USNO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the USNO clear sky chart from 2005-11-06 to 2025-08-05.

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 6% 4 4 5% 4 6% 8% 41%	125679
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	122388
Transparency	Day	25% 39% 8% 15% 11% 2	129272
	Night	25% 39% 7% 15% 12% 3	126864
Seeing	Day	20% 13% 28% 24% 12% 4	15948
	Night	18% 12% 29% 29% 11% 1	25431

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	15% 4 2 4 3 3 4 3 4 6% 52%	3689
	11th to 20th	Day	14% 10% 2 5% 4 4 3 3 3 5% 47%	4060
	21st to end	Day	15% 7% 3 7% 4 4 4 3 4 6% 44%	4466
February	1st to 10th	Day	15% 7% 1 7% 6% 4 5% 4 6% 7% 39%	1800
	11th to 20th	Day	18% 7% 2 5% 4 3 4 4 5% 6% 40%	1818
	21st to end	Day	17% 8% 3 6% 4 4 4 3 4 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 4 3 4 4 3 5% 7% 43%	4836
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	4980
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 5% 6% 41%	5573
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 6% 7% 42%	2400
	11th to 20th	Day	12% 6% 1 5% 5% 5% 3 3 5% 6% 47%	2374
	21st to end	Day	18% 6% 2 6% 3 3 4 3 7% 7% 41%	2400
May	1st to 10th	Day	20% 7% 3 7% 4 4 4 5% 7% 9% 31%	5726
	11th to 20th	Day	21% 7% 2 6% 4 4 5% 4 6% 10% 30%	6175
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 42%	2800
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	2800
	21st to end	Day	9% 4 2 4 4 4 6% 5% 7% 12% 42%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 4 5% 5% 8% 11% 42%	6360
	11th to 20th	Day	9% 7% 3 7% 6% 6% 6% 5% 10% 12% 29%	6182
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 10% 38%	6424
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 6% 7% 10% 32%	4156
	11th to 20th	Day	11% 5% 2 7% 5% 6% 7% 5% 7% 9% 38%	3858
	21st to end	Day	10% 5% 2 5% 5% 4 4 5% 7% 8% 46%	4180
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 4 6% 7% 44%	2280
	11th to 20th	Day	11% 6% 2 5% 3 4 4 4 6% 9% 46%	2280
	21st to end	Day	20% 7% 3 6% 4 3 4 4 6% 6% 37%	2001
October	1st to 10th	Day	16% 5% 3 5% 3 4 4 3 5% 7% 45%	1900
	11th to 20th	Day	17% 4 2 5% 2 3 3 3 6% 6% 49%	1900
	21st to end	Day	14% 5% 2 6% 3 3 4 3 5% 6% 50%	2090
November	1st to 10th	Day	12% 5% 2 4 3 4 4 4 5% 7% 51%	1847
	11th to 20th	Day	14% 6% 2 4 3 3 4 3 7% 6% 49%	1790
	21st to end	Day	15% 6% 2 5% 4 3 4 4 5% 6% 48%	1600
December	1st to 10th	Day	11% 5% 2 7% 3 3 5% 3 5% 6% 49%	2600
	11th to 20th	Day	18% 8% 3 4 3 3 3 3 5% 7% 43%	2990
	21st to end	Day	16% 8% 3 6% 4 4 4 3 5% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	6239
	11th to 20th	Night	14% 7% 2 4 3 3 3 3 4 6% 51%	5900
	21st to end	Night	15% 8% 3 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 5% 4 4 3 3 4 5% 43%	2467
	21st to end	Night	16% 8% 2 6% 3 4 3 3 5% 6% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 4 4 3 4 5% 48%	5012
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	12% 8% 2 7% 5% 4 4 3 5% 6% 44%	5358
April	1st to 10th	Night	15% 7% 3 6% 5% 4 4 5% 7% 6% 40%	2000
	11th to 20th	Night	15% 5% 2 6% 3 4 4 5% 6% 6% 45%	1975
	21st to end	Night	16% 5% 2 5% 4 4 4 4 7% 8% 42%	2000
May	1st to 10th	Night	21% 10% 3 7% 4 5% 5% 4 6% 7% 30%	4198
	11th to 20th	Night	23% 8% 3 5% 4 4 5% 4 6% 9% 31%	3718
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 7% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 5% 4 4 6% 4 7% 8% 45%	1600
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 7% 10% 40%	1600
	21st to end	Night	9% 4 2 6% 6% 5% 5% 5% 6% 10% 42%	1600
July	1st to 10th	Night	9% 5% 3 7% 5% 5% 6% 4 7% 9% 39%	3598
	11th to 20th	Night	9% 8% 3 8% 6% 7% 7% 6% 8% 9% 30%	3771
	21st to end	Night	9% 6% 2 6% 5% 4 5% 5% 8% 9% 41%	4427
August	1st to 10th	Night	17% 5% 3 7% 5% 5% 5% 4 7% 9% 33%	2861
	11th to 20th	Night	12% 6% 3 9% 6% 5% 5% 4 7% 8% 35%	2939
	21st to end	Night	10% 5% 2 5% 3 3 4 3 6% 8% 50%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 4 5% 2 6% 7% 45%	1900
	11th to 20th	Night	10% 6% 3 5% 4 4 3 3 5% 7% 50%	1900
	21st to end	Night	19% 6% 3 6% 5% 4 4 4 5% 6% 38%	2175
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 4 4 6% 45%	2280
	11th to 20th	Night	14% 5% 1 5% 3 3 3 3 4 6% 52%	2280
	21st to end	Night	14% 6% 1 4 4 3 4 3 5% 7% 50%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 4 5% 54%	2450
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 6% 50%	2605
	21st to end	Night	13% 4 2 4 4 3 4 4 5% 7% 48%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 7% 51%	4580
	11th to 20th	Night	21% 6% 2 5% 3 4 4 4 5% 6% 41%	5205
	21st to end	Night	17% 7% 2 6% 4 4 4 4 6% 7% 38%	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	24% 26% 4 21% 19% 6%	3690
	11th to 20th	Day	30% 26% 5% 16% 15% 7%	4060
	21st to end	Day	31% 29% 7% 21% 10% 2	4466
February	1st to 10th	Day	19% 63% 6% 8% 4	1800
	11th to 20th	Day	19% 61% 3 9% 7% 1	1827
	21st to end	Day	18% 61% 4 9% 7% 1	1650
March	1st to 10th	Day	30% 30% 6% 15% 12% 6%	4836
	11th to 20th	Day	24% 29% 7% 20% 17% 4	4980
	21st to end	Day	30% 29% 7% 14% 14% 5%	5573
April	1st to 10th	Day	18% 63% 3 8% 7% 1	2400
	11th to 20th	Day	15% 62% 5% 9% 7% 1	2374
	21st to end	Day	17% 62% 4 10% 7% 1	2400
May	1st to 10th	Day	37% 31% 11% 12% 6% 1	5759
	11th to 20th	Day	35% 31% 9% 19% 6%	6179
	21st to end	Day	31% 30% 10% 15% 12% 2	6949
June	1st to 10th	Day	16% 62% 4 10% 6% 1	2800
	11th to 20th	Day	15% 63% 4 10% 7% 1	2800
	21st to end	Day	12% 62% 7% 10% 8% 1	2800
July	1st to 10th	Day	25% 31% 15% 14% 11% 4	6290
	11th to 20th	Day	35% 31% 15% 15% 4	6107
	21st to end	Day	28% 28% 8% 19% 18%	6240
August	1st to 10th	Day	31% 39% 11% 16% 4	4143
	11th to 20th	Day	23% 40% 10% 20% 8%	3897
	21st to end	Day	19% 39% 9% 20% 12% 1	4180
September	1st to 10th	Day	16% 61% 7% 9% 7% 1	2280
	11th to 20th	Day	15% 61% 3 11% 8% 2	2280
	21st to end	Day	21% 60% 3 10% 4 1	2001
October	1st to 10th	Day	24% 38% 3 10% 21% 4	3160
	11th to 20th	Day	11% 37% 4 20% 19% 8%	3140
	21st to end	Day	8% 36% 7% 21% 21% 6%	3428
November	1st to 10th	Day	17% 56% 6% 10% 8% 3	1847
	11th to 20th	Day	14% 56% 3 12% 12% 2	1790
	21st to end	Day	15% 57% 4 15% 8% 1	1600
December	1st to 10th	Day	24% 36% 9% 18% 10% 4	2600
	11th to 20th	Day	30% 31% 9% 21% 8% 1	2986
	21st to end	Day	34% 29% 11% 15% 11%	3960
January	1st to 10th	Night	27% 26% 5% 18% 20% 4	6239
	11th to 20th	Night	25% 26% 5% 20% 17% 7%	5900
	21st to end	Night	30% 28% 5% 21% 15% 2	6490
February	1st to 10th	Night	17% 62% 7% 10% 4 1	2600
	11th to 20th	Night	18% 60% 3 8% 9% 2	2469
	21st to end	Night	16% 61% 5% 9% 6% 3	1980
March	1st to 10th	Night	31% 30% 4 18% 10% 8%	5012
	11th to 20th	Night	22% 30% 8% 21% 14% 6%	4980
	21st to end	Night	28% 29% 6% 15% 17% 4	5358
April	1st to 10th	Night	18% 62% 3 8% 8% 1	2000
	11th to 20th	Night	15% 63% 5% 10% 6% 1	1975
	21st to end	Night	16% 62% 5% 10% 7% 1	2000
May	1st to 10th	Night	41% 31% 9% 11% 5% 2	4227
	11th to 20th	Night	37% 32% 8% 16% 6%	3719
	21st to end	Night	35% 30% 9% 16% 8% 1	3927
June	1st to 10th	Night	15% 63% 4 11% 7% 1	1600
	11th to 20th	Night	15% 63% 6% 11% 5%	1600
	21st to end	Night	13% 62% 6% 10% 8% 1	1600
July	1st to 10th	Night	30% 31% 14% 12% 11% 2	3560
	11th to 20th	Night	38% 32% 15% 11% 4	3726
	21st to end	Night	28% 28% 10% 18% 15%	4297
August	1st to 10th	Night	33% 39% 12% 12% 3 1	2850
	11th to 20th	Night	28% 40% 8% 17% 6%	2966
	21st to end	Night	18% 38% 8% 23% 13%	3476
September	1st to 10th	Night	16% 60% 6% 11% 5% 2	1900
	11th to 20th	Night	14% 61% 4 12% 9% 2	1900
	21st to end	Night	22% 60% 5% 8% 5%	2175
October	1st to 10th	Night	26% 38% 7% 12% 13% 4	3800
	11th to 20th	Night	11% 37% 7% 14% 24% 7%	3779
	21st to end	Night	10% 36% 7% 23% 18% 7%	4120
November	1st to 10th	Night	16% 56% 6% 11% 8% 3	2450
	11th to 20th	Night	15% 56% 3 10% 12% 4	2605
	21st to end	Night	13% 57% 6% 14% 9% 1	2800
December	1st to 10th	Night	20% 36% 9% 18% 15% 2	4580
	11th to 20th	Night	32% 32% 9% 12% 12% 4	5208
	21st to end	Night	33% 28% 11% 15% 13%	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	32% 19% 33% 12% 4	220
	11th to 20th	Day	31% 24% 30% 11% 4	300
	21st to end	Day	24% 24% 35% 13% 4	330
February	1st to 10th	Day	27% 25% 34% 11% 4	300
	11th to 20th	Day	38% 21% 34% 6% 1	365
	21st to end	Day	29% 19% 33% 17% 2	414
March	1st to 10th	Day	28% 20% 32% 17% 3 1	500
	11th to 20th	Day	28% 20% 31% 16% 4 1	500
	21st to end	Day	26% 19% 33% 16% 5% 1	550
April	1st to 10th	Day	25% 21% 33% 16% 4 1	500
	11th to 20th	Day	20% 19% 35% 17% 7% 3	492
	21st to end	Day	27% 14% 33% 18% 8% 1	500
May	1st to 10th	Day	24% 17% 30% 19% 7% 3	503
	11th to 20th	Day	22% 15% 28% 23% 10% 2	527
	21st to end	Day	19% 10% 24% 29% 16% 3	573
June	1st to 10th	Day	14% 10% 28% 27% 16% 6%	530
	11th to 20th	Day	17% 8% 26% 29% 17% 3	530
	21st to end	Day	10% 7% 28% 30% 20% 5%	530
July	1st to 10th	Day	12% 3 19% 37% 19% 10%	525
	11th to 20th	Day	6% 3 23% 34% 28% 6%	524
	21st to end	Day	12% 3 22% 36% 21% 6%	576
August	1st to 10th	Day	13% 5% 22% 31% 20% 9%	502
	11th to 20th	Day	13% 5% 19% 31% 23% 9%	479
	21st to end	Day	10% 3 22% 35% 18% 13%	539
September	1st to 10th	Day	17% 6% 22% 27% 17% 12%	490
	11th to 20th	Day	12% 5% 22% 34% 17% 9%	490
	21st to end	Day	28% 9% 24% 26% 9% 5%	490
October	1st to 10th	Day	22% 8% 23% 26% 16% 5%	490
	11th to 20th	Day	18% 15% 28% 23% 13% 2	490
	21st to end	Day	16% 14% 36% 23% 9% 2	539
November	1st to 10th	Day	15% 15% 33% 23% 10% 3	440
	11th to 20th	Day	19% 16% 34% 21% 9% 1	390
	21st to end	Day	28% 20% 29% 15% 8% 1	200
December	1st to 10th	Day	22% 24% 35% 16% 3 2	200
	11th to 20th	Day	25% 21% 30% 18% 7%	200
	21st to end	Day	27% 16% 33% 19% 5%	220
January	1st to 10th	Night	20% 20% 39% 18% 3	800
	11th to 20th	Night	23% 17% 31% 25% 4	800
	21st to end	Night	21% 17% 36% 25% 1	880
February	1st to 10th	Night	20% 17% 34% 27% 3	800
	11th to 20th	Night	27% 21% 34% 17% 1	783
	21st to end	Night	25% 16% 35% 20% 2	660
March	1st to 10th	Night	20% 18% 37% 23% 2	800
	11th to 20th	Night	23% 17% 36% 19% 4 1	800
	21st to end	Night	19% 18% 38% 20% 4 1	880
April	1st to 10th	Night	21% 22% 32% 21% 4	800
	11th to 20th	Night	18% 17% 33% 26% 6%	791
	21st to end	Night	21% 12% 32% 27% 7%	800
May	1st to 10th	Night	21% 11% 30% 27% 11%	676
	11th to 20th	Night	21% 7% 29% 34% 9%	459
	21st to end	Night	17% 6% 24% 37% 15% 1	433
June	1st to 10th	Night	11% 5% 29% 36% 19% 1	400
	11th to 20th	Night	14% 6% 25% 41% 16%	400
	21st to end	Night	10% 5% 25% 44% 16% 1	400
July	1st to 10th	Night	8% 4 20% 43% 23% 2	390
	11th to 20th	Night	6% 3 16% 43% 29% 2	458
	21st to end	Night	9% 2 20% 42% 24% 3	657
August	1st to 10th	Night	11% 2 13% 42% 29% 2	579
	11th to 20th	Night	12% 4 14% 41% 26% 3	653
	21st to end	Night	12% 3 19% 36% 27% 4	836
September	1st to 10th	Night	16% 4 20% 31% 26% 4	760
	11th to 20th	Night	12% 4 16% 37% 26% 4	760
	21st to end	Night	24% 4 23% 30% 18% 1	760
October	1st to 10th	Night	21% 7% 21% 28% 19% 4	760
	11th to 20th	Night	17% 11% 26% 32% 14% 1	760
	21st to end	Night	18% 12% 30% 27% 12%	836
November	1st to 10th	Night	13% 11% 34% 29% 11% 1	780
	11th to 20th	Night	19% 14% 32% 27% 7% 1	800
	21st to end	Night	18% 14% 33% 26% 8% 1	800
December	1st to 10th	Night	22% 16% 32% 22% 7% 1	800
	11th to 20th	Night	19% 13% 36% 27% 5%	800
	21st to end	Night	19% 15% 30% 32% 4	880

Page generated at 2025-08-02 01:32:23. Version ac07

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