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 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	12% 6% 2 6% 4 4 4 4 6% 8% 43%	125685
	Night	14% 6% 2 5% 3 3 3 3 5% 7% 48%	122662
Transparency	Day	37% 7% 11% 20% 17% 9%	129415
	Night	35% 6% 11% 20% 17% 11%	126995
Seeing	Day	13% 7% 20% 27% 25% 9%	15850
	Night	15% 7% 25% 35% 17% 1	25702

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	19% 10% 3 6% 3 3 3 2 3 4 43%	4077
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 5% 5% 51%	4080
	21st to end	Day	29% 9% 2 5% 2 2 2 2 4 5% 38%	4602
February	1st to 10th	Day	21% 8% 3 6% 3 3 3 3 3 4 43%	2000
	11th to 20th	Day	21% 8% 2 6% 4 2 2 3 4 5% 42%	1949
	21st to end	Day	18% 8% 3 6% 4 3 4 3 4 5% 44%	1703
March	1st to 10th	Day	13% 6% 2 7% 4 4 4 3 5% 7% 45%	4936
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 3 5% 48%	4980
	21st to end	Day	17% 8% 3 6% 4 4 4 3 4 5% 42%	5462
April	1st to 10th	Day	14% 8% 3 6% 5% 4 4 4 6% 6% 40%	2200
	11th to 20th	Day	14% 8% 3 8% 5% 4 3 4 5% 5% 40%	2209
	21st to end	Day	13% 7% 3 6% 5% 4 5% 4 5% 6% 44%	2452
May	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 4 6% 7% 33%	5862
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 6% 8% 44%	5862
	21st to end	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 10% 36%	6434
June	1st to 10th	Day	5% 3 2 5% 4 6% 6% 5% 8% 9% 47%	2600
	11th to 20th	Day	6% 3 2 5% 5% 5% 5% 5% 7% 11% 45%	2600
	21st to end	Day	5% 6% 2 7% 7% 8% 7% 6% 9% 9% 35%	2600
July	1st to 10th	Day	9% 5% 3 7% 6% 5% 6% 5% 7% 10% 36%	5900
	11th to 20th	Day	6% 5% 2 6% 4 5% 6% 5% 8% 14% 38%	5900
	21st to end	Day	8% 6% 2 6% 5% 6% 5% 5% 8% 11% 37%	6422
August	1st to 10th	Day	1 1 1 3 3 4 4 4 6% 10% 64%	4157
	11th to 20th	Day	3 2 1 3 4 4 4 6% 9% 13% 50%	3930
	21st to end	Day	4 3 2 6% 5% 6% 6% 5% 9% 11% 44%	3870
September	1st to 10th	Day	6% 6% 3 7% 6% 6% 5% 4 6% 8% 43%	2090
	11th to 20th	Day	9% 6% 2 8% 5% 6% 5% 4 6% 8% 40%	2090
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 4 7% 7% 49%	2090
October	1st to 10th	Day	6% 3 1 4 4 4 5% 4 5% 8% 55%	2081
	11th to 20th	Day	13% 6% 2 5% 3 4 3 3 4 5% 52%	1900
	21st to end	Day	12% 4 2 5% 3 3 3 3 4 5% 56%	2090
November	1st to 10th	Day	14% 7% 2 5% 4 3 4 3 4 7% 47%	1958
	11th to 20th	Day	11% 6% 2 6% 3 3 4 3 5% 6% 52%	1980
	21st to end	Day	17% 8% 3 6% 3 4 4 3 5% 6% 43%	1800
December	1st to 10th	Day	26% 8% 3 6% 3 3 3 3 3 5% 36%	2940
	11th to 20th	Day	18% 7% 3 7% 4 4 4 3 4 5% 43%	3391
	21st to end	Day	14% 7% 2 6% 3 3 3 2 3 5% 51%	4488
January	1st to 10th	Night	21% 8% 2 4 2 3 3 2 3 5% 47%	5888
	11th to 20th	Night	13% 5% 1 3 2 3 3 2 4 5% 58%	5900
	21st to end	Night	30% 7% 2 5% 3 2 2 2 3 5% 38%	6418
February	1st to 10th	Night	19% 9% 2 6% 3 3 3 2 4 5% 45%	2400
	11th to 20th	Night	21% 7% 2 5% 3 3 2 2 3 6% 45%	2338
	21st to end	Night	22% 8% 2 5% 4 3 3 3 4 5% 42%	1925
March	1st to 10th	Night	18% 9% 2 5% 4 4 4 3 5% 6% 41%	4919
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 5% 46%	4980
	21st to end	Night	19% 10% 3 5% 3 3 3 2 4 5% 42%	5467
April	1st to 10th	Night	12% 8% 2 6% 4 3 3 4 6% 7% 45%	2200
	11th to 20th	Night	16% 6% 2 6% 4 3 4 3 5% 7% 44%	2139
	21st to end	Night	14% 8% 2 5% 4 3 4 3 4 7% 45%	1942
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 3 5% 7% 31%	4053
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 46%	4053
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 5% 7% 10% 36%	4443
June	1st to 10th	Night	4 3 2 5% 4 3 4 3 5% 11% 57%	1800
	11th to 20th	Night	4 3 1 3 4 4 4 4 7% 10% 56%	1800
	21st to end	Night	4 5% 2 5% 4 5% 5% 4 9% 10% 47%	1800
July	1st to 10th	Night	8% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	4078
	11th to 20th	Night	4 4 2 5% 4 3 5% 3 7% 11% 51%	4077
	21st to end	Night	7% 6% 2 6% 4 4 5% 5% 7% 11% 45%	4449
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 9% 72%	2872
	11th to 20th	Night	2 2 1 2 3 3 4 4 9% 11% 59%	2890
	21st to end	Night	3 3 1 3 4 4 4 5% 8% 11% 53%	3785
September	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	2090
	11th to 20th	Night	9% 6% 2 5% 4 5% 5% 4 5% 8% 48%	2090
	21st to end	Night	7% 4 2 6% 4 4 4 4 6% 7% 52%	2090
October	1st to 10th	Night	4 3 1 4 3 3 2 3 5% 8% 64%	2090
	11th to 20th	Night	11% 4 1 4 3 2 3 2 4 6% 58%	2267
	21st to end	Night	12% 5% 2 4 2 2 3 2 3 4 61%	2508
November	1st to 10th	Night	12% 5% 1 5% 4 3 3 3 5% 6% 51%	2340
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 6% 53%	2415
	21st to end	Night	15% 7% 3 5% 3 3 3 3 4 7% 47%	2600
December	1st to 10th	Night	25% 9% 3 6% 4 3 3 2 4 5% 35%	4250
	11th to 20th	Night	17% 8% 3 5% 3 3 3 3 4 5% 46%	4816
	21st to end	Night	15% 6% 2 5% 3 2 3 2 3 4 55%	6490

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	46% 4 5% 19% 19% 8%	4078
	11th to 20th	Day	36% 3 5% 18% 23% 15%	4080
	21st to end	Day	50% 3 3 9% 21% 14%	4602
February	1st to 10th	Day	46% 4 6% 12% 19% 14%	2000
	11th to 20th	Day	46% 4 7% 16% 16% 11%	1959
	21st to end	Day	44% 5% 8% 15% 16% 13%	1703
March	1st to 10th	Day	38% 5% 7% 18% 15% 17%	4936
	11th to 20th	Day	39% 5% 5% 9% 17% 25%	4980
	21st to end	Day	44% 5% 7% 18% 19% 7%	5464
April	1st to 10th	Day	44% 6% 7% 16% 15% 12%	2200
	11th to 20th	Day	45% 5% 8% 14% 18% 10%	2209
	21st to end	Day	40% 6% 8% 19% 18% 8%	2452
May	1st to 10th	Day	46% 8% 10% 20% 12% 4	5900
	11th to 20th	Day	36% 7% 11% 19% 20% 6%	5866
	21st to end	Day	38% 9% 13% 26% 13% 1	6447
June	1st to 10th	Day	29% 9% 16% 25% 19% 2	2600
	11th to 20th	Day	31% 8% 15% 24% 19% 4	2600
	21st to end	Day	40% 12% 14% 22% 11% 1	2600
July	1st to 10th	Day	40% 9% 23% 23% 4 1	5836
	11th to 20th	Day	33% 9% 22% 26% 9%	5831
	21st to end	Day	36% 10% 18% 24% 11% 1	6240
August	1st to 10th	Day	16% 6% 13% 43% 22% 1	4143
	11th to 20th	Day	21% 10% 29% 30% 8% 2	3969
	21st to end	Day	30% 12% 18% 24% 13% 2	3869
September	1st to 10th	Day	37% 9% 12% 20% 14% 7%	2090
	11th to 20th	Day	39% 9% 13% 17% 17% 4	2090
	21st to end	Day	31% 7% 11% 21% 16% 14%	2090
October	1st to 10th	Day	17% 4 7% 12% 21% 38%	3458
	11th to 20th	Day	22% 3 8% 18% 31% 17%	3140
	21st to end	Day	27% 6% 5% 18% 28% 16%	3428
November	1st to 10th	Day	36% 7% 10% 17% 19% 11%	1958
	11th to 20th	Day	33% 5% 11% 15% 21% 15%	1980
	21st to end	Day	41% 6% 9% 14% 18% 12%	1800
December	1st to 10th	Day	52% 4 6% 14% 17% 7%	2940
	11th to 20th	Day	43% 6% 7% 10% 22% 12%	3389
	21st to end	Day	36% 4 5% 17% 24% 14%	4488
January	1st to 10th	Night	40% 3 6% 19% 20% 11%	5888
	11th to 20th	Night	30% 3 4 19% 28% 15%	5900
	21st to end	Night	50% 3 3 11% 19% 14%	6418
February	1st to 10th	Night	43% 4 6% 11% 19% 16%	2400
	11th to 20th	Night	42% 4 7% 16% 18% 13%	2338
	21st to end	Night	47% 3 6% 14% 16% 14%	1925
March	1st to 10th	Night	43% 6% 7% 15% 16% 13%	4919
	11th to 20th	Night	40% 5% 6% 12% 13% 23%	4980
	21st to end	Night	45% 5% 7% 16% 18% 10%	5467
April	1st to 10th	Night	38% 6% 11% 17% 17% 12%	2200
	11th to 20th	Night	40% 5% 10% 15% 15% 15%	2139
	21st to end	Night	39% 7% 10% 20% 16% 8%	1942
May	1st to 10th	Night	52% 7% 9% 19% 10% 4	4080
	11th to 20th	Night	34% 6% 12% 23% 18% 7%	4054
	21st to end	Night	40% 9% 15% 27% 8% 1	4452
June	1st to 10th	Night	22% 6% 19% 30% 19% 2	1800
	11th to 20th	Night	22% 8% 15% 33% 19% 4	1800
	21st to end	Night	28% 12% 18% 27% 14% 2	1800
July	1st to 10th	Night	36% 9% 27% 25% 3	4035
	11th to 20th	Night	27% 8% 24% 32% 9%	4026
	21st to end	Night	33% 9% 21% 25% 10% 3	4320
August	1st to 10th	Night	9% 6% 17% 47% 20% 1	2862
	11th to 20th	Night	17% 8% 33% 34% 8% 1	2917
	21st to end	Night	22% 13% 24% 28% 12% 2	3785
September	1st to 10th	Night	30% 9% 18% 22% 14% 7%	2090
	11th to 20th	Night	33% 9% 15% 23% 15% 6%	2090
	21st to end	Night	29% 7% 15% 19% 17% 14%	2090
October	1st to 10th	Night	13% 4 8% 16% 19% 39%	3480
	11th to 20th	Night	17% 3 8% 20% 30% 21%	3766
	21st to end	Night	20% 4 5% 24% 30% 17%	4120
November	1st to 10th	Night	33% 6% 11% 18% 19% 13%	2340
	11th to 20th	Night	33% 5% 10% 17% 20% 15%	2415
	21st to end	Night	38% 4 9% 18% 18% 14%	2600
December	1st to 10th	Night	53% 4 5% 15% 15% 8%	4250
	11th to 20th	Night	40% 5% 9% 9% 21% 16%	4817
	21st to end	Night	34% 3 5% 16% 30% 13%	6490

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	23% 11% 35% 23% 6% 1	300
	11th to 20th	Day	24% 13% 28% 27% 7% 1	300
	21st to end	Day	25% 13% 32% 21% 8% 1	330
February	1st to 10th	Day	28% 14% 29% 21% 9%	300
	11th to 20th	Day	28% 18% 29% 18% 6%	300
	21st to end	Day	27% 15% 37% 16% 5%	249
March	1st to 10th	Day	17% 17% 40% 19% 7%	300
	11th to 20th	Day	17% 18% 40% 18% 7% 1	300
	21st to end	Day	18% 15% 35% 25% 7%	330
April	1st to 10th	Day	22% 17% 33% 22% 5% 1	300
	11th to 20th	Day	19% 14% 33% 24% 9% 1	327
	21st to end	Day	18% 13% 33% 26% 9% 1	552
May	1st to 10th	Day	15% 11% 37% 28% 9%	703
	11th to 20th	Day	13% 9% 27% 33% 16% 2	725
	21st to end	Day	12% 6% 26% 35% 18% 3	789
June	1st to 10th	Day	6% 2 13% 37% 33% 8%	730
	11th to 20th	Day	6% 2 13% 42% 30% 8%	730
	21st to end	Day	7% 2 9% 32% 35% 15%	730
July	1st to 10th	Day	7% 3 7% 29% 37% 16%	721
	11th to 20th	Day	5% 2 3 27% 48% 15%	718
	21st to end	Day	5% 2 5% 25% 45% 19%	794
August	1st to 10th	Day	2 3 24% 49% 22%	697
	11th to 20th	Day	5% 4 23% 46% 21%	606
	21st to end	Day	7% 3 3 18% 32% 37%	359
September	1st to 10th	Day	7% 1 5% 24% 37% 25%	300
	11th to 20th	Day	6% 1 5% 21% 43% 23%	300
	21st to end	Day	9% 2 6% 18% 43% 21%	300
October	1st to 10th	Day	6% 1 11% 31% 39% 13%	300
	11th to 20th	Day	11% 4 16% 32% 29% 8%	300
	21st to end	Day	18% 11% 22% 22% 21% 7%	330
November	1st to 10th	Day	18% 6% 22% 32% 16% 7%	300
	11th to 20th	Day	17% 7% 22% 30% 20% 4	300
	21st to end	Day	20% 15% 30% 20% 12% 3	300
December	1st to 10th	Day	21% 8% 31% 25% 11% 5%	300
	11th to 20th	Day	20% 14% 28% 26% 10% 2	300
	21st to end	Day	19% 10% 37% 24% 8% 2	330
January	1st to 10th	Night	22% 10% 30% 30% 8%	1000
	11th to 20th	Night	22% 12% 29% 29% 8%	1000
	21st to end	Night	24% 10% 30% 31% 6%	1067
February	1st to 10th	Night	24% 14% 30% 28% 4	800
	11th to 20th	Night	26% 9% 35% 26% 4	784
	21st to end	Night	27% 13% 38% 19% 3	605
March	1st to 10th	Night	22% 12% 36% 27% 4	600
	11th to 20th	Night	21% 12% 35% 28% 5%	600
	21st to end	Night	17% 10% 42% 27% 4	660
April	1st to 10th	Night	14% 12% 41% 31% 2	600
	11th to 20th	Night	17% 12% 40% 25% 6%	593
	21st to end	Night	18% 11% 38% 26% 7% 1	600
May	1st to 10th	Night	15% 10% 43% 28% 5%	600
	11th to 20th	Night	12% 7% 34% 36% 11% 1	591
	21st to end	Night	8% 6% 32% 40% 14%	650
June	1st to 10th	Night	4 2 18% 52% 23% 1	600
	11th to 20th	Night	4 2 24% 52% 17% 2	600
	21st to end	Night	5% 2 12% 45% 32% 4	600
July	1st to 10th	Night	6% 2 9% 47% 32% 4	585
	11th to 20th	Night	4 1 9% 47% 37% 2	582
	21st to end	Night	4 1 8% 49% 35% 4	657
August	1st to 10th	Night	1 5% 54% 37% 3	579
	11th to 20th	Night	4 1 6% 51% 34% 4	564
	21st to end	Night	5% 4 38% 48% 6%	627
September	1st to 10th	Night	8% 1 6% 37% 42% 5%	570
	11th to 20th	Night	10% 1 7% 36% 42% 4	570
	21st to end	Night	9% 1 13% 34% 38% 4	570
October	1st to 10th	Night	4 2 11% 46% 33% 5%	570
	11th to 20th	Night	11% 3 21% 43% 19% 2	747
	21st to end	Night	12% 8% 27% 32% 19% 2	836
November	1st to 10th	Night	11% 5% 23% 41% 19% 1	780
	11th to 20th	Night	12% 7% 25% 39% 15% 1	815
	21st to end	Night	18% 10% 28% 29% 14% 1	1000
December	1st to 10th	Night	20% 9% 28% 30% 12% 1	1000
	11th to 20th	Night	24% 12% 26% 31% 7% 1	1000
	21st to end	Night	19% 10% 29% 33% 8%	1100

Page generated at 2025-08-02 00:32:23. Version ac07

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