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 2025-04-26.

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% 50%	118152
	Night	8% 6% 2 6% 4 4 4 3 5% 7% 51%	119208
Transparency	Day	34% 5% 13% 26% 18% 3	117908
	Night	32% 5% 12% 27% 20% 4	119042
Seeing	Day	14% 10% 32% 30% 13% 2	16327
	Night	12% 9% 36% 33% 9%	26389

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% 13% 4 10% 7% 5% 4 3 4 5% 36%	3638
	11th to 20th	Day	5% 9% 4 8% 5% 4 5% 4 5% 6% 46%	3962
	21st to end	Day	5% 6% 4 12% 5% 4 4 3 4 5% 48%	4488
February	1st to 10th	Day	9% 11% 4 7% 5% 5% 5% 5% 5% 7% 38%	1800
	11th to 20th	Day	6% 9% 3 8% 4 4 4 4 6% 6% 46%	1931
	21st to end	Day	8% 8% 2 5% 4 4 4 3 4 6% 51%	1650
March	1st to 10th	Day	12% 7% 3 5% 3 2 3 2 3 4 56%	4632
	11th to 20th	Day	10% 7% 2 6% 3 3 3 3 4 6% 54%	4980
	21st to end	Day	13% 10% 3 8% 4 4 4 4 5% 8% 38%	5464
April	1st to 10th	Day	12% 7% 2 6% 4 3 4 4 5% 7% 48%	2278
	11th to 20th	Day	14% 8% 2 6% 4 4 3 3 5% 6% 45%	2514
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 40%	2536
May	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 7% 7% 42%	4120
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 5% 7% 47%	4082
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 7% 43%	4489
June	1st to 10th	Day	6% 4 1 4 4 3 4 4 6% 8% 55%	2496
	11th to 20th	Day	9% 5% 2 4 4 4 5% 3 6% 7% 52%	2660
	21st to end	Day	6% 4 1 4 3 4 4 4 7% 10% 53%	2660
July	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 48%	4430
	11th to 20th	Day	6% 4 2 5% 4 4 5% 5% 6% 8% 51%	4289
	21st to end	Day	5% 4 1 5% 4 4 4 3 6% 9% 54%	4466
August	1st to 10th	Day	7% 5% 2 7% 5% 5% 5% 4 7% 8% 45%	4103
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 6% 8% 43%	4041
	21st to end	Day	6% 4 2 5% 4 4 3 3 4 6% 58%	4436
September	1st to 10th	Day	12% 5% 2 5% 3 3 3 2 4 6% 55%	2147
	11th to 20th	Day	8% 4 2 5% 3 3 3 3 5% 7% 57%	2090
	21st to end	Day	9% 4 1 3 2 3 3 3 4 6% 61%	2090
October	1st to 10th	Day	8% 4 2 3 2 2 2 2 3 4 70%	3398
	11th to 20th	Day	6% 3 1 3 2 3 3 2 4 6% 66%	3140
	21st to end	Day	9% 5% 2 5% 3 2 4 2 5% 6% 57%	3246
November	1st to 10th	Day	8% 6% 1 4 4 4 4 4 5% 8% 53%	1763
	11th to 20th	Day	8% 6% 2 5% 4 3 2 3 3 6% 57%	1800
	21st to end	Day	10% 8% 2 5% 3 2 4 3 5% 8% 49%	1800
December	1st to 10th	Day	5% 5% 2 5% 4 3 4 4 4 7% 57%	2940
	11th to 20th	Day	6% 8% 3 7% 4 4 4 3 6% 7% 48%	3391
	21st to end	Day	12% 10% 3 12% 4 4 4 3 4 5% 38%	4202
January	1st to 10th	Night	8% 12% 5% 11% 5% 4 5% 4 6% 6% 34%	6327
	11th to 20th	Night	4 7% 3 7% 4 5% 6% 4 5% 6% 49%	6012
	21st to end	Night	5% 9% 4 12% 4 4 4 3 4 6% 46%	6490
February	1st to 10th	Night	8% 10% 4 8% 5% 5% 5% 3 4 5% 43%	2600
	11th to 20th	Night	6% 9% 3 7% 5% 5% 5% 5% 5% 6% 45%	2373
	21st to end	Night	6% 7% 2 5% 4 4 3 3 5% 6% 54%	1980
March	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 5% 55%	5236
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 4 6% 58%	4980
	21st to end	Night	12% 10% 3 6% 4 4 5% 4 6% 7% 40%	5467
April	1st to 10th	Night	11% 8% 2 5% 3 4 4 3 5% 7% 47%	2119
	11th to 20th	Night	12% 6% 2 5% 4 5% 4 3 6% 9% 44%	1834
	21st to end	Night	14% 5% 2 5% 3 4 3 3 6% 7% 48%	1760
May	1st to 10th	Night	19% 6% 2 4 3 4 4 3 5% 6% 44%	2850
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 6% 7% 48%	2823
	21st to end	Night	14% 5% 2 7% 3 3 4 4 6% 9% 41%	3098
June	1st to 10th	Night	6% 4 1 4 4 4 3 4 8% 10% 53%	1678
	11th to 20th	Night	8% 5% 2 4 3 4 4 3 6% 7% 54%	1520
	21st to end	Night	4 3 1 4 4 3 4 5% 6% 9% 58%	1520
July	1st to 10th	Night	9% 4 2 5% 4 3 4 3 5% 6% 55%	2540
	11th to 20th	Night	5% 3 2 4 3 4 4 4 6% 11% 55%	2664
	21st to end	Night	3 3 1 3 3 3 3 3 6% 9% 62%	3097
August	1st to 10th	Night	4 3 2 5% 4 3 5% 4 7% 12% 52%	2836
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 6% 9% 48%	2798
	21st to end	Night	4 3 2 5% 3 4 4 4 6% 9% 57%	3226
September	1st to 10th	Night	8% 3 1 3 3 3 3 2 5% 6% 64%	2033
	11th to 20th	Night	7% 4 2 4 4 3 3 3 5% 6% 59%	2090
	21st to end	Night	7% 4 1 4 3 3 4 3 5% 5% 61%	2090
October	1st to 10th	Night	9% 2 1 3 2 2 2 2 3 5% 70%	3529
	11th to 20th	Night	6% 3 1 4 2 3 3 2 4 7% 65%	3779
	21st to end	Night	9% 4 1 4 3 3 3 2 5% 7% 59%	4304
November	1st to 10th	Night	6% 5% 1 4 3 2 4 4 5% 7% 61%	2535
	11th to 20th	Night	8% 5% 2 5% 3 4 3 3 4 6% 57%	2600
	21st to end	Night	9% 6% 2 4 3 3 3 2 5% 7% 56%	2600
December	1st to 10th	Night	3 5% 2 4 3 3 4 3 5% 7% 61%	4250
	11th to 20th	Night	7% 4 3 6% 4 4 4 3 6% 8% 51%	4816
	21st to end	Night	13% 12% 4 9% 4 3 4 3 4 6% 39%	6754

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	50% 4 12% 22% 11% 1	3639
	11th to 20th	Day	39% 5% 14% 20% 19% 3	3962
	21st to end	Day	40% 4 13% 26% 14% 3	4488
February	1st to 10th	Day	45% 6% 15% 21% 11% 2	1800
	11th to 20th	Day	38% 5% 15% 23% 15% 4	1941
	21st to end	Day	34% 5% 11% 27% 17% 5%	1650
March	1st to 10th	Day	33% 3 9% 23% 26% 7%	4632
	11th to 20th	Day	32% 4 8% 25% 25% 7%	4980
	21st to end	Day	44% 6% 21% 20% 7% 2	5464
April	1st to 10th	Day	35% 6% 15% 26% 15% 2	2278
	11th to 20th	Day	39% 4 14% 23% 15% 3	2517
	21st to end	Day	43% 6% 12% 25% 13%	2536
May	1st to 10th	Day	37% 8% 14% 22% 15% 4	4120
	11th to 20th	Day	36% 6% 13% 28% 15% 1	4086
	21st to end	Day	39% 7% 13% 29% 12% 1	4489
June	1st to 10th	Day	25% 8% 18% 32% 15% 2	2496
	11th to 20th	Day	31% 7% 13% 29% 17% 3	2660
	21st to end	Day	24% 8% 15% 31% 17% 4	2660
July	1st to 10th	Day	35% 5% 11% 29% 19% 2	4361
	11th to 20th	Day	29% 7% 14% 29% 18% 3	4214
	21st to end	Day	24% 7% 15% 26% 22% 5%	4325
August	1st to 10th	Day	34% 7% 15% 25% 15% 4	4090
	11th to 20th	Day	39% 6% 14% 24% 16% 1	4080
	21st to end	Day	27% 6% 14% 24% 22% 7%	4435
September	1st to 10th	Day	32% 5% 11% 25% 21% 6%	2147
	11th to 20th	Day	27% 6% 12% 31% 22% 3	2090
	21st to end	Day	25% 4 9% 33% 27% 2	2090
October	1st to 10th	Day	21% 2 5% 22% 42% 8%	3398
	11th to 20th	Day	21% 3 11% 36% 20% 9%	3140
	21st to end	Day	29% 3 10% 30% 24% 4	3246
November	1st to 10th	Day	29% 5% 14% 28% 21% 3	1763
	11th to 20th	Day	30% 3 11% 27% 25% 4	1800
	21st to end	Day	35% 4 13% 27% 18% 3	1800
December	1st to 10th	Day	28% 4 13% 33% 20% 3	2940
	11th to 20th	Day	36% 4 19% 27% 13% 1	3389
	21st to end	Day	48% 5% 9% 27% 11% 1	4202
January	1st to 10th	Night	48% 5% 10% 22% 14% 1	6327
	11th to 20th	Night	34% 5% 13% 26% 20% 2	6012
	21st to end	Night	40% 5% 10% 22% 20% 4	6490
February	1st to 10th	Night	43% 5% 12% 23% 14% 3	2600
	11th to 20th	Night	38% 6% 13% 25% 15% 3	2373
	21st to end	Night	31% 5% 11% 23% 22% 7%	1980
March	1st to 10th	Night	31% 4 7% 28% 25% 6%	5236
	11th to 20th	Night	28% 3 6% 22% 32% 9%	4980
	21st to end	Night	42% 6% 19% 25% 8% 1	5467
April	1st to 10th	Night	36% 5% 14% 25% 19% 2	2119
	11th to 20th	Night	36% 6% 14% 26% 16% 3	1834
	21st to end	Night	35% 7% 12% 26% 17% 1	1760
May	1st to 10th	Night	39% 6% 11% 27% 15% 3	2850
	11th to 20th	Night	33% 8% 14% 26% 19%	2824
	21st to end	Night	36% 10% 15% 23% 13% 2	3099
June	1st to 10th	Night	23% 10% 18% 30% 17% 2	1678
	11th to 20th	Night	28% 7% 14% 28% 18% 5%	1520
	21st to end	Night	21% 7% 16% 32% 21% 3	1520
July	1st to 10th	Night	29% 5% 14% 32% 17% 3	2500
	11th to 20th	Night	23% 7% 15% 32% 20% 3	2616
	21st to end	Night	18% 6% 17% 28% 24% 7%	2999
August	1st to 10th	Night	24% 7% 18% 28% 18% 5%	2826
	11th to 20th	Night	33% 6% 17% 28% 14% 1	2825
	21st to end	Night	24% 6% 16% 28% 17% 9%	3226
September	1st to 10th	Night	23% 4 10% 28% 25% 10%	2033
	11th to 20th	Night	26% 4 9% 32% 24% 4	2090
	21st to end	Night	26% 5% 8% 28% 28% 5%	2090
October	1st to 10th	Night	19% 3 6% 17% 46% 9%	3529
	11th to 20th	Night	22% 3 10% 29% 26% 11%	3779
	21st to end	Night	26% 3 10% 29% 27% 4	4304
November	1st to 10th	Night	23% 5% 13% 29% 25% 6%	2535
	11th to 20th	Night	28% 3 10% 29% 24% 5%	2600
	21st to end	Night	29% 3 13% 28% 23% 3	2600
December	1st to 10th	Night	23% 4 15% 36% 20% 2	4250
	11th to 20th	Night	31% 4 15% 33% 16% 1	4817
	21st to end	Night	47% 5% 10% 26% 11% 1	6754

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% 36% 23% 7% 2	300
	11th to 20th	Day	17% 17% 35% 24% 7% 1	300
	21st to end	Day	15% 11% 45% 21% 7% 1	330
February	1st to 10th	Day	15% 16% 45% 19% 5%	300
	11th to 20th	Day	14% 12% 48% 20% 5% 1	300
	21st to end	Day	16% 17% 45% 18% 4	249
March	1st to 10th	Day	15% 16% 42% 22% 4 1	300
	11th to 20th	Day	23% 9% 39% 20% 9%	300
	21st to end	Day	28% 16% 37% 16% 4	330
April	1st to 10th	Day	20% 22% 37% 16% 4	378
	11th to 20th	Day	19% 19% 34% 24% 4	635
	21st to end	Day	22% 17% 30% 20% 8% 2	676
May	1st to 10th	Day	17% 13% 34% 26% 9% 1	653
	11th to 20th	Day	19% 11% 31% 24% 12% 3	675
	21st to end	Day	15% 10% 32% 30% 11% 2	734
June	1st to 10th	Day	6% 7% 27% 36% 20% 5%	680
	11th to 20th	Day	11% 7% 34% 32% 14% 3	680
	21st to end	Day	8% 8% 32% 33% 18% 2	680
July	1st to 10th	Day	10% 4 20% 37% 25% 4	671
	11th to 20th	Day	5% 3 20% 44% 22% 5%	668
	21st to end	Day	7% 3 20% 42% 24% 4	739
August	1st to 10th	Day	8% 3 19% 45% 21% 3	676
	11th to 20th	Day	8% 5% 22% 42% 22% 2	665
	21st to end	Day	10% 3 24% 38% 20% 4	691
September	1st to 10th	Day	14% 5% 27% 37% 15% 3	357
	11th to 20th	Day	6% 7% 33% 36% 14% 5%	300
	21st to end	Day	14% 11% 28% 30% 12% 5%	300
October	1st to 10th	Day	20% 11% 25% 32% 9% 4	300
	11th to 20th	Day	14% 14% 38% 24% 8% 2	300
	21st to end	Day	18% 16% 34% 24% 7% 1	330
November	1st to 10th	Day	15% 16% 39% 20% 8% 2	300
	11th to 20th	Day	13% 16% 44% 21% 6%	300
	21st to end	Day	21% 14% 36% 17% 10% 2	300
December	1st to 10th	Day	9% 13% 43% 29% 5% 1	300
	11th to 20th	Day	16% 10% 43% 26% 5%	300
	21st to end	Day	20% 13% 45% 21% 1	330
January	1st to 10th	Night	12% 11% 37% 32% 7% 1	1000
	11th to 20th	Night	11% 13% 41% 29% 6% 1	1000
	21st to end	Night	11% 9% 43% 30% 7%	1100
February	1st to 10th	Night	14% 12% 44% 26% 5%	1000
	11th to 20th	Night	12% 12% 41% 30% 5%	819
	21st to end	Night	10% 12% 47% 25% 5%	660
March	1st to 10th	Night	13% 13% 43% 27% 4 1	740
	11th to 20th	Night	17% 10% 41% 27% 5% 1	600
	21st to end	Night	20% 12% 39% 24% 5%	660
April	1st to 10th	Night	17% 15% 41% 24% 3	600
	11th to 20th	Night	16% 14% 43% 24% 3 1	593
	21st to end	Night	18% 12% 31% 31% 7% 1	587
May	1st to 10th	Night	17% 10% 36% 30% 7%	570
	11th to 20th	Night	15% 8% 34% 34% 8% 1	561
	21st to end	Night	16% 8% 34% 30% 11% 1	617
June	1st to 10th	Night	8% 5% 28% 42% 15% 2	570
	11th to 20th	Night	9% 7% 38% 36% 10%	570
	21st to end	Night	6% 4 34% 43% 13%	570
July	1st to 10th	Night	6% 4 21% 45% 23% 1	555
	11th to 20th	Night	3 2 22% 51% 21% 1	552
	21st to end	Night	5% 2 19% 52% 22% 1	624
August	1st to 10th	Night	5% 3 21% 47% 22% 2	567
	11th to 20th	Night	7% 2 25% 47% 18%	564
	21st to end	Night	7% 4 26% 43% 19% 1	627
September	1st to 10th	Night	9% 3 28% 44% 15% 1	570
	11th to 20th	Night	9% 6% 33% 38% 14% 1	570
	21st to end	Night	8% 7% 35% 34% 15% 1	570
October	1st to 10th	Night	16% 10% 30% 31% 13%	593
	11th to 20th	Night	13% 11% 37% 31% 8%	760
	21st to end	Night	16% 12% 32% 30% 10%	945
November	1st to 10th	Night	9% 12% 36% 37% 6% 1	975
	11th to 20th	Night	12% 11% 40% 30% 6%	1000
	21st to end	Night	11% 10% 38% 31% 9%	1000
December	1st to 10th	Night	10% 11% 45% 29% 5%	1000
	11th to 20th	Night	13% 10% 42% 30% 6%	1000
	21st to end	Night	16% 13% 37% 29% 5%	1100

Page generated at 2025-04-23 08:25:07. Version ac07

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