United States Naval Academy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the United States Naval Academy clear sky chart from 2005-11-06 to 2025-09-04.

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	14% 6% 2 5% 4 4 4 4 6% 8% 44%	126062
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 44%	122656
Transparency	Day	24% 39% 9% 15% 11% 2	129653
	Night	25% 39% 8% 15% 12% 3	127134
Seeing	Day	16% 9% 25% 26% 17% 6%	16096
	Night	17% 10% 28% 29% 14% 2	25502

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	14% 3 2 4 3 3 3 3 5% 6% 53%	3597
	11th to 20th	Day	14% 8% 3 6% 3 4 3 3 4 5% 47%	4037
	21st to end	Day	15% 7% 2 5% 4 3 4 3 4 6% 46%	4466
February	1st to 10th	Day	15% 7% 2 6% 3 4 6% 3 6% 7% 40%	1800
	11th to 20th	Day	20% 7% 2 4 4 4 3 3 6% 6% 42%	1853
	21st to end	Day	18% 8% 2 6% 4 4 3 3 5% 6% 41%	1650
March	1st to 10th	Day	19% 4 2 4 3 3 4 3 5% 6% 46%	4727
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 4 6% 51%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 43%	5642
April	1st to 10th	Day	14% 6% 2 5% 5% 4 4 4 7% 6% 43%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 47%	2374
	21st to end	Day	16% 6% 2 5% 4 4 4 3 6% 8% 42%	2400
May	1st to 10th	Day	21% 7% 2 6% 4 4 4 4 7% 9% 33%	5628
	11th to 20th	Day	21% 6% 2 6% 4 4 4 4 7% 8% 34%	6304
	21st to end	Day	16% 4 2 5% 4 4 4 4 6% 9% 41%	6934
June	1st to 10th	Day	10% 4 2 5% 4 4 5% 5% 7% 9% 44%	2800
	11th to 20th	Day	11% 5% 2 4 4 4 5% 4 7% 9% 44%	2800
	21st to end	Day	9% 4 1 4 3 4 5% 4 8% 10% 47%	2800
July	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 8% 10% 45%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 5% 9% 11% 34%	6324
	21st to end	Day	8% 5% 2 6% 4 4 6% 5% 7% 10% 43%	6424
August	1st to 10th	Day	16% 5% 2 6% 4 4 5% 5% 8% 11% 35%	4234
	11th to 20th	Day	10% 5% 1 6% 4 4 5% 5% 7% 9% 43%	3922
	21st to end	Day	10% 4 2 5% 3 4 4 4 7% 9% 50%	4312
September	1st to 10th	Day	14% 4 2 4 4 4 3 4 6% 7% 48%	2316
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 4 8% 50%	2280
	21st to end	Day	20% 7% 2 6% 3 3 4 4 5% 7% 38%	2024
October	1st to 10th	Day	17% 5% 2 5% 3 3 4 3 5% 7% 48%	1900
	11th to 20th	Day	17% 5% 1 3 2 3 3 3 5% 7% 52%	1900
	21st to end	Day	14% 5% 2 5% 3 4 4 3 5% 5% 51%	2090
November	1st to 10th	Day	12% 6% 1 4 3 4 4 3 4 6% 53%	1804
	11th to 20th	Day	14% 5% 1 4 3 4 4 4 5% 6% 49%	1800
	21st to end	Day	14% 6% 2 4 4 4 4 4 4 5% 49%	1630
December	1st to 10th	Day	12% 5% 2 6% 3 4 3 3 6% 7% 49%	2600
	11th to 20th	Day	18% 8% 3 4 3 3 3 3 4 6% 44%	2990
	21st to end	Day	14% 7% 2 6% 4 4 5% 3 5% 7% 43%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 49%	6349
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 5% 52%	5918
	21st to end	Night	15% 9% 2 6% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 4 6% 7% 41%	2600
	11th to 20th	Night	19% 7% 3 5% 3 4 3 3 4 5% 44%	2431
	21st to end	Night	17% 8% 2 5% 4 3 3 3 5% 6% 43%	1980
March	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 3 5% 50%	5108
	11th to 20th	Night	10% 7% 2 4 3 4 4 3 6% 7% 51%	4980
	21st to end	Night	14% 7% 2 6% 4 4 4 3 4 6% 45%	5289
April	1st to 10th	Night	17% 8% 2 5% 5% 4 4 3 5% 7% 41%	2000
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 6% 7% 45%	1975
	21st to end	Night	17% 5% 2 5% 3 4 4 3 7% 6% 44%	2000
May	1st to 10th	Night	19% 9% 3 7% 5% 5% 5% 4 7% 7% 30%	4309
	11th to 20th	Night	24% 6% 3 6% 4 4 5% 4 6% 8% 32%	3585
	21st to end	Night	18% 6% 2 7% 4 4 4 4 6% 9% 37%	3920
June	1st to 10th	Night	11% 5% 3 6% 5% 3 4 4 6% 10% 44%	1600
	11th to 20th	Night	11% 6% 3 7% 5% 5% 4 5% 5% 10% 41%	1600
	21st to end	Night	10% 4 2 6% 5% 5% 4 4 6% 11% 42%	1600
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 8% 10% 39%	3598
	11th to 20th	Night	10% 7% 3 9% 5% 5% 6% 5% 8% 12% 31%	3615
	21st to end	Night	9% 6% 3 5% 5% 4 6% 5% 7% 9% 42%	4427
August	1st to 10th	Night	17% 5% 2 7% 5% 5% 5% 4 7% 10% 34%	2915
	11th to 20th	Night	12% 7% 3 8% 5% 6% 6% 5% 7% 8% 34%	3093
	21st to end	Night	12% 4 2 5% 4 4 5% 4 5% 9% 47%	3586
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 3 5% 6% 45%	1931
	11th to 20th	Night	10% 7% 2 5% 4 3 4 4 5% 8% 50%	1900
	21st to end	Night	20% 7% 2 6% 4 4 4 3 6% 6% 38%	2150
October	1st to 10th	Night	19% 6% 2 5% 3 4 4 3 5% 5% 44%	2280
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 52%	2280
	21st to end	Night	16% 4 1 5% 3 3 3 3 5% 7% 49%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 5% 53%	2493
	11th to 20th	Night	16% 6% 2 5% 3 3 4 3 4 6% 49%	2600
	21st to end	Night	13% 5% 1 4 5% 4 4 4 6% 6% 48%	2765
December	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 52%	4580
	11th to 20th	Night	21% 5% 2 5% 4 4 4 3 5% 7% 40%	5205
	21st to end	Night	16% 7% 2 6% 4 4 5% 4 6% 8% 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	23% 27% 4 21% 21% 5%	3598
	11th to 20th	Day	30% 27% 4 17% 15% 7%	4037
	21st to end	Day	29% 28% 10% 21% 9% 2	4466
February	1st to 10th	Day	18% 63% 6% 9% 3	1800
	11th to 20th	Day	18% 61% 3 9% 8% 1	1862
	21st to end	Day	18% 61% 6% 8% 6% 1	1650
March	1st to 10th	Day	29% 30% 7% 15% 13% 7%	4727
	11th to 20th	Day	23% 30% 6% 21% 15% 4	4980
	21st to end	Day	30% 30% 7% 13% 14% 6%	5642
April	1st to 10th	Day	17% 63% 4 8% 7% 2	2400
	11th to 20th	Day	15% 63% 5% 9% 7% 1	2374
	21st to end	Day	16% 62% 4 10% 7% 1	2400
May	1st to 10th	Day	36% 32% 12% 13% 6% 1	5660
	11th to 20th	Day	34% 32% 10% 18% 6%	6308
	21st to end	Day	29% 31% 10% 16% 12% 1	6949
June	1st to 10th	Day	14% 64% 5% 10% 7% 1	2800
	11th to 20th	Day	12% 64% 5% 9% 8% 1	2800
	21st to end	Day	11% 62% 8% 11% 8% 1	2800
July	1st to 10th	Day	24% 32% 17% 13% 11% 3	6290
	11th to 20th	Day	32% 32% 16% 16% 4	6249
	21st to end	Day	25% 29% 9% 17% 19% 1	6240
August	1st to 10th	Day	28% 39% 12% 17% 3	4221
	11th to 20th	Day	22% 39% 9% 23% 7% 1	3960
	21st to end	Day	16% 38% 10% 22% 14% 1	4312
September	1st to 10th	Day	15% 60% 7% 11% 5% 2	2316
	11th to 20th	Day	14% 61% 5% 10% 8% 2	2280
	21st to end	Day	23% 60% 4 9% 4 1	2024
October	1st to 10th	Day	21% 38% 6% 11% 21% 4	3160
	11th to 20th	Day	11% 37% 5% 20% 20% 8%	3140
	21st to end	Day	8% 36% 7% 22% 21% 7%	3428
November	1st to 10th	Day	17% 56% 7% 10% 7% 3	1804
	11th to 20th	Day	13% 57% 4 13% 12% 2	1800
	21st to end	Day	15% 56% 4 15% 9% 1	1630
December	1st to 10th	Day	23% 37% 11% 17% 8% 4	2600
	11th to 20th	Day	29% 32% 8% 22% 7% 1	2986
	21st to end	Day	30% 29% 12% 16% 12% 1	3960
January	1st to 10th	Night	27% 28% 4 17% 20% 4	6349
	11th to 20th	Night	24% 27% 5% 20% 16% 7%	5918
	21st to end	Night	30% 29% 5% 20% 14% 2	6490
February	1st to 10th	Night	17% 62% 7% 9% 4 1	2600
	11th to 20th	Night	18% 60% 3 8% 10% 2	2433
	21st to end	Night	16% 62% 4 10% 6% 3	1980
March	1st to 10th	Night	30% 29% 5% 18% 9% 9%	5108
	11th to 20th	Night	21% 30% 9% 21% 13% 5%	4980
	21st to end	Night	29% 30% 6% 14% 18% 4	5289
April	1st to 10th	Night	18% 62% 4 9% 8% 1	2000
	11th to 20th	Night	16% 63% 4 10% 6% 1	1975
	21st to end	Night	16% 62% 5% 9% 8% 1	2000
May	1st to 10th	Night	40% 31% 9% 11% 6% 1	4339
	11th to 20th	Night	36% 32% 10% 15% 6%	3586
	21st to end	Night	34% 30% 10% 15% 9% 1	3927
June	1st to 10th	Night	15% 63% 5% 9% 8%	1600
	11th to 20th	Night	15% 63% 5% 11% 6% 1	1600
	21st to end	Night	13% 61% 7% 11% 8% 1	1600
July	1st to 10th	Night	29% 31% 15% 13% 11% 1	3560
	11th to 20th	Night	36% 31% 18% 12% 3	3570
	21st to end	Night	29% 28% 10% 18% 15%	4297
August	1st to 10th	Night	33% 38% 12% 13% 4	2904
	11th to 20th	Night	28% 40% 9% 17% 6%	3121
	21st to end	Night	19% 37% 7% 22% 14% 1	3586
September	1st to 10th	Night	15% 61% 6% 11% 5% 2	1931
	11th to 20th	Night	13% 60% 4 11% 9% 2	1900
	21st to end	Night	22% 61% 4 9% 4	2150
October	1st to 10th	Night	26% 39% 7% 11% 14% 3	3800
	11th to 20th	Night	10% 37% 8% 12% 27% 6%	3779
	21st to end	Night	9% 36% 7% 23% 18% 6%	4120
November	1st to 10th	Night	16% 57% 5% 11% 8% 3	2493
	11th to 20th	Night	15% 57% 4 9% 12% 4	2600
	21st to end	Night	13% 57% 6% 13% 10% 1	2765
December	1st to 10th	Night	20% 36% 9% 20% 13% 2	4580
	11th to 20th	Night	31% 32% 9% 13% 11% 4	5208
	21st to end	Night	32% 28% 11% 15% 13% 1	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	18% 22% 36% 18% 7% 1	200
	11th to 20th	Day	18% 24% 34% 22% 3	295
	21st to end	Day	18% 18% 40% 17% 6% 1	330
February	1st to 10th	Day	21% 12% 44% 15% 7%	300
	11th to 20th	Day	27% 19% 40% 14% 2	400
	21st to end	Day	24% 14% 37% 21% 5%	414
March	1st to 10th	Day	16% 19% 36% 21% 7% 1	500
	11th to 20th	Day	17% 18% 32% 23% 8% 2	500
	21st to end	Day	20% 11% 39% 23% 6% 1	550
April	1st to 10th	Day	20% 14% 35% 23% 6% 1	500
	11th to 20th	Day	14% 15% 34% 23% 11% 4	492
	21st to end	Day	19% 11% 29% 27% 12% 2	500
May	1st to 10th	Day	21% 9% 31% 26% 11% 2	503
	11th to 20th	Day	18% 6% 29% 27% 15% 5%	527
	21st to end	Day	19% 4 18% 26% 24% 8%	573
June	1st to 10th	Day	11% 6% 23% 28% 22% 10%	530
	11th to 20th	Day	13% 5% 18% 31% 25% 8%	530
	21st to end	Day	11% 2 16% 35% 26% 10%	530
July	1st to 10th	Day	10% 2 10% 32% 32% 14%	525
	11th to 20th	Day	4 1 8% 36% 36% 14%	524
	21st to end	Day	9% 3 13% 33% 32% 10%	576
August	1st to 10th	Day	12% 2 11% 32% 31% 12%	526
	11th to 20th	Day	12% 3 13% 30% 31% 12%	519
	21st to end	Day	9% 2 15% 31% 29% 14%	583
September	1st to 10th	Day	17% 3 16% 23% 28% 13%	502
	11th to 20th	Day	14% 3 14% 31% 26% 11%	490
	21st to end	Day	25% 6% 22% 28% 15% 3	490
October	1st to 10th	Day	21% 5% 18% 29% 19% 8%	490
	11th to 20th	Day	16% 9% 27% 27% 16% 5%	490
	21st to end	Day	16% 9% 30% 27% 13% 4	539
November	1st to 10th	Day	15% 10% 33% 25% 12% 5%	418
	11th to 20th	Day	14% 16% 35% 26% 9% 2	400
	21st to end	Day	13% 19% 33% 20% 14% 2	230
December	1st to 10th	Day	18% 15% 37% 24% 6% 2	200
	11th to 20th	Day	20% 17% 32% 24% 9%	200
	21st to end	Day	20% 12% 30% 28% 10%	220
January	1st to 10th	Night	19% 19% 39% 19% 4	800
	11th to 20th	Night	21% 18% 31% 25% 6%	800
	21st to end	Night	19% 17% 36% 26% 2	880
February	1st to 10th	Night	20% 13% 37% 27% 4	800
	11th to 20th	Night	24% 19% 36% 19% 1	783
	21st to end	Night	21% 14% 39% 22% 3	660
March	1st to 10th	Night	16% 17% 38% 24% 5%	800
	11th to 20th	Night	19% 15% 34% 23% 7% 2	800
	21st to end	Night	18% 12% 38% 26% 5% 1	880
April	1st to 10th	Night	20% 16% 36% 22% 6%	800
	11th to 20th	Night	15% 11% 34% 31% 9%	791
	21st to end	Night	19% 9% 33% 29% 10% 1	800
May	1st to 10th	Night	18% 9% 28% 32% 13% 1	723
	11th to 20th	Night	19% 7% 26% 32% 14% 1	404
	21st to end	Night	14% 3 20% 36% 24% 3	433
June	1st to 10th	Night	11% 3 22% 39% 24% 2	400
	11th to 20th	Night	11% 4 22% 40% 22% 1	400
	21st to end	Night	11% 1 19% 43% 22% 3	400
July	1st to 10th	Night	11% 3 15% 38% 31% 4	390
	11th to 20th	Night	6% 3 10% 40% 37% 4	393
	21st to end	Night	8% 2 15% 40% 31% 4	657
August	1st to 10th	Night	11% 3 12% 41% 30% 3	597
	11th to 20th	Night	12% 3 13% 34% 33% 5%	722
	21st to end	Night	12% 3 13% 35% 30% 7%	880
September	1st to 10th	Night	16% 3 15% 31% 28% 6%	773
	11th to 20th	Night	11% 3 14% 35% 30% 6%	760
	21st to end	Night	22% 6% 19% 31% 19% 3	760
October	1st to 10th	Night	21% 6% 21% 26% 21% 5%	760
	11th to 20th	Night	17% 10% 27% 28% 16% 2	760
	21st to end	Night	18% 12% 27% 28% 14% 1	836
November	1st to 10th	Night	14% 8% 31% 31% 13% 2	780
	11th to 20th	Night	19% 15% 30% 27% 9%	800
	21st to end	Night	16% 15% 33% 27% 9% 1	800
December	1st to 10th	Night	20% 18% 30% 22% 9% 1	800
	11th to 20th	Night	18% 14% 38% 25% 6%	800
	21st to end	Night	19% 12% 34% 31% 5%	880

Page generated at 2025-09-01 01:51:03. Version ac07

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