West Point Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Point Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 38%	138650
	Night	12% 10% 4 8% 5% 4 4 3 5% 6% 39%	134680
Transparency	Day	45% 4 11% 25% 14% 2	142119
	Night	46% 3 11% 23% 15% 2	139261
Seeing	Day	17% 10% 29% 25% 14% 5%	18298
	Night	18% 9% 30% 30% 11% 1	28108

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	10% 11% 8% 16% 7% 6% 5% 3 4 5% 26%	4691
	11th to 20th	Day	6% 11% 6% 11% 5% 4 4 6% 6% 7% 32%	4710
	21st to end	Day	9% 13% 6% 9% 5% 4 4 3 4 6% 38%	5514
February	1st to 10th	Day	12% 13% 5% 10% 5% 4 4 4 5% 6% 31%	1890
	11th to 20th	Day	12% 12% 4 7% 5% 4 5% 3 5% 7% 36%	1963
	21st to end	Day	12% 14% 4 10% 6% 5% 3 3 5% 5% 33%	1730
March	1st to 10th	Day	16% 8% 3 5% 4 3 3 3 4 6% 46%	4580
	11th to 20th	Day	12% 6% 3 8% 4 3 4 3 6% 8% 44%	4984
	21st to end	Day	22% 12% 4 9% 5% 4 4 3 4 5% 28%	5994
April	1st to 10th	Day	18% 12% 4 7% 5% 5% 4 4 5% 5% 32%	2520
	11th to 20th	Day	17% 11% 2 7% 5% 5% 5% 3 6% 5% 34%	2494
	21st to end	Day	18% 10% 3 8% 5% 4 4 4 5% 6% 34%	2560
May	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	7914
	11th to 20th	Day	12% 6% 2 7% 5% 4 5% 4 6% 8% 42%	8222
	21st to end	Day	14% 7% 3 6% 4 5% 5% 4 6% 8% 38%	9047
June	1st to 10th	Day	11% 5% 2 6% 4 5% 4 4 7% 9% 43%	2940
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 7% 8% 42%	2940
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 7% 7% 39%	2940
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 10% 38%	6500
	11th to 20th	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	6500
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	7065
August	1st to 10th	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 9% 41%	4434
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 7% 46%	3952
	21st to end	Day	9% 6% 2 6% 4 3 4 4 6% 9% 46%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 4 5% 3 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 1 5% 4 3 3 3 5% 8% 51%	2350
	21st to end	Day	12% 7% 2 7% 4 4 5% 3 5% 7% 45%	2200
October	1st to 10th	Day	13% 7% 2 6% 4 3 4 2 4 6% 49%	2018
	11th to 20th	Day	19% 8% 3 7% 4 4 3 3 5% 6% 38%	2000
	21st to end	Day	18% 10% 3 6% 4 3 3 3 4 6% 40%	2039
November	1st to 10th	Day	12% 8% 2 7% 4 4 5% 3 5% 6% 44%	1852
	11th to 20th	Day	14% 13% 4 8% 5% 4 4 4 4 6% 34%	1867
	21st to end	Day	14% 14% 4 10% 4 3 3 3 4 7% 35%	1680
December	1st to 10th	Day	8% 10% 5% 8% 5% 5% 5% 4 6% 7% 39%	2690
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 5% 3 6% 6% 25%	3088
	21st to end	Day	19% 23% 8% 12% 5% 4 4 4 4 5% 13%	4070
January	1st to 10th	Night	10% 12% 6% 14% 6% 4 3 3 4 5% 32%	8232
	11th to 20th	Night	8% 14% 6% 14% 6% 5% 5% 5% 6% 5% 24%	8280
	21st to end	Night	10% 13% 6% 10% 5% 5% 4 3 5% 6% 34%	8685
February	1st to 10th	Night	11% 13% 5% 9% 6% 5% 4 3 5% 5% 33%	2730
	11th to 20th	Night	11% 11% 3 8% 5% 4 5% 3 5% 5% 39%	2542
	21st to end	Night	15% 12% 4 8% 4 4 3 4 5% 6% 36%	2076
March	1st to 10th	Night	16% 9% 4 7% 4 3 4 3 4 4 42%	5493
	11th to 20th	Night	9% 8% 2 6% 4 3 4 3 5% 8% 48%	5241
	21st to end	Night	22% 12% 5% 11% 5% 4 4 3 4 5% 27%	5177
April	1st to 10th	Night	18% 9% 3 6% 4 3 4 3 5% 6% 37%	2100
	11th to 20th	Night	19% 10% 3 6% 5% 3 4 3 5% 5% 37%	2075
	21st to end	Night	16% 8% 3 7% 5% 5% 4 4 4 7% 37%	2058
May	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 8% 43%	4938
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 5% 8% 45%	4670
	21st to end	Night	13% 7% 3 7% 4 5% 4 4 6% 7% 41%	5139
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 8% 9% 46%	1680
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 5% 10% 44%	1680
	21st to end	Night	9% 6% 3 6% 3 4 4 3 5% 8% 48%	1680
July	1st to 10th	Night	12% 7% 2 6% 4 5% 4 4 6% 8% 42%	3698
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 5% 8% 52%	3697
	21st to end	Night	7% 5% 2 7% 4 4 4 3 6% 9% 48%	4027
August	1st to 10th	Night	8% 4 2 6% 4 4 5% 4 6% 7% 51%	2723
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 7% 50%	3098
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 7% 50%	3586
September	1st to 10th	Night	8% 5% 1 5% 3 4 4 2 5% 7% 55%	2000
	11th to 20th	Night	10% 4 2 4 4 4 4 4 5% 8% 52%	2030
	21st to end	Night	9% 5% 2 5% 3 4 4 3 5% 7% 51%	2200
October	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 51%	2379
	11th to 20th	Night	17% 7% 2 6% 4 3 3 3 4 6% 44%	2400
	21st to end	Night	18% 8% 3 7% 4 4 4 3 5% 7% 37%	2800
November	1st to 10th	Night	12% 8% 3 5% 4 4 3 3 4 6% 47%	2666
	11th to 20th	Night	14% 11% 3 9% 4 5% 4 3 5% 5% 36%	2751
	21st to end	Night	17% 12% 4 8% 4 3 3 2 4 6% 37%	2940
December	1st to 10th	Night	8% 9% 3 7% 5% 5% 5% 4 5% 6% 42%	4720
	11th to 20th	Night	16% 16% 5% 9% 5% 4 4 3 4 4 27%	5339
	21st to end	Night	20% 19% 6% 12% 5% 5% 4 3 4 4 17%	7150

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	62% 2 12% 16% 7% 1	4692
	11th to 20th	Day	50% 3 10% 17% 18% 2	4710
	21st to end	Day	50% 2 11% 26% 11%	5514
February	1st to 10th	Day	54% 4 11% 18% 12% 1	1890
	11th to 20th	Day	49% 4 11% 25% 9% 3	1972
	21st to end	Day	53% 4 9% 21% 10% 2	1730
March	1st to 10th	Day	41% 3 11% 30% 11% 4	4580
	11th to 20th	Day	39% 4 12% 30% 13% 2	4984
	21st to end	Day	59% 4 11% 15% 9% 2	5996
April	1st to 10th	Day	54% 5% 9% 20% 11%	2520
	11th to 20th	Day	51% 5% 10% 21% 11% 2	2494
	21st to end	Day	52% 5% 11% 19% 12% 1	2560
May	1st to 10th	Day	43% 3 8% 26% 20% 1	7949
	11th to 20th	Day	42% 4 9% 28% 16% 1	8226
	21st to end	Day	44% 4 9% 24% 15% 4	9061
June	1st to 10th	Day	37% 6% 13% 26% 16% 1	2940
	11th to 20th	Day	37% 7% 13% 24% 18% 2	2940
	21st to end	Day	42% 6% 13% 24% 13% 2	2940
July	1st to 10th	Day	42% 3 14% 30% 10% 1	6431
	11th to 20th	Day	36% 4 13% 32% 15% 1	6425
	21st to end	Day	43% 4 14% 23% 15% 2	6869
August	1st to 10th	Day	39% 5% 14% 28% 13% 1	4419
	11th to 20th	Day	37% 4 14% 31% 14% 1	3990
	21st to end	Day	35% 4 13% 32% 16% 1	4311
September	1st to 10th	Day	38% 6% 9% 27% 18% 2	2400
	11th to 20th	Day	31% 4 12% 33% 17% 2	2350
	21st to end	Day	39% 4 9% 29% 19% 1	2200
October	1st to 10th	Day	24% 3 5% 39% 27% 1	3278
	11th to 20th	Day	36% 3 8% 27% 24% 3	3240
	21st to end	Day	43% 2 9% 29% 14% 3	3264
November	1st to 10th	Day	42% 4 13% 26% 14% 2	1852
	11th to 20th	Day	51% 4 12% 21% 10% 1	1867
	21st to end	Day	52% 4 14% 23% 7% 1	1680
December	1st to 10th	Day	45% 3 18% 26% 7% 1	2690
	11th to 20th	Day	60% 3 10% 18% 7% 2	3085
	21st to end	Day	75% 3 8% 9% 6%	4070
January	1st to 10th	Night	56% 3 12% 20% 8% 1	8233
	11th to 20th	Night	61% 3 9% 12% 15% 1	8280
	21st to end	Night	53% 2 11% 22% 11%	8685
February	1st to 10th	Night	53% 4 11% 18% 12% 2	2730
	11th to 20th	Night	47% 4 11% 24% 13% 1	2543
	21st to end	Night	50% 3 12% 21% 11% 3	2076
March	1st to 10th	Night	47% 3 11% 23% 11% 7%	5493
	11th to 20th	Night	36% 3 11% 33% 14% 4	5241
	21st to end	Night	62% 3 8% 16% 10% 1	5177
April	1st to 10th	Night	47% 5% 10% 22% 14% 2	2100
	11th to 20th	Night	49% 4 7% 19% 18% 3	2075
	21st to end	Night	46% 6% 11% 22% 12% 2	2058
May	1st to 10th	Night	40% 4 9% 22% 24% 1	4965
	11th to 20th	Night	38% 5% 13% 23% 19% 2	4671
	21st to end	Night	42% 5% 8% 24% 16% 4	5149
June	1st to 10th	Night	33% 6% 14% 26% 20% 2	1680
	11th to 20th	Night	35% 5% 11% 24% 22% 3	1680
	21st to end	Night	35% 6% 15% 23% 18% 3	1680
July	1st to 10th	Night	42% 2 13% 26% 14% 2	3660
	11th to 20th	Night	32% 3 12% 32% 19% 2	3652
	21st to end	Night	34% 4 16% 24% 18% 3	3912
August	1st to 10th	Night	32% 4 13% 30% 20% 1	2714
	11th to 20th	Night	34% 3 12% 32% 17% 1	3126
	21st to end	Night	34% 4 11% 29% 18% 4	3586
September	1st to 10th	Night	29% 5% 12% 28% 24% 2	2000
	11th to 20th	Night	32% 5% 11% 32% 18% 2	2030
	21st to end	Night	34% 4 9% 31% 21% 1	2200
October	1st to 10th	Night	25% 3 5% 37% 29% 2	3886
	11th to 20th	Night	34% 3 9% 27% 25% 3	3899
	21st to end	Night	44% 3 8% 27% 15% 2	4512
November	1st to 10th	Night	40% 4 10% 27% 18% 2	2666
	11th to 20th	Night	50% 4 12% 21% 12% 2	2751
	21st to end	Night	50% 4 12% 23% 10% 1	2940
December	1st to 10th	Night	44% 3 15% 28% 10% 1	4720
	11th to 20th	Night	60% 3 11% 16% 9% 1	5341
	21st to end	Night	71% 3 7% 13% 5%	7150

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% 15% 32% 27% 8% 2	330
	11th to 20th	Day	19% 17% 40% 19% 5% 1	330
	21st to end	Day	20% 19% 35% 21% 5%	363
February	1st to 10th	Day	16% 22% 38% 19% 5% 1	330
	11th to 20th	Day	23% 18% 36% 19% 4	330
	21st to end	Day	23% 20% 41% 15% 1	273
March	1st to 10th	Day	20% 17% 44% 15% 4	330
	11th to 20th	Day	22% 21% 37% 14% 5% 2	330
	21st to end	Day	25% 16% 34% 18% 5% 1	541
April	1st to 10th	Day	27% 16% 33% 19% 4	540
	11th to 20th	Day	28% 15% 27% 18% 10% 1	532
	21st to end	Day	22% 11% 32% 24% 9% 1	540
May	1st to 10th	Day	19% 11% 32% 27% 9% 2	647
	11th to 20th	Day	19% 12% 29% 23% 12% 5%	775
	21st to end	Day	17% 9% 26% 25% 16% 7%	844
June	1st to 10th	Day	11% 7% 27% 30% 19% 5%	780
	11th to 20th	Day	14% 7% 31% 29% 14% 5%	780
	21st to end	Day	15% 7% 29% 31% 14% 4	780
July	1st to 10th	Day	9% 2 20% 29% 26% 13%	771
	11th to 20th	Day	11% 5% 24% 27% 24% 9%	768
	21st to end	Day	10% 3 26% 32% 20% 8%	845
August	1st to 10th	Day	8% 3 19% 36% 25% 10%	726
	11th to 20th	Day	14% 4 19% 27% 23% 12%	586
	21st to end	Day	11% 5% 20% 30% 24% 9%	583
September	1st to 10th	Day	13% 7% 22% 30% 20% 9%	530
	11th to 20th	Day	11% 3 20% 29% 27% 10%	530
	21st to end	Day	14% 9% 26% 26% 18% 6%	530
October	1st to 10th	Day	16% 9% 28% 28% 15% 4	348
	11th to 20th	Day	25% 13% 34% 18% 9%	330
	21st to end	Day	27% 14% 29% 23% 5% 1	363
November	1st to 10th	Day	18% 15% 40% 18% 8% 2	330
	11th to 20th	Day	20% 17% 36% 18% 7%	330
	21st to end	Day	24% 13% 38% 21% 4	330
December	1st to 10th	Day	16% 17% 42% 17% 7% 1	330
	11th to 20th	Day	20% 14% 39% 20% 7%	330
	21st to end	Day	28% 15% 37% 17% 4	363
January	1st to 10th	Night	17% 14% 37% 24% 6% 1	1050
	11th to 20th	Night	20% 14% 37% 24% 4 1	1050
	21st to end	Night	21% 12% 36% 26% 5%	1155
February	1st to 10th	Night	19% 12% 38% 24% 6%	1050
	11th to 20th	Night	18% 15% 37% 25% 4	907
	21st to end	Night	22% 13% 40% 20% 4	692
March	1st to 10th	Night	23% 13% 35% 23% 5%	840
	11th to 20th	Night	18% 11% 34% 28% 8%	703
	21st to end	Night	24% 11% 33% 25% 6% 1	693
April	1st to 10th	Night	22% 14% 36% 24% 4	630
	11th to 20th	Night	25% 12% 28% 28% 7%	623
	21st to end	Night	20% 10% 32% 29% 9% 1	630
May	1st to 10th	Night	17% 7% 35% 31% 9%	630
	11th to 20th	Night	19% 6% 31% 33% 11%	621
	21st to end	Night	15% 4 28% 36% 16% 1	683
June	1st to 10th	Night	10% 4 24% 46% 14% 1	630
	11th to 20th	Night	15% 5% 29% 39% 13%	630
	21st to end	Night	13% 3 26% 40% 17% 1	630
July	1st to 10th	Night	10% 3 13% 40% 31% 4	615
	11th to 20th	Night	9% 3 19% 42% 25% 2	612
	21st to end	Night	8% 1 19% 45% 24% 2	687
August	1st to 10th	Night	7% 2 15% 45% 28% 3	597
	11th to 20th	Night	12% 2 13% 39% 28% 6%	594
	21st to end	Night	10% 2 15% 45% 26% 3	660
September	1st to 10th	Night	10% 2 21% 39% 23% 5%	600
	11th to 20th	Night	12% 2 20% 35% 26% 4	630
	21st to end	Night	13% 7% 23% 35% 21% 1	800
October	1st to 10th	Night	16% 5% 27% 33% 18% 1	800
	11th to 20th	Night	20% 11% 29% 31% 9%	800
	21st to end	Night	22% 10% 33% 27% 7% 1	880
November	1st to 10th	Night	18% 10% 37% 26% 9% 1	820
	11th to 20th	Night	20% 14% 36% 23% 6% 1	861
	21st to end	Night	23% 13% 34% 23% 7%	1050
December	1st to 10th	Night	21% 12% 37% 25% 5%	1050
	11th to 20th	Night	23% 12% 35% 24% 6%	1050
	21st to end	Night	29% 12% 33% 23% 3	1155

Page generated at 2026-07-28 02:25:25. Version ac07

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