NEAF
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NEAF clear sky chart from 2005-11-06 to 2025-10-04.

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	17% 7% 2 6% 4 4 4 4 6% 7% 39%	126500
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	122883
Transparency	Day	28% 38% 7% 15% 11% 2	130014
	Night	27% 38% 7% 15% 11% 2	127456
Seeing	Day	21% 16% 30% 22% 10% 3	16141
	Night	20% 12% 30% 27% 9% 1	25230

By Month

Percentage Cloud Cover NE (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	12% 7% 1 5% 4 4 5% 4 5% 6% 46%	3597
	11th to 20th	Day	14% 9% 3 6% 5% 5% 5% 3 4 5% 41%	3570
	21st to end	Day	18% 8% 3 7% 4 4 4 4 5% 4 39%	4282
February	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 36%	1800
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 3 4 5% 43%	1948
	21st to end	Day	19% 9% 2 5% 5% 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	22% 6% 2 5% 4 3 4 3 5% 5% 41%	4476
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 6% 43%	4783
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 5% 41%	5922
April	1st to 10th	Day	19% 8% 2 5% 4 4 4 4 5% 7% 38%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 40%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 6% 6% 39%	2430
May	1st to 10th	Day	31% 7% 3 5% 3 3 4 3 6% 8% 28%	5886
	11th to 20th	Day	23% 8% 2 7% 4 3 3 3 4 6% 34%	6313
	21st to end	Day	21% 6% 2 5% 4 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	17% 7% 2 5% 5% 4 5% 4 7% 9% 36%	2800
	11th to 20th	Day	17% 7% 2 5% 4 4 6% 4 7% 8% 35%	2800
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 9% 38%	2800
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 6% 5% 6% 9% 36%	6360
	11th to 20th	Day	11% 7% 2 7% 5% 6% 7% 5% 8% 11% 32%	6360
	21st to end	Day	13% 6% 2 7% 5% 4 4 4 6% 8% 39%	6926
August	1st to 10th	Day	21% 6% 2 7% 5% 5% 5% 4 6% 7% 32%	4344
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 7% 9% 40%	3850
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 47%	4312
September	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 4 5% 6% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2315
	21st to end	Day	18% 6% 3 7% 4 5% 4 4 6% 6% 39%	2200
October	1st to 10th	Day	21% 6% 2 5% 3 4 3 3 5% 6% 42%	2013
	11th to 20th	Day	15% 5% 2 4 3 4 4 3 5% 7% 48%	1900
	21st to end	Day	21% 5% 2 4 4 4 4 3 5% 7% 41%	1933
November	1st to 10th	Day	14% 6% 3 4 3 3 3 3 5% 5% 52%	1761
	11th to 20th	Day	18% 6% 1 4 4 3 3 4 5% 6% 47%	1800
	21st to end	Day	16% 7% 2 4 4 4 4 4 6% 7% 42%	1700
December	1st to 10th	Day	16% 7% 3 5% 4 4 4 3 6% 7% 42%	2600
	11th to 20th	Day	23% 7% 1 4 3 3 3 3 4 5% 45%	2986
	21st to end	Day	18% 7% 3 7% 4 4 4 3 4 7% 39%	3960
January	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 48%	6349
	11th to 20th	Night	13% 7% 3 7% 4 4 4 3 4 6% 46%	6315
	21st to end	Night	17% 9% 3 5% 4 4 4 3 5% 6% 40%	6656
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 6% 41%	2600
	11th to 20th	Night	17% 8% 2 6% 4 3 5% 4 5% 5% 41%	2338
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	20% 8% 2 4 3 3 4 2 5% 7% 43%	5364
	11th to 20th	Night	14% 9% 3 5% 3 3 3 3 4 6% 45%	5173
	21st to end	Night	19% 9% 3 7% 4 3 3 3 4 5% 40%	5009
April	1st to 10th	Night	19% 8% 3 6% 4 4 4 3 5% 7% 37%	2000
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 5% 7% 40%	1975
	21st to end	Night	19% 7% 2 6% 3 3 3 2 5% 6% 43%	1969
May	1st to 10th	Night	30% 8% 3 6% 4 3 4 3 5% 6% 27%	4054
	11th to 20th	Night	23% 9% 3 6% 4 3 3 3 5% 5% 36%	3622
	21st to end	Night	21% 8% 2 6% 4 4 3 3 5% 7% 37%	3926
June	1st to 10th	Night	17% 6% 2 5% 3 3 6% 4 5% 8% 41%	1600
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 6% 9% 40%	1600
	21st to end	Night	12% 5% 1 6% 6% 4 4 4 7% 9% 44%	1600
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	3600
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 4 6% 9% 34%	3600
	21st to end	Night	13% 6% 2 5% 3 3 4 3 6% 7% 45%	3926
August	1st to 10th	Night	22% 6% 2 7% 5% 5% 4 4 4 8% 33%	2770
	11th to 20th	Night	15% 7% 2 7% 5% 4 5% 4 6% 6% 40%	3197
	21st to end	Night	11% 4 2 5% 4 3 3 3 5% 7% 53%	3586
September	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 6% 45%	2000
	11th to 20th	Night	13% 6% 2 4 4 3 3 3 5% 6% 53%	2084
	21st to end	Night	18% 7% 2 5% 4 3 4 3 4 6% 45%	2200
October	1st to 10th	Night	21% 7% 2 6% 4 3 3 3 4 5% 42%	2231
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 5% 6% 49%	2280
	21st to end	Night	22% 6% 3 5% 2 3 3 3 4 5% 43%	2663
November	1st to 10th	Night	13% 5% 3 5% 3 3 3 3 5% 6% 51%	2537
	11th to 20th	Night	19% 7% 2 4 3 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 5% 2 5% 4 4 5% 3 4 7% 46%	2695
December	1st to 10th	Night	14% 7% 2 6% 4 3 3 3 4 6% 48%	4580
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 42%	5208
	21st to end	Night	19% 9% 3 6% 4 4 3 2 4 5% 40%	6996

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	27% 26% 6% 23% 15% 3	3598
	11th to 20th	Day	33% 26% 6% 18% 12% 5%	3600
	21st to end	Day	37% 28% 6% 16% 11% 1	4282
February	1st to 10th	Day	19% 62% 7% 8% 4 1	1800
	11th to 20th	Day	17% 60% 2 10% 9% 1	1957
	21st to end	Day	17% 61% 5% 10% 6% 2	1650
March	1st to 10th	Day	35% 29% 7% 12% 9% 8%	4479
	11th to 20th	Day	27% 31% 9% 24% 7% 4	4783
	21st to end	Day	34% 29% 6% 8% 19% 4	5922
April	1st to 10th	Day	20% 61% 4 8% 6% 2	2400
	11th to 20th	Day	16% 62% 4 8% 9% 1	2374
	21st to end	Day	18% 61% 3 10% 7% 1	2430
May	1st to 10th	Day	42% 30% 11% 10% 6%	5897
	11th to 20th	Day	38% 29% 8% 14% 10% 1	6317
	21st to end	Day	36% 30% 6% 17% 10% 1	6949
June	1st to 10th	Day	17% 62% 3 10% 8%	2800
	11th to 20th	Day	17% 62% 4 10% 7%	2800
	21st to end	Day	16% 60% 5% 9% 9% 1	2800
July	1st to 10th	Day	34% 30% 8% 15% 12% 1	6290
	11th to 20th	Day	34% 29% 12% 16% 7% 1	6285
	21st to end	Day	32% 27% 6% 17% 16% 2	6728
August	1st to 10th	Day	35% 37% 8% 14% 6% 1	4330
	11th to 20th	Day	25% 37% 9% 22% 6%	3886
	21st to end	Day	19% 36% 6% 23% 14% 1	4312
September	1st to 10th	Day	17% 57% 5% 13% 8% 1	2400
	11th to 20th	Day	17% 57% 3 10% 12% 1	2315
	21st to end	Day	22% 57% 5% 9% 6% 1	2200
October	1st to 10th	Day	23% 37% 5% 10% 19% 6%	3325
	11th to 20th	Day	13% 37% 5% 17% 25% 3	3140
	21st to end	Day	13% 36% 10% 17% 20% 3	3158
November	1st to 10th	Day	17% 55% 6% 10% 10% 1	1761
	11th to 20th	Day	15% 56% 2 12% 13% 2	1800
	21st to end	Day	16% 57% 7% 14% 5% 2	1700
December	1st to 10th	Day	27% 37% 10% 19% 6% 1	2600
	11th to 20th	Day	30% 31% 9% 17% 12% 2	2986
	21st to end	Day	36% 27% 4 17% 13% 3	3960
January	1st to 10th	Night	28% 26% 6% 20% 17% 3	6349
	11th to 20th	Night	28% 26% 12% 15% 14% 5%	6360
	21st to end	Night	36% 28% 7% 13% 12% 4	6656
February	1st to 10th	Night	17% 61% 5% 11% 4 1	2600
	11th to 20th	Night	17% 60% 2 9% 10% 2	2340
	21st to end	Night	18% 61% 3 8% 8% 2	1980
March	1st to 10th	Night	34% 29% 7% 12% 10% 7%	5374
	11th to 20th	Night	28% 30% 12% 18% 10% 3	5174
	21st to end	Night	36% 29% 4 9% 19% 4	5009
April	1st to 10th	Night	20% 62% 3 9% 5% 1	2000
	11th to 20th	Night	15% 62% 5% 9% 9% 1	1975
	21st to end	Night	18% 61% 2 9% 8% 1	1969
May	1st to 10th	Night	45% 30% 9% 10% 6%	4060
	11th to 20th	Night	38% 30% 10% 10% 10% 1	3623
	21st to end	Night	37% 29% 8% 17% 7% 1	3927
June	1st to 10th	Night	16% 62% 2 10% 10% 1	1600
	11th to 20th	Night	15% 62% 6% 11% 6%	1600
	21st to end	Night	14% 61% 5% 11% 8% 2	1600
July	1st to 10th	Night	32% 29% 10% 15% 12% 2	3560
	11th to 20th	Night	37% 28% 11% 16% 8%	3552
	21st to end	Night	28% 27% 8% 19% 16% 1	3809
August	1st to 10th	Night	38% 39% 8% 10% 5% 1	2760
	11th to 20th	Night	27% 38% 8% 20% 6%	3227
	21st to end	Night	17% 36% 7% 27% 12% 1	3586
September	1st to 10th	Night	16% 57% 4 15% 8% 1	2000
	11th to 20th	Night	15% 57% 5% 11% 11% 2	2084
	21st to end	Night	21% 57% 4 9% 8% 1	2200
October	1st to 10th	Night	24% 37% 7% 15% 15% 2	3699
	11th to 20th	Night	11% 37% 4 20% 25% 3	3779
	21st to end	Night	17% 36% 7% 23% 15% 1	4388
November	1st to 10th	Night	17% 56% 5% 11% 10% 1	2537
	11th to 20th	Night	16% 56% 3 10% 12% 3	2600
	21st to end	Night	15% 57% 6% 14% 7% 1	2695
December	1st to 10th	Night	25% 35% 8% 23% 7% 1	4580
	11th to 20th	Night	34% 31% 6% 14% 12% 3	5208
	21st to end	Night	36% 27% 7% 13% 13% 4	6996

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	27% 27% 30% 14% 3	200
	11th to 20th	Day	30% 25% 33% 11% 2 1	200
	21st to end	Day	20% 27% 37% 10% 4 2	290
February	1st to 10th	Day	28% 24% 35% 11% 2	300
	11th to 20th	Day	27% 25% 33% 14% 1	496
	21st to end	Day	27% 19% 34% 16% 3	414
March	1st to 10th	Day	24% 26% 28% 18% 3	500
	11th to 20th	Day	21% 24% 33% 16% 4 1	500
	21st to end	Day	29% 21% 29% 16% 3 1	550
April	1st to 10th	Day	28% 25% 26% 13% 7% 1	500
	11th to 20th	Day	25% 23% 30% 14% 7% 1	492
	21st to end	Day	23% 21% 33% 16% 4 1	500
May	1st to 10th	Day	31% 17% 30% 13% 6% 2	503
	11th to 20th	Day	21% 17% 33% 20% 8% 1	527
	21st to end	Day	21% 12% 25% 29% 11% 3	573
June	1st to 10th	Day	18% 12% 31% 24% 12% 2	530
	11th to 20th	Day	19% 15% 29% 26% 10% 1	530
	21st to end	Day	12% 13% 30% 27% 16% 2	530
July	1st to 10th	Day	15% 7% 27% 30% 16% 5%	525
	11th to 20th	Day	11% 6% 26% 34% 18% 5%	524
	21st to end	Day	13% 8% 28% 34% 14% 1	576
August	1st to 10th	Day	15% 5% 26% 32% 15% 7%	526
	11th to 20th	Day	14% 5% 24% 34% 17% 6%	519
	21st to end	Day	12% 10% 28% 26% 17% 7%	583
September	1st to 10th	Day	17% 11% 28% 22% 17% 6%	530
	11th to 20th	Day	14% 8% 28% 24% 15% 12%	530
	21st to end	Day	21% 9% 28% 25% 11% 5%	530
October	1st to 10th	Day	25% 8% 29% 19% 14% 5%	502
	11th to 20th	Day	16% 18% 29% 23% 11% 2	490
	21st to end	Day	26% 14% 29% 18% 11% 1	456
November	1st to 10th	Day	16% 18% 31% 23% 9% 2	395
	11th to 20th	Day	25% 19% 32% 20% 5% 1	400
	21st to end	Day	21% 21% 32% 20% 4 1	300
December	1st to 10th	Day	26% 24% 31% 15% 5%	200
	11th to 20th	Day	32% 22% 33% 11% 3	200
	21st to end	Day	25% 17% 37% 15% 5%	220
January	1st to 10th	Night	19% 19% 39% 20% 3	800
	11th to 20th	Night	24% 17% 31% 25% 3	800
	21st to end	Night	20% 18% 36% 24% 3	880
February	1st to 10th	Night	22% 17% 35% 25% 2	800
	11th to 20th	Night	23% 21% 34% 20% 1	783
	21st to end	Night	23% 17% 36% 20% 4	660
March	1st to 10th	Night	23% 19% 35% 19% 4	800
	11th to 20th	Night	24% 17% 33% 22% 4	800
	21st to end	Night	23% 19% 33% 20% 5%	880
April	1st to 10th	Night	22% 21% 33% 21% 4	800
	11th to 20th	Night	23% 16% 34% 22% 5%	791
	21st to end	Night	21% 17% 32% 25% 5%	769
May	1st to 10th	Night	27% 15% 30% 22% 6%	600
	11th to 20th	Night	24% 12% 30% 28% 6%	415
	21st to end	Night	20% 9% 21% 40% 9% 2	433
June	1st to 10th	Night	18% 5% 28% 38% 11% 1	400
	11th to 20th	Night	16% 6% 28% 37% 13% 1	400
	21st to end	Night	13% 3 27% 43% 14%	400
July	1st to 10th	Night	14% 6% 19% 43% 18% 1	390
	11th to 20th	Night	12% 3 24% 43% 17% 1	388
	21st to end	Night	13% 4 20% 43% 18% 1	438
August	1st to 10th	Night	14% 1 20% 42% 20% 2	504
	11th to 20th	Night	13% 5% 19% 37% 25% 2	790
	21st to end	Night	16% 4 21% 36% 21% 2	880
September	1st to 10th	Night	17% 5% 25% 35% 16% 3	800
	11th to 20th	Night	17% 5% 21% 33% 21% 4	800
	21st to end	Night	21% 4 24% 33% 16% 2	800
October	1st to 10th	Night	23% 9% 28% 22% 15% 2	773
	11th to 20th	Night	18% 11% 30% 29% 11% 1	760
	21st to end	Night	25% 10% 32% 23% 9%	836
November	1st to 10th	Night	14% 12% 33% 28% 11% 1	780
	11th to 20th	Night	23% 13% 32% 24% 8% 1	800
	21st to end	Night	20% 16% 31% 26% 7%	800
December	1st to 10th	Night	25% 15% 32% 21% 5% 1	800
	11th to 20th	Night	24% 16% 34% 23% 4	800
	21st to end	Night	24% 15% 35% 24% 3	880

Page generated at 2025-10-01 01:39:06. Version ac07

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