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 2024-03-28.

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% 6% 3 5% 5% 3 5% 5% 6% 7% 41%	79261
	Night	13% 6% 3 5% 5% 3 4 4 5% 7% 45%	80747
Transparency	Day	40% 7% 11% 19% 18% 5%	78987
	Night	38% 6% 10% 20% 20% 6%	80540
Seeing	Day	17% 11% 23% 23% 18% 8%	13650
	Night	16% 9% 24% 28% 20% 4	23772

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 6% 5% 3 3 4 5% 4 38%	2836
	11th to 20th	Day	16% 9% 4 6% 4 3 3 3 3 4 46%	2944
	21st to end	Day	23% 11% 3 4 3 2 2 3 3 6% 41%	3476
February	1st to 10th	Day	19% 10% 3 6% 4 3 3 4 4 6% 39%	1900
	11th to 20th	Day	24% 8% 2 5% 4 2 3 4 4 5% 40%	1848
	21st to end	Day	22% 8% 3 5% 4 3 4 4 4 5% 37%	1570
March	1st to 10th	Day	16% 8% 2 5% 5% 3 4 4 5% 6% 43%	1900
	11th to 20th	Day	18% 7% 4 7% 5% 3 4 5% 5% 6% 37%	2000
	21st to end	Day	14% 7% 3 6% 6% 3 5% 5% 5% 7% 38%	2366
April	1st to 10th	Day	11% 6% 3 5% 5% 3 4 4 6% 7% 47%	2160
	11th to 20th	Day	13% 6% 2 5% 5% 3 4 4 4 6% 47%	2134
	21st to end	Day	9% 5% 2 5% 5% 3 4 5% 6% 8% 48%	2160
May	1st to 10th	Day	12% 5% 3 5% 4 3 5% 3 5% 8% 47%	2312
	11th to 20th	Day	10% 6% 2 6% 5% 3 5% 5% 7% 9% 41%	2301
	21st to end	Day	8% 4 2 6% 6% 4 6% 6% 6% 10% 41%	2531
June	1st to 10th	Day	9% 6% 2 7% 7% 4 7% 7% 7% 9% 34%	2340
	11th to 20th	Day	5% 4 2 5% 5% 4 6% 7% 9% 10% 43%	2340
	21st to end	Day	7% 4 2 5% 6% 4 7% 6% 7% 10% 40%	2340
July	1st to 10th	Day	8% 5% 3 7% 6% 5% 8% 8% 7% 11% 32%	2340
	11th to 20th	Day	9% 5% 2 6% 6% 5% 8% 8% 8% 12% 31%	2340
	21st to end	Day	7% 4 2 6% 7% 5% 9% 8% 10% 12% 30%	2507
August	1st to 10th	Day	6% 5% 2 5% 8% 5% 7% 8% 9% 12% 33%	2148
	11th to 20th	Day	9% 6% 3 7% 7% 4 7% 6% 8% 10% 32%	2125
	21st to end	Day	11% 6% 3 6% 7% 4 7% 6% 7% 9% 34%	2376
September	1st to 10th	Day	9% 5% 3 6% 6% 4 6% 5% 8% 8% 41%	2120
	11th to 20th	Day	11% 7% 2 5% 5% 3 6% 4 6% 7% 43%	1980
	21st to end	Day	10% 4 2 4 5% 3 5% 4 5% 6% 52%	1980
October	1st to 10th	Day	9% 4 2 4 4 3 4 4 5% 6% 54%	1980
	11th to 20th	Day	11% 5% 2 4 3 2 3 4 5% 7% 53%	1980
	21st to end	Day	14% 4 2 4 3 3 4 3 4 5% 54%	1848
November	1st to 10th	Day	12% 5% 2 5% 4 2 4 3 4 4 55%	1672
	11th to 20th	Day	13% 5% 2 4 4 2 4 4 4 6% 52%	1710
	21st to end	Day	15% 6% 2 5% 4 2 4 5% 6% 6% 45%	1710
December	1st to 10th	Day	22% 8% 2 5% 4 2 4 4 5% 5% 38%	1710
	11th to 20th	Day	17% 7% 3 6% 6% 3 4 4 3 4 43%	2153
	21st to end	Day	19% 9% 5% 6% 5% 3 5% 4 5% 6% 34%	3124
January	1st to 10th	Night	18% 12% 3 5% 4 3 4 3 4 5% 40%	4109
	11th to 20th	Night	19% 8% 4 5% 4 2 3 4 4 5% 43%	4014
	21st to end	Night	21% 13% 3 4 4 2 3 3 4 4 39%	4180
February	1st to 10th	Night	18% 8% 3 5% 4 2 3 4 4 6% 42%	2280
	11th to 20th	Night	21% 8% 3 5% 4 2 2 3 4 4 43%	2218
	21st to end	Night	20% 7% 3 5% 5% 3 4 5% 5% 6% 38%	1884
March	1st to 10th	Night	17% 7% 2 5% 4 2 3 4 4 6% 45%	2280
	11th to 20th	Night	13% 7% 3 5% 5% 3 4 5% 6% 7% 41%	2178
	21st to end	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 44%	2159
April	1st to 10th	Night	11% 5% 2 4 5% 3 5% 5% 6% 7% 48%	1800
	11th to 20th	Night	14% 6% 2 4 4 2 4 4 5% 6% 50%	1775
	21st to end	Night	10% 4 3 5% 4 2 4 5% 5% 8% 51%	1800
May	1st to 10th	Night	10% 4 2 4 4 2 5% 5% 5% 8% 52%	1648
	11th to 20th	Night	8% 5% 2 6% 6% 3 5% 6% 6% 10% 44%	1592
	21st to end	Night	5% 4 3 5% 4 3 6% 6% 6% 11% 48%	1744
June	1st to 10th	Night	6% 4 3 6% 6% 4 6% 7% 9% 10% 40%	1620
	11th to 20th	Night	4 2 1 3 4 2 5% 5% 6% 10% 55%	1620
	21st to end	Night	5% 4 3 4 6% 4 6% 5% 6% 11% 46%	1620
July	1st to 10th	Night	8% 3 2 5% 6% 4 7% 6% 9% 13% 37%	1620
	11th to 20th	Night	5% 4 2 4 6% 4 6% 6% 8% 12% 43%	1620
	21st to end	Night	5% 5% 3 6% 7% 4 6% 8% 7% 11% 39%	1807
August	1st to 10th	Night	4 3 1 4 6% 4 6% 7% 9% 13% 42%	1790
	11th to 20th	Night	7% 4 3 5% 6% 4 6% 6% 7% 11% 42%	1776
	21st to end	Night	10% 6% 2 5% 6% 3 5% 6% 6% 8% 41%	1980
September	1st to 10th	Night	8% 5% 2 5% 6% 4 5% 6% 6% 8% 46%	1822
	11th to 20th	Night	10% 5% 2 6% 6% 4 4 5% 6% 8% 45%	1980
	21st to end	Night	6% 4 2 4 4 2 5% 4 5% 8% 55%	1980
October	1st to 10th	Night	9% 4 2 4 4 2 3 3 4 5% 61%	1980
	11th to 20th	Night	11% 6% 2 4 4 2 3 4 5% 6% 53%	1980
	21st to end	Night	15% 6% 3 4 4 2 2 3 3 4 55%	2501
November	1st to 10th	Night	11% 4 2 4 4 3 4 4 4 6% 56%	2406
	11th to 20th	Night	13% 6% 2 3 3 2 3 4 4 6% 54%	2470
	21st to end	Night	14% 5% 2 5% 4 3 4 4 5% 6% 47%	2470
December	1st to 10th	Night	20% 6% 2 6% 5% 3 4 4 5% 6% 40%	2470
	11th to 20th	Night	18% 7% 3 5% 4 2 3 3 4 5% 47%	3042
	21st to end	Night	19% 8% 4 6% 5% 4 4 5% 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	43% 4 8% 15% 20% 10%	1900
	11th to 20th	Day	48% 5% 9% 15% 18% 5%	2000
	21st to end	Day	45% 6% 9% 21% 17% 2	2366
April	1st to 10th	Day	37% 5% 11% 21% 19% 7%	2160
	11th to 20th	Day	39% 5% 7% 20% 23% 7%	2134
	21st to end	Day	34% 5% 9% 25% 23% 3	2160
May	1st to 10th	Day	36% 6% 11% 23% 21% 4	2312
	11th to 20th	Day	38% 9% 14% 22% 16% 2	2305
	21st to end	Day	36% 8% 13% 24% 16% 2	2531
June	1st to 10th	Day	43% 10% 13% 17% 16% 1	2340
	11th to 20th	Day	31% 10% 17% 25% 15% 2	2340
	21st to end	Day	36% 11% 16% 22% 11% 4	2340
July	1st to 10th	Day	40% 13% 18% 19% 8% 1	2275
	11th to 20th	Day	40% 14% 20% 16% 8% 2	2271
	21st to end	Day	38% 16% 18% 19% 9%	2367
August	1st to 10th	Day	38% 13% 20% 22% 7% 1	2136
	11th to 20th	Day	43% 12% 15% 17% 10% 3	2125
	21st to end	Day	45% 10% 14% 18% 9% 4	2376
September	1st to 10th	Day	38% 10% 14% 17% 17% 5%	2120
	11th to 20th	Day	39% 7% 12% 18% 20% 4	1980
	21st to end	Day	33% 6% 10% 22% 22% 7%	1980
October	1st to 10th	Day	30% 7% 8% 21% 26% 9%	1980
	11th to 20th	Day	30% 4 9% 22% 26% 9%	1980
	21st to end	Day	33% 5% 5% 17% 31% 10%	1848
November	1st to 10th	Day	33% 3 7% 18% 28% 10%	1672
	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	41% 4 7% 17% 20% 11%	2280
	11th to 20th	Night	41% 6% 10% 16% 20% 6%	2178
	21st to end	Night	40% 6% 8% 24% 18% 4	2159
April	1st to 10th	Night	35% 6% 12% 22% 19% 6%	1800
	11th to 20th	Night	35% 5% 6% 21% 21% 12%	1775
	21st to end	Night	32% 5% 10% 26% 22% 5%	1800
May	1st to 10th	Night	29% 8% 11% 22% 26% 5%	1648
	11th to 20th	Night	34% 9% 17% 20% 16% 3	1593
	21st to end	Night	29% 10% 13% 27% 17% 4	1745
June	1st to 10th	Night	33% 11% 15% 20% 18% 2	1620
	11th to 20th	Night	23% 9% 19% 29% 17% 3	1620
	21st to end	Night	31% 9% 16% 22% 18% 4	1620
July	1st to 10th	Night	35% 12% 21% 20% 9% 3	1575
	11th to 20th	Night	31% 13% 22% 22% 11% 2	1566
	21st to end	Night	36% 13% 19% 21% 10% 1	1706
August	1st to 10th	Night	27% 14% 20% 29% 9% 1	1778
	11th to 20th	Night	34% 11% 21% 18% 12% 3	1776
	21st to end	Night	38% 10% 15% 20% 12% 5%	1980
September	1st to 10th	Night	34% 9% 17% 19% 16% 6%	1822
	11th to 20th	Night	37% 8% 11% 18% 21% 5%	1980
	21st to end	Night	27% 6% 11% 22% 25% 9%	1980
October	1st to 10th	Night	27% 4 7% 24% 26% 12%	1980
	11th to 20th	Night	32% 5% 7% 20% 25% 12%	1980
	21st to end	Night	35% 3 5% 15% 31% 11%	2501
November	1st to 10th	Night	30% 4 7% 18% 26% 14%	2406
	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	24% 20% 33% 18% 6%	490
April	1st to 10th	Day	20% 15% 30% 21% 12% 1	420
	11th to 20th	Day	18% 16% 29% 22% 13% 1	412
	21st to end	Day	15% 16% 31% 25% 12% 1	420
May	1st to 10th	Day	16% 8% 31% 24% 15% 5%	423
	11th to 20th	Day	8% 6% 31% 28% 20% 7%	447
	21st to end	Day	11% 5% 21% 30% 19% 13%	485
June	1st to 10th	Day	13% 5% 18% 29% 23% 12%	450
	11th to 20th	Day	6% 5% 16% 27% 26% 19%	450
	21st to end	Day	13% 2 15% 28% 27% 15%	450
July	1st to 10th	Day	8% 2 12% 27% 31% 21%	445
	11th to 20th	Day	8% 2 9% 24% 30% 27%	444
	21st to end	Day	8% 2 11% 19% 38% 21%	488
August	1st to 10th	Day	10% 2 11% 25% 28% 24%	446
	11th to 20th	Day	11% 1 10% 31% 33% 14%	439
	21st to end	Day	16% 2 9% 26% 31% 16%	495
September	1st to 10th	Day	10% 3 10% 33% 32% 12%	450
	11th to 20th	Day	11% 3 16% 26% 34% 11%	450
	21st to end	Day	13% 4 18% 30% 27% 9%	450
October	1st to 10th	Day	11% 5% 27% 31% 18% 8%	450
	11th to 20th	Day	16% 8% 26% 32% 16% 2	450
	21st to end	Day	18% 14% 25% 29% 13% 1	307
November	1st to 10th	Day	21% 9% 33% 24% 10% 3	270
	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	21% 12% 32% 25% 9%	724
April	1st to 10th	Night	14% 12% 34% 27% 13% 1	540
	11th to 20th	Night	17% 11% 33% 24% 14% 2	533
	21st to end	Night	13% 9% 30% 32% 16% 1	540
May	1st to 10th	Night	11% 6% 28% 30% 22% 3	540
	11th to 20th	Night	9% 4 25% 33% 24% 5%	531
	21st to end	Night	6% 2 14% 38% 33% 7%	584
June	1st to 10th	Night	6% 3 14% 35% 34% 8%	540
	11th to 20th	Night	4 1 10% 31% 39% 15%	540
	21st to end	Night	7% 2 11% 27% 42% 10%	540
July	1st to 10th	Night	7% 2 7% 27% 41% 17%	525
	11th to 20th	Night	6% 1 6% 25% 43% 19%	522
	21st to end	Night	6% 1 10% 25% 42% 15%	591
August	1st to 10th	Night	4 1 5% 29% 46% 16%	537
	11th to 20th	Night	8% 1 9% 31% 41% 10%	534
	21st to end	Night	12% 1 7% 32% 36% 11%	594
September	1st to 10th	Night	9% 2 10% 31% 38% 9%	562
	11th to 20th	Night	12% 3 13% 30% 37% 5%	720
	21st to end	Night	8% 2 15% 36% 35% 4	720
October	1st to 10th	Night	11% 4 22% 33% 26% 6%	720
	11th to 20th	Night	14% 7% 24% 33% 20% 3	720
	21st to end	Night	16% 8% 22% 34% 18% 2	792
November	1st to 10th	Night	13% 8% 21% 36% 18% 3	740
	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 2024-03-24 22:36:57. Version ac07

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