Great Neck South High Big Blue Rebel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Neck South High Big Blue Rebel Observatory clear sky chart from 2005-11-06 to 2026-07-01.

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% 2 6% 4 4 4 4 5% 7% 40%	137695
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 42%	134656
Transparency	Day	40% 7% 12% 22% 16% 2	141224
	Night	42% 6% 11% 22% 16% 3	139203
Seeing	Day	19% 13% 31% 24% 11% 2	17127
	Night	20% 11% 31% 27% 9% 1	26255

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	13% 6% 2 5% 4 4 3 3 5% 6% 49%	4661
	11th to 20th	Day	16% 8% 2 6% 3 4 4 3 4 6% 45%	4650
	21st to end	Day	15% 8% 3 7% 4 4 4 3 5% 6% 41%	5518
February	1st to 10th	Day	16% 8% 2 7% 4 4 4 3 5% 6% 41%	1927
	11th to 20th	Day	15% 7% 3 6% 4 3 4 3 4 6% 44%	2048
	21st to end	Day	20% 8% 2 5% 4 4 3 3 5% 6% 40%	1730
March	1st to 10th	Day	22% 7% 2 5% 3 3 3 3 5% 5% 42%	4576
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 6% 8% 42%	4808
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	6097
April	1st to 10th	Day	19% 8% 3 6% 3 3 4 3 5% 8% 38%	2520
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 5% 8% 41%	2494
	21st to end	Day	18% 6% 2 5% 4 5% 4 3 6% 7% 40%	2566
May	1st to 10th	Day	25% 8% 2 5% 3 4 4 3 5% 8% 33%	7639
	11th to 20th	Day	19% 9% 2 6% 4 4 4 3 5% 6% 38%	8074
	21st to end	Day	24% 7% 2 5% 4 4 4 3 5% 8% 34%	9060
June	1st to 10th	Day	15% 6% 2 5% 4 4 4 4 7% 9% 39%	2940
	11th to 20th	Day	15% 7% 2 6% 4 4 5% 4 6% 8% 39%	2940
	21st to end	Day	11% 6% 2 7% 4 5% 5% 4 7% 9% 41%	2940
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 11% 36%	6360
	11th to 20th	Day	10% 7% 2 7% 5% 5% 6% 6% 8% 11% 34%	6360
	21st to end	Day	13% 7% 2 6% 4 4 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	19% 6% 2 6% 4 4 5% 5% 7% 8% 34%	4295
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 4 7% 8% 41%	3850
	21st to end	Day	9% 4 2 5% 4 4 4 4 6% 9% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 3 4 5% 4 5% 7% 46%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 7% 50%	2295
	21st to end	Day	19% 6% 2 7% 4 4 4 4 5% 6% 39%	2200
October	1st to 10th	Day	18% 7% 2 5% 3 4 4 3 5% 6% 43%	2116
	11th to 20th	Day	15% 5% 2 6% 4 4 4 3 5% 6% 47%	2000
	21st to end	Day	19% 5% 2 6% 3 4 4 3 4 6% 44%	2021
November	1st to 10th	Day	13% 6% 2 6% 4 2 3 3 5% 5% 52%	1851
	11th to 20th	Day	16% 6% 2 4 3 3 4 3 5% 6% 48%	1890
	21st to end	Day	16% 7% 2 5% 4 4 4 4 5% 7% 44%	1837
December	1st to 10th	Day	16% 7% 2 5% 4 3 3 3 5% 7% 45%	2680
	11th to 20th	Day	22% 7% 2 4 2 2 3 2 3 6% 46%	3066
	21st to end	Day	17% 9% 2 6% 4 4 4 3 5% 6% 39%	4048
January	1st to 10th	Night	13% 8% 3 6% 3 3 3 3 5% 6% 47%	8234
	11th to 20th	Night	16% 8% 2 6% 3 3 4 3 4 5% 46%	8235
	21st to end	Night	16% 8% 2 5% 4 4 4 4 5% 5% 43%	8658
February	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 4 6% 41%	2691
	11th to 20th	Night	17% 8% 3 6% 4 4 3 3 5% 5% 41%	2458
	21st to end	Night	19% 8% 2 5% 4 4 4 3 5% 5% 41%	2076
March	1st to 10th	Night	19% 9% 2 5% 3 3 3 2 4 5% 43%	5484
	11th to 20th	Night	14% 9% 2 7% 3 4 3 3 5% 6% 43%	5353
	21st to end	Night	18% 9% 3 7% 4 4 4 3 5% 5% 39%	5076
April	1st to 10th	Night	20% 6% 3 7% 4 4 5% 4 5% 7% 37%	2100
	11th to 20th	Night	19% 8% 2 6% 3 4 5% 4 4 7% 39%	2075
	21st to end	Night	18% 6% 3 6% 4 3 4 4 5% 6% 42%	2053
May	1st to 10th	Night	23% 9% 3 7% 4 4 4 3 5% 7% 31%	5259
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 5% 6% 36%	4841
	21st to end	Night	25% 9% 3 6% 4 4 4 3 4 6% 33%	5114
June	1st to 10th	Night	17% 7% 2 4 4 4 4 4 5% 7% 42%	1680
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 6% 10% 38%	1680
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 7% 10% 39%	1680
July	1st to 10th	Night	10% 8% 3 8% 6% 5% 5% 5% 5% 9% 35%	3600
	11th to 20th	Night	14% 10% 3 8% 5% 5% 5% 4 6% 8% 32%	3600
	21st to end	Night	13% 7% 2 5% 4 3 4 4 6% 8% 42%	3926
August	1st to 10th	Night	23% 7% 2 6% 4 4 4 4 6% 7% 33%	2804
	11th to 20th	Night	14% 8% 2 7% 4 5% 5% 4 6% 7% 39%	3197
	21st to end	Night	8% 5% 2 5% 3 4 5% 3 6% 7% 52%	3586
September	1st to 10th	Night	14% 5% 2 7% 4 5% 5% 4 5% 7% 43%	2000
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 5% 6% 51%	2103
	21st to end	Night	17% 6% 2 7% 4 4 3 3 5% 7% 42%	2200
October	1st to 10th	Night	18% 6% 2 6% 5% 3 4 2 4 6% 43%	2280
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 3 6% 50%	2400
	21st to end	Night	20% 7% 3 5% 4 3 3 3 4 6% 44%	2817
November	1st to 10th	Night	12% 5% 2 6% 4 4 3 3 4 6% 51%	2667
	11th to 20th	Night	18% 6% 2 4 3 3 3 2 4 5% 49%	2730
	21st to end	Night	15% 6% 2 5% 4 4 3 4 5% 6% 46%	2781
December	1st to 10th	Night	16% 7% 2 4 3 3 4 3 4 6% 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 43%	5348
	21st to end	Night	19% 8% 3 6% 4 3 4 3 5% 5% 41%	7150

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	36% 4 10% 31% 18% 2	4662
	11th to 20th	Day	40% 4 10% 23% 18% 6%	4680
	21st to end	Day	43% 5% 11% 25% 15% 1	5518
February	1st to 10th	Day	44% 5% 13% 23% 13% 2	1927
	11th to 20th	Day	42% 5% 9% 26% 16% 2	2057
	21st to end	Day	45% 5% 12% 22% 14% 3	1730
March	1st to 10th	Day	42% 5% 12% 20% 13% 8%	4579
	11th to 20th	Day	36% 8% 15% 27% 11% 3	4808
	21st to end	Day	41% 6% 11% 14% 23% 5%	6097
April	1st to 10th	Day	44% 6% 11% 20% 16% 4	2520
	11th to 20th	Day	41% 5% 14% 21% 16% 2	2494
	21st to end	Day	41% 7% 11% 21% 16% 4	2566
May	1st to 10th	Day	50% 7% 13% 17% 13% 1	7651
	11th to 20th	Day	45% 7% 11% 23% 14% 1	8078
	21st to end	Day	47% 7% 13% 20% 12% 1	9061
June	1st to 10th	Day	39% 9% 14% 21% 16% 2	2940
	11th to 20th	Day	40% 9% 13% 23% 12% 2	2940
	21st to end	Day	37% 9% 15% 22% 15% 1	2940
July	1st to 10th	Day	42% 10% 12% 21% 14% 1	6290
	11th to 20th	Day	40% 10% 19% 21% 9% 1	6285
	21st to end	Day	39% 8% 12% 20% 20% 1	6728
August	1st to 10th	Day	45% 9% 15% 22% 8% 1	4281
	11th to 20th	Day	37% 8% 14% 30% 10% 1	3886
	21st to end	Day	31% 8% 12% 28% 19% 2	4312
September	1st to 10th	Day	37% 7% 13% 22% 19% 2	2400
	11th to 20th	Day	32% 6% 12% 25% 24% 2	2295
	21st to end	Day	45% 7% 11% 17% 18% 3	2200
October	1st to 10th	Day	35% 6% 11% 19% 23% 5%	3441
	11th to 20th	Day	26% 5% 11% 22% 31% 5%	3240
	21st to end	Day	28% 4 15% 25% 25% 3	3246
November	1st to 10th	Day	34% 5% 13% 22% 22% 3	1851
	11th to 20th	Day	37% 5% 10% 22% 22% 4	1890
	21st to end	Day	39% 5% 14% 29% 11% 2	1837
December	1st to 10th	Day	39% 4 16% 27% 12% 1	2680
	11th to 20th	Day	42% 3 12% 23% 17% 2	3066
	21st to end	Day	44% 7% 9% 22% 15% 2	4048
January	1st to 10th	Night	38% 4 11% 24% 20% 3	8234
	11th to 20th	Night	41% 4 12% 21% 17% 6%	8280
	21st to end	Night	43% 4 8% 21% 20% 4	8658
February	1st to 10th	Night	44% 5% 13% 23% 13% 2	2691
	11th to 20th	Night	45% 5% 9% 20% 17% 4	2460
	21st to end	Night	44% 4 8% 24% 16% 3	2076
March	1st to 10th	Night	43% 4 10% 21% 13% 9%	5494
	11th to 20th	Night	40% 7% 14% 24% 11% 4	5354
	21st to end	Night	47% 5% 6% 14% 23% 5%	5076
April	1st to 10th	Night	46% 7% 10% 18% 15% 4	2100
	11th to 20th	Night	44% 7% 10% 17% 20% 2	2075
	21st to end	Night	42% 7% 9% 19% 18% 4	2053
May	1st to 10th	Night	52% 6% 12% 16% 12% 1	5265
	11th to 20th	Night	48% 6% 13% 19% 13% 2	4842
	21st to end	Night	52% 6% 11% 20% 10% 1	5115
June	1st to 10th	Night	41% 7% 9% 21% 21% 1	1680
	11th to 20th	Night	41% 8% 14% 25% 11% 1	1680
	21st to end	Night	38% 9% 14% 24% 14% 2	1680
July	1st to 10th	Night	47% 9% 11% 20% 12% 1	3560
	11th to 20th	Night	48% 9% 13% 21% 9% 1	3552
	21st to end	Night	38% 7% 15% 21% 18% 1	3809
August	1st to 10th	Night	50% 7% 16% 17% 9% 1	2794
	11th to 20th	Night	40% 9% 13% 27% 10% 1	3227
	21st to end	Night	30% 7% 13% 32% 18% 1	3586
September	1st to 10th	Night	39% 9% 10% 22% 19% 1	2000
	11th to 20th	Night	33% 6% 11% 24% 23% 3	2103
	21st to end	Night	43% 7% 9% 20% 19% 2	2200
October	1st to 10th	Night	37% 5% 12% 23% 20% 3	3722
	11th to 20th	Night	24% 4 10% 27% 30% 4	3899
	21st to end	Night	30% 5% 12% 29% 22% 2	4542
November	1st to 10th	Night	35% 5% 12% 23% 21% 4	2667
	11th to 20th	Night	39% 5% 8% 23% 21% 5%	2730
	21st to end	Night	38% 5% 12% 27% 16% 1	2781
December	1st to 10th	Night	39% 4 12% 29% 13% 2	4720
	11th to 20th	Night	45% 4 10% 23% 15% 3	5348
	21st to end	Night	44% 5% 11% 20% 16% 4	7150

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	25% 26% 29% 17% 3	220
	11th to 20th	Day	20% 31% 33% 13% 2	220
	21st to end	Day	19% 22% 39% 13% 5% 1	314
February	1st to 10th	Day	23% 18% 40% 16% 3	367
	11th to 20th	Day	24% 19% 37% 18% 1	536
	21st to end	Day	24% 19% 37% 17% 4	446
March	1st to 10th	Day	20% 22% 34% 19% 4	540
	11th to 20th	Day	19% 18% 40% 16% 6% 1	540
	21st to end	Day	24% 22% 33% 16% 5% 1	594
April	1st to 10th	Day	24% 20% 34% 15% 6% 1	540
	11th to 20th	Day	21% 17% 34% 20% 8% 1	532
	21st to end	Day	25% 15% 34% 20% 5% 1	540
May	1st to 10th	Day	30% 15% 31% 17% 6% 1	543
	11th to 20th	Day	20% 11% 34% 27% 8% 1	567
	21st to end	Day	22% 7% 26% 26% 16% 3	617
June	1st to 10th	Day	16% 9% 30% 30% 13% 2	570
	11th to 20th	Day	17% 11% 29% 31% 11% 2	570
	21st to end	Day	15% 7% 31% 29% 16% 2	570
July	1st to 10th	Day	13% 4 24% 36% 19% 5%	525
	11th to 20th	Day	9% 5% 24% 35% 23% 4	524
	21st to end	Day	13% 5% 26% 37% 17% 2	576
August	1st to 10th	Day	12% 3 25% 35% 18% 6%	526
	11th to 20th	Day	13% 5% 19% 36% 22% 3	519
	21st to end	Day	11% 6% 27% 35% 15% 7%	583
September	1st to 10th	Day	18% 5% 28% 25% 19% 5%	530
	11th to 20th	Day	14% 4 27% 31% 15% 9%	530
	21st to end	Day	22% 9% 26% 26% 13% 4	530
October	1st to 10th	Day	22% 10% 24% 24% 15% 5%	530
	11th to 20th	Day	18% 13% 29% 25% 12% 2	530
	21st to end	Day	20% 11% 36% 21% 9% 2	484
November	1st to 10th	Day	15% 14% 32% 27% 11% 2	425
	11th to 20th	Day	20% 20% 32% 23% 5%	430
	21st to end	Day	21% 16% 33% 23% 6% 1	377
December	1st to 10th	Day	21% 19% 37% 17% 6%	220
	11th to 20th	Day	29% 21% 32% 14% 3	220
	21st to end	Day	27% 17% 37% 10% 9%	242
January	1st to 10th	Night	19% 19% 40% 19% 3	840
	11th to 20th	Night	25% 18% 32% 22% 3	840
	21st to end	Night	19% 17% 41% 20% 2	924
February	1st to 10th	Night	22% 15% 38% 24% 2	840
	11th to 20th	Night	23% 17% 40% 18% 1	823
	21st to end	Night	22% 16% 38% 20% 3	692
March	1st to 10th	Night	23% 14% 37% 21% 5%	840
	11th to 20th	Night	22% 14% 38% 22% 4 1	840
	21st to end	Night	22% 14% 39% 22% 4	924
April	1st to 10th	Night	23% 14% 35% 22% 5%	840
	11th to 20th	Night	20% 12% 35% 26% 6%	831
	21st to end	Night	21% 10% 36% 28% 5% 1	793
May	1st to 10th	Night	23% 11% 33% 25% 7%	630
	11th to 20th	Night	22% 9% 34% 28% 6%	488
	21st to end	Night	22% 5% 19% 36% 16% 2	455
June	1st to 10th	Night	19% 4 27% 36% 13% 1	420
	11th to 20th	Night	15% 6% 24% 41% 14%	420
	21st to end	Night	12% 5% 21% 48% 15%	420
July	1st to 10th	Night	11% 4 21% 43% 19% 1	390
	11th to 20th	Night	14% 3 16% 44% 20% 2	388
	21st to end	Night	11% 4 21% 45% 19% 1	438
August	1st to 10th	Night	14% 3 19% 43% 21% 1	525
	11th to 20th	Night	14% 4 18% 37% 25% 2	790
	21st to end	Night	14% 4 19% 37% 23% 3	880
September	1st to 10th	Night	15% 4 24% 35% 19% 3	800
	11th to 20th	Night	15% 4 21% 35% 21% 4	800
	21st to end	Night	20% 5% 25% 31% 17% 2	800
October	1st to 10th	Night	21% 10% 25% 25% 17% 3	800
	11th to 20th	Night	18% 11% 28% 31% 12% 1	800
	21st to end	Night	23% 11% 34% 24% 8%	880
November	1st to 10th	Night	15% 10% 35% 27% 11% 1	820
	11th to 20th	Night	22% 15% 32% 23% 8%	840
	21st to end	Night	18% 16% 34% 26% 5% 1	840
December	1st to 10th	Night	24% 15% 36% 20% 5% 1	840
	11th to 20th	Night	24% 16% 34% 23% 3	840
	21st to end	Night	23% 15% 38% 21% 3	924

Page generated at 2026-06-28 02:10:50. Version ac07

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