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 2026-07-01.

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	16% 7% 2 6% 4 4 5% 4 5% 7% 38%	137822
	Night	16% 8% 3 6% 4 4 4 3 5% 6% 40%	134562
Transparency	Day	43% 5% 11% 23% 15% 2	141338
	Night	42% 5% 11% 23% 16% 3	139135
Seeing	Day	23% 16% 29% 20% 9% 2	17033
	Night	22% 13% 30% 26% 8% 1	26157

By Month

Percentage Cloud Cover NE (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	12% 7% 3 8% 5% 4 5% 4 5% 6% 41%	4661
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 4 5% 41%	4650
	21st to end	Day	15% 8% 3 6% 4 4 4 3 5% 7% 41%	5497
February	1st to 10th	Day	15% 8% 3 8% 5% 4 5% 4 4 7% 36%	1890
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 3 5% 7% 38%	2048
	21st to end	Day	18% 9% 2 6% 4 4 4 3 4 6% 38%	1730
March	1st to 10th	Day	21% 8% 2 6% 5% 4 4 3 4 5% 38%	4576
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 3 5% 7% 42%	4851
	21st to end	Day	17% 8% 2 7% 5% 4 4 4 5% 7% 38%	6073
April	1st to 10th	Day	18% 9% 3 6% 5% 4 4 3 5% 6% 37%	2520
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 3 5% 7% 40%	2494
	21st to end	Day	19% 7% 2 6% 4 4 5% 4 5% 7% 38%	2567
May	1st to 10th	Day	25% 8% 2 6% 4 4 4 3 5% 8% 32%	7639
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 6% 38%	8218
	21st to end	Day	22% 7% 2 6% 4 4 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 7% 9% 37%	2940
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 8% 36%	2940
	21st to end	Day	12% 7% 2 7% 4 4 6% 4 7% 9% 37%	2940
July	1st to 10th	Day	10% 8% 3 7% 6% 5% 6% 5% 6% 9% 35%	6360
	11th to 20th	Day	9% 7% 3 8% 5% 6% 7% 5% 8% 11% 32%	6360
	21st to end	Day	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	6926
August	1st to 10th	Day	19% 7% 3 6% 5% 5% 5% 4 6% 8% 33%	4379
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 4 7% 8% 41%	3850
	21st to end	Day	10% 6% 2 6% 4 3 4 3 6% 9% 46%	4312
September	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	2400
	11th to 20th	Day	12% 4 2 5% 4 4 3 3 5% 8% 48%	2310
	21st to end	Day	17% 6% 2 7% 5% 5% 4 4 5% 7% 40%	2200
October	1st to 10th	Day	19% 8% 1 5% 3 3 4 4 4 5% 42%	2090
	11th to 20th	Day	14% 6% 3 5% 3 4 5% 3 6% 8% 44%	2000
	21st to end	Day	20% 6% 2 6% 4 3 5% 4 5% 6% 39%	2024
November	1st to 10th	Day	13% 6% 2 5% 4 4 3 2 5% 6% 50%	1851
	11th to 20th	Day	16% 6% 2 5% 4 4 4 4 4 7% 44%	1890
	21st to end	Day	17% 8% 2 6% 4 4 4 4 5% 6% 40%	1782
December	1st to 10th	Day	14% 9% 3 6% 5% 5% 4 4 6% 6% 39%	2680
	11th to 20th	Day	20% 9% 2 5% 2 3 3 2 4 6% 43%	3066
	21st to end	Day	17% 10% 4 7% 4 4 4 4 5% 7% 35%	4048
January	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 5% 7% 40%	8234
	11th to 20th	Night	15% 9% 3 9% 4 4 4 3 5% 6% 38%	8235
	21st to end	Night	14% 8% 2 6% 4 4 4 3 5% 6% 43%	8681
February	1st to 10th	Night	15% 10% 3 6% 4 4 4 4 5% 6% 40%	2728
	11th to 20th	Night	15% 8% 3 6% 5% 5% 4 4 6% 6% 38%	2458
	21st to end	Night	16% 8% 2 8% 5% 4 4 3 4 4 41%	2076
March	1st to 10th	Night	19% 9% 3 6% 3 3 3 3 4 6% 42%	5484
	11th to 20th	Night	12% 9% 4 7% 4 4 4 3 5% 7% 42%	5324
	21st to end	Night	18% 9% 3 7% 4 4 3 4 4 6% 38%	5101
April	1st to 10th	Night	18% 8% 2 7% 6% 4 5% 5% 5% 6% 34%	2100
	11th to 20th	Night	18% 9% 3 5% 4 4 4 4 5% 7% 38%	2075
	21st to end	Night	19% 7% 3 6% 3 4 4 3 5% 6% 40%	2051
May	1st to 10th	Night	26% 8% 3 7% 5% 3 4 3 5% 7% 30%	5259
	11th to 20th	Night	19% 7% 3 7% 5% 4 4 3 6% 7% 35%	4698
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 7% 34%	5114
June	1st to 10th	Night	16% 5% 2 5% 4 4 5% 3 5% 8% 42%	1680
	11th to 20th	Night	13% 7% 2 7% 6% 3 5% 4 6% 8% 40%	1680
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 7% 9% 43%	1680
July	1st to 10th	Night	11% 8% 3 7% 4 4 5% 4 7% 9% 39%	3600
	11th to 20th	Night	13% 8% 3 8% 5% 5% 6% 4 7% 9% 33%	3600
	21st to end	Night	12% 7% 2 5% 3 4 4 4 6% 7% 46%	3926
August	1st to 10th	Night	20% 7% 3 7% 4 4 5% 4 5% 7% 33%	2735
	11th to 20th	Night	13% 8% 3 7% 5% 4 5% 4 5% 6% 41%	3197
	21st to end	Night	12% 5% 2 4 4 3 4 3 5% 7% 52%	3586
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 5% 6% 52%	2090
	21st to end	Night	17% 6% 2 5% 4 3 4 3 4 6% 45%	2200
October	1st to 10th	Night	19% 8% 2 6% 4 3 3 3 3 6% 43%	2305
	11th to 20th	Night	15% 5% 2 6% 3 4 4 2 5% 7% 48%	2400
	21st to end	Night	20% 6% 2 6% 4 3 4 3 5% 6% 41%	2815
November	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2667
	11th to 20th	Night	17% 7% 2 5% 4 4 4 3 5% 6% 45%	2730
	21st to end	Night	15% 6% 2 6% 4 4 3 4 6% 7% 43%	2835
December	1st to 10th	Night	15% 8% 3 6% 3 3 4 2 5% 6% 44%	4720
	11th to 20th	Night	23% 9% 2 4 3 3 3 3 5% 6% 39%	5348
	21st to end	Night	18% 10% 3 7% 4 3 3 3 5% 7% 36%	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	39% 6% 10% 28% 15% 2	4662
	11th to 20th	Day	43% 4 9% 22% 16% 6%	4680
	21st to end	Day	43% 4 10% 25% 16% 1	5497
February	1st to 10th	Day	47% 5% 11% 22% 14% 2	1890
	11th to 20th	Day	45% 5% 9% 23% 16% 2	2057
	21st to end	Day	46% 4 11% 22% 14% 2	1730
March	1st to 10th	Day	46% 5% 10% 18% 12% 9%	4579
	11th to 20th	Day	38% 7% 14% 29% 9% 4	4851
	21st to end	Day	46% 5% 9% 14% 22% 6%	6073
April	1st to 10th	Day	46% 5% 9% 21% 15% 4	2520
	11th to 20th	Day	42% 5% 12% 21% 17% 2	2494
	21st to end	Day	45% 5% 9% 22% 16% 3	2567
May	1st to 10th	Day	51% 5% 12% 17% 14% 1	7651
	11th to 20th	Day	45% 5% 11% 23% 14% 1	8222
	21st to end	Day	48% 6% 12% 21% 12% 1	9061
June	1st to 10th	Day	42% 6% 13% 22% 16% 1	2940
	11th to 20th	Day	43% 6% 13% 24% 12% 1	2940
	21st to end	Day	42% 7% 12% 24% 14% 1	2940
July	1st to 10th	Day	44% 8% 11% 21% 15% 1	6290
	11th to 20th	Day	42% 8% 17% 21% 11% 1	6285
	21st to end	Day	42% 5% 11% 22% 19% 1	6728
August	1st to 10th	Day	49% 7% 12% 21% 10% 1	4365
	11th to 20th	Day	38% 6% 16% 29% 10%	3886
	21st to end	Day	34% 5% 11% 31% 17% 1	4312
September	1st to 10th	Day	40% 6% 10% 24% 18% 2	2400
	11th to 20th	Day	35% 5% 9% 25% 25% 1	2310
	21st to end	Day	45% 5% 9% 22% 16% 3	2200
October	1st to 10th	Day	38% 5% 9% 19% 25% 5%	3402
	11th to 20th	Day	29% 4 9% 27% 26% 4	3240
	21st to end	Day	30% 3 14% 25% 23% 4	3249
November	1st to 10th	Day	35% 4 12% 26% 20% 3	1851
	11th to 20th	Day	40% 4 10% 24% 19% 3	1890
	21st to end	Day	42% 4 13% 28% 10% 2	1782
December	1st to 10th	Day	43% 5% 14% 27% 10% 1	2680
	11th to 20th	Day	43% 2 14% 22% 15% 3	3066
	21st to end	Day	48% 5% 8% 22% 15% 2	4048
January	1st to 10th	Night	41% 4 12% 24% 17% 2	8234
	11th to 20th	Night	44% 4 11% 20% 15% 6%	8280
	21st to end	Night	42% 4 9% 22% 20% 4	8681
February	1st to 10th	Night	45% 5% 13% 23% 13% 2	2728
	11th to 20th	Night	45% 4 10% 20% 17% 4	2460
	21st to end	Night	45% 4 7% 24% 16% 4	2076
March	1st to 10th	Night	43% 4 12% 20% 15% 7%	5494
	11th to 20th	Night	41% 5% 15% 25% 11% 4	5325
	21st to end	Night	47% 4 6% 15% 23% 4	5101
April	1st to 10th	Night	48% 4 11% 20% 14% 3	2100
	11th to 20th	Night	45% 6% 10% 19% 18% 2	2075
	21st to end	Night	43% 5% 8% 22% 17% 4	2051
May	1st to 10th	Night	53% 5% 11% 18% 12% 1	5265
	11th to 20th	Night	46% 6% 13% 21% 13% 2	4699
	21st to end	Night	50% 5% 12% 22% 10% 1	5115
June	1st to 10th	Night	39% 7% 10% 24% 19% 2	1680
	11th to 20th	Night	41% 7% 15% 26% 11% 1	1680
	21st to end	Night	37% 6% 16% 27% 14% 2	1680
July	1st to 10th	Night	42% 8% 13% 21% 14% 2	3560
	11th to 20th	Night	47% 7% 14% 21% 11%	3552
	21st to end	Night	35% 6% 13% 24% 20% 1	3809
August	1st to 10th	Night	50% 7% 14% 18% 9% 1	2725
	11th to 20th	Night	42% 6% 14% 28% 9%	3227
	21st to end	Night	31% 6% 11% 34% 17% 1	3586
September	1st to 10th	Night	39% 5% 11% 26% 18% 1	2000
	11th to 20th	Night	32% 5% 11% 25% 25% 2	2090
	21st to end	Night	41% 5% 9% 22% 21% 2	2200
October	1st to 10th	Night	39% 5% 11% 22% 21% 2	3773
	11th to 20th	Night	27% 3 9% 30% 27% 4	3899
	21st to end	Night	34% 4 12% 29% 19% 2	4540
November	1st to 10th	Night	35% 4 12% 24% 21% 3	2667
	11th to 20th	Night	40% 3 8% 24% 21% 4	2730
	21st to end	Night	39% 4 14% 30% 13% 1	2835
December	1st to 10th	Night	40% 3 12% 32% 11% 1	4720
	11th to 20th	Night	47% 3 11% 20% 16% 3	5348
	21st to end	Night	47% 4 10% 18% 16% 4	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	31% 27% 26% 12% 5%	220
	11th to 20th	Day	38% 23% 25% 12% 1	220
	21st to end	Day	25% 27% 34% 11% 2 2	310
February	1st to 10th	Day	31% 25% 33% 9% 1	330
	11th to 20th	Day	33% 23% 31% 12% 1	536
	21st to end	Day	31% 22% 33% 11% 3	446
March	1st to 10th	Day	31% 21% 29% 16% 4	540
	11th to 20th	Day	27% 26% 29% 14% 3 1	540
	21st to end	Day	31% 23% 29% 13% 3 2	594
April	1st to 10th	Day	34% 20% 28% 11% 6% 1	540
	11th to 20th	Day	27% 24% 29% 13% 6% 1	532
	21st to end	Day	27% 19% 31% 17% 5% 1	540
May	1st to 10th	Day	33% 19% 27% 14% 6% 1	543
	11th to 20th	Day	22% 15% 36% 19% 6% 1	567
	21st to end	Day	24% 13% 26% 24% 12% 2	617
June	1st to 10th	Day	21% 12% 30% 26% 10% 2	570
	11th to 20th	Day	20% 15% 29% 24% 11% 1	570
	21st to end	Day	16% 13% 28% 28% 12% 3	570
July	1st to 10th	Day	14% 8% 29% 29% 15% 5%	525
	11th to 20th	Day	13% 8% 27% 32% 17% 3	524
	21st to end	Day	14% 7% 32% 32% 13% 2	576
August	1st to 10th	Day	14% 8% 24% 31% 16% 6%	526
	11th to 20th	Day	14% 8% 24% 33% 17% 5%	519
	21st to end	Day	13% 10% 28% 27% 15% 7%	583
September	1st to 10th	Day	18% 11% 26% 24% 15% 7%	530
	11th to 20th	Day	14% 8% 30% 24% 15% 10%	530
	21st to end	Day	22% 9% 31% 23% 10% 5%	530
October	1st to 10th	Day	25% 11% 23% 20% 15% 5%	530
	11th to 20th	Day	20% 18% 29% 21% 10% 1	530
	21st to end	Day	27% 15% 30% 20% 8% 1	486
November	1st to 10th	Day	20% 16% 29% 23% 9% 2	425
	11th to 20th	Day	27% 21% 27% 19% 5% 1	430
	21st to end	Day	26% 22% 30% 18% 3 1	322
December	1st to 10th	Day	27% 21% 33% 14% 5%	220
	11th to 20th	Day	38% 20% 31% 10% 2	220
	21st to end	Day	26% 24% 29% 16% 4	242
January	1st to 10th	Night	23% 17% 38% 19% 3	840
	11th to 20th	Night	25% 19% 30% 23% 2	840
	21st to end	Night	20% 19% 37% 21% 2	924
February	1st to 10th	Night	23% 18% 35% 22% 2	840
	11th to 20th	Night	25% 20% 37% 17% 1	823
	21st to end	Night	25% 18% 36% 20% 2	692
March	1st to 10th	Night	25% 20% 33% 19% 3	840
	11th to 20th	Night	24% 21% 33% 19% 3	840
	21st to end	Night	24% 20% 35% 17% 4 1	924
April	1st to 10th	Night	26% 19% 33% 19% 4	840
	11th to 20th	Night	24% 16% 36% 21% 4	831
	21st to end	Night	22% 15% 37% 21% 5%	791
May	1st to 10th	Night	27% 13% 33% 21% 5%	630
	11th to 20th	Night	22% 13% 33% 27% 5%	435
	21st to end	Night	21% 8% 25% 36% 9% 1	455
June	1st to 10th	Night	18% 7% 28% 36% 11%	420
	11th to 20th	Night	17% 5% 30% 35% 12%	420
	21st to end	Night	14% 5% 27% 41% 12%	420
July	1st to 10th	Night	16% 4 20% 40% 18% 1	390
	11th to 20th	Night	14% 2 24% 42% 16% 1	388
	21st to end	Night	13% 4 21% 42% 19% 1	438
August	1st to 10th	Night	14% 2 21% 43% 20% 1	482
	11th to 20th	Night	15% 5% 19% 36% 23% 2	790
	21st to end	Night	16% 6% 19% 37% 20% 2	880
September	1st to 10th	Night	18% 5% 25% 36% 15% 2	800
	11th to 20th	Night	17% 5% 23% 33% 19% 4	800
	21st to end	Night	20% 7% 24% 32% 16% 2	800
October	1st to 10th	Night	22% 9% 26% 26% 16% 2	800
	11th to 20th	Night	20% 12% 30% 27% 10%	800
	21st to end	Night	25% 13% 32% 22% 8%	880
November	1st to 10th	Night	17% 12% 35% 25% 10% 1	820
	11th to 20th	Night	23% 17% 31% 23% 6%	840
	21st to end	Night	21% 18% 29% 25% 6%	840
December	1st to 10th	Night	26% 17% 32% 19% 5%	840
	11th to 20th	Night	27% 16% 32% 23% 3	840
	21st to end	Night	26% 17% 35% 20% 2	924

Page generated at 2026-06-28 02:10:44. Version ac07

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