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 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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	101213
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	97751
Transparency	Day	22% 47% 7% 14% 9% 2	104850
	Night	22% 47% 6% 14% 9% 2	102253
Seeing	Day	20% 12% 28% 24% 12% 4	15006
	Night	18% 12% 29% 29% 12% 1	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	17% 5% 2 4 3 3 4 3 4 6% 48%	2581
	11th to 20th	Day	15% 12% 3 6% 4 4 4 3 4 6% 40%	2840
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 3 4 6% 36%	3124
February	1st to 10th	Day	15% 7% 1 7% 6% 4 5% 4 6% 7% 39%	1710
	11th to 20th	Day	18% 7% 2 5% 4 3 4 4 5% 6% 41%	1724
	21st to end	Day	18% 8% 3 7% 5% 4 4 3 4 5% 40%	1570
March	1st to 10th	Day	22% 7% 2 5% 4 4 4 4 5% 8% 36%	3364
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 6% 7% 50%	3480
	21st to end	Day	20% 7% 2 6% 4 3 4 3 5% 6% 40%	3895
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 7% 7% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 5% 5% 3 3 5% 6% 48%	2254
	21st to end	Day	18% 6% 2 6% 4 3 4 3 7% 7% 41%	2280
May	1st to 10th	Day	23% 7% 3 7% 4 4 4 4 6% 8% 28%	3988
	11th to 20th	Day	21% 7% 2 6% 5% 4 5% 4 6% 9% 29%	4297
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 6% 9% 44%	4823
June	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 42%	2660
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	2660
	21st to end	Day	9% 4 2 4 5% 5% 6% 5% 7% 12% 41%	2660
July	1st to 10th	Day	6% 4 2 6% 6% 5% 6% 5% 8% 11% 41%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 9% 11% 37%	4318
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 5% 7% 9% 35%	4466
August	1st to 10th	Day	16% 6% 2 6% 5% 5% 6% 5% 7% 10% 32%	4104
	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 3 5% 7% 51%	1783
	11th to 20th	Day	14% 6% 2 4 2 3 4 3 7% 6% 49%	1701
	21st to end	Day	15% 5% 2 5% 4 3 4 4 5% 6% 48%	1520
December	1st to 10th	Day	15% 6% 1 6% 4 4 5% 4 6% 7% 41%	1520
	11th to 20th	Day	16% 6% 2 5% 4 4 3 3 5% 7% 46%	1910
	21st to end	Day	16% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	2772
January	1st to 10th	Night	15% 7% 2 5% 3 4 4 3 5% 7% 44%	4361
	11th to 20th	Night	19% 7% 2 4 4 3 3 3 4 6% 46%	4120
	21st to end	Night	20% 9% 3 5% 3 3 3 3 4 5% 41%	4532
February	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 6% 5% 42%	2470
	11th to 20th	Night	18% 7% 2 6% 4 4 3 2 4 5% 44%	2341
	21st to end	Night	17% 8% 2 6% 3 4 3 3 5% 6% 42%	1884
March	1st to 10th	Night	21% 7% 3 6% 4 4 4 3 4 6% 39%	3484
	11th to 20th	Night	11% 4 2 5% 4 4 4 4 6% 8% 49%	3480
	21st to end	Night	14% 9% 3 7% 4 3 3 3 4 5% 46%	3736
April	1st to 10th	Night	15% 7% 2 6% 4 4 4 4 7% 6% 40%	1900
	11th to 20th	Night	14% 5% 2 6% 3 4 4 4 5% 6% 46%	1875
	21st to end	Night	17% 5% 2 5% 3 4 4 4 6% 9% 41%	1900
May	1st to 10th	Night	22% 9% 2 6% 4 4 5% 3 5% 7% 32%	2922
	11th to 20th	Night	22% 9% 3 5% 5% 4 5% 4 6% 7% 31%	2596
	21st to end	Night	10% 5% 2 5% 4 5% 5% 4 7% 11% 43%	2732
June	1st to 10th	Night	10% 5% 2 5% 4 4 6% 4 8% 8% 45%	1520
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 7% 10% 41%	1520
	21st to end	Night	9% 4 2 6% 6% 5% 5% 5% 6% 10% 43%	1520
July	1st to 10th	Night	7% 4 3 7% 5% 5% 7% 4 7% 9% 41%	2520
	11th to 20th	Night	8% 6% 2 7% 5% 6% 6% 6% 7% 9% 38%	2638
	21st to end	Night	11% 7% 2 7% 5% 4 5% 5% 8% 8% 37%	3087
August	1st to 10th	Night	17% 5% 3 7% 5% 5% 5% 4 7% 9% 33%	2825
	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	12% 5% 2 5% 4 3 3 3 4 5% 54%	2367
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 6% 50%	2474
	21st to end	Night	13% 4 2 4 4 3 4 4 5% 7% 48%	2660
December	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 7% 41%	2660
	11th to 20th	Night	16% 5% 2 5% 4 4 4 4 6% 6% 44%	3285
	21st to end	Night	16% 7% 2 6% 5% 5% 5% 5% 6% 7% 37%	4884

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% 37% 5% 21% 10% 4	2582
	11th to 20th	Day	31% 37% 5% 13% 10% 3	2840
	21st to end	Day	32% 40% 7% 14% 5% 1	3124
February	1st to 10th	Day	17% 66% 5% 7% 4	1710
	11th to 20th	Day	16% 64% 3 9% 7% 2	1733
	21st to end	Day	18% 64% 4 7% 6% 1	1570
March	1st to 10th	Day	33% 42% 5% 12% 5% 2	3364
	11th to 20th	Day	17% 40% 5% 20% 16% 2	3480
	21st to end	Day	27% 40% 6% 13% 11% 2	3895
April	1st to 10th	Day	16% 66% 3 7% 7% 1	2280
	11th to 20th	Day	13% 65% 5% 9% 7% 1	2254
	21st to end	Day	16% 65% 4 9% 6% 1	2280
May	1st to 10th	Day	37% 42% 5% 11% 4 1	4024
	11th to 20th	Day	32% 43% 10% 14% 2	4301
	21st to end	Day	20% 41% 10% 15% 12% 2	4837
June	1st to 10th	Day	15% 65% 4 10% 5%	2660
	11th to 20th	Day	13% 66% 4 9% 7%	2660
	21st to end	Day	11% 65% 7% 9% 7% 1	2660
July	1st to 10th	Day	21% 40% 15% 12% 7% 4	4370
	11th to 20th	Day	25% 39% 12% 18% 6%	4243
	21st to end	Day	30% 37% 7% 15% 11%	4324
August	1st to 10th	Day	31% 39% 11% 16% 3	4091
	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% 57% 6% 10% 8% 3	1783
	11th to 20th	Day	13% 59% 3 11% 12% 2	1701
	21st to end	Day	14% 59% 4 14% 8% 1	1520
December	1st to 10th	Day	20% 60% 6% 9% 4 2	1520
	11th to 20th	Day	18% 48% 5% 20% 8% 2	1906
	21st to end	Day	30% 40% 8% 15% 6%	2772
January	1st to 10th	Night	26% 37% 5% 18% 10% 3	4361
	11th to 20th	Night	26% 36% 4 18% 13% 3	4120
	21st to end	Night	31% 39% 4 16% 9% 1	4532
February	1st to 10th	Night	15% 65% 6% 9% 3 1	2470
	11th to 20th	Night	16% 63% 2 8% 9% 2	2343
	21st to end	Night	15% 64% 5% 8% 5% 3	1884
March	1st to 10th	Night	34% 41% 4 14% 5% 1	3484
	11th to 20th	Night	17% 41% 6% 22% 13% 2	3480
	21st to end	Night	24% 40% 4 14% 14% 3	3736
April	1st to 10th	Night	15% 65% 3 8% 8% 1	1900
	11th to 20th	Night	13% 66% 5% 9% 5% 1	1875
	21st to end	Night	15% 65% 4 9% 6% 1	1900
May	1st to 10th	Night	35% 42% 6% 11% 5% 1	2951
	11th to 20th	Night	34% 43% 7% 13% 3	2597
	21st to end	Night	21% 42% 11% 16% 9% 2	2739
June	1st to 10th	Night	13% 66% 4 11% 6%	1520
	11th to 20th	Night	12% 66% 6% 11% 5%	1520
	21st to end	Night	12% 65% 6% 9% 7% 1	1520
July	1st to 10th	Night	24% 40% 15% 11% 8% 2	2480
	11th to 20th	Night	27% 38% 14% 15% 5%	2590
	21st to end	Night	31% 38% 9% 13% 10%	2988
August	1st to 10th	Night	33% 39% 12% 12% 3 1	2814
	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	15% 58% 6% 11% 8% 3	2367
	11th to 20th	Night	13% 59% 3 9% 13% 3	2474
	21st to end	Night	12% 60% 5% 13% 8% 1	2660
December	1st to 10th	Night	20% 59% 6% 9% 4 1	2660
	11th to 20th	Night	20% 49% 4 12% 11% 3	3288
	21st to end	Night	30% 39% 10% 14% 7%	4884

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	29% 20% 33% 13% 4 1	198
	11th to 20th	Day	31% 23% 29% 12% 4	270
	21st to end	Day	24% 24% 34% 14% 4	297
February	1st to 10th	Day	25% 24% 35% 12% 4	270
	11th to 20th	Day	35% 21% 36% 7% 2	331
	21st to end	Day	30% 18% 31% 18% 2	382
March	1st to 10th	Day	27% 20% 31% 17% 3 1	460
	11th to 20th	Day	28% 20% 32% 15% 4 1	460
	21st to end	Day	26% 18% 33% 16% 6% 1	506
April	1st to 10th	Day	23% 20% 35% 16% 5% 1	460
	11th to 20th	Day	19% 18% 35% 18% 7% 3	452
	21st to end	Day	28% 14% 33% 17% 7% 1	460
May	1st to 10th	Day	25% 17% 28% 20% 7% 3	463
	11th to 20th	Day	21% 14% 29% 23% 10% 2	487
	21st to end	Day	18% 8% 24% 31% 17% 3	529
June	1st to 10th	Day	14% 9% 28% 27% 16% 6%	490
	11th to 20th	Day	16% 8% 27% 29% 18% 4	490
	21st to end	Day	10% 7% 30% 31% 18% 4	490
July	1st to 10th	Day	12% 3 19% 37% 19% 10%	485
	11th to 20th	Day	6% 3 23% 34% 29% 6%	484
	21st to end	Day	12% 3 23% 35% 20% 6%	532
August	1st to 10th	Day	13% 5% 21% 32% 21% 9%	486
	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	16% 15% 32% 23% 11% 3	418
	11th to 20th	Day	20% 15% 33% 21% 9% 1	361
	21st to end	Day	26% 20% 29% 17% 8% 1	180
December	1st to 10th	Day	21% 24% 33% 17% 3 2	180
	11th to 20th	Day	25% 22% 29% 17% 7%	180
	21st to end	Day	27% 16% 32% 19% 6%	198
January	1st to 10th	Night	20% 18% 41% 19% 3	760
	11th to 20th	Night	23% 16% 31% 25% 4	760
	21st to end	Night	21% 17% 35% 25% 2	836
February	1st to 10th	Night	19% 17% 33% 28% 3	760
	11th to 20th	Night	26% 21% 34% 17% 1	743
	21st to end	Night	26% 17% 35% 20% 3	628
March	1st to 10th	Night	20% 17% 38% 23% 3	760
	11th to 20th	Night	24% 17% 36% 19% 4 1	760
	21st to end	Night	19% 18% 38% 21% 4 1	836
April	1st to 10th	Night	20% 22% 32% 21% 4	760
	11th to 20th	Night	17% 16% 33% 26% 7%	751
	21st to end	Night	22% 12% 32% 27% 8%	760
May	1st to 10th	Night	22% 10% 30% 27% 11%	642
	11th to 20th	Night	20% 7% 29% 34% 10%	436
	21st to end	Night	16% 5% 24% 38% 16% 1	411
June	1st to 10th	Night	11% 4 29% 36% 19% 1	380
	11th to 20th	Night	12% 6% 25% 41% 16%	380
	21st to end	Night	10% 5% 26% 44% 15% 1	380
July	1st to 10th	Night	8% 4 20% 43% 24% 2	370
	11th to 20th	Night	6% 3 17% 42% 30% 2	434
	21st to end	Night	9% 3 20% 41% 24% 3	624
August	1st to 10th	Night	11% 2 12% 42% 30% 2	567
	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	14% 10% 34% 29% 11% 1	755
	11th to 20th	Night	19% 14% 32% 28% 7% 1	760
	21st to end	Night	17% 14% 33% 27% 9% 1	760
December	1st to 10th	Night	22% 17% 32% 21% 8% 1	760
	11th to 20th	Night	18% 13% 37% 27% 5%	760
	21st to end	Night	20% 15% 30% 31% 3	836

Page generated at 2024-11-01 22:47:32. Version ac07

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