Grundland Astronomy Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grundland Astronomy Park clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 4 5% 4 6% 8% 43%	126234
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 43%	123479
Transparency	Day	24% 39% 9% 15% 11% 2	129825
	Night	25% 39% 7% 15% 12% 3	127958
Seeing	Day	15% 6% 22% 30% 21% 6%	16514
	Night	16% 7% 25% 32% 18% 3	26046

By Month

Percentage Cloud Cover SE (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	14% 4 1 4 3 3 4 3 4 4 56%	3781
	11th to 20th	Day	11% 7% 2 6% 4 3 3 3 4 5% 53%	4060
	21st to end	Day	10% 8% 2 8% 5% 5% 5% 4 5% 6% 44%	4466
February	1st to 10th	Day	12% 9% 3 5% 5% 5% 5% 4 6% 7% 40%	1800
	11th to 20th	Day	21% 7% 2 4 3 3 3 4 5% 6% 43%	1911
	21st to end	Day	16% 8% 2 5% 4 3 5% 3 7% 7% 40%	1650
March	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 4 7% 41%	4748
	11th to 20th	Day	9% 7% 3 6% 4 4 3 3 4 7% 51%	4980
	21st to end	Day	13% 7% 2 5% 3 4 4 4 5% 7% 46%	5643
April	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 7% 49%	2400
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 7% 49%	2374
	21st to end	Day	12% 5% 2 6% 5% 3 4 4 6% 7% 45%	2400
May	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 7% 10% 34%	5405
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 4 6% 8% 33%	6111
	21st to end	Day	13% 6% 2 5% 3 4 4 4 6% 10% 43%	6934
June	1st to 10th	Day	10% 5% 2 5% 4 5% 4 5% 8% 11% 43%	2800
	11th to 20th	Day	10% 6% 2 4 4 4 5% 3 7% 11% 44%	2800
	21st to end	Day	6% 3 2 5% 5% 5% 5% 5% 8% 12% 44%	2800
July	1st to 10th	Day	7% 4 2 6% 4 4 5% 5% 9% 13% 40%	6360
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 8% 12% 32%	6067
	21st to end	Day	5% 5% 2 6% 5% 6% 6% 5% 8% 12% 39%	6424
August	1st to 10th	Day	13% 8% 4 7% 5% 6% 6% 5% 8% 11% 27%	4203
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 4 8% 9% 37%	3850
	21st to end	Day	8% 5% 2 4 4 4 5% 4 7% 11% 47%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 9% 43%	2400
	11th to 20th	Day	15% 6% 2 7% 4 4 5% 4 5% 6% 43%	2400
	21st to end	Day	19% 7% 3 7% 4 4 4 3 5% 7% 38%	2143
October	1st to 10th	Day	16% 6% 2 6% 4 3 3 3 4 5% 48%	2000
	11th to 20th	Day	14% 5% 2 6% 3 3 4 2 3 6% 51%	1980
	21st to end	Day	11% 6% 2 5% 5% 4 4 2 4 6% 51%	2090
November	1st to 10th	Day	14% 5% 2 5% 3 4 3 3 4 7% 49%	1852
	11th to 20th	Day	13% 7% 3 4 4 3 3 3 5% 7% 49%	1800
	21st to end	Day	14% 4 2 4 4 4 3 3 4 6% 51%	1740
December	1st to 10th	Day	10% 5% 2 5% 3 3 4 3 4 5% 54%	2600
	11th to 20th	Day	15% 8% 2 6% 4 3 4 3 5% 5% 45%	2990
	21st to end	Day	12% 5% 2 6% 3 3 4 3 5% 8% 49%	3960
January	1st to 10th	Night	12% 6% 3 5% 3 3 4 3 4 6% 52%	6147
	11th to 20th	Night	13% 6% 2 6% 4 3 3 2 4 5% 52%	5900
	21st to end	Night	14% 7% 2 7% 4 3 3 2 4 6% 48%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	2600
	11th to 20th	Night	21% 8% 2 5% 4 3 4 3 4 7% 40%	2373
	21st to end	Night	16% 8% 2 5% 3 4 3 2 4 5% 47%	1980
March	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 4 5% 43%	5102
	11th to 20th	Night	12% 6% 3 6% 4 4 3 3 5% 6% 47%	4980
	21st to end	Night	14% 8% 2 5% 3 4 4 3 5% 6% 46%	5288
April	1st to 10th	Night	13% 7% 2 5% 5% 4 4 5% 6% 6% 43%	2000
	11th to 20th	Night	11% 6% 2 6% 3 3 5% 2 5% 7% 49%	1975
	21st to end	Night	13% 6% 2 5% 4 4 5% 3 5% 7% 46%	2000
May	1st to 10th	Night	13% 8% 3 9% 5% 5% 6% 5% 7% 9% 32%	4490
	11th to 20th	Night	20% 8% 3 7% 6% 5% 4 3 6% 7% 31%	3825
	21st to end	Night	14% 8% 3 7% 4 5% 5% 4 6% 9% 36%	3920
June	1st to 10th	Night	12% 5% 1 5% 6% 4 6% 4 7% 11% 40%	1600
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	1600
	21st to end	Night	7% 4 2 6% 6% 5% 4 6% 8% 8% 44%	1600
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 7% 6% 8% 12% 36%	3598
	11th to 20th	Night	8% 7% 3 8% 6% 5% 6% 6% 9% 12% 31%	3871
	21st to end	Night	9% 7% 3 8% 5% 5% 6% 5% 8% 10% 35%	4427
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 6% 4 7% 10% 29%	2915
	11th to 20th	Night	9% 7% 3 8% 5% 5% 4 4 5% 9% 39%	3194
	21st to end	Night	9% 5% 2 5% 4 4 4 4 7% 10% 44%	3586
September	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 8% 45%	2000
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 5% 6% 43%	2000
	21st to end	Night	19% 8% 3 7% 4 5% 4 4 4 6% 35%	2253
October	1st to 10th	Night	16% 6% 2 6% 3 4 4 3 5% 6% 47%	2400
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 5% 6% 49%	2377
	21st to end	Night	13% 5% 2 5% 3 4 4 3 4 6% 51%	2508
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 7% 47%	2444
	11th to 20th	Night	15% 6% 2 5% 4 3 4 3 5% 6% 46%	2600
	21st to end	Night	11% 4 2 5% 4 4 4 3 6% 6% 50%	2655
December	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 4 6% 53%	4580
	11th to 20th	Night	16% 8% 2 7% 5% 4 4 4 5% 5% 39%	5205
	21st to end	Night	11% 6% 2 6% 4 4 4 3 6% 7% 47%	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	22% 27% 3 20% 17% 11%	3782
	11th to 20th	Day	24% 27% 4 15% 25% 5%	4060
	21st to end	Day	31% 30% 7% 22% 10% 1	4466
February	1st to 10th	Day	17% 62% 5% 10% 5% 1	1800
	11th to 20th	Day	18% 62% 4 7% 6% 3	1920
	21st to end	Day	15% 63% 5% 8% 7% 2	1650
March	1st to 10th	Day	35% 30% 5% 11% 14% 5%	4748
	11th to 20th	Day	23% 29% 6% 13% 20% 7%	4980
	21st to end	Day	26% 30% 10% 13% 16% 5%	5643
April	1st to 10th	Day	15% 63% 4 9% 8% 1	2400
	11th to 20th	Day	14% 63% 5% 10% 7% 1	2374
	21st to end	Day	14% 63% 6% 9% 6% 2	2400
May	1st to 10th	Day	33% 34% 13% 13% 7% 1	5437
	11th to 20th	Day	34% 33% 13% 15% 5%	6115
	21st to end	Day	27% 31% 11% 19% 10% 2	6949
June	1st to 10th	Day	13% 64% 7% 9% 6% 1	2800
	11th to 20th	Day	11% 64% 8% 10% 8%	2800
	21st to end	Day	11% 62% 7% 12% 7%	2800
July	1st to 10th	Day	26% 29% 18% 17% 8% 2	6290
	11th to 20th	Day	35% 31% 21% 12% 2	5994
	21st to end	Day	28% 28% 11% 20% 12%	6240
August	1st to 10th	Day	37% 37% 12% 13% 2	4190
	11th to 20th	Day	26% 38% 10% 17% 9%	3886
	21st to end	Day	18% 37% 9% 22% 13% 1	4312
September	1st to 10th	Day	17% 60% 7% 10% 5% 2	2400
	11th to 20th	Day	19% 59% 6% 9% 6% 2	2400
	21st to end	Day	24% 58% 5% 10% 3	2143
October	1st to 10th	Day	18% 36% 6% 20% 18% 2	3260
	11th to 20th	Day	13% 35% 4 12% 29% 6%	3220
	21st to end	Day	9% 36% 6% 21% 18% 9%	3428
November	1st to 10th	Day	18% 56% 5% 13% 7% 2	1852
	11th to 20th	Day	16% 57% 5% 8% 11% 3	1800
	21st to end	Day	15% 57% 4 12% 10% 1	1740
December	1st to 10th	Day	21% 36% 5% 20% 16% 2	2600
	11th to 20th	Day	30% 32% 11% 12% 12% 4	2986
	21st to end	Day	24% 28% 11% 23% 14%	3960
January	1st to 10th	Night	25% 27% 4 15% 22% 8%	6147
	11th to 20th	Night	25% 27% 2 18% 24% 5%	5900
	21st to end	Night	27% 30% 5% 21% 14% 3	6490
February	1st to 10th	Night	16% 63% 6% 10% 5% 1	2600
	11th to 20th	Night	20% 61% 4 8% 6% 2	2375
	21st to end	Night	15% 61% 2 10% 7% 4	1980
March	1st to 10th	Night	33% 31% 6% 12% 12% 7%	5102
	11th to 20th	Night	26% 29% 6% 14% 18% 8%	4980
	21st to end	Night	30% 29% 5% 18% 12% 6%	5288
April	1st to 10th	Night	17% 63% 4 6% 8% 1	2000
	11th to 20th	Night	14% 63% 5% 9% 7% 1	1975
	21st to end	Night	16% 63% 6% 8% 7% 1	2000
May	1st to 10th	Night	35% 33% 13% 13% 6%	4520
	11th to 20th	Night	38% 32% 9% 16% 4	3826
	21st to end	Night	33% 32% 10% 17% 7%	3927
June	1st to 10th	Night	16% 64% 6% 7% 6% 1	1600
	11th to 20th	Night	14% 63% 7% 10% 6%	1600
	21st to end	Night	12% 62% 6% 12% 8% 1	1600
July	1st to 10th	Night	31% 30% 14% 16% 8% 1	3560
	11th to 20th	Night	36% 31% 19% 12% 1	3825
	21st to end	Night	34% 27% 9% 17% 12%	4297
August	1st to 10th	Night	36% 37% 12% 13% 2	2904
	11th to 20th	Night	26% 37% 11% 18% 8%	3224
	21st to end	Night	20% 36% 5% 19% 19% 1	3586
September	1st to 10th	Night	16% 60% 9% 10% 4 1	2000
	11th to 20th	Night	19% 58% 5% 9% 8% 2	2000
	21st to end	Night	26% 58% 5% 8% 3	2253
October	1st to 10th	Night	24% 36% 6% 16% 17% 1	3920
	11th to 20th	Night	16% 35% 3 13% 26% 7%	3876
	21st to end	Night	9% 36% 3 23% 20% 9%	4120
November	1st to 10th	Night	20% 56% 4 11% 8% 1	2444
	11th to 20th	Night	17% 57% 4 8% 9% 5%	2600
	21st to end	Night	13% 58% 4 13% 11% 1	2655
December	1st to 10th	Night	20% 36% 3 20% 18% 3	4580
	11th to 20th	Night	35% 31% 8% 12% 9% 4	5208
	21st to end	Night	26% 29% 12% 22% 11% 1	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	18% 13% 39% 24% 6%	240
	11th to 20th	Day	16% 14% 41% 26% 3	300
	21st to end	Day	17% 8% 37% 32% 6%	330
February	1st to 10th	Day	17% 12% 33% 30% 7% 1	300
	11th to 20th	Day	27% 10% 41% 20% 3	459
	21st to end	Day	19% 10% 38% 28% 5%	414
March	1st to 10th	Day	13% 13% 37% 30% 7%	500
	11th to 20th	Day	17% 12% 35% 27% 8% 1	500
	21st to end	Day	15% 9% 35% 32% 9%	550
April	1st to 10th	Day	13% 12% 37% 28% 9% 1	500
	11th to 20th	Day	10% 8% 30% 36% 13% 4	492
	21st to end	Day	13% 7% 29% 32% 17% 2	500
May	1st to 10th	Day	16% 6% 26% 35% 16% 2	503
	11th to 20th	Day	17% 4 19% 35% 22% 3	527
	21st to end	Day	18% 3 15% 33% 25% 7%	573
June	1st to 10th	Day	11% 3 13% 35% 29% 9%	530
	11th to 20th	Day	13% 4 11% 31% 31% 10%	530
	21st to end	Day	8% 2 11% 33% 35% 10%	530
July	1st to 10th	Day	10% 1 6% 28% 42% 14%	525
	11th to 20th	Day	8% 1 6% 25% 45% 15%	524
	21st to end	Day	9% 1 7% 32% 38% 14%	576
August	1st to 10th	Day	14% 2 9% 31% 34% 10%	526
	11th to 20th	Day	13% 2 8% 29% 35% 13%	519
	21st to end	Day	10% 2 10% 27% 36% 14%	583
September	1st to 10th	Day	15% 2 11% 27% 32% 14%	530
	11th to 20th	Day	16% 2 14% 28% 28% 12%	530
	21st to end	Day	22% 5% 18% 28% 22% 5%	530
October	1st to 10th	Day	18% 3 16% 25% 25% 12%	530
	11th to 20th	Day	15% 8% 20% 32% 21% 4	522
	21st to end	Day	14% 6% 27% 31% 16% 7%	539
November	1st to 10th	Day	16% 6% 25% 29% 18% 6%	442
	11th to 20th	Day	17% 11% 30% 29% 11% 3	400
	21st to end	Day	16% 13% 25% 27% 16% 3	340
December	1st to 10th	Day	21% 4 42% 22% 10% 2	200
	11th to 20th	Day	20% 6% 35% 28% 12%	200
	21st to end	Day	18% 8% 32% 26% 15%	220
January	1st to 10th	Night	16% 16% 37% 25% 7%	800
	11th to 20th	Night	19% 12% 34% 29% 6%	800
	21st to end	Night	21% 10% 36% 30% 3	880
February	1st to 10th	Night	21% 6% 39% 29% 5%	800
	11th to 20th	Night	24% 12% 41% 20% 3	783
	21st to end	Night	20% 11% 34% 29% 5%	660
March	1st to 10th	Night	16% 9% 38% 30% 8%	800
	11th to 20th	Night	20% 9% 36% 26% 8% 1	800
	21st to end	Night	19% 8% 37% 31% 6%	880
April	1st to 10th	Night	18% 8% 38% 29% 7% 1	800
	11th to 20th	Night	13% 7% 30% 37% 12%	791
	21st to end	Night	15% 6% 27% 39% 12% 1	800
May	1st to 10th	Night	14% 8% 28% 38% 11% 1	800
	11th to 20th	Night	15% 4 21% 42% 17% 2	514
	21st to end	Night	13% 3 15% 35% 28% 4	433
June	1st to 10th	Night	12% 1 16% 42% 25% 5%	400
	11th to 20th	Night	11% 2 14% 41% 29% 4	400
	21st to end	Night	9% 2 11% 41% 34% 4	400
July	1st to 10th	Night	8% 1 8% 43% 36% 4	390
	11th to 20th	Night	9% 1 5% 33% 45% 8%	505
	21st to end	Night	9% 2 8% 41% 35% 5%	657
August	1st to 10th	Night	12% 2 10% 38% 34% 6%	597
	11th to 20th	Night	13% 1 8% 35% 37% 7%	787
	21st to end	Night	11% 1 10% 31% 39% 8%	880
September	1st to 10th	Night	14% 1 11% 34% 31% 9%	800
	11th to 20th	Night	15% 2 12% 31% 35% 6%	800
	21st to end	Night	23% 4 14% 31% 25% 3	800
October	1st to 10th	Night	17% 4 15% 28% 28% 8%	800
	11th to 20th	Night	18% 7% 23% 31% 18% 3	793
	21st to end	Night	15% 8% 24% 28% 20% 4	836
November	1st to 10th	Night	18% 7% 22% 31% 18% 4	780
	11th to 20th	Night	17% 9% 30% 29% 14% 1	800
	21st to end	Night	14% 11% 29% 32% 14% 2	800
December	1st to 10th	Night	20% 8% 37% 23% 10% 1	800
	11th to 20th	Night	20% 8% 35% 29% 9%	800
	21st to end	Night	18% 9% 31% 32% 9%	880

Page generated at 2025-10-16 01:50:27. Version ac07

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