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 2025-03-27.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 38%	109097
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	110435
Transparency	Day	45% 5% 11% 24% 14% 1	112611
	Night	46% 4 11% 23% 15% 2	115035
Seeing	Day	17% 10% 29% 25% 14% 5%	16320
	Night	18% 9% 31% 30% 11% 1	26374

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	8% 10% 8% 17% 7% 7% 5% 3 5% 5% 27%	3617
	11th to 20th	Day	8% 10% 7% 11% 5% 4 4 7% 6% 7% 31%	3620
	21st to end	Day	12% 16% 7% 9% 3 3 3 2 4 6% 35%	4258
February	1st to 10th	Day	12% 13% 6% 10% 5% 4 4 4 5% 6% 31%	1800
	11th to 20th	Day	13% 12% 4 7% 5% 4 5% 3 5% 7% 35%	1867
	21st to end	Day	12% 14% 4 10% 6% 4 3 3 5% 6% 33%	1650
March	1st to 10th	Day	14% 8% 3 6% 4 3 4 3 4 7% 44%	3220
	11th to 20th	Day	11% 7% 3 8% 4 3 4 3 6% 7% 44%	3513
	21st to end	Day	22% 13% 3 9% 5% 4 5% 3 5% 5% 26%	4161
April	1st to 10th	Day	19% 11% 4 7% 4 5% 4 4 5% 5% 32%	2280
	11th to 20th	Day	18% 11% 3 7% 5% 4 5% 3 5% 5% 35%	2254
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 5% 6% 33%	2316
May	1st to 10th	Day	16% 8% 2 7% 5% 5% 4 4 6% 7% 37%	4241
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 5% 7% 42%	4399
	21st to end	Day	13% 7% 2 6% 4 5% 5% 4 7% 8% 38%	4824
June	1st to 10th	Day	11% 4 2 6% 4 5% 4 4 7% 9% 44%	2660
	11th to 20th	Day	12% 5% 2 5% 5% 4 5% 4 7% 9% 42%	2660
	21st to end	Day	12% 6% 3 7% 5% 5% 5% 5% 7% 7% 39%	2660
July	1st to 10th	Day	10% 8% 3 8% 5% 5% 6% 5% 7% 9% 34%	4440
	11th to 20th	Day	5% 4 2 5% 4 5% 5% 4 6% 9% 51%	4440
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	4813
August	1st to 10th	Day	9% 5% 2 7% 5% 6% 5% 5% 7% 9% 40%	4298
	11th to 20th	Day	11% 4 2 5% 4 4 5% 4 6% 7% 47%	3829
	21st to end	Day	9% 6% 2 6% 4 3 4 4 6% 9% 46%	4180
September	1st to 10th	Day	12% 6% 2 6% 4 4 4 3 6% 7% 45%	2280
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 5% 8% 51%	2233
	21st to end	Day	12% 7% 3 7% 4 4 4 3 5% 7% 45%	2090
October	1st to 10th	Day	13% 7% 2 6% 4 3 4 2 3 6% 49%	1917
	11th to 20th	Day	19% 8% 3 7% 4 3 3 3 5% 6% 38%	1900
	21st to end	Day	18% 10% 2 6% 4 4 4 3 4 6% 40%	1937
November	1st to 10th	Day	12% 8% 2 8% 4 4 5% 3 5% 6% 44%	1762
	11th to 20th	Day	14% 13% 4 8% 5% 4 4 4 4 6% 33%	1778
	21st to end	Day	14% 13% 4 9% 4 3 3 3 4 6% 36%	1600
December	1st to 10th	Day	8% 10% 5% 7% 5% 4 5% 4 6% 7% 40%	2610
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 5% 3 6% 6% 24%	3008
	21st to end	Day	19% 23% 8% 12% 5% 4 4 4 5% 5% 13%	3982
January	1st to 10th	Night	9% 11% 6% 15% 6% 4 3 3 4 5% 32%	6348
	11th to 20th	Night	10% 13% 6% 13% 6% 5% 4 5% 7% 6% 24%	6360
	21st to end	Night	13% 15% 6% 10% 4 4 4 3 4 5% 31%	6705
February	1st to 10th	Night	11% 14% 5% 9% 6% 5% 4 3 5% 5% 33%	2600
	11th to 20th	Night	11% 11% 3 8% 5% 4 5% 3 5% 5% 39%	2418
	21st to end	Night	16% 12% 4 8% 4 4 3 4 5% 6% 36%	1980
March	1st to 10th	Night	17% 10% 3 6% 4 4 3 3 5% 4 41%	3853
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 3 6% 8% 47%	3699
	21st to end	Night	24% 12% 4 11% 4 3 4 3 4 5% 26%	3608
April	1st to 10th	Night	17% 10% 3 6% 4 3 4 3 5% 6% 38%	1900
	11th to 20th	Night	19% 10% 3 5% 5% 3 4 3 5% 6% 37%	1875
	21st to end	Night	17% 8% 3 7% 4 5% 4 4 5% 7% 38%	1862
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 5% 8% 36%	2643
	11th to 20th	Night	12% 7% 2 6% 6% 4 4 4 5% 6% 42%	2506
	21st to end	Night	10% 7% 2 6% 3 4 4 4 6% 8% 46%	2743
June	1st to 10th	Night	9% 4 2 5% 4 4 4 4 8% 9% 46%	1520
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 5% 10% 45%	1520
	21st to end	Night	9% 6% 3 6% 3 4 4 3 5% 8% 49%	1520
July	1st to 10th	Night	14% 7% 2 6% 4 4 4 4 6% 8% 41%	2530
	11th to 20th	Night	4 4 1 5% 4 4 5% 4 5% 9% 55%	2530
	21st to end	Night	6% 5% 2 7% 4 4 5% 4 6% 9% 48%	2750
August	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 6% 7% 51%	2639
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 7% 51%	3001
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 7% 50%	3476
September	1st to 10th	Night	8% 5% 1 5% 3 4 4 2 5% 7% 56%	1900
	11th to 20th	Night	10% 5% 2 4 4 4 4 4 4 8% 52%	1928
	21st to end	Night	10% 6% 2 5% 3 4 4 3 5% 7% 51%	2090
October	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 51%	2260
	11th to 20th	Night	16% 7% 2 6% 4 3 3 3 4 6% 45%	2280
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 7% 37%	2660
November	1st to 10th	Night	12% 8% 3 5% 4 4 3 3 4 6% 47%	2536
	11th to 20th	Night	14% 11% 3 9% 4 5% 4 3 5% 5% 36%	2620
	21st to end	Night	17% 11% 4 7% 4 3 3 3 4 6% 38%	2800
December	1st to 10th	Night	8% 8% 3 7% 5% 5% 5% 4 5% 6% 42%	4580
	11th to 20th	Night	16% 17% 5% 9% 5% 4 4 3 4 4 28%	5199
	21st to end	Night	20% 19% 6% 12% 5% 5% 4 3 4 4 17%	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	61% 3 9% 19% 7%	3618
	11th to 20th	Day	51% 4 13% 12% 20% 2	3620
	21st to end	Day	52% 3 13% 20% 12%	4258
February	1st to 10th	Day	54% 5% 11% 18% 12% 1	1800
	11th to 20th	Day	49% 4 11% 24% 9% 2	1876
	21st to end	Day	53% 4 9% 21% 11% 2	1650
March	1st to 10th	Day	42% 5% 10% 28% 11% 3	3220
	11th to 20th	Day	39% 5% 11% 31% 13% 1	3513
	21st to end	Day	60% 5% 11% 15% 7% 1	4161
April	1st to 10th	Day	54% 5% 9% 20% 11%	2280
	11th to 20th	Day	51% 5% 9% 21% 12% 2	2254
	21st to end	Day	52% 5% 10% 18% 13% 2	2316
May	1st to 10th	Day	46% 5% 11% 21% 16% 1	4279
	11th to 20th	Day	41% 6% 9% 31% 12%	4403
	21st to end	Day	41% 7% 11% 23% 15% 3	4837
June	1st to 10th	Day	36% 7% 13% 26% 16% 1	2660
	11th to 20th	Day	37% 8% 13% 23% 18% 2	2660
	21st to end	Day	42% 7% 12% 23% 13% 2	2660
July	1st to 10th	Day	46% 4 14% 26% 9% 1	4371
	11th to 20th	Day	29% 5% 15% 32% 16% 1	4365
	21st to end	Day	41% 5% 14% 21% 17% 2	4662
August	1st to 10th	Day	40% 5% 14% 27% 13% 1	4283
	11th to 20th	Day	36% 4 13% 31% 14% 1	3867
	21st to end	Day	35% 4 13% 31% 16% 2	4179
September	1st to 10th	Day	38% 7% 10% 26% 19% 2	2280
	11th to 20th	Day	32% 4 12% 33% 17% 2	2233
	21st to end	Day	39% 4 9% 29% 17% 1	2090
October	1st to 10th	Day	24% 3 5% 39% 27% 1	3177
	11th to 20th	Day	35% 3 8% 27% 25% 2	3140
	21st to end	Day	43% 2 9% 28% 14% 3	3162
November	1st to 10th	Day	43% 4 13% 25% 13% 2	1762
	11th to 20th	Day	52% 4 12% 20% 10% 1	1778
	21st to end	Day	51% 4 15% 22% 7% 1	1600
December	1st to 10th	Day	44% 3 19% 27% 7% 1	2610
	11th to 20th	Day	61% 3 10% 17% 7% 2	3005
	21st to end	Day	75% 3 8% 9% 6%	3982
January	1st to 10th	Night	55% 3 9% 21% 10% 1	6348
	11th to 20th	Night	59% 3 9% 11% 16% 1	6360
	21st to end	Night	56% 3 11% 18% 11%	6705
February	1st to 10th	Night	53% 4 11% 18% 12% 2	2600
	11th to 20th	Night	47% 4 11% 23% 14% 1	2419
	21st to end	Night	50% 4 12% 20% 11% 3	1980
March	1st to 10th	Night	47% 3 10% 24% 10% 5%	3853
	11th to 20th	Night	35% 4 11% 31% 17% 2	3699
	21st to end	Night	62% 4 6% 17% 10% 1	3608
April	1st to 10th	Night	46% 5% 11% 22% 14% 2	1900
	11th to 20th	Night	49% 4 6% 19% 19% 3	1875
	21st to end	Night	45% 6% 11% 22% 12% 2	1862
May	1st to 10th	Night	46% 6% 11% 16% 19% 2	2670
	11th to 20th	Night	41% 7% 11% 23% 17% 1	2507
	21st to end	Night	34% 8% 13% 22% 20% 3	2751
June	1st to 10th	Night	33% 7% 14% 26% 20% 1	1520
	11th to 20th	Night	34% 6% 10% 25% 23% 3	1520
	21st to end	Night	35% 6% 15% 23% 18% 3	1520
July	1st to 10th	Night	43% 3 14% 24% 13% 3	2490
	11th to 20th	Night	28% 3 13% 35% 18% 2	2482
	21st to end	Night	31% 6% 17% 21% 20% 5%	2662
August	1st to 10th	Night	32% 4 13% 30% 20% 1	2630
	11th to 20th	Night	34% 3 12% 33% 18% 1	3029
	21st to end	Night	34% 4 11% 28% 18% 4	3476
September	1st to 10th	Night	29% 5% 12% 27% 25% 2	1900
	11th to 20th	Night	32% 5% 11% 31% 19% 2	1928
	21st to end	Night	33% 4 10% 31% 20% 1	2090
October	1st to 10th	Night	25% 3 6% 37% 29% 2	3767
	11th to 20th	Night	33% 3 9% 28% 25% 3	3779
	21st to end	Night	44% 3 8% 27% 15% 2	4372
November	1st to 10th	Night	40% 4 10% 27% 18% 2	2536
	11th to 20th	Night	50% 4 12% 20% 12% 2	2620
	21st to end	Night	49% 4 12% 23% 11% 1	2800
December	1st to 10th	Night	43% 3 15% 29% 10% 1	4580
	11th to 20th	Night	60% 3 11% 16% 9% 1	5201
	21st to end	Night	72% 3 7% 13% 5%	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	17% 15% 30% 28% 8% 2	300
	11th to 20th	Day	18% 17% 39% 20% 5% 1	300
	21st to end	Day	21% 16% 36% 22% 5%	330
February	1st to 10th	Day	16% 23% 38% 18% 5% 1	300
	11th to 20th	Day	23% 19% 35% 20% 4	300
	21st to end	Day	24% 20% 40% 16% 1	249
March	1st to 10th	Day	18% 17% 44% 16% 4	300
	11th to 20th	Day	21% 22% 36% 14% 5% 2	300
	21st to end	Day	27% 15% 33% 18% 6% 1	480
April	1st to 10th	Day	26% 15% 32% 21% 5%	460
	11th to 20th	Day	28% 13% 26% 19% 11% 2	452
	21st to end	Day	22% 11% 31% 24% 10% 2	460
May	1st to 10th	Day	21% 10% 32% 26% 10% 2	557
	11th to 20th	Day	20% 10% 29% 24% 12% 4	675
	21st to end	Day	17% 9% 25% 25% 16% 7%	734
June	1st to 10th	Day	10% 7% 29% 28% 20% 6%	680
	11th to 20th	Day	13% 6% 30% 31% 15% 5%	680
	21st to end	Day	15% 7% 28% 29% 16% 5%	680
July	1st to 10th	Day	10% 3 19% 29% 27% 13%	671
	11th to 20th	Day	10% 5% 25% 28% 23% 9%	668
	21st to end	Day	9% 3 26% 32% 22% 8%	739
August	1st to 10th	Day	8% 3 19% 36% 25% 9%	676
	11th to 20th	Day	13% 5% 19% 28% 23% 13%	543
	21st to end	Day	12% 5% 19% 30% 25% 9%	539
September	1st to 10th	Day	13% 6% 22% 30% 20% 8%	490
	11th to 20th	Day	11% 3 21% 29% 26% 9%	490
	21st to end	Day	15% 10% 26% 26% 18% 5%	490
October	1st to 10th	Day	17% 9% 28% 28% 15% 3	317
	11th to 20th	Day	26% 14% 33% 18% 9%	300
	21st to end	Day	28% 15% 29% 22% 5% 2	330
November	1st to 10th	Day	18% 13% 40% 19% 8% 2	300
	11th to 20th	Day	20% 16% 39% 17% 8%	300
	21st to end	Day	23% 13% 40% 21% 4	300
December	1st to 10th	Day	17% 17% 41% 17% 7% 1	300
	11th to 20th	Day	20% 13% 39% 20% 8%	300
	21st to end	Day	27% 15% 35% 18% 4	330
January	1st to 10th	Night	17% 15% 37% 24% 7% 1	1000
	11th to 20th	Night	19% 14% 37% 25% 4 1	1000
	21st to end	Night	21% 11% 36% 26% 5%	1100
February	1st to 10th	Night	19% 12% 39% 24% 6%	1000
	11th to 20th	Night	18% 16% 37% 25% 4	863
	21st to end	Night	23% 12% 41% 20% 4	660
March	1st to 10th	Night	23% 13% 35% 23% 5%	800
	11th to 20th	Night	17% 11% 34% 29% 8%	670
	21st to end	Night	25% 11% 32% 26% 6% 1	647
April	1st to 10th	Night	21% 14% 36% 25% 4	570
	11th to 20th	Night	25% 12% 28% 28% 7%	563
	21st to end	Night	20% 10% 32% 28% 10% 1	570
May	1st to 10th	Night	18% 8% 35% 29% 9% 1	570
	11th to 20th	Night	19% 6% 31% 34% 11% 1	561
	21st to end	Night	14% 4 29% 36% 17% 1	617
June	1st to 10th	Night	10% 5% 25% 45% 14% 1	570
	11th to 20th	Night	14% 4 28% 39% 14% 1	570
	21st to end	Night	13% 3 26% 40% 17% 1	570
July	1st to 10th	Night	10% 3 12% 40% 32% 3	555
	11th to 20th	Night	8% 3 21% 42% 24% 3	552
	21st to end	Night	8% 1 19% 45% 25% 2	624
August	1st to 10th	Night	7% 2 16% 44% 28% 3	567
	11th to 20th	Night	12% 2 14% 38% 29% 6%	564
	21st to end	Night	10% 2 15% 44% 26% 3	627
September	1st to 10th	Night	10% 2 21% 39% 23% 5%	570
	11th to 20th	Night	12% 3 21% 35% 26% 4	598
	21st to end	Night	13% 8% 24% 35% 19% 1	760
October	1st to 10th	Night	16% 5% 26% 33% 18% 1	760
	11th to 20th	Night	20% 11% 29% 31% 9%	760
	21st to end	Night	22% 11% 33% 27% 7% 1	836
November	1st to 10th	Night	18% 9% 37% 26% 9% 1	780
	11th to 20th	Night	20% 14% 36% 23% 6% 1	820
	21st to end	Night	22% 13% 35% 23% 7%	1000
December	1st to 10th	Night	21% 12% 36% 25% 5%	1000
	11th to 20th	Night	23% 12% 35% 24% 6% 1	1000
	21st to end	Night	29% 12% 32% 24% 3	1100

Page generated at 2025-03-24 08:04:29. Version ac07

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