Grand Island
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Island clear sky chart from 2005-11-06 to 2026-08-30.

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	9% 6% 2 6% 4 4 4 3 5% 7% 49%	152971
	Night	8% 6% 2 6% 4 4 4 3 5% 7% 51%	146643
Transparency	Day	34% 5% 13% 26% 18% 4	147694
	Night	33% 5% 12% 26% 20% 4	142932
Seeing	Day	14% 10% 32% 29% 12% 2	18338
	Night	12% 10% 36% 33% 9%	28119

By Month

Percentage Cloud Cover SW (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	10% 12% 4 10% 6% 5% 3 3 4 5% 39%	4720
	11th to 20th	Day	4 9% 4 8% 5% 4 5% 4 5% 6% 45%	5153
	21st to end	Day	5% 9% 4 11% 5% 4 4 3 4 5% 46%	5812
February	1st to 10th	Day	9% 11% 4 7% 5% 5% 5% 5% 6% 7% 38%	1890
	11th to 20th	Day	6% 9% 3 8% 4 4 4 3 6% 7% 46%	2030
	21st to end	Day	8% 7% 2 5% 4 4 4 3 5% 6% 52%	1730
March	1st to 10th	Day	12% 7% 3 5% 3 2 3 2 3 4 56%	4735
	11th to 20th	Day	9% 7% 2 6% 3 3 3 3 4 6% 54%	5090
	21st to end	Day	13% 10% 3 8% 4 4 4 4 5% 8% 38%	5585
April	1st to 10th	Day	12% 7% 2 6% 4 3 4 4 5% 7% 47%	2392
	11th to 20th	Day	13% 7% 2 6% 4 4 3 3 5% 6% 46%	2641
	21st to end	Day	19% 7% 2 5% 4 4 4 3 5% 6% 40%	2730
May	1st to 10th	Day	11% 6% 2 5% 3 4 4 4 6% 8% 48%	7655
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 52%	7627
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 6% 40%	8404
June	1st to 10th	Day	7% 5% 1 4 3 3 4 4 6% 8% 53%	2759
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 7% 52%	2940
	21st to end	Day	8% 4 1 4 3 4 4 4 7% 9% 52%	2940
July	1st to 10th	Day	7% 5% 2 6% 4 4 4 4 7% 9% 49%	8249
	11th to 20th	Day	5% 4 2 4 3 4 4 3 5% 7% 59%	8005
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 7% 9% 43%	8339
August	1st to 10th	Day	7% 5% 2 7% 5% 5% 5% 5% 7% 9% 44%	5931
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 10% 46%	5951
	21st to end	Day	11% 6% 2 7% 5% 4 4 3 5% 7% 47%	6469
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 2 4 6% 55%	2260
	11th to 20th	Day	8% 4 2 5% 4 3 4 3 5% 7% 55%	2200
	21st to end	Day	9% 4 1 3 2 3 3 3 4 6% 61%	2200
October	1st to 10th	Day	8% 4 2 3 2 2 2 2 3 4 70%	3506
	11th to 20th	Day	7% 3 1 3 3 3 3 2 4 6% 65%	3240
	21st to end	Day	10% 5% 2 5% 3 2 4 2 5% 6% 56%	3350
November	1st to 10th	Day	7% 5% 1 5% 3 4 3 3 5% 7% 55%	1853
	11th to 20th	Day	8% 6% 2 5% 4 3 3 3 3 6% 57%	1890
	21st to end	Day	10% 8% 2 5% 3 2 3 3 5% 7% 49%	1890
December	1st to 10th	Day	5% 5% 2 6% 5% 3 4 4 4 7% 56%	3030
	11th to 20th	Day	6% 8% 3 7% 4 4 4 4 6% 7% 48%	3481
	21st to end	Day	12% 10% 3 11% 4 4 4 3 4 5% 38%	4294
January	1st to 10th	Night	7% 11% 4 10% 5% 4 5% 4 6% 6% 38%	8201
	11th to 20th	Night	4 6% 3 8% 4 5% 6% 4 6% 7% 48%	7818
	21st to end	Night	5% 9% 4 11% 4 4 4 3 4 5% 46%	8402
February	1st to 10th	Night	8% 10% 4 7% 5% 4 5% 3 4 5% 44%	2730
	11th to 20th	Night	6% 9% 3 7% 5% 5% 5% 5% 5% 6% 45%	2495
	21st to end	Night	6% 7% 3 6% 4 4 3 3 5% 6% 54%	2076
March	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 5% 55%	5353
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 4 6% 58%	5090
	21st to end	Night	12% 9% 3 6% 4 4 5% 4 6% 7% 40%	5588
April	1st to 10th	Night	11% 8% 2 5% 3 4 4 3 5% 7% 47%	2225
	11th to 20th	Night	12% 6% 2 5% 4 5% 4 3 6% 9% 45%	1927
	21st to end	Night	16% 5% 2 5% 3 4 4 2 5% 6% 48%	1890
May	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 5% 6% 49%	5292
	11th to 20th	Night	8% 6% 2 5% 3 3 4 3 6% 7% 54%	5265
	21st to end	Night	16% 6% 3 7% 4 4 4 3 5% 7% 42%	5803
June	1st to 10th	Night	6% 4 1 5% 4 4 3 4 7% 9% 52%	1854
	11th to 20th	Night	8% 5% 1 4 3 4 4 3 6% 7% 56%	1680
	21st to end	Night	6% 4 1 5% 4 3 4 5% 6% 9% 56%	1680
July	1st to 10th	Night	7% 5% 3 6% 4 3 4 4 6% 9% 50%	4740
	11th to 20th	Night	6% 3 2 5% 3 3 4 3 5% 8% 58%	4968
	21st to end	Night	7% 6% 2 6% 4 4 5% 4 7% 9% 48%	5770
August	1st to 10th	Night	4 3 2 5% 3 3 4 4 7% 13% 51%	4101
	11th to 20th	Night	7% 4 2 4 4 4 4 3 7% 10% 51%	4118
	21st to end	Night	9% 4 2 6% 4 4 5% 4 7% 9% 47%	4702
September	1st to 10th	Night	8% 3 1 3 3 3 3 2 5% 5% 63%	2140
	11th to 20th	Night	7% 4 2 4 4 3 3 3 5% 6% 59%	2200
	21st to end	Night	7% 4 1 4 3 4 4 3 5% 6% 60%	2200
October	1st to 10th	Night	9% 2 1 3 2 2 2 2 3 5% 70%	3640
	11th to 20th	Night	6% 3 1 4 2 3 3 2 4 7% 65%	3899
	21st to end	Night	10% 4 1 4 3 2 3 2 5% 7% 58%	4442
November	1st to 10th	Night	6% 5% 1 4 3 2 3 4 5% 6% 61%	2665
	11th to 20th	Night	7% 5% 2 5% 3 4 3 3 4 7% 57%	2730
	21st to end	Night	9% 6% 2 5% 4 3 3 2 5% 7% 54%	2730
December	1st to 10th	Night	3 5% 2 4 3 3 4 4 5% 7% 60%	4380
	11th to 20th	Night	8% 4 3 5% 4 4 4 3 6% 8% 51%	4946
	21st to end	Night	13% 12% 3 9% 4 3 4 3 4 6% 39%	6903

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	49% 4 11% 22% 13% 2	4723
	11th to 20th	Day	39% 5% 15% 20% 19% 3	5153
	21st to end	Day	42% 4 12% 25% 14% 2	5812
February	1st to 10th	Day	44% 6% 15% 21% 11% 2	1890
	11th to 20th	Day	37% 5% 14% 24% 15% 4	2040
	21st to end	Day	33% 5% 11% 28% 18% 5%	1730
March	1st to 10th	Day	33% 3 9% 23% 25% 7%	4735
	11th to 20th	Day	32% 4 8% 25% 25% 7%	5090
	21st to end	Day	44% 6% 21% 20% 7% 2	5585
April	1st to 10th	Day	36% 6% 15% 26% 15% 2	2392
	11th to 20th	Day	38% 4 14% 23% 17% 4	2644
	21st to end	Day	44% 6% 11% 25% 14% 1	2730
May	1st to 10th	Day	32% 6% 11% 25% 23% 2	7655
	11th to 20th	Day	32% 5% 11% 25% 21% 6%	7631
	21st to end	Day	44% 6% 11% 26% 13% 1	8405
June	1st to 10th	Day	27% 8% 18% 31% 15% 2	2759
	11th to 20th	Day	30% 6% 14% 28% 18% 3	2940
	21st to end	Day	26% 8% 15% 29% 18% 4	2940
July	1st to 10th	Day	31% 6% 13% 31% 18% 1	8181
	11th to 20th	Day	24% 6% 11% 30% 21% 8%	7930
	21st to end	Day	35% 7% 16% 24% 15% 3	8155
August	1st to 10th	Day	33% 7% 15% 25% 15% 4	4350
	11th to 20th	Day	38% 7% 15% 24% 16% 1	4340
	21st to end	Day	28% 6% 14% 23% 21% 7%	4692
September	1st to 10th	Day	32% 5% 12% 25% 21% 6%	2260
	11th to 20th	Day	28% 6% 12% 31% 21% 3	2200
	21st to end	Day	25% 4 9% 33% 27% 3	2200
October	1st to 10th	Day	21% 2 5% 22% 42% 7%	3506
	11th to 20th	Day	22% 3 11% 35% 20% 9%	3240
	21st to end	Day	30% 3 10% 29% 24% 4	3350
November	1st to 10th	Day	28% 5% 14% 28% 23% 3	1853
	11th to 20th	Day	30% 3 12% 28% 24% 4	1890
	21st to end	Day	35% 4 13% 27% 18% 3	1890
December	1st to 10th	Day	29% 4 13% 33% 19% 2	3030
	11th to 20th	Day	36% 4 19% 27% 13% 1	3479
	21st to end	Day	48% 5% 9% 27% 11% 1	4294
January	1st to 10th	Night	46% 4 11% 21% 16% 2	8202
	11th to 20th	Night	33% 5% 14% 27% 19% 2	7818
	21st to end	Night	41% 4 11% 22% 19% 3	8402
February	1st to 10th	Night	42% 5% 13% 23% 14% 3	2730
	11th to 20th	Night	38% 6% 13% 25% 14% 3	2495
	21st to end	Night	31% 5% 10% 23% 24% 7%	2076
March	1st to 10th	Night	31% 4 7% 27% 25% 6%	5353
	11th to 20th	Night	28% 3 7% 21% 31% 9%	5090
	21st to end	Night	41% 6% 18% 25% 9% 1	5588
April	1st to 10th	Night	36% 5% 14% 25% 18% 2	2225
	11th to 20th	Night	35% 6% 13% 26% 17% 3	1927
	21st to end	Night	36% 7% 12% 26% 17% 1	1890
May	1st to 10th	Night	35% 5% 9% 28% 22% 2	5293
	11th to 20th	Night	28% 6% 13% 24% 23% 6%	5266
	21st to end	Night	41% 7% 12% 24% 14% 1	5805
June	1st to 10th	Night	25% 10% 18% 28% 17% 2	1854
	11th to 20th	Night	27% 6% 13% 29% 19% 5%	1680
	21st to end	Night	24% 7% 15% 30% 21% 3	1680
July	1st to 10th	Night	31% 5% 15% 33% 14% 2	4700
	11th to 20th	Night	25% 6% 11% 29% 21% 9%	4920
	21st to end	Night	31% 6% 16% 24% 19% 4	5641
August	1st to 10th	Night	25% 7% 18% 28% 18% 5%	3006
	11th to 20th	Night	33% 6% 18% 28% 14% 1	3005
	21st to end	Night	25% 6% 16% 27% 17% 9%	3410
September	1st to 10th	Night	24% 4 11% 28% 24% 9%	2140
	11th to 20th	Night	26% 5% 10% 32% 23% 4	2200
	21st to end	Night	26% 5% 8% 28% 28% 5%	2200
October	1st to 10th	Night	19% 3 6% 18% 45% 8%	3640
	11th to 20th	Night	22% 3 10% 29% 26% 10%	3899
	21st to end	Night	27% 3 10% 29% 27% 4	4442
November	1st to 10th	Night	23% 5% 12% 29% 25% 6%	2665
	11th to 20th	Night	28% 3 11% 30% 24% 5%	2730
	21st to end	Night	31% 4 13% 27% 23% 3	2730
December	1st to 10th	Night	24% 4 15% 36% 20% 2	4380
	11th to 20th	Night	31% 4 14% 33% 16% 1	4947
	21st to end	Night	46% 5% 10% 26% 12% 1	6903

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	18% 13% 35% 25% 7% 2	330
	11th to 20th	Day	17% 18% 36% 23% 6% 1	330
	21st to end	Day	15% 12% 45% 20% 7% 1	363
February	1st to 10th	Day	14% 15% 46% 19% 5%	330
	11th to 20th	Day	15% 11% 48% 20% 5% 1	330
	21st to end	Day	15% 16% 47% 18% 4	273
March	1st to 10th	Day	16% 15% 43% 21% 4 1	330
	11th to 20th	Day	23% 10% 38% 20% 9%	330
	21st to end	Day	27% 17% 37% 15% 3	363
April	1st to 10th	Day	20% 22% 38% 16% 4	412
	11th to 20th	Day	19% 19% 35% 23% 4	682
	21st to end	Day	24% 17% 29% 20% 8% 2	750
May	1st to 10th	Day	15% 12% 35% 26% 10% 2	753
	11th to 20th	Day	19% 12% 31% 24% 11% 2	775
	21st to end	Day	17% 10% 32% 28% 11% 2	844
June	1st to 10th	Day	7% 8% 28% 35% 18% 5%	780
	11th to 20th	Day	10% 7% 34% 32% 14% 3	780
	21st to end	Day	9% 7% 33% 32% 16% 2	780
July	1st to 10th	Day	10% 4 21% 36% 24% 5%	771
	11th to 20th	Day	5% 4 20% 42% 24% 7%	768
	21st to end	Day	7% 4 21% 41% 23% 4	849
August	1st to 10th	Day	7% 3 21% 45% 20% 3	776
	11th to 20th	Day	7% 5% 22% 43% 21% 2	765
	21st to end	Day	10% 4 24% 38% 19% 4	788
September	1st to 10th	Day	13% 6% 27% 36% 16% 3	390
	11th to 20th	Day	7% 7% 33% 35% 14% 4	330
	21st to end	Day	14% 10% 27% 30% 13% 5%	330
October	1st to 10th	Day	18% 11% 27% 32% 9% 4	330
	11th to 20th	Day	15% 15% 38% 22% 8% 2	330
	21st to end	Day	19% 18% 33% 22% 7% 1	363
November	1st to 10th	Day	14% 16% 41% 20% 7% 2	330
	11th to 20th	Day	13% 15% 43% 22% 6%	330
	21st to end	Day	23% 13% 36% 17% 9% 2	330
December	1st to 10th	Day	9% 14% 43% 28% 5% 1	330
	11th to 20th	Day	16% 12% 42% 25% 5%	330
	21st to end	Day	19% 12% 45% 21% 2	363
January	1st to 10th	Night	12% 11% 38% 32% 6% 1	1050
	11th to 20th	Night	11% 14% 41% 29% 5% 1	1050
	21st to end	Night	11% 10% 44% 29% 6%	1155
February	1st to 10th	Night	13% 12% 44% 26% 5%	1050
	11th to 20th	Night	12% 13% 40% 30% 5%	861
	21st to end	Night	10% 12% 47% 26% 5%	692
March	1st to 10th	Night	13% 13% 42% 26% 4 1	777
	11th to 20th	Night	17% 10% 41% 26% 5%	630
	21st to end	Night	19% 12% 41% 24% 4	693
April	1st to 10th	Night	17% 16% 41% 23% 3	630
	11th to 20th	Night	16% 15% 43% 23% 2	623
	21st to end	Night	19% 12% 32% 30% 6% 1	630
May	1st to 10th	Night	16% 10% 34% 31% 7%	630
	11th to 20th	Night	14% 10% 34% 33% 8%	621
	21st to end	Night	17% 8% 33% 31% 10% 1	683
June	1st to 10th	Night	8% 6% 28% 41% 15% 2	630
	11th to 20th	Night	9% 7% 38% 37% 10%	630
	21st to end	Night	7% 5% 35% 41% 12%	630
July	1st to 10th	Night	6% 4 22% 45% 23% 1	615
	11th to 20th	Night	4 3 21% 50% 22%	612
	21st to end	Night	5% 2 20% 52% 21% 1	690
August	1st to 10th	Night	4 3 23% 47% 21% 2	627
	11th to 20th	Night	7% 2 24% 47% 19%	624
	21st to end	Night	8% 4 25% 43% 18% 1	687
September	1st to 10th	Night	10% 3 28% 44% 15% 1	600
	11th to 20th	Night	9% 6% 33% 38% 13% 1	600
	21st to end	Night	8% 7% 34% 35% 15% 1	600
October	1st to 10th	Night	16% 10% 30% 31% 13%	624
	11th to 20th	Night	13% 13% 37% 30% 8%	800
	21st to end	Night	17% 12% 31% 30% 9%	995
November	1st to 10th	Night	8% 12% 37% 36% 6% 1	1025
	11th to 20th	Night	12% 11% 41% 30% 6%	1050
	21st to end	Night	12% 11% 37% 31% 9%	1050
December	1st to 10th	Night	10% 11% 45% 29% 5%	1050
	11th to 20th	Night	13% 10% 42% 29% 6%	1050
	21st to end	Night	16% 13% 37% 29% 5%	1155

Page generated at 2026-08-27 03:07:11. Version ac07

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