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 2005-11-05 to 2024-10-14.

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% 3 6% 4 4 4 4 5% 7% 38%	101133
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	97438
Transparency	Day	44% 6% 12% 23% 14% 2	100889
	Night	43% 5% 12% 23% 14% 2	97307
Seeing	Day	23% 15% 29% 21% 9% 3	14877
	Night	21% 13% 30% 26% 8% 1	23964

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	14% 7% 3 6% 4 4 3 3 5% 6% 45%	2517
	11th to 20th	Day	14% 10% 4 9% 6% 5% 5% 3 5% 4 35%	2520
	21st to end	Day	20% 12% 5% 8% 4 4 4 2 4 5% 30%	2965
February	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 4 4 7% 35%	1710
	11th to 20th	Day	14% 9% 3 7% 5% 4 4 3 5% 7% 39%	1848
	21st to end	Day	19% 9% 2 7% 4 4 4 3 4 6% 37%	1570
March	1st to 10th	Day	25% 10% 3 7% 5% 3 3 3 4 5% 33%	3116
	11th to 20th	Day	9% 6% 3 6% 6% 5% 5% 4 6% 8% 41%	3316
	21st to end	Day	20% 8% 2 6% 4 4 3 3 4 7% 39%	4152
April	1st to 10th	Day	18% 9% 3 6% 5% 4 4 3 5% 7% 37%	2280
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 7% 41%	2254
	21st to end	Day	19% 6% 2 6% 4 4 5% 4 5% 6% 38%	2323
May	1st to 10th	Day	26% 9% 2 6% 4 4 3 3 5% 8% 31%	4106
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 3 5% 6% 30%	4395
	21st to end	Day	12% 5% 2 6% 3 4 5% 4 6% 10% 43%	4837
June	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 6% 9% 36%	2660
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 5% 7% 8% 35%	2660
	21st to end	Day	12% 7% 2 7% 4 4 6% 4 7% 9% 38%	2660
July	1st to 10th	Day	9% 7% 3 8% 7% 6% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	11% 7% 2 7% 4 5% 6% 5% 7% 10% 38%	4440
	21st to end	Day	13% 8% 3 8% 5% 6% 5% 4 6% 6% 34%	4814
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 8% 32%	4245
	11th to 20th	Day	12% 7% 2 6% 4 5% 5% 4 7% 8% 40%	3730
	21st to end	Day	11% 6% 2 6% 4 4 4 3 6% 9% 46%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 5% 7% 42%	2280
	11th to 20th	Day	13% 4 2 5% 4 4 3 3 5% 8% 48%	2195
	21st to end	Day	17% 6% 2 7% 4 4 4 4 5% 6% 40%	2090
October	1st to 10th	Day	19% 8% 2 6% 3 4 4 4 4 5% 41%	1986
	11th to 20th	Day	15% 6% 3 5% 3 4 5% 3 6% 7% 44%	1830
	21st to end	Day	22% 7% 2 6% 4 3 5% 4 5% 6% 36%	1822
November	1st to 10th	Day	14% 7% 2 5% 4 3 3 2 4 6% 50%	1671
	11th to 20th	Day	17% 6% 2 5% 3 4 4 3 4 7% 43%	1710
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 41%	1613
December	1st to 10th	Day	18% 8% 4 6% 5% 5% 4 4 5% 6% 34%	1520
	11th to 20th	Day	21% 7% 2 5% 3 4 2 3 5% 7% 41%	1906
	21st to end	Day	18% 9% 5% 9% 5% 5% 4 4 5% 7% 29%	2772
January	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 7% 44%	4429
	11th to 20th	Night	14% 8% 3 9% 5% 4 4 3 5% 5% 38%	4440
	21st to end	Night	23% 11% 3 7% 4 4 3 3 4 5% 33%	4688
February	1st to 10th	Night	16% 10% 2 6% 4 4 4 4 5% 5% 40%	2469
	11th to 20th	Night	15% 8% 3 6% 4 5% 4 4 5% 6% 39%	2218
	21st to end	Night	17% 8% 2 7% 5% 4 4 3 4 4 41%	1884
March	1st to 10th	Night	23% 11% 3 7% 3 3 3 3 4 5% 35%	3724
	11th to 20th	Night	12% 6% 3 6% 5% 4 4 4 6% 8% 42%	3639
	21st to end	Night	17% 9% 3 7% 4 4 3 3 4 6% 40%	3480
April	1st to 10th	Night	17% 8% 2 7% 5% 4 5% 5% 5% 7% 34%	1900
	11th to 20th	Night	19% 9% 2 5% 4 4 4 4 5% 6% 39%	1875
	21st to end	Night	19% 7% 3 6% 4 4 4 3 5% 6% 40%	1856
May	1st to 10th	Night	26% 8% 3 6% 5% 3 4 3 5% 8% 28%	2834
	11th to 20th	Night	24% 9% 3 7% 4 4 4 3 5% 6% 32%	2527
	21st to end	Night	14% 6% 2 5% 3 3 4 4 7% 8% 43%	2738
June	1st to 10th	Night	17% 5% 2 5% 4 4 5% 3 5% 8% 41%	1520
	11th to 20th	Night	13% 6% 2 7% 6% 3 5% 4 6% 9% 41%	1520
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 7% 9% 43%	1520
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 8% 9% 39%	2520
	11th to 20th	Night	12% 6% 3 6% 5% 5% 6% 4 7% 9% 37%	2520
	21st to end	Night	15% 9% 3 6% 4 4 4 4 6% 7% 38%	2739
August	1st to 10th	Night	20% 7% 3 8% 4 4 5% 4 6% 7% 32%	2650
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 4 5% 6% 41%	3097
	21st to end	Night	11% 5% 2 4 4 3 4 3 5% 7% 52%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 5% 46%	1900
	11th to 20th	Night	12% 5% 2 4 4 3 2 3 5% 6% 53%	1985
	21st to end	Night	17% 6% 2 5% 4 3 4 3 4 6% 45%	2090
October	1st to 10th	Night	19% 8% 2 6% 4 3 3 3 4 6% 42%	2190
	11th to 20th	Night	16% 6% 2 6% 3 4 4 2 5% 7% 47%	2203
	21st to end	Night	22% 7% 3 6% 4 3 4 3 5% 6% 38%	2533
November	1st to 10th	Night	13% 6% 2 5% 5% 3 4 3 5% 6% 47%	2407
	11th to 20th	Night	18% 7% 2 5% 4 4 3 3 5% 6% 45%	2470
	21st to end	Night	14% 6% 2 6% 4 4 3 4 6% 7% 44%	2565
December	1st to 10th	Night	20% 8% 2 6% 3 4 4 3 5% 7% 38%	2660
	11th to 20th	Night	19% 8% 2 5% 4 4 4 4 6% 6% 37%	3288
	21st to end	Night	21% 10% 3 6% 4 4 4 3 5% 6% 35%	4884

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	38% 6% 9% 34% 10% 2	2518
	11th to 20th	Day	50% 5% 10% 23% 10% 1	2520
	21st to end	Day	56% 4 9% 21% 10% 1	2965
February	1st to 10th	Day	48% 5% 12% 21% 13% 2	1710
	11th to 20th	Day	44% 5% 9% 24% 16% 3	1857
	21st to end	Day	48% 4 11% 22% 13% 3	1570
March	1st to 10th	Day	52% 6% 11% 20% 10% 2	3119
	11th to 20th	Day	35% 8% 14% 31% 11% 2	3316
	21st to end	Day	45% 4 7% 17% 22% 5%	4152
April	1st to 10th	Day	46% 5% 9% 21% 14% 4	2280
	11th to 20th	Day	43% 5% 11% 21% 17% 3	2254
	21st to end	Day	45% 5% 9% 22% 16% 3	2323
May	1st to 10th	Day	53% 5% 13% 16% 12% 1	4120
	11th to 20th	Day	52% 6% 12% 20% 9% 1	4399
	21st to end	Day	35% 8% 13% 27% 17% 2	4837
June	1st to 10th	Day	43% 6% 13% 23% 14% 1	2660
	11th to 20th	Day	43% 7% 14% 24% 11% 1	2660
	21st to end	Day	42% 7% 12% 25% 14% 1	2660
July	1st to 10th	Day	43% 7% 11% 22% 16% 1	4370
	11th to 20th	Day	40% 7% 17% 22% 13% 1	4365
	21st to end	Day	48% 6% 11% 18% 15% 2	4661
August	1st to 10th	Day	49% 7% 12% 21% 10%	4231
	11th to 20th	Day	39% 5% 16% 29% 10%	3766
	21st to end	Day	35% 5% 11% 31% 17% 1	4180
September	1st to 10th	Day	41% 6% 10% 24% 18% 2	2280
	11th to 20th	Day	35% 5% 8% 25% 25% 2	2195
	21st to end	Day	44% 5% 9% 23% 16% 3	2090
October	1st to 10th	Day	45% 5% 8% 22% 16% 3	1986
	11th to 20th	Day	38% 5% 13% 28% 16% 1	1831
	21st to end	Day	47% 4 9% 22% 14% 3	1822
November	1st to 10th	Day	36% 4 13% 27% 18% 3	1671
	11th to 20th	Day	40% 4 10% 24% 18% 3	1710
	21st to end	Day	41% 5% 13% 29% 11% 2	1613
December	1st to 10th	Day	49% 6% 13% 19% 13% 1	1520
	11th to 20th	Day	43% 3 16% 23% 13% 2	1906
	21st to end	Day	53% 6% 10% 22% 8% 1	2772
January	1st to 10th	Night	39% 4 12% 30% 12% 3	4429
	11th to 20th	Night	45% 5% 11% 25% 11% 3	4440
	21st to end	Night	53% 4 8% 22% 11% 1	4688
February	1st to 10th	Night	45% 5% 13% 23% 12% 2	2469
	11th to 20th	Night	44% 4 10% 20% 18% 4	2220
	21st to end	Night	45% 4 7% 24% 16% 4	1884
March	1st to 10th	Night	50% 5% 11% 20% 12% 2	3734
	11th to 20th	Night	37% 6% 14% 28% 13% 2	3640
	21st to end	Night	45% 4 7% 16% 23% 5%	3480
April	1st to 10th	Night	46% 5% 11% 21% 14% 3	1900
	11th to 20th	Night	45% 6% 10% 19% 19% 2	1875
	21st to end	Night	44% 5% 8% 22% 17% 4	1856
May	1st to 10th	Night	54% 7% 11% 18% 10% 1	2840
	11th to 20th	Night	52% 6% 13% 17% 11% 1	2528
	21st to end	Night	38% 7% 14% 27% 12% 2	2739
June	1st to 10th	Night	40% 8% 10% 23% 18% 2	1520
	11th to 20th	Night	40% 7% 16% 27% 10% 1	1520
	21st to end	Night	36% 6% 16% 27% 14% 1	1520
July	1st to 10th	Night	40% 8% 15% 20% 14% 2	2480
	11th to 20th	Night	42% 6% 14% 24% 12% 1	2472
	21st to end	Night	44% 7% 14% 15% 18% 2	2651
August	1st to 10th	Night	51% 8% 15% 17% 9% 1	2640
	11th to 20th	Night	43% 6% 13% 28% 9% 1	3127
	21st to end	Night	31% 6% 12% 34% 17% 1	3476
September	1st to 10th	Night	39% 6% 11% 26% 18% 1	1900
	11th to 20th	Night	32% 4 11% 26% 25% 2	1985
	21st to end	Night	41% 5% 9% 22% 22% 1	2090
October	1st to 10th	Night	44% 5% 11% 22% 15% 2	2190
	11th to 20th	Night	38% 4 13% 30% 14% 1	2207
	21st to end	Night	47% 4 11% 23% 14% 1	2533
November	1st to 10th	Night	36% 5% 12% 23% 21% 3	2407
	11th to 20th	Night	41% 3 9% 24% 19% 4	2470
	21st to end	Night	38% 4 14% 30% 12% 1	2565
December	1st to 10th	Night	46% 5% 11% 23% 14% 1	2660
	11th to 20th	Night	45% 5% 13% 20% 15% 3	3288
	21st to end	Night	49% 5% 12% 22% 11%	4884

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% 23% 29% 13% 5%	180
	11th to 20th	Day	37% 21% 28% 13% 2	180
	21st to end	Day	22% 25% 36% 13% 3 2	253
February	1st to 10th	Day	33% 25% 32% 9% 1	270
	11th to 20th	Day	30% 24% 33% 13%	456
	21st to end	Day	31% 20% 32% 12% 3	382
March	1st to 10th	Day	30% 20% 30% 17% 4	460
	11th to 20th	Day	25% 28% 29% 15% 3 1	460
	21st to end	Day	31% 19% 31% 14% 3 2	506
April	1st to 10th	Day	32% 19% 28% 12% 7% 1	460
	11th to 20th	Day	26% 23% 29% 14% 7% 1	452
	21st to end	Day	28% 18% 31% 17% 5% 1	460
May	1st to 10th	Day	31% 19% 27% 15% 6% 1	463
	11th to 20th	Day	23% 12% 37% 20% 6% 1	487
	21st to end	Day	19% 12% 25% 28% 13% 2	529
June	1st to 10th	Day	21% 11% 31% 24% 11% 2	490
	11th to 20th	Day	16% 15% 29% 26% 12% 1	490
	21st to end	Day	16% 13% 27% 28% 12% 3	490
July	1st to 10th	Day	14% 8% 29% 28% 16% 5%	485
	11th to 20th	Day	13% 8% 27% 33% 16% 3	484
	21st to end	Day	14% 7% 31% 32% 14% 2	532
August	1st to 10th	Day	15% 8% 24% 31% 16% 5%	486
	11th to 20th	Day	14% 8% 24% 33% 16% 5%	479
	21st to end	Day	14% 10% 26% 27% 16% 7%	539
September	1st to 10th	Day	18% 11% 25% 23% 16% 7%	490
	11th to 20th	Day	14% 8% 30% 23% 14% 11%	490
	21st to end	Day	22% 9% 30% 23% 10% 6%	490
October	1st to 10th	Day	27% 11% 23% 20% 14% 4	490
	11th to 20th	Day	20% 16% 30% 22% 11% 1	462
	21st to end	Day	29% 14% 29% 19% 8%	416
November	1st to 10th	Day	20% 14% 29% 24% 11% 2	365
	11th to 20th	Day	28% 20% 28% 19% 6% 1	370
	21st to end	Day	22% 19% 33% 21% 4 1	273
December	1st to 10th	Day	26% 21% 34% 14% 5%	180
	11th to 20th	Day	38% 19% 32% 10% 2	180
	21st to end	Day	26% 25% 28% 17% 5%	198
January	1st to 10th	Night	22% 16% 39% 19% 3	760
	11th to 20th	Night	24% 19% 30% 23% 3	760
	21st to end	Night	21% 19% 36% 22% 3	836
February	1st to 10th	Night	24% 18% 35% 22% 2	760
	11th to 20th	Night	24% 21% 38% 16% 1	743
	21st to end	Night	25% 18% 34% 19% 3	628
March	1st to 10th	Night	25% 20% 34% 18% 3	760
	11th to 20th	Night	23% 21% 34% 19% 3	760
	21st to end	Night	24% 20% 33% 18% 4 1	836
April	1st to 10th	Night	24% 18% 35% 19% 4	760
	11th to 20th	Night	24% 15% 35% 22% 4	751
	21st to end	Night	22% 15% 36% 21% 5%	716
May	1st to 10th	Night	26% 14% 33% 21% 6%	570
	11th to 20th	Night	22% 12% 34% 26% 5% 1	393
	21st to end	Night	19% 8% 24% 38% 10% 1	411
June	1st to 10th	Night	19% 6% 29% 35% 11%	380
	11th to 20th	Night	15% 5% 29% 37% 13% 1	380
	21st to end	Night	14% 5% 27% 42% 12% 1	380
July	1st to 10th	Night	16% 4 21% 39% 18% 1	370
	11th to 20th	Night	14% 2 24% 42% 16% 1	368
	21st to end	Night	14% 4 20% 42% 19% 1	416
August	1st to 10th	Night	14% 2 21% 43% 19% 1	457
	11th to 20th	Night	16% 5% 18% 36% 23% 2	750
	21st to end	Night	16% 6% 18% 37% 21% 2	836
September	1st to 10th	Night	18% 5% 25% 36% 15% 2	760
	11th to 20th	Night	17% 4 24% 32% 19% 4	760
	21st to end	Night	20% 7% 24% 31% 16% 2	760
October	1st to 10th	Night	23% 9% 26% 26% 14% 2	760
	11th to 20th	Night	20% 12% 29% 29% 10%	735
	21st to end	Night	26% 13% 32% 21% 8%	792
November	1st to 10th	Night	16% 12% 35% 26% 10% 1	740
	11th to 20th	Night	23% 16% 31% 23% 7% 1	760
	21st to end	Night	20% 18% 30% 25% 7%	760
December	1st to 10th	Night	27% 17% 31% 19% 5% 1	760
	11th to 20th	Night	25% 16% 33% 23% 3	760
	21st to end	Night	25% 16% 36% 20% 2	836

Page generated at 2024-10-11 10:50:01. Version ac07

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