% 9% 2	270
	21st to end	Day	35% 25% 34% 5% 1	225
March	1st to 10th	Day	28% 19% 36% 12% 4 1	270
	11th to 20th	Day	25% 21% 31% 16% 5% 1	370
	21st to end	Day	23% 19% 33% 18% 6%	506
April	1st to 10th	Day	22% 15% 31% 20% 11% 1	460
	11th to 20th	Day	19% 16% 30% 22% 12% 1	452
	21st to end	Day	15% 15% 32% 25% 12% 1	460
May	1st to 10th	Day	15% 9% 32% 25% 14% 5%	463
	11th to 20th	Day	9% 7% 31% 27% 19% 7%	487
	21st to end	Day	11% 5% 24% 29% 18% 12%	529
June	1st to 10th	Day	13% 5% 20% 29% 22% 11%	490
	11th to 20th	Day	6% 5% 17% 27% 26% 19%	490
	21st to end	Day	12% 2 15% 28% 28% 14%	490
July	1st to 10th	Day	7% 2 12% 27% 31% 21%	485
	11th to 20th	Day	8% 2 9% 24% 29% 27%	484
	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	20% 9% 34% 24% 10% 3	279
	11th to 20th	Day	16% 20% 39% 19% 7%	270
	21st to end	Day	18% 20% 33% 20% 9% 1	270
December	1st to 10th	Day	30% 20% 29% 17% 3	270
	11th to 20th	Day	27% 22% 31% 13% 6% 1	270
	21st to end	Day	28% 21% 31% 17% 3	297
January	1st to 10th	Night	23% 16% 36% 20% 5%	760
	11th to 20th	Night	25% 18% 27% 23% 7%	760
	21st to end	Night	23% 17% 36% 22% 3	836
February	1st to 10th	Night	24% 13% 33% 26% 3	760
	11th to 20th	Night	26% 16% 32% 22% 3	743
	21st to end	Night	26% 16% 34% 22% 1	628
March	1st to 10th	Night	21% 16% 31% 28% 5%	760
	11th to 20th	Night	17% 15% 35% 23% 8% 1	760
	21st to end	Night	20% 13% 32% 25% 9%	736
April	1st to 10th	Night	14% 13% 34% 26% 12% 1	570
	11th to 20th	Night	17% 11% 33% 24% 13% 1	563
	21st to end	Night	13% 9% 30% 31% 15% 1	570
May	1st to 10th	Night	12% 6% 28% 30% 21% 3	570
	11th to 20th	Night	10% 4 25% 33% 23% 4	561
	21st to end	Night	6% 2 15% 39% 32% 6%	617
June	1st to 10th	Night	7% 3 15% 35% 33% 7%	570
	11th to 20th	Night	4 1 11% 31% 38% 15%	570
	21st to end	Night	7% 2 12% 27% 42% 10%	570
July	1st to 10th	Night	6% 2 7% 28% 41% 16%	555
	11th to 20th	Night	6% 1 6% 24% 44% 19%	552
	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	13% 8% 21% 36% 19% 3	755
	11th to 20th	Night	16% 10% 30% 32% 12% 1	760
	21st to end	Night	19% 10% 30% 29% 11% 1	760
December	1st to 10th	Night	25% 13% 28% 25% 8% 1	760
	11th to 20th	Night	24% 13% 29% 24% 9%	760
	21st to end	Night	26% 12% 31% 25% 6%	836

Page generated at 2024-11-01 21:47:46. Version ac07

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