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 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	12% 6% 2 6% 4 4 4 4 6% 8% 43%	101005
	Night	13% 6% 2 5% 4 3 4 3 5% 7% 48%	98103
Transparency	Day	36% 7% 12% 20% 17% 8%	104774
	Night	33% 6% 12% 20% 18% 10%	102461
Seeing	Day	12% 7% 19% 27% 25% 9%	14820
	Night	14% 7% 25% 35% 17% 1	24698

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	16% 9% 2 6% 4 3 4 3 4 5% 45%	2847
	11th to 20th	Day	17% 6% 2 5% 3 2 2 2 4 5% 53%	2850
	21st to end	Day	23% 8% 3 5% 3 3 2 2 5% 7% 38%	3207
February	1st to 10th	Day	21% 8% 3 6% 3 3 3 3 3 5% 42%	1900
	11th to 20th	Day	21% 8% 2 5% 4 2 3 3 4 5% 43%	1849
	21st to end	Day	18% 8% 2 5% 4 3 4 3 4 5% 44%	1620
March	1st to 10th	Day	15% 8% 3 8% 5% 5% 4 3 5% 8% 36%	3436
	11th to 20th	Day	20% 9% 3 8% 5% 6% 5% 4 4 6% 31%	3480
	21st to end	Day	17% 7% 2 6% 4 3 4 3 5% 6% 43%	3814
April	1st to 10th	Day	14% 8% 3 6% 5% 4 4 4 6% 6% 39%	2090
	11th to 20th	Day	15% 8% 3 8% 4 4 3 4 5% 5% 40%	2097
	21st to end	Day	13% 7% 3 5% 5% 4 5% 3 5% 6% 45%	2329
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 31%	4082
	11th to 20th	Day	14% 7% 2 7% 5% 5% 4 4 6% 7% 40%	4082
	21st to end	Day	9% 6% 2 6% 5% 6% 6% 6% 8% 10% 37%	4477
June	1st to 10th	Day	6% 3 2 6% 4 6% 6% 5% 7% 9% 47%	2470
	11th to 20th	Day	6% 3 2 5% 5% 5% 5% 5% 7% 11% 46%	2470
	21st to end	Day	5% 6% 2 7% 7% 8% 7% 6% 9% 9% 35%	2470
July	1st to 10th	Day	8% 4 2 6% 4 5% 6% 4 7% 10% 44%	4120
	11th to 20th	Day	6% 5% 3 6% 5% 6% 6% 5% 8% 13% 38%	4120
	21st to end	Day	10% 7% 3 7% 6% 6% 6% 5% 7% 11% 32%	4465
August	1st to 10th	Day	1 1 1 3 3 4 4 4 6% 10% 64%	4104
	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% 6% 2 5% 4 3 4 3 4 7% 47%	1888
	11th to 20th	Day	11% 6% 2 6% 3 3 4 3 5% 6% 51%	1881
	21st to end	Day	17% 8% 3 6% 3 4 4 3 5% 5% 42%	1710
December	1st to 10th	Day	18% 7% 3 6% 4 4 3 3 4 6% 43%	1710
	11th to 20th	Day	17% 7% 3 6% 4 4 4 3 5% 4 43%	2161
	21st to end	Day	16% 7% 3 6% 3 3 3 2 4 5% 49%	3135
January	1st to 10th	Night	20% 8% 2 4 3 3 3 2 3 5% 47%	4108
	11th to 20th	Night	15% 6% 2 4 2 2 2 2 2 4 60%	4120
	21st to end	Night	23% 7% 3 6% 4 3 2 2 4 6% 41%	4488
February	1st to 10th	Night	19% 9% 2 6% 3 3 3 2 4 5% 44%	2280
	11th to 20th	Night	20% 7% 2 5% 3 3 2 2 3 6% 46%	2218
	21st to end	Night	21% 8% 2 5% 4 3 3 3 4 5% 42%	1832
March	1st to 10th	Night	21% 11% 3 6% 4 4 4 3 5% 6% 34%	3419
	11th to 20th	Night	26% 9% 3 7% 4 4 4 3 5% 6% 30%	3480
	21st to end	Night	16% 7% 3 6% 4 4 4 3 4 5% 44%	3817
April	1st to 10th	Night	11% 8% 2 6% 4 3 3 4 6% 8% 44%	2090
	11th to 20th	Night	16% 6% 2 6% 4 3 4 3 5% 7% 43%	2031
	21st to end	Night	14% 8% 2 4 4 3 4 3 4 6% 46%	1845
May	1st to 10th	Night	24% 8% 2 7% 4 4 4 3 5% 7% 33%	2823
	11th to 20th	Night	13% 6% 2 6% 3 4 4 3 5% 7% 46%	2823
	21st to end	Night	10% 7% 2 6% 4 4 4 5% 7% 10% 41%	3090
June	1st to 10th	Night	4 3 1 5% 4 3 4 3 5% 10% 58%	1710
	11th to 20th	Night	4 3 1 3 4 4 4 4 7% 10% 56%	1710
	21st to end	Night	4 5% 2 5% 4 5% 6% 4 9% 10% 46%	1710
July	1st to 10th	Night	5% 4 1 4 3 4 4 4 6% 10% 54%	2850
	11th to 20th	Night	4 4 2 5% 4 3 5% 3 7% 10% 52%	2850
	21st to end	Night	8% 7% 2 7% 4 4 5% 5% 7% 11% 41%	3098
August	1st to 10th	Night	1 1 2 2 2 3 3 5% 9% 72%	2836
	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	11% 5% 2 5% 4 3 3 3 5% 6% 52%	2263
	11th to 20th	Night	10% 7% 2 6% 4 4 3 3 5% 6% 52%	2294
	21st to end	Night	16% 7% 3 5% 4 3 3 3 4 7% 46%	2470
December	1st to 10th	Night	17% 7% 2 4 4 3 3 3 5% 5% 46%	2470
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 6% 42%	3036
	21st to end	Night	16% 7% 2 5% 3 3 3 2 4 5% 51%	4532

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	41% 5% 5% 15% 22% 11%	2848
	11th to 20th	Day	34% 3 6% 18% 26% 12%	2850
	21st to end	Day	46% 4 4 11% 24% 11%	3207
February	1st to 10th	Day	46% 4 6% 12% 19% 13%	1900
	11th to 20th	Day	45% 4 7% 15% 17% 11%	1859
	21st to end	Day	43% 5% 9% 15% 16% 13%	1620
March	1st to 10th	Day	46% 6% 9% 16% 16% 7%	3436
	11th to 20th	Day	53% 8% 7% 9% 16% 7%	3480
	21st to end	Day	41% 6% 8% 17% 21% 7%	3814
April	1st to 10th	Day	44% 6% 8% 16% 15% 12%	2090
	11th to 20th	Day	46% 5% 8% 14% 18% 10%	2097
	21st to end	Day	40% 6% 8% 19% 19% 8%	2329
May	1st to 10th	Day	49% 9% 11% 16% 11% 5%	4120
	11th to 20th	Day	41% 8% 9% 17% 20% 5%	4086
	21st to end	Day	36% 11% 14% 28% 11% 1	4489
June	1st to 10th	Day	29% 9% 16% 24% 19% 3	2470
	11th to 20th	Day	30% 8% 15% 24% 19% 4	2470
	21st to end	Day	40% 12% 13% 22% 11% 1	2470
July	1st to 10th	Day	32% 8% 23% 31% 6% 1	4056
	11th to 20th	Day	35% 9% 19% 25% 12% 1	4051
	21st to end	Day	42% 11% 18% 23% 5% 1	4325
August	1st to 10th	Day	16% 6% 13% 43% 22% 1	4090
	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% 10%	1888
	11th to 20th	Day	34% 5% 11% 16% 21% 14%	1881
	21st to end	Day	42% 6% 8% 14% 18% 12%	1710
December	1st to 10th	Day	43% 5% 9% 17% 19% 7%	1710
	11th to 20th	Day	42% 6% 7% 10% 23% 11%	2159
	21st to end	Day	39% 5% 5% 15% 24% 12%	3135
January	1st to 10th	Night	39% 4 7% 15% 19% 16%	4108
	11th to 20th	Night	31% 3 5% 17% 29% 15%	4120
	21st to end	Night	46% 4 4 12% 23% 11%	4488
February	1st to 10th	Night	44% 4 6% 12% 18% 16%	2280
	11th to 20th	Night	41% 4 7% 16% 18% 14%	2218
	21st to end	Night	46% 3 6% 15% 16% 14%	1832
March	1st to 10th	Night	49% 7% 9% 13% 14% 7%	3419
	11th to 20th	Night	54% 7% 9% 12% 11% 7%	3480
	21st to end	Night	41% 5% 9% 14% 21% 10%	3817
April	1st to 10th	Night	37% 7% 11% 17% 17% 11%	2090
	11th to 20th	Night	41% 5% 10% 14% 15% 15%	2031
	21st to end	Night	38% 7% 9% 21% 17% 8%	1845
May	1st to 10th	Night	50% 8% 10% 18% 9% 5%	2850
	11th to 20th	Night	36% 8% 12% 20% 17% 8%	2824
	21st to end	Night	35% 10% 16% 29% 9% 1	3099
June	1st to 10th	Night	22% 7% 19% 29% 20% 2	1710
	11th to 20th	Night	22% 7% 15% 33% 19% 4	1710
	21st to end	Night	28% 12% 17% 28% 14% 2	1710
July	1st to 10th	Night	25% 8% 27% 35% 5%	2805
	11th to 20th	Night	27% 8% 22% 30% 12% 1	2796
	21st to end	Night	37% 9% 23% 25% 6%	2999
August	1st to 10th	Night	9% 6% 17% 47% 21% 1	2826
	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	32% 6% 11% 19% 20% 13%	2263
	11th to 20th	Night	34% 5% 10% 18% 19% 14%	2294
	21st to end	Night	39% 4 9% 17% 17% 13%	2470
December	1st to 10th	Night	40% 5% 8% 21% 16% 10%	2470
	11th to 20th	Night	42% 6% 6% 11% 19% 16%	3037
	21st to end	Night	37% 4 6% 14% 27% 12%	4532

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% 9% 36% 24% 7% 1	270
	11th to 20th	Day	24% 12% 29% 27% 7% 1	270
	21st to end	Day	22% 13% 33% 23% 8% 1	297
February	1st to 10th	Day	30% 14% 29% 18% 9%	270
	11th to 20th	Day	26% 18% 29% 19% 7%	270
	21st to end	Day	27% 16% 37% 17% 3	225
March	1st to 10th	Day	19% 16% 39% 19% 7%	270
	11th to 20th	Day	18% 17% 38% 19% 7% 1	270
	21st to end	Day	18% 16% 35% 25% 7%	297
April	1st to 10th	Day	23% 16% 35% 21% 4 1	270
	11th to 20th	Day	19% 15% 33% 23% 9% 1	295
	21st to end	Day	18% 11% 34% 27% 10% 1	509
May	1st to 10th	Day	15% 10% 37% 28% 9%	653
	11th to 20th	Day	14% 8% 26% 33% 17% 2	675
	21st to end	Day	12% 5% 26% 35% 18% 3	734
June	1st to 10th	Day	6% 2 13% 37% 33% 9%	680
	11th to 20th	Day	6% 2 13% 42% 30% 8%	680
	21st to end	Day	7% 2 9% 33% 35% 14%	680
July	1st to 10th	Day	6% 3 7% 29% 38% 17%	671
	11th to 20th	Day	4 1 3 26% 49% 16%	668
	21st to end	Day	4 2 5% 26% 46% 18%	739
August	1st to 10th	Day	2 3 24% 49% 21%	676
	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% 5% 22% 33% 16% 7%	279
	11th to 20th	Day	18% 7% 21% 32% 18% 4	270
	21st to end	Day	22% 15% 30% 19% 11% 3	270
December	1st to 10th	Day	19% 9% 30% 26% 12% 5%	270
	11th to 20th	Day	20% 15% 29% 25% 9% 1	270
	21st to end	Day	20% 9% 39% 23% 7% 2	297
January	1st to 10th	Night	22% 10% 30% 30% 9%	950
	11th to 20th	Night	23% 12% 28% 29% 8%	950
	21st to end	Night	22% 10% 30% 32% 6%	1014
February	1st to 10th	Night	24% 14% 30% 28% 4	760
	11th to 20th	Night	26% 9% 35% 26% 5%	744
	21st to end	Night	27% 14% 39% 18% 3	576
March	1st to 10th	Night	23% 11% 36% 27% 4	570
	11th to 20th	Night	22% 11% 34% 29% 5%	570
	21st to end	Night	16% 10% 43% 27% 4	627
April	1st to 10th	Night	13% 12% 42% 31% 2	570
	11th to 20th	Night	18% 10% 40% 25% 6%	563
	21st to end	Night	17% 11% 38% 26% 7% 1	570
May	1st to 10th	Night	14% 10% 42% 29% 4	570
	11th to 20th	Night	12% 5% 34% 37% 11% 1	561
	21st to end	Night	8% 5% 31% 40% 15%	617
June	1st to 10th	Night	4 2 17% 52% 24% 1	570
	11th to 20th	Night	4 2 25% 52% 16% 2	570
	21st to end	Night	5% 2 13% 45% 31% 4	570
July	1st to 10th	Night	5% 1 9% 48% 32% 4	555
	11th to 20th	Night	3 1 8% 47% 38% 2	552
	21st to end	Night	4 8% 50% 34% 4	624
August	1st to 10th	Night	1 5% 54% 36% 3	567
	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	755
	11th to 20th	Night	13% 7% 25% 38% 16% 1	774
	21st to end	Night	19% 10% 27% 29% 14% 1	950
December	1st to 10th	Night	19% 9% 28% 30% 13% 1	950
	11th to 20th	Night	24% 12% 26% 30% 7% 1	950
	21st to end	Night	20% 10% 30% 33% 8%	1045

Page generated at 2024-11-01 21:47:49. Version ac07

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