GrandIsland Sky Watchers
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GrandIsland Sky Watchers clear sky chart from 2005-11-06 to 2025-12-03.

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% 11% 4 9% 5% 5% 4 3 5% 7% 31%	127201
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 30%	123479
Transparency	Day	51% 7% 12% 19% 10% 1	130794
	Night	53% 7% 11% 17% 10% 1	127972
Seeing	Day	22% 16% 28% 21% 10% 3	15859
	Night	22% 15% 28% 24% 10% 1	25277

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% 20% 7% 18% 10% 4 4 3 3 4 15%	3597
	11th to 20th	Day	12% 21% 9% 19% 9% 6% 4 3 4 4 9%	3916
	21st to end	Day	19% 16% 8% 14% 8% 7% 5% 3 4 4 12%	4466
February	1st to 10th	Day	18% 19% 4 14% 7% 7% 4 3 4 5% 15%	1800
	11th to 20th	Day	14% 18% 7% 16% 9% 5% 5% 3 3 4 16%	1753
	21st to end	Day	20% 15% 5% 12% 7% 5% 4 4 4 4 19%	1558
March	1st to 10th	Day	16% 12% 6% 12% 6% 5% 3 3 4 5% 28%	4932
	11th to 20th	Day	11% 17% 4 8% 5% 5% 5% 3 5% 6% 31%	4980
	21st to end	Day	19% 13% 3 9% 4 4 4 3 4 6% 30%	5465
April	1st to 10th	Day	18% 14% 4 8% 4 4 4 3 4 6% 30%	2380
	11th to 20th	Day	20% 11% 2 7% 4 4 4 3 4 6% 35%	2399
	21st to end	Day	20% 9% 3 8% 5% 4 4 3 4 5% 34%	2600
May	1st to 10th	Day	21% 10% 3 8% 5% 4 4 3 5% 6% 31%	5888
	11th to 20th	Day	15% 11% 4 9% 5% 5% 5% 4 5% 7% 32%	6290
	21st to end	Day	19% 8% 2 6% 4 4 4 3 6% 7% 37%	6949
June	1st to 10th	Day	15% 6% 3 6% 4 4 4 5% 6% 7% 40%	2800
	11th to 20th	Day	12% 6% 2 6% 6% 5% 5% 4 7% 9% 38%	2800
	21st to end	Day	14% 5% 2 5% 5% 4 4 4 7% 8% 42%	2800
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 44%	6360
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 4 6% 9% 41%	6360
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 47%	6475
August	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	4234
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 5% 7% 11% 35%	4174
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 9% 45%	4426
September	1st to 10th	Day	14% 7% 2 5% 4 4 4 3 7% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 6% 7% 46%	2313
	21st to end	Day	15% 8% 3 7% 6% 6% 5% 4 7% 8% 33%	2139
October	1st to 10th	Day	20% 7% 2 6% 5% 4 4 3 5% 7% 37%	2000
	11th to 20th	Day	21% 10% 4 9% 4 4 5% 3 6% 7% 29%	2000
	21st to end	Day	25% 13% 4 10% 5% 4 4 3 4 5% 24%	2200
November	1st to 10th	Day	15% 11% 3 9% 6% 6% 5% 4 5% 6% 31%	1825
	11th to 20th	Day	14% 14% 5% 9% 5% 5% 4 3 5% 7% 29%	1680
	21st to end	Day	19% 18% 4 10% 6% 5% 4 4 4 6% 20%	1680
December	1st to 10th	Day	21% 18% 6% 11% 7% 6% 4 4 5% 5% 13%	2616
	11th to 20th	Day	21% 18% 5% 10% 5% 4 4 3 5% 5% 21%	2986
	21st to end	Day	23% 21% 6% 12% 7% 5% 4 3 3 4 12%	3960
January	1st to 10th	Night	15% 16% 6% 16% 10% 5% 4 3 4 4 17%	6349
	11th to 20th	Night	11% 19% 8% 18% 8% 6% 5% 3 5% 5% 14%	5954
	21st to end	Night	21% 17% 6% 16% 7% 5% 4 3 4 4 14%	6490
February	1st to 10th	Night	17% 17% 6% 13% 6% 4 4 3 5% 4 20%	2600
	11th to 20th	Night	15% 16% 5% 12% 7% 5% 5% 3 5% 5% 21%	2533
	21st to end	Night	18% 15% 5% 14% 6% 5% 4 3 4 6% 19%	2067
March	1st to 10th	Night	18% 12% 5% 10% 8% 4 4 3 4 5% 28%	4908
	11th to 20th	Night	14% 12% 3 7% 4 4 5% 4 5% 7% 35%	4979
	21st to end	Night	19% 10% 3 9% 5% 4 3 3 4 5% 35%	5467
April	1st to 10th	Night	20% 12% 4 8% 6% 5% 3 3 4 4 30%	2020
	11th to 20th	Night	19% 8% 3 6% 5% 4 4 4 5% 6% 35%	1967
	21st to end	Night	17% 9% 4 6% 4 4 6% 5% 5% 6% 35%	1800
May	1st to 10th	Night	24% 12% 4 8% 4 4 3 3 4 5% 30%	4053
	11th to 20th	Night	12% 8% 2 7% 6% 5% 6% 4 7% 8% 36%	3603
	21st to end	Night	17% 7% 3 6% 4 4 4 4 5% 7% 38%	3926
June	1st to 10th	Night	13% 6% 3 7% 6% 5% 6% 5% 7% 7% 36%	1600
	11th to 20th	Night	10% 6% 2 6% 6% 5% 5% 4 6% 7% 42%	1600
	21st to end	Night	13% 6% 3 6% 4 5% 5% 4 5% 9% 41%	1600
July	1st to 10th	Night	9% 6% 3 7% 4 5% 5% 4 7% 9% 43%	3600
	11th to 20th	Night	10% 6% 2 6% 4 5% 5% 3 6% 8% 46%	3600
	21st to end	Night	9% 5% 2 6% 4 4 4 3 6% 7% 50%	4373
August	1st to 10th	Night	10% 6% 2 6% 5% 5% 4 4 7% 8% 43%	2915
	11th to 20th	Night	14% 6% 3 7% 4 4 5% 3 6% 7% 40%	2875
	21st to end	Night	8% 5% 2 6% 4 4 4 4 7% 10% 46%	3437
September	1st to 10th	Night	13% 6% 2 6% 3 4 5% 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 52%	2083
	21st to end	Night	17% 7% 3 7% 4 4 4 4 5% 7% 37%	2260
October	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 6% 41%	2400
	11th to 20th	Night	22% 10% 3 7% 5% 4 4 3 5% 6% 31%	2400
	21st to end	Night	25% 10% 3 8% 4 4 4 4 5% 5% 28%	2640
November	1st to 10th	Night	16% 11% 3 7% 5% 4 4 3 4 6% 38%	2687
	11th to 20th	Night	15% 13% 4 9% 6% 5% 5% 4 6% 6% 29%	2940
	21st to end	Night	18% 16% 5% 9% 5% 4 5% 4 5% 6% 22%	2940
December	1st to 10th	Night	17% 21% 7% 12% 6% 5% 5% 3 5% 5% 14%	4609
	11th to 20th	Night	18% 21% 5% 10% 4 4 4 3 4 5% 21%	5208
	21st to end	Night	25% 17% 6% 10% 6% 6% 4 3 4 5% 14%	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	74% 9% 7% 7% 4	3598
	11th to 20th	Day	78% 7% 6% 6% 3 1	3950
	21st to end	Day	76% 7% 10% 4 3	4466
February	1st to 10th	Day	73% 7% 9% 10% 2	1800
	11th to 20th	Day	71% 7% 9% 11% 2	1762
	21st to end	Day	67% 8% 10% 10% 5%	1558
March	1st to 10th	Day	58% 6% 13% 15% 8%	4935
	11th to 20th	Day	51% 7% 19% 14% 7% 1	4980
	21st to end	Day	55% 8% 8% 20% 8% 1	5465
April	1st to 10th	Day	56% 5% 12% 17% 9% 1	2380
	11th to 20th	Day	50% 6% 11% 19% 13% 1	2399
	21st to end	Day	52% 6% 11% 17% 12% 2	2600
May	1st to 10th	Day	53% 7% 11% 15% 10% 4	5897
	11th to 20th	Day	50% 9% 10% 18% 12% 1	6294
	21st to end	Day	45% 8% 11% 22% 12% 2	6949
June	1st to 10th	Day	42% 8% 14% 19% 16% 2	2800
	11th to 20th	Day	40% 8% 14% 23% 13% 1	2800
	21st to end	Day	38% 9% 14% 25% 14% 1	2800
July	1st to 10th	Day	35% 7% 13% 27% 17% 1	6290
	11th to 20th	Day	37% 8% 13% 27% 14% 1	6285
	21st to end	Day	33% 8% 15% 26% 18% 1	6289
August	1st to 10th	Day	42% 8% 15% 21% 13% 1	4221
	11th to 20th	Day	42% 9% 15% 24% 9% 1	4213
	21st to end	Day	35% 7% 12% 34% 12% 1	4426
September	1st to 10th	Day	39% 8% 15% 25% 12% 1	2400
	11th to 20th	Day	35% 8% 12% 31% 14% 1	2313
	21st to end	Day	47% 8% 15% 21% 9% 1	2139
October	1st to 10th	Day	37% 7% 14% 27% 15% 1	3260
	11th to 20th	Day	46% 6% 16% 20% 12%	3240
	21st to end	Day	51% 5% 15% 21% 7% 1	3538
November	1st to 10th	Day	53% 7% 12% 18% 8% 1	1825
	11th to 20th	Day	55% 8% 15% 15% 7% 1	1680
	21st to end	Day	64% 8% 13% 11% 4	1680
December	1st to 10th	Day	71% 7% 15% 6% 1	2616
	11th to 20th	Day	66% 9% 9% 10% 7%	2986
	21st to end	Day	76% 6% 9% 7% 2	3960
January	1st to 10th	Night	67% 9% 11% 10% 3	6349
	11th to 20th	Night	72% 11% 7% 7% 3	5995
	21st to end	Night	73% 8% 8% 5% 6% 1	6490
February	1st to 10th	Night	68% 8% 9% 11% 4	2600
	11th to 20th	Night	64% 7% 12% 13% 4	2535
	21st to end	Night	66% 7% 10% 12% 5%	2067
March	1st to 10th	Night	58% 5% 11% 17% 8% 1	4918
	11th to 20th	Night	47% 8% 16% 19% 9% 2	4980
	21st to end	Night	50% 6% 10% 20% 14% 1	5467
April	1st to 10th	Night	57% 6% 9% 17% 10% 1	2020
	11th to 20th	Night	49% 6% 10% 19% 14% 2	1967
	21st to end	Night	47% 8% 10% 18% 14% 2	1800
May	1st to 10th	Night	57% 6% 8% 13% 11% 6%	4060
	11th to 20th	Night	44% 10% 14% 20% 11% 1	3604
	21st to end	Night	44% 8% 10% 21% 15% 2	3927
June	1st to 10th	Night	44% 9% 10% 20% 16% 2	1600
	11th to 20th	Night	39% 9% 12% 21% 18% 2	1600
	21st to end	Night	40% 8% 13% 21% 16% 2	1600
July	1st to 10th	Night	37% 6% 12% 24% 19% 2	3560
	11th to 20th	Night	35% 7% 14% 28% 15% 1	3552
	21st to end	Night	33% 7% 13% 24% 22% 1	4244
August	1st to 10th	Night	37% 7% 15% 26% 13% 2	2904
	11th to 20th	Night	41% 8% 14% 27% 10%	2902
	21st to end	Night	32% 8% 14% 32% 13% 1	3437
September	1st to 10th	Night	36% 7% 14% 27% 14% 1	2000
	11th to 20th	Night	31% 5% 13% 32% 18% 1	2083
	21st to end	Night	45% 8% 12% 21% 13% 1	2260
October	1st to 10th	Night	40% 6% 9% 25% 20% 1	3920
	11th to 20th	Night	49% 5% 13% 21% 12% 1	3899
	21st to end	Night	45% 5% 17% 24% 9% 1	4252
November	1st to 10th	Night	47% 7% 12% 22% 11% 2	2687
	11th to 20th	Night	54% 7% 14% 16% 8% 1	2940
	21st to end	Night	61% 9% 13% 14% 3	2940
December	1st to 10th	Night	70% 8% 13% 7% 1	4609
	11th to 20th	Night	66% 7% 7% 11% 8% 1	5208
	21st to end	Night	72% 6% 10% 9% 3	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	27% 28% 29% 12% 4 2	200
	11th to 20th	Day	27% 27% 31% 11% 3 1	277
	21st to end	Day	25% 25% 29% 17% 3	330
February	1st to 10th	Day	32% 26% 29% 11% 2	300
	11th to 20th	Day	29% 30% 31% 8% 1	300
	21st to end	Day	40% 16% 30% 13% 1	302
March	1st to 10th	Day	27% 19% 34% 15% 5% 1	500
	11th to 20th	Day	28% 17% 33% 17% 4 1	500
	21st to end	Day	31% 17% 29% 18% 5% 1	550
April	1st to 10th	Day	25% 23% 29% 16% 5% 1	500
	11th to 20th	Day	30% 21% 24% 16% 7% 2	492
	21st to end	Day	25% 19% 32% 16% 7% 2	500
May	1st to 10th	Day	26% 15% 29% 18% 8% 3	503
	11th to 20th	Day	19% 13% 28% 24% 13% 3	527
	21st to end	Day	17% 12% 29% 25% 12% 5%	573
June	1st to 10th	Day	18% 17% 32% 23% 9% 1	530
	11th to 20th	Day	14% 14% 29% 27% 12% 3	530
	21st to end	Day	17% 12% 32% 28% 9% 2	530
July	1st to 10th	Day	10% 9% 25% 32% 20% 5%	525
	11th to 20th	Day	15% 10% 28% 27% 15% 5%	524
	21st to end	Day	11% 8% 32% 31% 14% 5%	576
August	1st to 10th	Day	13% 7% 23% 31% 19% 8%	526
	11th to 20th	Day	13% 9% 22% 30% 17% 9%	519
	21st to end	Day	12% 9% 30% 25% 16% 8%	583
September	1st to 10th	Day	14% 12% 23% 24% 19% 8%	530
	11th to 20th	Day	10% 7% 20% 24% 26% 13%	530
	21st to end	Day	17% 13% 29% 22% 12% 6%	530
October	1st to 10th	Day	21% 13% 25% 19% 16% 5%	530
	11th to 20th	Day	23% 20% 31% 19% 5% 2	530
	21st to end	Day	34% 20% 23% 17% 6%	583
November	1st to 10th	Day	25% 21% 31% 15% 5% 2	365
	11th to 20th	Day	30% 27% 25% 11% 4 3	220
	21st to end	Day	32% 28% 26% 11% 2	220
December	1st to 10th	Day	29% 25% 32% 10% 3	204
	11th to 20th	Day	33% 25% 27% 14% 2	200
	21st to end	Day	30% 34% 22% 10% 3 1	220
January	1st to 10th	Night	29% 24% 31% 12% 5%	800
	11th to 20th	Night	31% 24% 27% 16% 3	800
	21st to end	Night	24% 20% 32% 18% 5%	880
February	1st to 10th	Night	27% 24% 31% 15% 3	800
	11th to 20th	Night	26% 22% 32% 17% 3	783
	21st to end	Night	28% 21% 32% 18% 2	660
March	1st to 10th	Night	23% 17% 34% 21% 5%	800
	11th to 20th	Night	25% 19% 31% 20% 5% 1	800
	21st to end	Night	24% 13% 32% 23% 8% 1	880
April	1st to 10th	Night	25% 17% 33% 22% 3	800
	11th to 20th	Night	23% 15% 32% 22% 7% 1	783
	21st to end	Night	21% 11% 31% 29% 9%	600
May	1st to 10th	Night	21% 9% 25% 31% 12% 2	600
	11th to 20th	Night	14% 7% 31% 40% 8%	409
	21st to end	Night	14% 8% 27% 34% 15% 2	433
June	1st to 10th	Night	13% 7% 33% 36% 12%	400
	11th to 20th	Night	14% 7% 32% 33% 14%	400
	21st to end	Night	15% 6% 31% 37% 11%	400
July	1st to 10th	Night	7% 7% 25% 42% 19% 1	390
	11th to 20th	Night	10% 6% 29% 36% 18% 1	388
	21st to end	Night	11% 4 25% 38% 21% 1	628
August	1st to 10th	Night	9% 5% 18% 42% 24% 2	597
	11th to 20th	Night	10% 5% 20% 33% 29% 3	594
	21st to end	Night	12% 5% 23% 36% 22% 2	783
September	1st to 10th	Night	15% 7% 23% 32% 20% 3	800
	11th to 20th	Night	13% 5% 20% 29% 29% 5%	800
	21st to end	Night	18% 11% 22% 30% 16% 4	800
October	1st to 10th	Night	18% 10% 27% 26% 17% 2	800
	11th to 20th	Night	25% 16% 27% 23% 8% 1	800
	21st to end	Night	30% 14% 28% 23% 4	880
November	1st to 10th	Night	24% 15% 28% 23% 9% 1	820
	11th to 20th	Night	24% 20% 30% 17% 7% 1	840
	21st to end	Night	26% 22% 30% 16% 5%	840
December	1st to 10th	Night	32% 23% 25% 17% 4	809
	11th to 20th	Night	28% 25% 28% 16% 3	800
	21st to end	Night	32% 19% 31% 16% 2	880

Page generated at 2025-11-30 01:00:53. Version ac07

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