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 2024-11-30.

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	13% 10% 4 9% 5% 5% 5% 4 5% 7% 34%	101874
	Night	12% 11% 4 9% 5% 5% 4 4 5% 6% 34%	97877
Transparency	Day	48% 8% 14% 21% 9% 1	105355
	Night	49% 8% 13% 20% 9% 1	102483
Seeing	Day	19% 18% 35% 19% 8% 2	16179
	Night	18% 15% 32% 25% 9% 1	26092

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	9% 17% 9% 19% 6% 5% 4 3 4 5% 20%	2527
	11th to 20th	Day	6% 16% 6% 17% 11% 8% 8% 5% 6% 6% 12%	2530
	21st to end	Day	8% 17% 9% 18% 7% 5% 5% 4 4 5% 17%	2861
February	1st to 10th	Day	7% 13% 5% 13% 6% 6% 5% 5% 6% 5% 30%	1710
	11th to 20th	Day	8% 13% 4 10% 7% 5% 5% 4 6% 6% 31%	1664
	21st to end	Day	10% 13% 5% 11% 7% 6% 5% 4 5% 7% 27%	1541
March	1st to 10th	Day	9% 11% 3 8% 5% 5% 5% 3 5% 7% 37%	3140
	11th to 20th	Day	8% 13% 6% 12% 6% 4 4 4 6% 7% 32%	3480
	21st to end	Day	18% 14% 4 13% 4 4 3 3 5% 5% 26%	4005
April	1st to 10th	Day	15% 16% 5% 10% 6% 4 3 3 4 6% 28%	2280
	11th to 20th	Day	19% 13% 5% 7% 5% 3 4 3 4 5% 31%	2254
	21st to end	Day	20% 13% 4 7% 4 4 4 3 4 5% 31%	2459
May	1st to 10th	Day	19% 9% 3 6% 4 4 4 3 4 6% 39%	4428
	11th to 20th	Day	15% 8% 2 6% 5% 5% 5% 4 6% 6% 39%	4399
	21st to end	Day	18% 9% 3 8% 5% 5% 4 4 5% 6% 33%	4837
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 7% 8% 40%	2660
	11th to 20th	Day	13% 8% 3 7% 5% 4 4 4 6% 7% 37%	2660
	21st to end	Day	13% 6% 2 6% 5% 5% 6% 4 6% 10% 36%	2660
July	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 43%	4440
	11th to 20th	Day	6% 5% 2 5% 4 4 5% 5% 6% 10% 47%	4440
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 50%	4814
August	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 4 7% 7% 38%	4392
	11th to 20th	Day	15% 5% 1 5% 3 3 4 3 5% 8% 48%	4041
	21st to end	Day	12% 7% 3 7% 4 4 5% 4 6% 8% 40%	4222
September	1st to 10th	Day	12% 6% 3 7% 4 3 4 4 6% 8% 44%	2280
	11th to 20th	Day	11% 7% 3 5% 4 4 5% 4 7% 9% 40%	2280
	21st to end	Day	18% 9% 3 6% 4 5% 3 2 4 7% 39%	2017
October	1st to 10th	Day	17% 9% 3 7% 4 4 4 3 5% 7% 36%	1900
	11th to 20th	Day	17% 16% 4 8% 5% 4 5% 2 5% 6% 28%	1900
	21st to end	Day	19% 16% 3 8% 5% 5% 4 3 4 5% 29%	1929
November	1st to 10th	Day	11% 17% 4 9% 5% 4 4 4 6% 6% 30%	1710
	11th to 20th	Day	11% 19% 6% 13% 7% 6% 5% 3 4 5% 21%	1600
	21st to end	Day	13% 14% 6% 14% 6% 6% 4 4 4 5% 23%	1597
December	1st to 10th	Day	10% 17% 6% 16% 8% 6% 5% 3 4 6% 18%	1520
	11th to 20th	Day	11% 15% 6% 13% 9% 6% 6% 5% 5% 5% 18%	1914
	21st to end	Day	19% 17% 7% 17% 6% 5% 4 3 3 4 15%	2783
January	1st to 10th	Night	12% 15% 7% 13% 6% 6% 5% 4 5% 5% 23%	4428
	11th to 20th	Night	7% 13% 7% 17% 9% 9% 6% 5% 7% 6% 16%	4440
	21st to end	Night	8% 19% 9% 15% 6% 6% 4 4 4 5% 20%	4792
February	1st to 10th	Night	6% 14% 6% 11% 8% 5% 4 4 5% 6% 30%	2470
	11th to 20th	Night	8% 9% 4 9% 8% 7% 5% 4 6% 6% 33%	2403
	21st to end	Night	10% 14% 3 11% 6% 6% 5% 3 4 6% 31%	1912
March	1st to 10th	Night	8% 8% 4 8% 4 4 4 4 5% 8% 43%	3722
	11th to 20th	Night	8% 9% 4 16% 6% 5% 4 4 5% 5% 32%	3486
	21st to end	Night	19% 15% 6% 12% 4 4 3 3 3 3 28%	3626
April	1st to 10th	Night	17% 13% 4 9% 5% 5% 4 3 5% 6% 31%	1900
	11th to 20th	Night	19% 11% 4 8% 4 4 4 3 5% 5% 32%	1875
	21st to end	Night	19% 12% 3 7% 4 4 4 4 5% 5% 34%	1716
May	1st to 10th	Night	16% 8% 3 8% 4 5% 4 3 5% 7% 37%	2523
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 46%	2506
	21st to end	Night	11% 9% 3 9% 6% 5% 5% 5% 6% 8% 34%	2750
June	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 7% 8% 42%	1520
	11th to 20th	Night	10% 6% 3 7% 4 4 5% 4 7% 9% 40%	1520
	21st to end	Night	10% 7% 3 6% 5% 4 5% 3 7% 8% 42%	1520
July	1st to 10th	Night	9% 4 2 5% 3 4 5% 4 5% 9% 49%	2530
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 7% 10% 54%	2530
	21st to end	Night	7% 7% 2 5% 3 3 3 4 4 7% 54%	2750
August	1st to 10th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 44%	2544
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 6% 7% 50%	2798
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 6% 42%	3435
September	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 6% 7% 51%	1900
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 49%	1900
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 42%	2142
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 5% 6% 43%	2280
	11th to 20th	Night	16% 10% 4 8% 5% 4 5% 4 5% 6% 33%	2280
	21st to end	Night	19% 14% 4 7% 4 5% 5% 3 4 6% 29%	2667
November	1st to 10th	Night	12% 13% 5% 9% 5% 4 5% 3 4 8% 32%	2586
	11th to 20th	Night	12% 15% 5% 12% 7% 6% 4 3 4 6% 25%	2800
	21st to end	Night	13% 13% 5% 12% 8% 6% 4 4 4 6% 25%	2799
December	1st to 10th	Night	9% 14% 6% 14% 8% 5% 5% 5% 5% 7% 22%	2660
	11th to 20th	Night	10% 14% 6% 13% 8% 7% 5% 4 7% 6% 20%	3283
	21st to end	Night	22% 21% 6% 11% 5% 4 3 2 3 4 18%	4884

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	68% 6% 9% 13% 3	2528
	11th to 20th	Day	71% 7% 9% 11% 2	2530
	21st to end	Day	69% 7% 11% 10% 4	2861
February	1st to 10th	Day	54% 8% 11% 16% 10% 1	1710
	11th to 20th	Day	52% 8% 11% 20% 7%	1673
	21st to end	Day	57% 8% 10% 15% 10% 1	1541
March	1st to 10th	Day	44% 10% 15% 22% 9% 1	3142
	11th to 20th	Day	49% 8% 16% 18% 9%	3480
	21st to end	Day	58% 7% 11% 18% 5% 1	4005
April	1st to 10th	Day	57% 6% 10% 16% 10% 1	2280
	11th to 20th	Day	55% 5% 10% 19% 9% 2	2254
	21st to end	Day	54% 7% 10% 18% 10% 1	2459
May	1st to 10th	Day	44% 8% 11% 22% 14% 1	4440
	11th to 20th	Day	40% 10% 15% 25% 9% 1	4403
	21st to end	Day	47% 9% 11% 17% 14% 1	4837
June	1st to 10th	Day	37% 9% 16% 24% 12% 1	2660
	11th to 20th	Day	43% 9% 13% 19% 15% 1	2660
	21st to end	Day	41% 10% 17% 24% 8% 1	2660
July	1st to 10th	Day	36% 7% 20% 25% 12% 1	4371
	11th to 20th	Day	29% 9% 17% 32% 12% 1	4365
	21st to end	Day	30% 7% 15% 31% 16% 1	4662
August	1st to 10th	Day	42% 6% 16% 23% 11% 1	4377
	11th to 20th	Day	34% 6% 19% 32% 8%	4080
	21st to end	Day	39% 7% 18% 25% 10% 1	4221
September	1st to 10th	Day	37% 8% 17% 28% 10%	2280
	11th to 20th	Day	38% 11% 18% 25% 8%	2280
	21st to end	Day	45% 7% 13% 25% 9%	2017
October	1st to 10th	Day	37% 5% 15% 30% 13% 1	3160
	11th to 20th	Day	50% 5% 10% 24% 11%	3140
	21st to end	Day	52% 6% 15% 21% 6%	3154
November	1st to 10th	Day	53% 7% 13% 18% 7% 1	1710
	11th to 20th	Day	66% 6% 11% 12% 5%	1600
	21st to end	Day	63% 7% 13% 13% 4 1	1597
December	1st to 10th	Day	68% 8% 11% 11% 2	1520
	11th to 20th	Day	65% 11% 10% 10% 3	1915
	21st to end	Day	75% 6% 7% 10% 2	2783
January	1st to 10th	Night	62% 9% 12% 15% 3	4428
	11th to 20th	Night	65% 11% 14% 9% 2	4440
	21st to end	Night	65% 8% 10% 14% 3	4792
February	1st to 10th	Night	55% 7% 10% 18% 9% 1	2470
	11th to 20th	Night	50% 8% 15% 18% 8% 1	2404
	21st to end	Night	53% 7% 13% 19% 7% 2	1912
March	1st to 10th	Night	37% 9% 17% 20% 15% 2	3733
	11th to 20th	Night	49% 9% 13% 21% 7% 1	3487
	21st to end	Night	62% 5% 8% 16% 9% 1	3626
April	1st to 10th	Night	54% 7% 10% 16% 13% 1	1900
	11th to 20th	Night	51% 8% 9% 20% 10% 2	1875
	21st to end	Night	50% 6% 10% 22% 12% 1	1716
May	1st to 10th	Night	44% 8% 15% 21% 12% 1	2530
	11th to 20th	Night	37% 7% 12% 26% 15% 3	2507
	21st to end	Night	44% 12% 15% 17% 11% 1	2751
June	1st to 10th	Night	37% 9% 18% 24% 11% 2	1520
	11th to 20th	Night	37% 9% 15% 20% 18% 1	1520
	21st to end	Night	38% 8% 15% 26% 12% 1	1520
July	1st to 10th	Night	32% 8% 21% 24% 11% 3	2490
	11th to 20th	Night	24% 8% 18% 36% 13% 1	2482
	21st to end	Night	29% 7% 13% 29% 21% 2	2662
August	1st to 10th	Night	37% 6% 15% 25% 15% 2	2535
	11th to 20th	Night	34% 6% 16% 34% 10% 1	2825
	21st to end	Night	40% 8% 17% 24% 10% 1	3435
September	1st to 10th	Night	31% 8% 16% 32% 12% 1	1900
	11th to 20th	Night	32% 8% 19% 32% 8%	1900
	21st to end	Night	42% 8% 15% 26% 9%	2142
October	1st to 10th	Night	30% 5% 10% 33% 21% 2	3800
	11th to 20th	Night	46% 6% 13% 21% 14%	3779
	21st to end	Night	55% 5% 11% 21% 7%	4392
November	1st to 10th	Night	52% 8% 13% 19% 8%	2586
	11th to 20th	Night	60% 8% 12% 12% 7%	2800
	21st to end	Night	60% 8% 15% 13% 4 1	2799
December	1st to 10th	Night	63% 8% 13% 12% 3	2660
	11th to 20th	Night	62% 9% 13% 13% 3	3281
	21st to end	Night	71% 6% 8% 11% 4	4884

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	20% 23% 33% 19% 4 1	270
	11th to 20th	Day	20% 20% 38% 20% 3	270
	21st to end	Day	20% 15% 41% 20% 4	297
February	1st to 10th	Day	19% 23% 43% 9% 4 1	270
	11th to 20th	Day	20% 32% 33% 10% 4 1	270
	21st to end	Day	20% 30% 41% 8%	225
March	1st to 10th	Day	19% 23% 37% 17% 4	270
	11th to 20th	Day	23% 19% 41% 14% 4	270
	21st to end	Day	25% 21% 32% 14% 6% 2	411
April	1st to 10th	Day	24% 22% 35% 13% 5%	460
	11th to 20th	Day	28% 20% 28% 16% 7% 1	452
	21st to end	Day	28% 18% 29% 18% 6% 1	576
May	1st to 10th	Day	23% 22% 34% 14% 5% 1	653
	11th to 20th	Day	23% 17% 34% 18% 5% 3	675
	21st to end	Day	22% 16% 33% 19% 8% 2	734
June	1st to 10th	Day	13% 19% 37% 20% 9% 2	680
	11th to 20th	Day	18% 17% 41% 17% 6% 1	680
	21st to end	Day	17% 13% 37% 19% 11% 3	680
July	1st to 10th	Day	9% 10% 36% 26% 14% 5%	671
	11th to 20th	Day	9% 15% 35% 25% 11% 4	668
	21st to end	Day	9% 11% 37% 24% 14% 5%	739
August	1st to 10th	Day	14% 9% 32% 30% 11% 3	676
	11th to 20th	Day	17% 7% 30% 28% 15% 3	665
	21st to end	Day	18% 12% 30% 24% 14% 3	568
September	1st to 10th	Day	16% 13% 32% 23% 12% 2	490
	11th to 20th	Day	15% 13% 34% 27% 9% 3	490
	21st to end	Day	23% 19% 26% 19% 12% 1	403
October	1st to 10th	Day	25% 18% 28% 18% 9% 2	300
	11th to 20th	Day	27% 23% 38% 8% 3	300
	21st to end	Day	29% 25% 35% 9% 2	330
November	1st to 10th	Day	22% 24% 40% 10% 3	300
	11th to 20th	Day	24% 26% 39% 8% 3	300
	21st to end	Day	23% 24% 40% 10% 2 1	299
December	1st to 10th	Day	20% 23% 40% 12% 4	270
	11th to 20th	Day	25% 22% 36% 14% 2	270
	21st to end	Day	30% 25% 34% 8% 2	297
January	1st to 10th	Night	17% 21% 36% 20% 7%	950
	11th to 20th	Night	17% 18% 41% 18% 5% 1	950
	21st to end	Night	18% 18% 40% 20% 4	1045
February	1st to 10th	Night	15% 21% 37% 20% 6% 1	950
	11th to 20th	Night	15% 22% 36% 20% 6% 1	929
	21st to end	Night	20% 19% 34% 23% 4	656
March	1st to 10th	Night	18% 17% 36% 23% 6% 1	760
	11th to 20th	Night	19% 20% 33% 20% 7% 1	577
	21st to end	Night	20% 13% 32% 24% 8% 3	627
April	1st to 10th	Night	23% 15% 33% 24% 4 1	570
	11th to 20th	Night	28% 11% 29% 25% 7% 1	563
	21st to end	Night	25% 10% 27% 26% 9% 2	570
May	1st to 10th	Night	20% 13% 32% 27% 6% 1	570
	11th to 20th	Night	16% 11% 32% 30% 9% 2	561
	21st to end	Night	17% 12% 28% 30% 13%	617
June	1st to 10th	Night	17% 8% 31% 33% 11% 1	570
	11th to 20th	Night	15% 8% 31% 35% 10% 1	570
	21st to end	Night	14% 11% 27% 34% 14% 2	570
July	1st to 10th	Night	6% 7% 27% 37% 21% 2	555
	11th to 20th	Night	9% 9% 33% 30% 18% 1	552
	21st to end	Night	8% 4 30% 38% 17% 2	624
August	1st to 10th	Night	12% 4 27% 42% 14% 1	567
	11th to 20th	Night	16% 5% 24% 34% 19% 2	564
	21st to end	Night	15% 7% 26% 30% 20% 2	627
September	1st to 10th	Night	11% 8% 28% 33% 17% 2	570
	11th to 20th	Night	11% 9% 31% 31% 17% 2	570
	21st to end	Night	19% 14% 28% 26% 11% 2	727
October	1st to 10th	Night	22% 14% 29% 24% 11% 1	760
	11th to 20th	Night	21% 20% 33% 19% 7%	760
	21st to end	Night	29% 13% 29% 24% 6%	836
November	1st to 10th	Night	21% 15% 35% 23% 5% 1	830
	11th to 20th	Night	21% 22% 32% 18% 6%	1000
	21st to end	Night	21% 22% 34% 19% 4	1000
December	1st to 10th	Night	19% 19% 37% 19% 5% 1	950
	11th to 20th	Night	20% 21% 32% 20% 5% 1	950
	21st to end	Night	29% 18% 35% 14% 4	1045

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