Long Island Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Long Island Lake 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	10% 9% 3 9% 6% 5% 6% 5% 6% 8% 33%	131113
	Night	8% 10% 4 10% 6% 5% 5% 5% 6% 8% 33%	126598
Transparency	Day	46% 7% 17% 22% 7%	130804
	Night	47% 7% 16% 21% 9% 1	126486
Seeing	Day	13% 7% 27% 29% 18% 5%	17330
	Night	13% 6% 28% 35% 16% 2	25617

By Month

Percentage Cloud Cover NW (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	4 14% 6% 12% 10% 5% 6% 6% 7% 8% 21%	2921
	11th to 20th	Day	3 8% 4 12% 6% 7% 6% 6% 7% 9% 30%	3159
	21st to end	Day	6% 13% 4 9% 6% 6% 6% 5% 6% 8% 32%	3700
February	1st to 10th	Day	7% 12% 4 11% 9% 6% 7% 5% 8% 8% 24%	1600
	11th to 20th	Day	5% 11% 5% 13% 7% 6% 6% 5% 8% 8% 26%	1665
	21st to end	Day	4 11% 5% 11% 8% 8% 8% 6% 7% 8% 24%	1534
March	1st to 10th	Day	5% 9% 5% 12% 8% 8% 7% 5% 6% 7% 26%	4518
	11th to 20th	Day	4 12% 5% 13% 7% 6% 5% 3 5% 8% 31%	4926
	21st to end	Day	9% 14% 5% 18% 6% 5% 5% 4 6% 7% 21%	5609
April	1st to 10th	Day	7% 13% 4 8% 6% 5% 6% 6% 6% 7% 30%	2500
	11th to 20th	Day	9% 11% 3 8% 6% 7% 6% 5% 6% 7% 32%	2570
	21st to end	Day	11% 11% 4 9% 7% 6% 6% 5% 7% 8% 28%	2616
May	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 4 5% 7% 37%	6734
	11th to 20th	Day	15% 12% 4 9% 6% 6% 5% 5% 6% 8% 26%	6734
	21st to end	Day	10% 7% 3 8% 5% 5% 6% 4 6% 9% 36%	7405
June	1st to 10th	Day	15% 8% 3 7% 6% 6% 5% 5% 7% 8% 32%	3066
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 5% 6% 8% 23%	3200
	21st to end	Day	10% 6% 2 7% 6% 6% 6% 6% 7% 10% 35%	3200
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 10% 41%	7240
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 41%	6858
	21st to end	Day	12% 5% 2 5% 4 5% 5% 4 7% 9% 42%	7376
August	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	4712
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 9% 41%	4346
	21st to end	Day	11% 5% 2 7% 4 5% 5% 4 7% 9% 41%	4664
September	1st to 10th	Day	11% 7% 2 8% 5% 5% 5% 4 6% 7% 41%	2553
	11th to 20th	Day	14% 7% 2 6% 4 5% 5% 4 6% 9% 40%	2184
	21st to end	Day	9% 6% 3 7% 5% 4 5% 4 6% 9% 42%	2090
October	1st to 10th	Day	11% 7% 3 9% 5% 5% 6% 4 6% 10% 33%	3301
	11th to 20th	Day	12% 10% 4 8% 4 5% 5% 5% 5% 7% 34%	2846
	21st to end	Day	9% 12% 5% 9% 5% 5% 5% 4 6% 8% 32%	3024
November	1st to 10th	Day	10% 15% 4 9% 8% 7% 5% 4 5% 7% 25%	1556
	11th to 20th	Day	7% 12% 6% 12% 7% 5% 5% 5% 6% 7% 28%	1400
	21st to end	Day	6% 13% 4 14% 8% 5% 6% 4 6% 8% 27%	1400
December	1st to 10th	Day	7% 15% 4 11% 9% 7% 7% 5% 6% 8% 22%	2290
	11th to 20th	Day	9% 15% 4 10% 6% 7% 7% 5% 6% 8% 22%	2624
	21st to end	Day	3 7% 4 10% 7% 7% 6% 6% 7% 8% 34%	2992
January	1st to 10th	Night	4 11% 5% 15% 8% 7% 6% 5% 7% 7% 24%	7020
	11th to 20th	Night	3 10% 4 11% 7% 6% 6% 4 6% 6% 38%	6739
	21st to end	Night	6% 13% 5% 11% 7% 5% 5% 4 6% 7% 32%	7301
February	1st to 10th	Night	6% 12% 6% 12% 8% 7% 6% 5% 6% 8% 26%	2800
	11th to 20th	Night	6% 11% 4 11% 9% 7% 7% 5% 6% 7% 28%	2620
	21st to end	Night	4 10% 5% 10% 7% 6% 6% 6% 7% 8% 30%	2095
March	1st to 10th	Night	3 8% 4 11% 8% 6% 6% 5% 7% 10% 32%	5376
	11th to 20th	Night	5% 9% 4 11% 6% 6% 6% 5% 7% 9% 31%	5073
	21st to end	Night	9% 15% 7% 15% 6% 5% 4 4 5% 5% 25%	5363
April	1st to 10th	Night	7% 10% 3 9% 7% 5% 6% 5% 6% 8% 34%	1899
	11th to 20th	Night	10% 11% 3 9% 7% 6% 6% 5% 6% 7% 30%	1781
	21st to end	Night	10% 11% 4 8% 5% 5% 5% 5% 6% 7% 34%	1781
May	1st to 10th	Night	14% 9% 3 7% 4 3 4 3 6% 8% 40%	3226
	11th to 20th	Night	14% 10% 3 10% 7% 6% 7% 4 7% 7% 26%	3159
	21st to end	Night	12% 7% 3 7% 5% 4 5% 5% 7% 9% 36%	3470
June	1st to 10th	Night	15% 7% 3 9% 5% 6% 6% 4 6% 7% 32%	1330
	11th to 20th	Night	17% 9% 3 8% 4 6% 6% 6% 8% 8% 25%	1200
	21st to end	Night	8% 7% 2 8% 6% 5% 6% 3 8% 9% 37%	1200
July	1st to 10th	Night	8% 4 2 6% 5% 5% 5% 5% 7% 9% 43%	2720
	11th to 20th	Night	8% 7% 2 7% 6% 5% 5% 5% 6% 9% 40%	3088
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 6% 8% 44%	3467
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 4 4 6% 8% 43%	2435
	11th to 20th	Night	14% 6% 3 6% 4 4 5% 3 7% 8% 40%	2704
	21st to end	Night	12% 5% 2 6% 5% 4 4 4 7% 8% 44%	3245
September	1st to 10th	Night	11% 6% 3 7% 3 4 4 5% 6% 8% 44%	1845
	11th to 20th	Night	10% 7% 2 6% 5% 3 4 4 6% 8% 46%	2172
	21st to end	Night	9% 8% 2 6% 4 4 5% 4 5% 8% 44%	2090
October	1st to 10th	Night	7% 6% 3 9% 7% 5% 6% 4 7% 10% 35%	3672
	11th to 20th	Night	9% 7% 3 8% 4 5% 5% 4 7% 10% 36%	4081
	21st to end	Night	9% 8% 4 8% 5% 5% 6% 5% 8% 10% 33%	4502
November	1st to 10th	Night	11% 14% 5% 10% 6% 5% 5% 4 6% 6% 27%	2740
	11th to 20th	Night	8% 14% 5% 12% 7% 6% 5% 4 6% 7% 26%	3000
	21st to end	Night	6% 14% 5% 13% 8% 5% 5% 4 6% 8% 26%	3000
December	1st to 10th	Night	9% 15% 6% 14% 7% 7% 5% 4 6% 7% 22%	4890
	11th to 20th	Night	9% 16% 5% 12% 7% 6% 6% 5% 7% 7% 19%	5550
	21st to end	Night	3 7% 4 9% 6% 6% 7% 5% 7% 9% 38%	7964

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	58% 6% 11% 15% 9% 1	2921
	11th to 20th	Day	48% 7% 13% 14% 17% 2	3180
	21st to end	Day	47% 7% 22% 17% 7%	3700
February	1st to 10th	Day	54% 7% 14% 15% 10%	1600
	11th to 20th	Day	53% 8% 13% 18% 8%	1673
	21st to end	Day	53% 7% 16% 17% 6%	1534
March	1st to 10th	Day	54% 6% 13% 19% 9%	4518
	11th to 20th	Day	49% 7% 15% 22% 7%	4927
	21st to end	Day	59% 7% 11% 18% 5%	5609
April	1st to 10th	Day	48% 7% 18% 19% 7%	2500
	11th to 20th	Day	49% 6% 14% 21% 10% 1	2570
	21st to end	Day	51% 7% 13% 21% 7% 1	2616
May	1st to 10th	Day	45% 5% 9% 25% 15% 1	6734
	11th to 20th	Day	53% 7% 11% 23% 6%	6738
	21st to end	Day	42% 9% 18% 22% 8% 1	7406
June	1st to 10th	Day	48% 8% 13% 21% 8% 1	3066
	11th to 20th	Day	57% 9% 13% 17% 4	3200
	21st to end	Day	41% 11% 19% 21% 7%	3200
July	1st to 10th	Day	34% 9% 22% 25% 9% 1	7161
	11th to 20th	Day	37% 8% 14% 31% 10%	6775
	21st to end	Day	37% 8% 22% 27% 6%	7169
August	1st to 10th	Day	39% 9% 20% 30% 2	4694
	11th to 20th	Day	37% 9% 22% 24% 8%	4388
	21st to end	Day	38% 7% 28% 23% 4	4663
September	1st to 10th	Day	41% 8% 19% 25% 6% 1	2553
	11th to 20th	Day	40% 8% 21% 24% 7%	2184
	21st to end	Day	38% 10% 17% 26% 9% 1	2090
October	1st to 10th	Day	46% 5% 24% 21% 4	3301
	11th to 20th	Day	48% 6% 15% 25% 5%	2846
	21st to end	Day	50% 5% 18% 24% 3	3024
November	1st to 10th	Day	56% 8% 15% 16% 6%	1556
	11th to 20th	Day	53% 7% 18% 16% 5%	1400
	21st to end	Day	54% 9% 14% 18% 5% 1	1400
December	1st to 10th	Day	58% 8% 18% 12% 5% 1	2290
	11th to 20th	Day	57% 7% 17% 14% 4	2626
	21st to end	Day	44% 7% 11% 28% 9% 1	2992
January	1st to 10th	Night	57% 6% 11% 15% 10% 1	7020
	11th to 20th	Night	46% 5% 13% 16% 19% 1	6780
	21st to end	Night	50% 6% 22% 16% 6%	7301
February	1st to 10th	Night	55% 6% 12% 16% 10% 1	2800
	11th to 20th	Night	52% 7% 14% 17% 9% 1	2622
	21st to end	Night	48% 6% 14% 21% 9% 1	2095
March	1st to 10th	Night	46% 5% 17% 23% 9%	5376
	11th to 20th	Night	45% 7% 17% 22% 8% 1	5073
	21st to end	Night	59% 7% 9% 15% 10%	5363
April	1st to 10th	Night	46% 9% 12% 21% 12% 1	1899
	11th to 20th	Night	49% 7% 9% 19% 13% 2	1781
	21st to end	Night	47% 6% 10% 24% 13% 1	1781
May	1st to 10th	Night	43% 4 11% 23% 17% 3	3225
	11th to 20th	Night	53% 7% 11% 20% 7%	3159
	21st to end	Night	41% 9% 20% 21% 8%	3470
June	1st to 10th	Night	49% 8% 13% 19% 10% 1	1330
	11th to 20th	Night	52% 10% 14% 19% 5%	1200
	21st to end	Night	40% 9% 19% 25% 6% 1	1200
July	1st to 10th	Night	35% 8% 21% 21% 14% 1	2690
	11th to 20th	Night	39% 7% 14% 31% 9%	3048
	21st to end	Night	37% 6% 23% 29% 5%	3369
August	1st to 10th	Night	39% 8% 19% 33% 2	2428
	11th to 20th	Night	41% 8% 19% 22% 10%	2728
	21st to end	Night	36% 7% 25% 26% 6%	3245
September	1st to 10th	Night	36% 8% 18% 29% 7% 2	1845
	11th to 20th	Night	36% 6% 17% 30% 10% 1	2172
	21st to end	Night	37% 8% 15% 27% 12% 2	2090
October	1st to 10th	Night	40% 8% 22% 23% 7%	3672
	11th to 20th	Night	43% 6% 15% 31% 5%	4081
	21st to end	Night	44% 6% 24% 20% 5%	4502
November	1st to 10th	Night	56% 7% 12% 20% 5%	2740
	11th to 20th	Night	56% 8% 14% 16% 6%	3000
	21st to end	Night	55% 8% 14% 18% 4 1	3000
December	1st to 10th	Night	61% 6% 13% 14% 5%	4890
	11th to 20th	Night	61% 7% 16% 12% 4	5547
	21st to end	Night	41% 6% 15% 26% 10% 1	7964

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	8% 5% 32% 32% 22% 3	200
	11th to 20th	Day	10% 8% 36% 28% 15% 4	200
	21st to end	Day	11% 10% 33% 28% 15% 4	220
February	1st to 10th	Day	11% 10% 33% 28% 13% 6%	200
	11th to 20th	Day	10% 7% 31% 32% 15% 4	254
	21st to end	Day	5% 11% 32% 34% 13% 5%	298
March	1st to 10th	Day	9% 10% 30% 33% 15% 3	500
	11th to 20th	Day	10% 7% 32% 32% 14% 4	500
	21st to end	Day	17% 6% 31% 29% 16% 2	550
April	1st to 10th	Day	9% 13% 34% 27% 13% 3	500
	11th to 20th	Day	13% 11% 28% 31% 12% 4	492
	21st to end	Day	14% 11% 32% 27% 13% 3	500
May	1st to 10th	Day	15% 10% 27% 29% 15% 4	683
	11th to 20th	Day	13% 8% 24% 30% 20% 5%	725
	21st to end	Day	17% 5% 21% 29% 20% 7%	789
June	1st to 10th	Day	19% 8% 28% 29% 13% 3	796
	11th to 20th	Day	21% 6% 21% 25% 20% 6%	930
	21st to end	Day	11% 4 20% 32% 25% 8%	930
July	1st to 10th	Day	12% 4 20% 31% 27% 6%	916
	11th to 20th	Day	12% 4 23% 29% 24% 8%	755
	21st to end	Day	11% 4 19% 31% 24% 10%	794
August	1st to 10th	Day	13% 5% 17% 27% 24% 13%	633
	11th to 20th	Day	9% 6% 29% 32% 19% 5%	517
	21st to end	Day	11% 8% 21% 30% 23% 6%	583
September	1st to 10th	Day	15% 9% 27% 25% 16% 8%	530
	11th to 20th	Day	19% 7% 23% 30% 17% 3	523
	21st to end	Day	12% 12% 33% 24% 18% 2	490
October	1st to 10th	Day	17% 11% 27% 26% 17% 3	379
	11th to 20th	Day	15% 7% 34% 25% 16% 2	300
	21st to end	Day	18% 10% 37% 21% 11% 3	330
November	1st to 10th	Day	18% 6% 30% 30% 13% 2	293
	11th to 20th	Day	10% 8% 38% 28% 14% 4	200
	21st to end	Day	6% 12% 37% 29% 11% 6%	200
December	1st to 10th	Day	10% 9% 38% 26% 14% 5%	200
	11th to 20th	Day	13% 7% 32% 33% 14% 3	200
	21st to end	Day	9% 4 25% 37% 21% 5%	220
January	1st to 10th	Night	12% 5% 32% 34% 14% 2	1055
	11th to 20th	Night	11% 8% 31% 36% 12% 1	1000
	21st to end	Night	15% 9% 35% 28% 10% 2	1034
February	1st to 10th	Night	9% 7% 28% 35% 19% 3	800
	11th to 20th	Night	11% 4 32% 35% 17% 2	784
	21st to end	Night	6% 6% 30% 37% 17% 4	660
March	1st to 10th	Night	10% 5% 29% 39% 14% 3	800
	11th to 20th	Night	13% 4 31% 36% 14% 3	800
	21st to end	Night	15% 6% 28% 33% 16% 2	880
April	1st to 10th	Night	10% 7% 28% 35% 18% 3	699
	11th to 20th	Night	12% 8% 29% 34% 14% 3	593
	21st to end	Night	15% 8% 26% 34% 17% 1	581
May	1st to 10th	Night	14% 8% 23% 35% 18% 3	400
	11th to 20th	Night	13% 4 26% 39% 16% 3	394
	21st to end	Night	21% 5% 21% 33% 19% 2	433
June	1st to 10th	Night	20% 7% 22% 35% 15% 2	400
	11th to 20th	Night	17% 4 19% 34% 24% 3	400
	21st to end	Night	10% 4 15% 42% 27% 4	400
July	1st to 10th	Night	12% 3 18% 41% 25% 2	390
	11th to 20th	Night	13% 3 19% 39% 22% 3	388
	21st to end	Night	9% 3 16% 39% 28% 4	438
August	1st to 10th	Night	9% 4 15% 37% 30% 6%	398
	11th to 20th	Night	12% 2 21% 37% 25% 2	471
	21st to end	Night	13% 3 19% 41% 22% 2	660
September	1st to 10th	Night	15% 4 19% 37% 23% 2	645
	11th to 20th	Night	13% 3 20% 42% 20% 1	792
	21st to end	Night	12% 5% 31% 36% 14% 3	760
October	1st to 10th	Night	13% 4 30% 37% 14% 2	760
	11th to 20th	Night	14% 5% 29% 37% 13% 2	760
	21st to end	Night	15% 7% 33% 31% 13% 1	857
November	1st to 10th	Night	18% 6% 30% 34% 10% 2	975
	11th to 20th	Night	15% 6% 37% 30% 10% 3	1000
	21st to end	Night	10% 7% 38% 32% 11% 2	1000
December	1st to 10th	Night	13% 7% 35% 29% 12% 4	1000
	11th to 20th	Night	12% 8% 31% 36% 11% 2	1000
	21st to end	Night	10% 4 28% 38% 17% 4	1210

Page generated at 2025-09-15 23:49:15. Version ac07

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