Primland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Primland Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	126914
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	123458
Transparency	Day	40% 7% 12% 20% 16% 5%	130533
	Night	37% 6% 11% 22% 19% 6%	127908
Seeing	Day	21% 16% 25% 21% 12% 5%	16687
	Night	18% 14% 25% 25% 15% 2	26745

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% 6% 2 5% 3 2 3 3 4 6% 52%	4056
	11th to 20th	Day	12% 6% 2 5% 3 3 3 2 4 5% 55%	4060
	21st to end	Day	17% 6% 3 7% 3 3 3 3 3 5% 46%	4466
February	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 4 6% 7% 42%	1800
	11th to 20th	Day	20% 6% 2 5% 4 3 4 4 6% 6% 40%	1840
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 4 6% 40%	1714
March	1st to 10th	Day	18% 7% 3 7% 4 4 4 4 5% 7% 38%	4935
	11th to 20th	Day	12% 5% 2 4 3 3 4 3 5% 7% 52%	4980
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 6% 43%	5465
April	1st to 10th	Day	12% 5% 2 5% 4 4 5% 4 7% 8% 47%	2245
	11th to 20th	Day	14% 6% 2 5% 3 4 4 4 5% 8% 44%	2374
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 5% 6% 40%	2550
May	1st to 10th	Day	13% 6% 3 8% 6% 6% 7% 5% 8% 8% 30%	5862
	11th to 20th	Day	21% 7% 3 8% 5% 6% 6% 5% 7% 7% 26%	5861
	21st to end	Day	14% 6% 2 6% 5% 4 5% 5% 6% 9% 38%	6433
June	1st to 10th	Day	11% 5% 2 7% 6% 6% 5% 5% 8% 9% 35%	2600
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 8% 11% 35%	2600
	21st to end	Day	8% 5% 3 6% 5% 6% 7% 6% 7% 10% 37%	2600
July	1st to 10th	Day	7% 5% 3 8% 6% 6% 6% 5% 8% 10% 34%	5900
	11th to 20th	Day	7% 6% 3 9% 7% 7% 8% 6% 10% 11% 26%	5900
	21st to end	Day	10% 7% 3 9% 7% 7% 8% 6% 8% 9% 24%	6424
August	1st to 10th	Day	15% 8% 3 8% 6% 5% 6% 5% 8% 10% 26%	4234
	11th to 20th	Day	8% 6% 3 7% 5% 6% 7% 6% 9% 12% 32%	4174
	21st to end	Day	6% 5% 2 5% 4 4 5% 5% 7% 11% 46%	4363
September	1st to 10th	Day	11% 6% 2 7% 4 5% 4 4 6% 9% 43%	2317
	11th to 20th	Day	15% 6% 2 7% 6% 4 5% 4 7% 8% 36%	2200
	21st to end	Day	21% 8% 3 7% 5% 5% 4 3 5% 6% 34%	2200
October	1st to 10th	Day	14% 5% 2 5% 4 3 4 2 4 6% 51%	2035
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 4 6% 54%	2000
	21st to end	Day	15% 5% 2 5% 3 3 2 2 3 6% 53%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 4 3 4 5% 54%	2057
	11th to 20th	Day	13% 4 2 4 3 3 4 4 4 6% 52%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 3 6% 7% 49%	1800
December	1st to 10th	Day	12% 8% 3 5% 3 3 3 3 5% 5% 50%	2930
	11th to 20th	Day	17% 8% 2 4 3 3 4 4 5% 6% 44%	3373
	21st to end	Day	22% 7% 2 4 3 4 4 4 5% 8% 36%	4466
January	1st to 10th	Night	12% 5% 2 4 3 3 3 3 5% 5% 56%	5889
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 4 5% 56%	5900
	21st to end	Night	18% 6% 2 5% 3 2 3 2 4 6% 50%	6490
February	1st to 10th	Night	15% 7% 2 4 4 4 5% 4 6% 6% 43%	2600
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 5% 5% 42%	2443
	21st to end	Night	15% 6% 3 5% 4 4 4 4 5% 7% 42%	1915
March	1st to 10th	Night	18% 7% 2 5% 4 4 3 3 5% 5% 44%	4918
	11th to 20th	Night	10% 4 1 4 3 3 3 3 5% 7% 56%	4980
	21st to end	Night	15% 5% 2 5% 4 4 5% 4 6% 7% 44%	5467
April	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 5% 7% 45%	2153
	11th to 20th	Night	14% 5% 2 4 5% 3 4 3 4 7% 47%	1975
	21st to end	Night	15% 5% 2 4 4 4 5% 4 5% 6% 47%	1866
May	1st to 10th	Night	15% 8% 2 6% 5% 4 5% 4 5% 9% 38%	4033
	11th to 20th	Night	20% 7% 3 7% 4 4 5% 3 5% 8% 34%	4032
	21st to end	Night	17% 6% 2 5% 3 3 3 3 6% 10% 40%	4421
June	1st to 10th	Night	11% 5% 1 6% 5% 5% 7% 4 6% 9% 41%	1800
	11th to 20th	Night	11% 7% 2 5% 5% 4 4 4 6% 10% 44%	1800
	21st to end	Night	7% 4 2 4 6% 4 5% 4 8% 10% 44%	1800
July	1st to 10th	Night	9% 7% 3 7% 5% 5% 5% 4 8% 9% 37%	4058
	11th to 20th	Night	8% 6% 3 8% 5% 5% 6% 6% 9% 12% 33%	4057
	21st to end	Night	9% 8% 3 8% 6% 6% 6% 6% 8% 11% 30%	4427
August	1st to 10th	Night	16% 9% 4 8% 5% 4 5% 5% 8% 9% 29%	2915
	11th to 20th	Night	9% 6% 2 7% 5% 5% 6% 4 8% 9% 39%	2875
	21st to end	Night	6% 4 2 6% 4 4 4 4 6% 9% 51%	3487
September	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 5% 8% 47%	2079
	11th to 20th	Night	15% 6% 3 6% 5% 5% 5% 3 5% 7% 40%	2200
	21st to end	Night	20% 7% 3 8% 5% 5% 5% 3 5% 5% 36%	2200
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 4 6% 48%	2363
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 4 5% 53%	2400
	21st to end	Night	15% 5% 2 4 3 3 3 3 4 5% 56%	2640
November	1st to 10th	Night	14% 7% 2 5% 3 3 3 2 4 5% 52%	2461
	11th to 20th	Night	13% 4 2 4 3 3 3 3 5% 6% 55%	2652
	21st to end	Night	13% 5% 2 4 3 3 4 5% 5% 6% 50%	2600
December	1st to 10th	Night	12% 5% 3 4 4 4 3 3 5% 6% 51%	4250
	11th to 20th	Night	19% 10% 3 6% 3 3 3 2 4 5% 40%	4822
	21st to end	Night	21% 5% 2 4 3 3 4 4 6% 7% 42%	6490

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	33% 3 7% 20% 28% 9%	4057
	11th to 20th	Day	32% 3 6% 18% 31% 10%	4060
	21st to end	Day	42% 3 7% 22% 22% 5%	4466
February	1st to 10th	Day	40% 6% 11% 20% 18% 4	1800
	11th to 20th	Day	43% 5% 8% 21% 18% 5%	1849
	21st to end	Day	45% 4 11% 16% 17% 7%	1714
March	1st to 10th	Day	45% 3 8% 17% 15% 12%	4935
	11th to 20th	Day	31% 5% 6% 22% 22% 13%	4980
	21st to end	Day	42% 4 9% 22% 18% 5%	5465
April	1st to 10th	Day	34% 6% 11% 22% 19% 7%	2245
	11th to 20th	Day	37% 5% 11% 21% 19% 6%	2374
	21st to end	Day	43% 6% 12% 23% 13% 3	2550
May	1st to 10th	Day	46% 10% 14% 18% 11% 2	5897
	11th to 20th	Day	52% 9% 10% 19% 9% 1	5865
	21st to end	Day	40% 8% 14% 19% 16% 3	6447
June	1st to 10th	Day	40% 10% 14% 24% 11% 1	2600
	11th to 20th	Day	38% 12% 15% 19% 13% 2	2600
	21st to end	Day	38% 10% 16% 22% 12% 2	2600
July	1st to 10th	Day	40% 13% 16% 19% 10% 3	5835
	11th to 20th	Day	44% 15% 20% 15% 4 1	5831
	21st to end	Day	50% 13% 14% 18% 5%	6240
August	1st to 10th	Day	49% 10% 16% 21% 4	4221
	11th to 20th	Day	38% 12% 19% 20% 8% 2	4213
	21st to end	Day	31% 9% 18% 29% 12% 2	4363
September	1st to 10th	Day	37% 8% 13% 20% 19% 3	2317
	11th to 20th	Day	43% 8% 11% 21% 15% 2	2200
	21st to end	Day	50% 5% 10% 18% 15% 2	2200
October	1st to 10th	Day	31% 5% 6% 20% 28% 10%	3308
	11th to 20th	Day	24% 2 6% 23% 27% 17%	3240
	21st to end	Day	27% 3 9% 25% 21% 15%	3538
November	1st to 10th	Day	31% 4 9% 22% 26% 7%	2057
	11th to 20th	Day	33% 4 8% 22% 26% 7%	1900
	21st to end	Day	35% 3 9% 22% 27% 4	1800
December	1st to 10th	Day	36% 4 7% 20% 25% 7%	2930
	11th to 20th	Day	40% 3 12% 18% 21% 6%	3370
	21st to end	Day	45% 6% 11% 21% 14% 2	4466
January	1st to 10th	Night	30% 3 8% 19% 27% 13%	5889
	11th to 20th	Night	31% 4 6% 17% 32% 9%	5900
	21st to end	Night	37% 4 6% 23% 26% 5%	6490
February	1st to 10th	Night	40% 5% 10% 23% 19% 3	2600
	11th to 20th	Night	44% 4 9% 19% 17% 7%	2445
	21st to end	Night	40% 5% 11% 21% 17% 6%	1915
March	1st to 10th	Night	40% 5% 7% 19% 20% 10%	4918
	11th to 20th	Night	27% 4 7% 20% 26% 15%	4980
	21st to end	Night	39% 4 10% 23% 18% 5%	5467
April	1st to 10th	Night	38% 5% 12% 19% 21% 5%	2153
	11th to 20th	Night	37% 5% 10% 21% 21% 6%	1975
	21st to end	Night	36% 7% 11% 27% 16% 3	1866
May	1st to 10th	Night	42% 10% 14% 21% 12% 1	4060
	11th to 20th	Night	48% 9% 12% 21% 10% 1	4033
	21st to end	Night	38% 8% 16% 25% 10% 3	4429
June	1st to 10th	Night	36% 10% 15% 24% 14% 1	1800
	11th to 20th	Night	34% 9% 18% 23% 13% 1	1800
	21st to end	Night	32% 11% 17% 23% 14% 3	1800
July	1st to 10th	Night	39% 12% 15% 22% 10% 2	4015
	11th to 20th	Night	39% 14% 24% 18% 5% 1	4006
	21st to end	Night	43% 12% 18% 21% 7%	4297
August	1st to 10th	Night	48% 9% 18% 18% 6%	2904
	11th to 20th	Night	34% 11% 19% 26% 10% 1	2902
	21st to end	Night	28% 8% 15% 37% 10% 2	3487
September	1st to 10th	Night	35% 9% 14% 20% 17% 5%	2079
	11th to 20th	Night	42% 6% 10% 20% 20% 1	2200
	21st to end	Night	50% 6% 8% 18% 15% 3	2200
October	1st to 10th	Night	36% 5% 7% 19% 24% 9%	3870
	11th to 20th	Night	23% 3 7% 23% 27% 17%	3899
	21st to end	Night	24% 4 7% 26% 24% 16%	4252
November	1st to 10th	Night	36% 3 9% 21% 24% 7%	2461
	11th to 20th	Night	31% 4 8% 21% 27% 8%	2652
	21st to end	Night	33% 4 8% 24% 24% 6%	2600
December	1st to 10th	Night	33% 4 8% 25% 28% 3	4250
	11th to 20th	Night	46% 4 9% 17% 17% 8%	4824
	21st to end	Night	39% 4 15% 22% 17% 2	6490

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	34% 33% 23% 9% 2	300
	11th to 20th	Day	38% 29% 25% 6% 1	300
	21st to end	Day	32% 35% 27% 4 2	330
February	1st to 10th	Day	27% 36% 26% 11% 1	300
	11th to 20th	Day	47% 25% 24% 3 1	300
	21st to end	Day	36% 29% 24% 10% 1	313
March	1st to 10th	Day	28% 27% 29% 12% 4	500
	11th to 20th	Day	27% 27% 30% 11% 3 1	500
	21st to end	Day	29% 24% 33% 11% 4	550
April	1st to 10th	Day	25% 28% 24% 16% 7%	500
	11th to 20th	Day	25% 19% 27% 20% 8% 2	492
	21st to end	Day	22% 19% 31% 19% 8% 2	500
May	1st to 10th	Day	21% 20% 33% 15% 7% 4	503
	11th to 20th	Day	20% 16% 28% 21% 12% 5%	527
	21st to end	Day	21% 10% 22% 25% 16% 6%	573
June	1st to 10th	Day	13% 11% 28% 28% 14% 6%	530
	11th to 20th	Day	10% 10% 22% 31% 19% 8%	530
	21st to end	Day	10% 7% 25% 33% 18% 6%	530
July	1st to 10th	Day	11% 4 18% 33% 23% 11%	525
	11th to 20th	Day	6% 4 18% 32% 28% 12%	524
	21st to end	Day	12% 4 20% 29% 22% 13%	576
August	1st to 10th	Day	18% 7% 23% 22% 19% 12%	526
	11th to 20th	Day	11% 5% 20% 27% 24% 13%	519
	21st to end	Day	12% 4 17% 30% 22% 14%	583
September	1st to 10th	Day	12% 5% 23% 28% 20% 12%	530
	11th to 20th	Day	17% 5% 18% 28% 24% 8%	530
	21st to end	Day	24% 13% 22% 25% 12% 4	530
October	1st to 10th	Day	18% 11% 23% 26% 16% 5%	530
	11th to 20th	Day	16% 17% 28% 23% 14% 2	530
	21st to end	Day	23% 16% 29% 21% 8% 3	583
November	1st to 10th	Day	22% 16% 31% 20% 10% 1	535
	11th to 20th	Day	24% 24% 29% 16% 6% 1	358
	21st to end	Day	26% 23% 31% 15% 4 1	300
December	1st to 10th	Day	26% 33% 31% 9% 1	300
	11th to 20th	Day	25% 23% 35% 14% 3	300
	21st to end	Day	31% 24% 30% 12% 4	330
January	1st to 10th	Night	23% 27% 31% 15% 4	800
	11th to 20th	Night	23% 23% 33% 18% 3	800
	21st to end	Night	20% 23% 37% 18% 2	880
February	1st to 10th	Night	20% 23% 34% 22% 2	800
	11th to 20th	Night	29% 27% 30% 13% 1	783
	21st to end	Night	23% 22% 34% 19% 1	660
March	1st to 10th	Night	20% 23% 33% 21% 3	800
	11th to 20th	Night	24% 20% 32% 17% 6% 1	800
	21st to end	Night	22% 20% 31% 21% 5%	880
April	1st to 10th	Night	22% 20% 31% 21% 6%	800
	11th to 20th	Night	19% 19% 30% 22% 10% 1	791
	21st to end	Night	18% 13% 32% 29% 8%	666
May	1st to 10th	Night	15% 13% 34% 24% 13% 1	600
	11th to 20th	Night	17% 10% 25% 29% 17% 2	591
	21st to end	Night	19% 6% 18% 27% 25% 5%	650
June	1st to 10th	Night	12% 4 21% 34% 27% 3	600
	11th to 20th	Night	13% 4 20% 27% 32% 5%	600
	21st to end	Night	9% 2 17% 37% 31% 5%	600
July	1st to 10th	Night	11% 3 10% 36% 35% 6%	585
	11th to 20th	Night	6% 1 15% 32% 40% 6%	582
	21st to end	Night	9% 2 14% 37% 33% 6%	657
August	1st to 10th	Night	17% 3 12% 31% 32% 4	597
	11th to 20th	Night	13% 2 14% 33% 33% 5%	594
	21st to end	Night	10% 2 14% 29% 36% 8%	820
September	1st to 10th	Night	13% 5% 15% 30% 30% 9%	800
	11th to 20th	Night	19% 4 16% 27% 28% 6%	800
	21st to end	Night	24% 9% 19% 27% 20% 2	800
October	1st to 10th	Night	17% 11% 22% 21% 24% 5%	800
	11th to 20th	Night	18% 11% 28% 26% 15% 2	800
	21st to end	Night	19% 12% 25% 30% 12% 2	880
November	1st to 10th	Night	20% 12% 28% 25% 12% 2	820
	11th to 20th	Night	18% 18% 29% 27% 8% 1	829
	21st to end	Night	18% 19% 29% 23% 10% 1	800
December	1st to 10th	Night	22% 23% 27% 22% 6%	800
	11th to 20th	Night	18% 23% 30% 23% 6%	800
	21st to end	Night	27% 18% 27% 23% 5%	880

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