West Haven Pass Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West Haven Pass Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	126342
	Night	17% 11% 4 9% 5% 5% 5% 3 5% 6% 30%	122721
Transparency	Day	32% 40% 8% 13% 7% 1	129823
	Night	34% 40% 7% 11% 7% 1	127327
Seeing	Day	23% 17% 30% 20% 8% 2	15873
	Night	22% 13% 29% 27% 8% 1	24809

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% 11% 9% 16% 7% 5% 5% 3 5% 4 22%	3597
	11th to 20th	Day	13% 13% 6% 16% 6% 5% 5% 4 6% 5% 21%	3570
	21st to end	Day	13% 14% 5% 8% 6% 5% 5% 4 5% 6% 29%	3960
February	1st to 10th	Day	12% 14% 5% 10% 7% 6% 5% 4 6% 6% 25%	1799
	11th to 20th	Day	12% 13% 6% 11% 7% 5% 5% 3 5% 6% 28%	1923
	21st to end	Day	17% 13% 3 9% 7% 5% 4 3 4 6% 28%	1650
March	1st to 10th	Day	20% 13% 5% 13% 5% 5% 4 3 4 4 26%	4476
	11th to 20th	Day	14% 14% 3 7% 4 4 4 4 6% 6% 34%	4689
	21st to end	Day	20% 10% 3 9% 6% 4 4 3 4 5% 32%	5970
April	1st to 10th	Day	20% 11% 4 8% 4 3 3 3 4 6% 34%	2400
	11th to 20th	Day	17% 10% 2 7% 5% 4 5% 4 5% 5% 36%	2374
	21st to end	Day	21% 9% 4 7% 4 5% 4 3 5% 6% 31%	2539
May	1st to 10th	Day	31% 9% 3 8% 5% 4 4 3 5% 5% 22%	6019
	11th to 20th	Day	17% 9% 3 9% 5% 5% 5% 4 6% 7% 30%	6318
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 5% 8% 34%	6949
June	1st to 10th	Day	18% 8% 3 6% 6% 5% 5% 4 7% 8% 30%	2800
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 5% 5% 8% 35%	2800
	21st to end	Day	15% 8% 3 8% 5% 6% 5% 5% 6% 7% 33%	2800
July	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 5% 8% 11% 35%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 11% 36%	6360
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 41%	6926
August	1st to 10th	Day	13% 7% 2 7% 5% 5% 7% 5% 7% 8% 35%	4550
	11th to 20th	Day	13% 7% 3 7% 6% 5% 5% 5% 7% 10% 33%	3949
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 3 6% 8% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 7% 51%	2268
	21st to end	Day	14% 6% 2 7% 3 4 5% 4 7% 8% 40%	2090
October	1st to 10th	Day	20% 9% 2 8% 5% 4 5% 4 5% 6% 33%	2005
	11th to 20th	Day	16% 8% 3 9% 4 4 4 3 5% 8% 36%	1871
	21st to end	Day	23% 10% 3 7% 4 5% 4 4 5% 6% 29%	1881
November	1st to 10th	Day	13% 10% 3 8% 5% 4 5% 4 5% 7% 36%	1761
	11th to 20th	Day	16% 10% 2 8% 4 4 5% 4 7% 6% 34%	1800
	21st to end	Day	18% 10% 5% 8% 5% 5% 5% 4 6% 6% 28%	1630
December	1st to 10th	Day	15% 14% 6% 12% 8% 6% 4 3 5% 6% 20%	2600
	11th to 20th	Day	19% 13% 4 8% 4 4 4 3 5% 7% 28%	2986
	21st to end	Day	22% 15% 4 11% 5% 4 4 3 4 5% 23%	3960
January	1st to 10th	Night	14% 13% 8% 16% 6% 4 4 3 4 5% 23%	6349
	11th to 20th	Night	10% 15% 7% 17% 8% 6% 5% 4 6% 6% 18%	6315
	21st to end	Night	15% 12% 5% 11% 6% 5% 5% 4 6% 6% 24%	6996
February	1st to 10th	Night	14% 14% 4 12% 7% 7% 5% 4 5% 4 24%	2600
	11th to 20th	Night	12% 12% 4 10% 8% 6% 5% 4 6% 6% 28%	2363
	21st to end	Night	14% 14% 4 10% 7% 4 4 4 5% 5% 30%	1980
March	1st to 10th	Night	21% 12% 4 11% 5% 5% 4 4 5% 5% 27%	5364
	11th to 20th	Night	15% 13% 4 9% 5% 5% 4 3 4 7% 33%	5269
	21st to end	Night	21% 13% 4 9% 5% 4 4 3 3 4 31%	4962
April	1st to 10th	Night	19% 10% 3 8% 5% 5% 5% 3 5% 5% 31%	2000
	11th to 20th	Night	20% 10% 4 7% 5% 4 4 3 4 6% 33%	1975
	21st to end	Night	19% 9% 3 8% 5% 4 3 3 5% 5% 36%	1860
May	1st to 10th	Night	31% 12% 4 8% 5% 3 4 3 5% 5% 21%	3964
	11th to 20th	Night	17% 11% 3 11% 6% 5% 4 3 5% 6% 29%	3575
	21st to end	Night	20% 8% 3 8% 5% 4 5% 4 6% 7% 32%	3926
June	1st to 10th	Night	19% 8% 2 7% 5% 4 5% 3 6% 6% 35%	1600
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 4 5% 7% 38%	1600
	21st to end	Night	16% 6% 3 7% 6% 5% 4 4 5% 7% 36%	1600
July	1st to 10th	Night	10% 6% 3 7% 6% 6% 6% 5% 7% 9% 36%	3600
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	3600
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 6% 7% 45%	3926
August	1st to 10th	Night	17% 8% 3 8% 5% 4 4 3 4 7% 37%	2586
	11th to 20th	Night	13% 8% 2 7% 5% 4 5% 4 5% 7% 38%	3062
	21st to end	Night	9% 4 2 6% 5% 5% 6% 4 7% 9% 41%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 4 6% 52%	2088
	21st to end	Night	15% 6% 2 5% 4 4 6% 4 5% 6% 44%	2090
October	1st to 10th	Night	19% 9% 3 8% 6% 4 3 4 4 6% 32%	2171
	11th to 20th	Night	17% 8% 3 7% 5% 4 5% 3 5% 5% 37%	2308
	21st to end	Night	24% 9% 3 8% 5% 4 5% 3 5% 6% 28%	2717
November	1st to 10th	Night	14% 11% 4 8% 5% 4 4 3 5% 5% 36%	2537
	11th to 20th	Night	18% 9% 3 8% 5% 4 4 3 6% 6% 35%	2600
	21st to end	Night	17% 12% 4 8% 5% 5% 5% 4 5% 6% 30%	2768
December	1st to 10th	Night	16% 16% 5% 13% 7% 5% 5% 4 5% 6% 17%	4580
	11th to 20th	Night	24% 12% 4 9% 5% 4 4 3 4 5% 28%	5208
	21st to end	Night	22% 17% 5% 8% 6% 5% 5% 3 4 4 20%	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	48% 29% 8% 12% 3	3598
	11th to 20th	Day	47% 29% 7% 9% 5% 2	3600
	21st to end	Day	39% 31% 8% 12% 9% 2	3960
February	1st to 10th	Day	22% 63% 5% 5% 4 1	1799
	11th to 20th	Day	23% 61% 4 8% 5%	1932
	21st to end	Day	20% 63% 4 6% 6% 1	1650
March	1st to 10th	Day	47% 31% 5% 7% 7% 4	4479
	11th to 20th	Day	34% 32% 13% 13% 6% 2	4689
	21st to end	Day	41% 31% 4 9% 13% 2	5970
April	1st to 10th	Day	23% 62% 3 7% 5% 1	2400
	11th to 20th	Day	19% 63% 3 8% 7% 1	2374
	21st to end	Day	21% 63% 3 7% 6%	2539
May	1st to 10th	Day	49% 32% 6% 10% 3	6030
	11th to 20th	Day	39% 33% 12% 11% 5%	6322
	21st to end	Day	37% 32% 9% 16% 6%	6949
June	1st to 10th	Day	19% 63% 4 8% 6% 1	2800
	11th to 20th	Day	18% 62% 5% 8% 6% 1	2800
	21st to end	Day	18% 61% 4 10% 6% 1	2800
July	1st to 10th	Day	31% 29% 13% 18% 9%	6290
	11th to 20th	Day	29% 30% 12% 20% 9% 1	6285
	21st to end	Day	29% 28% 9% 27% 7%	6728
August	1st to 10th	Day	30% 39% 9% 14% 7%	4536
	11th to 20th	Day	31% 40% 10% 16% 3	3988
	21st to end	Day	23% 38% 13% 18% 8%	4312
September	1st to 10th	Day	13% 59% 5% 15% 8%	2400
	11th to 20th	Day	14% 58% 5% 13% 10% 1	2268
	21st to end	Day	16% 61% 6% 11% 5% 1	2090
October	1st to 10th	Day	28% 39% 6% 18% 9%	3330
	11th to 20th	Day	25% 38% 9% 15% 13%	3087
	21st to end	Day	20% 39% 8% 25% 7%	3081
November	1st to 10th	Day	23% 57% 6% 10% 4	1761
	11th to 20th	Day	20% 57% 4 9% 9% 1	1800
	21st to end	Day	24% 58% 8% 7% 3	1630
December	1st to 10th	Day	42% 38% 7% 10% 2	2600
	11th to 20th	Day	38% 33% 9% 12% 7%	2986
	21st to end	Day	45% 28% 5% 12% 9% 1	3960
January	1st to 10th	Night	48% 29% 10% 9% 4	6349
	11th to 20th	Night	48% 30% 7% 9% 5% 1	6360
	21st to end	Night	42% 32% 7% 7% 9% 2	6996
February	1st to 10th	Night	23% 63% 5% 6% 3	2600
	11th to 20th	Night	21% 61% 4 7% 6% 1	2365
	21st to end	Night	20% 63% 5% 8% 4 2	1980
March	1st to 10th	Night	41% 33% 7% 7% 10% 2	5374
	11th to 20th	Night	35% 32% 15% 10% 7% 1	5270
	21st to end	Night	45% 30% 4 9% 11% 1	4962
April	1st to 10th	Night	23% 62% 4 7% 5%	2000
	11th to 20th	Night	20% 63% 3 7% 6% 1	1975
	21st to end	Night	20% 62% 3 7% 7% 1	1860
May	1st to 10th	Night	49% 32% 7% 9% 3	3970
	11th to 20th	Night	42% 33% 8% 13% 5%	3576
	21st to end	Night	40% 32% 9% 11% 7% 1	3927
June	1st to 10th	Night	18% 62% 4 8% 7% 1	1600
	11th to 20th	Night	18% 63% 5% 7% 7%	1600
	21st to end	Night	18% 61% 4 12% 4 1	1600
July	1st to 10th	Night	32% 29% 14% 17% 8%	3560
	11th to 20th	Night	37% 30% 8% 16% 9%	3552
	21st to end	Night	28% 28% 10% 26% 8%	3809
August	1st to 10th	Night	35% 38% 8% 13% 6%	2576
	11th to 20th	Night	29% 39% 12% 14% 5%	3089
	21st to end	Night	22% 39% 12% 19% 8%	3586
September	1st to 10th	Night	13% 59% 6% 14% 7% 1	2000
	11th to 20th	Night	13% 58% 6% 13% 10% 1	2088
	21st to end	Night	13% 60% 6% 13% 6% 1	2090
October	1st to 10th	Night	34% 39% 7% 15% 5%	3626
	11th to 20th	Night	24% 38% 8% 17% 12%	3831
	21st to end	Night	27% 38% 7% 23% 4	4467
November	1st to 10th	Night	22% 57% 6% 9% 4 1	2537
	11th to 20th	Night	21% 57% 5% 8% 7% 1	2600
	21st to end	Night	23% 59% 7% 7% 4	2768
December	1st to 10th	Night	44% 40% 8% 7% 1	4580
	11th to 20th	Night	43% 33% 5% 8% 11%	5208
	21st to end	Night	49% 30% 5% 10% 5% 1	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	24% 29% 25% 21% 3 1	200
	11th to 20th	Day	27% 28% 29% 13% 3 1	200
	21st to end	Day	22% 29% 30% 15% 4	220
February	1st to 10th	Day	31% 30% 30% 8% 1	300
	11th to 20th	Day	29% 26% 32% 12% 1	471
	21st to end	Day	29% 21% 33% 15% 2	414
March	1st to 10th	Day	29% 17% 33% 17% 3	500
	11th to 20th	Day	28% 25% 31% 13% 3	500
	21st to end	Day	30% 23% 27% 13% 6% 1	550
April	1st to 10th	Day	36% 19% 23% 16% 5% 1	500
	11th to 20th	Day	37% 20% 24% 12% 5% 1	492
	21st to end	Day	30% 20% 29% 14% 6% 1	500
May	1st to 10th	Day	32% 19% 26% 14% 8% 1	503
	11th to 20th	Day	20% 16% 33% 24% 6% 2	527
	21st to end	Day	20% 16% 31% 22% 9% 1	573
June	1st to 10th	Day	21% 15% 33% 22% 8% 1	530
	11th to 20th	Day	20% 15% 33% 23% 8% 2	530
	21st to end	Day	19% 15% 36% 21% 8% 1	530
July	1st to 10th	Day	14% 10% 34% 27% 13% 2	525
	11th to 20th	Day	13% 9% 35% 29% 12% 2	524
	21st to end	Day	14% 9% 39% 28% 9% 1	576
August	1st to 10th	Day	13% 7% 31% 29% 17% 3	526
	11th to 20th	Day	16% 7% 28% 31% 15% 4	519
	21st to end	Day	13% 12% 28% 28% 14% 4	583
September	1st to 10th	Day	13% 12% 28% 27% 15% 5%	530
	11th to 20th	Day	13% 11% 26% 20% 19% 10%	522
	21st to end	Day	18% 12% 30% 21% 13% 6%	490
October	1st to 10th	Day	25% 14% 26% 20% 11% 4	490
	11th to 20th	Day	20% 17% 29% 22% 10% 1	474
	21st to end	Day	31% 15% 29% 20% 4	429
November	1st to 10th	Day	25% 20% 26% 20% 8%	395
	11th to 20th	Day	27% 18% 29% 20% 6% 1	400
	21st to end	Day	30% 17% 32% 17% 3	230
December	1st to 10th	Day	29% 24% 28% 19% 1 1	200
	11th to 20th	Day	30% 25% 34% 12% 1	200
	21st to end	Day	32% 15% 38% 12% 2	220
January	1st to 10th	Night	24% 18% 35% 21% 2	800
	11th to 20th	Night	26% 19% 30% 22% 3	800
	21st to end	Night	19% 16% 37% 25% 3	880
February	1st to 10th	Night	23% 17% 39% 20% 2	800
	11th to 20th	Night	20% 20% 38% 20% 2	783
	21st to end	Night	24% 17% 34% 21% 3	660
March	1st to 10th	Night	25% 16% 32% 24% 4	800
	11th to 20th	Night	23% 20% 32% 21% 4	800
	21st to end	Night	26% 18% 29% 21% 6%	880
April	1st to 10th	Night	26% 17% 30% 25% 3	800
	11th to 20th	Night	27% 19% 28% 19% 6%	791
	21st to end	Night	27% 14% 27% 26% 5%	660
May	1st to 10th	Night	32% 12% 24% 24% 7%	562
	11th to 20th	Night	19% 12% 31% 31% 7% 1	394
	21st to end	Night	20% 7% 27% 36% 10%	433
June	1st to 10th	Night	21% 7% 29% 32% 12%	400
	11th to 20th	Night	19% 7% 28% 35% 12%	400
	21st to end	Night	18% 8% 25% 39% 10% 1	400
July	1st to 10th	Night	13% 5% 20% 45% 16% 1	390
	11th to 20th	Night	15% 3 24% 40% 17% 1	388
	21st to end	Night	11% 4 22% 48% 15% 1	438
August	1st to 10th	Night	12% 4 20% 44% 19% 1	398
	11th to 20th	Night	15% 5% 20% 40% 19% 2	703
	21st to end	Night	14% 5% 21% 39% 19% 1	880
September	1st to 10th	Night	15% 6% 26% 35% 16% 2	800
	11th to 20th	Night	16% 7% 20% 30% 23% 3	793
	21st to end	Night	18% 7% 28% 28% 18% 2	760
October	1st to 10th	Night	25% 11% 20% 28% 14% 1	760
	11th to 20th	Night	21% 14% 24% 29% 10% 1	760
	21st to end	Night	28% 12% 30% 24% 6%	836
November	1st to 10th	Night	21% 14% 33% 25% 7%	780
	11th to 20th	Night	23% 15% 30% 24% 8% 1	800
	21st to end	Night	23% 16% 33% 22% 6%	800
December	1st to 10th	Night	28% 18% 30% 19% 4	800
	11th to 20th	Night	26% 15% 33% 22% 4	800
	21st to end	Night	32% 12% 33% 19% 3	880

Page generated at 2025-09-16 01:49:14. Version ac07

Copyright © 2025 Attilla Danko.