Gronlund Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gronlund Observing Site clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	131265
	Night	11% 11% 4 10% 6% 5% 5% 4 5% 6% 32%	129888
Transparency	Day	48% 7% 13% 22% 10% 1	134729
	Night	51% 6% 13% 20% 9% 1	134510
Seeing	Day	20% 18% 35% 19% 7% 2	17809
	Night	18% 15% 32% 25% 9% 1	27848

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	7% 13% 6% 19% 8% 7% 4 3 5% 6% 20%	4691
	11th to 20th	Day	5% 15% 5% 16% 8% 9% 9% 7% 7% 5% 13%	4679
	21st to end	Day	4 12% 7% 15% 7% 7% 8% 4 6% 7% 22%	5318
February	1st to 10th	Day	7% 13% 5% 13% 7% 6% 5% 5% 5% 5% 29%	1890
	11th to 20th	Day	8% 13% 4 10% 7% 5% 5% 4 6% 6% 32%	1799
	21st to end	Day	10% 14% 5% 11% 7% 5% 5% 4 5% 7% 27%	1620
March	1st to 10th	Day	12% 10% 3 7% 5% 4 5% 3 6% 8% 38%	4514
	11th to 20th	Day	8% 13% 5% 11% 5% 4 4 4 6% 7% 32%	4980
	21st to end	Day	20% 13% 4 11% 4 3 3 3 4 5% 30%	5739
April	1st to 10th	Day	15% 16% 5% 10% 6% 4 3 3 4 6% 28%	2400
	11th to 20th	Day	18% 13% 5% 7% 5% 3 4 3 5% 5% 31%	2374
	21st to end	Day	20% 13% 4 7% 5% 4 4 3 4 5% 31%	2589
May	1st to 10th	Day	16% 8% 2 5% 3 3 3 3 4 5% 48%	6348
	11th to 20th	Day	14% 9% 3 6% 5% 5% 5% 3 5% 6% 39%	6319
	21st to end	Day	14% 9% 3 7% 5% 5% 4 4 5% 6% 37%	6949
June	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 7% 8% 40%	2800
	11th to 20th	Day	13% 8% 3 8% 6% 4 5% 4 6% 8% 36%	2800
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 6% 10% 36%	2800
July	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 6% 9% 45%	6359
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 10% 46%	6360
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 47%	6926
August	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 4 7% 8% 38%	4531
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 5% 8% 47%	4171
	21st to end	Day	12% 7% 3 7% 4 4 5% 4 6% 8% 41%	4355
September	1st to 10th	Day	12% 6% 3 7% 4 3 4 4 6% 8% 43%	2400
	11th to 20th	Day	12% 7% 3 6% 4 4 5% 4 7% 9% 40%	2400
	21st to end	Day	17% 9% 3 6% 4 4 3 2 4 7% 40%	2123
October	1st to 10th	Day	16% 9% 3 7% 4 4 4 3 5% 7% 38%	2000
	11th to 20th	Day	16% 16% 4 7% 5% 5% 5% 2 5% 6% 29%	2000
	21st to end	Day	18% 16% 3 8% 5% 5% 4 3 4 5% 29%	2030
November	1st to 10th	Day	11% 17% 4 9% 6% 4 4 4 5% 6% 30%	1797
	11th to 20th	Day	11% 19% 6% 13% 7% 5% 5% 3 4 5% 23%	1680
	21st to end	Day	13% 14% 6% 15% 6% 6% 4 4 4 5% 23%	1680
December	1st to 10th	Day	8% 16% 7% 27% 9% 6% 4 3 4 4 13%	2690
	11th to 20th	Day	9% 16% 7% 13% 10% 6% 5% 5% 4 5% 20%	3084
	21st to end	Day	16% 17% 8% 18% 7% 5% 5% 3 3 4 15%	4070
January	1st to 10th	Night	8% 13% 7% 16% 8% 6% 6% 4 5% 5% 22%	8233
	11th to 20th	Night	5% 15% 7% 17% 9% 8% 8% 8% 5% 4 14%	8236
	21st to end	Night	5% 13% 6% 12% 8% 8% 7% 4 6% 6% 25%	8882
February	1st to 10th	Night	7% 14% 6% 11% 8% 5% 5% 4 5% 6% 29%	2730
	11th to 20th	Night	7% 9% 4 10% 8% 7% 6% 4 6% 6% 33%	2599
	21st to end	Night	10% 14% 3 11% 6% 6% 5% 3 5% 7% 30%	2009
March	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 5% 7% 45%	5362
	11th to 20th	Night	11% 10% 4 14% 7% 5% 5% 4 6% 6% 28%	4986
	21st to end	Night	20% 15% 5% 10% 4 4 3 3 3 4 29%	5192
April	1st to 10th	Night	18% 13% 4 8% 4 4 4 3 5% 6% 31%	2000
	11th to 20th	Night	20% 11% 4 8% 4 4 4 3 5% 5% 33%	1975
	21st to end	Night	19% 11% 3 7% 4 4 4 4 5% 5% 35%	1806
May	1st to 10th	Night	13% 7% 3 6% 4 4 3 3 4 6% 47%	3613
	11th to 20th	Night	13% 6% 3 6% 4 4 4 3 5% 5% 46%	3596
	21st to end	Night	10% 9% 3 8% 5% 5% 5% 4 6% 8% 38%	3949
June	1st to 10th	Night	13% 6% 3 6% 4 4 4 4 7% 8% 42%	1600
	11th to 20th	Night	11% 6% 3 7% 4 4 5% 4 7% 9% 39%	1600
	21st to end	Night	11% 7% 3 6% 5% 4 5% 3 7% 8% 41%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 5% 9% 48%	3620
	11th to 20th	Night	10% 4 2 4 3 3 3 4 6% 9% 52%	3620
	21st to end	Night	9% 7% 2 6% 4 4 4 4 5% 7% 47%	3948
August	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	2625
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 6% 7% 49%	2888
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 7% 42%	3544
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 6% 7% 50%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 8% 48%	2000
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 43%	2255
October	1st to 10th	Night	15% 7% 3 6% 4 3 4 4 5% 6% 43%	2400
	11th to 20th	Night	16% 10% 4 8% 5% 4 5% 4 5% 7% 34%	2400
	21st to end	Night	18% 13% 4 8% 4 5% 5% 3 4 6% 30%	2808
November	1st to 10th	Night	12% 13% 5% 9% 5% 4 5% 3 4 8% 32%	2719
	11th to 20th	Night	11% 15% 5% 12% 7% 6% 4 3 4 6% 27%	2940
	21st to end	Night	12% 13% 5% 13% 8% 6% 4 4 4 6% 24%	2940
December	1st to 10th	Night	8% 13% 7% 22% 10% 6% 5% 4 5% 5% 16%	4720
	11th to 20th	Night	9% 15% 8% 15% 7% 6% 7% 4 5% 5% 19%	5343
	21st to end	Night	19% 19% 7% 13% 6% 5% 4 3 4 5% 18%	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	66% 5% 11% 14% 5%	4692
	11th to 20th	Day	69% 6% 8% 12% 5%	4710
	21st to end	Day	60% 5% 10% 19% 6%	5318
February	1st to 10th	Day	56% 7% 11% 16% 10% 1	1890
	11th to 20th	Day	51% 8% 11% 21% 8%	1808
	21st to end	Day	56% 7% 11% 15% 10% 1	1620
March	1st to 10th	Day	43% 7% 15% 22% 10% 2	4516
	11th to 20th	Day	49% 6% 15% 18% 11% 1	4980
	21st to end	Day	56% 6% 11% 16% 11%	5739
April	1st to 10th	Day	57% 6% 10% 16% 10% 1	2400
	11th to 20th	Day	55% 5% 10% 19% 9% 2	2374
	21st to end	Day	55% 6% 10% 18% 10% 1	2589
May	1st to 10th	Day	38% 6% 13% 24% 18% 1	6357
	11th to 20th	Day	43% 8% 14% 21% 9% 5%	6323
	21st to end	Day	44% 8% 10% 22% 16% 1	6949
June	1st to 10th	Day	38% 9% 15% 25% 12% 1	2800
	11th to 20th	Day	44% 9% 14% 19% 14% 1	2800
	21st to end	Day	41% 9% 16% 24% 9% 1	2800
July	1st to 10th	Day	34% 6% 20% 30% 9% 1	6291
	11th to 20th	Day	32% 7% 19% 31% 10% 1	6285
	21st to end	Day	32% 6% 17% 30% 15%	6728
August	1st to 10th	Day	42% 6% 17% 23% 11% 1	4516
	11th to 20th	Day	35% 6% 19% 32% 8%	4210
	21st to end	Day	39% 7% 18% 26% 10% 1	4354
September	1st to 10th	Day	38% 7% 17% 28% 10%	2400
	11th to 20th	Day	39% 11% 18% 25% 7%	2400
	21st to end	Day	45% 7% 13% 25% 10%	2123
October	1st to 10th	Day	36% 5% 15% 31% 13% 1	3260
	11th to 20th	Day	50% 5% 10% 24% 11%	3240
	21st to end	Day	52% 6% 15% 21% 6%	3255
November	1st to 10th	Day	54% 7% 13% 17% 8% 1	1797
	11th to 20th	Day	64% 6% 11% 13% 5%	1680
	21st to end	Day	64% 7% 12% 12% 3 1	1680
December	1st to 10th	Day	76% 6% 8% 9% 1	2690
	11th to 20th	Day	65% 8% 7% 12% 7%	3085
	21st to end	Day	73% 6% 7% 12% 2	4070
January	1st to 10th	Night	64% 5% 12% 13% 5%	8233
	11th to 20th	Night	71% 8% 10% 9% 3	8280
	21st to end	Night	58% 6% 11% 17% 8%	8882
February	1st to 10th	Night	55% 7% 11% 18% 9% 1	2730
	11th to 20th	Night	50% 8% 14% 18% 9% 1	2600
	21st to end	Night	53% 7% 13% 19% 7% 1	2009
March	1st to 10th	Night	37% 7% 17% 19% 17% 3	5373
	11th to 20th	Night	53% 7% 14% 19% 7%	4987
	21st to end	Night	60% 4 9% 15% 12% 1	5192
April	1st to 10th	Night	54% 7% 9% 16% 13% 1	2000
	11th to 20th	Night	51% 8% 9% 20% 10% 2	1975
	21st to end	Night	50% 5% 10% 22% 12% 1	1806
May	1st to 10th	Night	37% 6% 15% 26% 14% 1	3620
	11th to 20th	Night	39% 7% 11% 23% 14% 6%	3597
	21st to end	Night	41% 9% 13% 23% 13% 1	3950
June	1st to 10th	Night	37% 9% 17% 24% 11% 2	1600
	11th to 20th	Night	38% 9% 15% 20% 17% 1	1600
	21st to end	Night	39% 8% 14% 26% 13% 1	1600
July	1st to 10th	Night	34% 7% 21% 28% 9% 2	3580
	11th to 20th	Night	29% 6% 20% 33% 11% 1	3572
	21st to end	Night	36% 6% 14% 26% 17% 1	3832
August	1st to 10th	Night	37% 6% 15% 25% 15% 2	2616
	11th to 20th	Night	34% 6% 16% 34% 10% 1	2915
	21st to end	Night	39% 8% 17% 24% 10% 1	3544
September	1st to 10th	Night	32% 7% 16% 32% 12% 1	2000
	11th to 20th	Night	34% 8% 19% 31% 8%	2000
	21st to end	Night	41% 8% 15% 26% 10%	2255
October	1st to 10th	Night	30% 5% 10% 33% 20% 2	3920
	11th to 20th	Night	46% 6% 13% 21% 14%	3899
	21st to end	Night	55% 5% 11% 21% 7%	4533
November	1st to 10th	Night	52% 7% 13% 19% 8%	2719
	11th to 20th	Night	58% 8% 13% 14% 7% 1	2940
	21st to end	Night	61% 8% 15% 12% 4 1	2940
December	1st to 10th	Night	72% 6% 10% 10% 2	4720
	11th to 20th	Night	67% 6% 9% 12% 4	5341
	21st to end	Night	69% 6% 8% 13% 5%	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	18% 24% 36% 18% 4 1	330
	11th to 20th	Day	20% 21% 36% 20% 2	330
	21st to end	Day	19% 18% 41% 18% 4	363
February	1st to 10th	Day	20% 25% 41% 9% 4 1	330
	11th to 20th	Day	18% 30% 35% 13% 4 1	315
	21st to end	Day	22% 30% 41% 8%	249
March	1st to 10th	Day	20% 24% 35% 16% 4	300
	11th to 20th	Day	24% 19% 40% 13% 4	300
	21st to end	Day	26% 22% 32% 13% 5% 2	450
April	1st to 10th	Day	26% 22% 34% 13% 5%	500
	11th to 20th	Day	29% 20% 28% 15% 7% 1	492
	21st to end	Day	28% 19% 29% 17% 6% 1	622
May	1st to 10th	Day	22% 21% 33% 16% 6% 2	703
	11th to 20th	Day	23% 18% 33% 18% 5% 3	725
	21st to end	Day	22% 16% 33% 20% 8% 2	789
June	1st to 10th	Day	13% 20% 37% 20% 8% 2	730
	11th to 20th	Day	18% 16% 41% 17% 6% 1	730
	21st to end	Day	18% 14% 38% 19% 10% 2	730
July	1st to 10th	Day	9% 10% 37% 26% 13% 5%	721
	11th to 20th	Day	10% 14% 35% 26% 11% 4	718
	21st to end	Day	9% 11% 38% 23% 13% 5%	794
August	1st to 10th	Day	14% 10% 32% 30% 11% 3	726
	11th to 20th	Day	17% 8% 31% 27% 14% 3	715
	21st to end	Day	17% 12% 30% 24% 14% 3	613
September	1st to 10th	Day	16% 14% 31% 23% 12% 3	530
	11th to 20th	Day	16% 13% 33% 27% 9% 3	530
	21st to end	Day	21% 18% 26% 21% 13% 2	438
October	1st to 10th	Day	23% 19% 29% 17% 9% 2	330
	11th to 20th	Day	27% 25% 37% 8% 3	330
	21st to end	Day	28% 25% 35% 10% 2 1	363
November	1st to 10th	Day	24% 24% 38% 11% 3	330
	11th to 20th	Day	24% 25% 38% 9% 4	330
	21st to end	Day	24% 24% 38% 12% 2 1	330
December	1st to 10th	Day	18% 23% 43% 11% 4	330
	11th to 20th	Day	24% 24% 37% 13% 2	330
	21st to end	Day	29% 24% 35% 9% 2 1	363
January	1st to 10th	Night	16% 21% 37% 20% 6%	1050
	11th to 20th	Night	17% 19% 41% 18% 5% 1	1050
	21st to end	Night	17% 19% 40% 20% 4	1155
February	1st to 10th	Night	16% 22% 36% 20% 6% 1	1050
	11th to 20th	Night	14% 22% 36% 20% 6% 1	1005
	21st to end	Night	20% 19% 35% 23% 4	689
March	1st to 10th	Night	18% 16% 37% 23% 5% 1	800
	11th to 20th	Night	19% 20% 33% 20% 6% 1	607
	21st to end	Night	21% 13% 32% 23% 8% 3	660
April	1st to 10th	Night	24% 15% 33% 24% 4 1	600
	11th to 20th	Night	27% 11% 28% 25% 7% 1	593
	21st to end	Night	26% 10% 28% 26% 9% 2	600
May	1st to 10th	Night	19% 13% 32% 27% 7% 2	600
	11th to 20th	Night	16% 11% 32% 30% 8% 2	591
	21st to end	Night	16% 11% 28% 31% 12%	650
June	1st to 10th	Night	17% 8% 31% 33% 11% 1	600
	11th to 20th	Night	16% 8% 31% 35% 10% 1	600
	21st to end	Night	14% 11% 27% 34% 13% 2	600
July	1st to 10th	Night	6% 7% 27% 38% 20% 2	585
	11th to 20th	Night	10% 8% 33% 31% 17% 1	582
	21st to end	Night	9% 5% 30% 38% 16% 2	657
August	1st to 10th	Night	12% 4 26% 41% 15% 1	597
	11th to 20th	Night	16% 5% 25% 34% 19% 2	594
	21st to end	Night	14% 7% 26% 30% 20% 2	660
September	1st to 10th	Night	12% 8% 28% 32% 17% 2	600
	11th to 20th	Night	12% 9% 30% 31% 17% 2	600
	21st to end	Night	18% 14% 27% 27% 12% 2	765
October	1st to 10th	Night	21% 14% 29% 24% 11% 1	800
	11th to 20th	Night	21% 20% 33% 20% 7%	800
	21st to end	Night	28% 13% 29% 24% 6%	880
November	1st to 10th	Night	22% 15% 35% 22% 4 1	873
	11th to 20th	Night	21% 22% 32% 18% 7%	1050
	21st to end	Night	22% 22% 33% 19% 4	1050
December	1st to 10th	Night	19% 19% 38% 19% 5%	1050
	11th to 20th	Night	22% 22% 31% 20% 5% 1	1050
	21st to end	Night	28% 18% 35% 14% 4 1	1155

Page generated at 2026-02-13 00:42:10. Version ac07

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