Radomski Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Radomski Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 30%	126054
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	122965
Transparency	Day	52% 4 10% 21% 11% 1	129657
	Night	54% 3 10% 21% 11% 2	127431
Seeing	Day	25% 12% 25% 21% 12% 5%	15858
	Night	24% 10% 24% 28% 12% 1	25489

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	16% 20% 8% 23% 6% 3 3 2 3 4 14%	3987
	11th to 20th	Day	12% 23% 10% 18% 10% 6% 2 1 2 2 13%	4060
	21st to end	Day	20% 20% 6% 12% 6% 5% 4 3 3 4 18%	4466
February	1st to 10th	Day	20% 17% 6% 10% 8% 5% 4 3 3 5% 20%	1800
	11th to 20th	Day	18% 17% 7% 12% 5% 4 3 3 5% 6% 20%	1753
	21st to end	Day	21% 15% 5% 12% 4 3 4 3 5% 5% 23%	1653
March	1st to 10th	Day	16% 14% 5% 9% 5% 4 4 3 4 6% 31%	4935
	11th to 20th	Day	13% 10% 4 9% 5% 5% 5% 4 5% 6% 34%	4980
	21st to end	Day	17% 11% 4 10% 6% 5% 4 3 5% 5% 30%	5465
April	1st to 10th	Day	17% 13% 3 8% 5% 5% 4 5% 5% 6% 29%	2325
	11th to 20th	Day	16% 12% 3 7% 5% 4 4 3 5% 7% 33%	2374
	21st to end	Day	20% 9% 3 8% 5% 5% 4 3 4 7% 33%	2595
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 5% 6% 7% 26%	5862
	11th to 20th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 29%	5861
	21st to end	Day	20% 8% 2 7% 4 5% 5% 4 6% 8% 30%	6884
June	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 5% 7% 8% 34%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 6% 5% 5% 7% 9% 34%	2800
	21st to end	Day	14% 6% 2 6% 6% 4 6% 4 7% 9% 37%	2800
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 5% 6% 8% 41%	6236
	11th to 20th	Day	11% 8% 3 8% 6% 6% 6% 5% 7% 11% 31%	5900
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	6424
August	1st to 10th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 8% 30%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 4 7% 8% 36%	4174
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4411
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 6% 7% 46%	2218
	21st to end	Day	21% 8% 3 6% 4 4 4 3 5% 7% 35%	2068
October	1st to 10th	Day	18% 6% 2 6% 5% 4 5% 4 5% 7% 40%	1900
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 35%	1900
	21st to end	Day	23% 11% 2 6% 4 3 3 3 4 6% 34%	2090
November	1st to 10th	Day	12% 10% 3 6% 6% 4 3 3 5% 7% 42%	1912
	11th to 20th	Day	16% 16% 3 8% 4 4 5% 3 5% 7% 30%	1637
	21st to end	Day	19% 18% 6% 8% 5% 4 4 3 5% 6% 25%	1600
December	1st to 10th	Day	17% 14% 7% 19% 6% 4 4 3 4 4 18%	2600
	11th to 20th	Day	24% 18% 7% 10% 4 3 3 2 3 5% 22%	2990
	21st to end	Day	21% 21% 6% 11% 6% 5% 4 3 5% 5% 15%	3960
January	1st to 10th	Night	15% 18% 7% 24% 6% 4 3 2 3 4 14%	5953
	11th to 20th	Night	14% 20% 9% 15% 9% 5% 4 3 3 4 15%	5900
	21st to end	Night	21% 16% 6% 12% 7% 4 4 3 3 3 20%	6490
February	1st to 10th	Night	20% 16% 6% 11% 5% 4 3 3 4 4 24%	2600
	11th to 20th	Night	19% 15% 5% 11% 6% 5% 3 3 4 6% 24%	2533
	21st to end	Night	20% 14% 5% 11% 5% 4 4 3 5% 4 25%	1970
March	1st to 10th	Night	20% 13% 4 8% 5% 3 3 2 4 5% 32%	4918
	11th to 20th	Night	14% 11% 4 6% 4 3 3 2 4 7% 41%	4980
	21st to end	Night	15% 11% 4 8% 5% 5% 5% 4 5% 5% 34%	5467
April	1st to 10th	Night	19% 14% 4 7% 4 3 4 3 5% 5% 34%	2073
	11th to 20th	Night	17% 8% 2 6% 4 4 5% 3 5% 8% 37%	1975
	21st to end	Night	16% 9% 3 7% 5% 3 4 4 5% 7% 37%	1821
May	1st to 10th	Night	26% 12% 4 8% 5% 4 4 3 5% 6% 24%	4033
	11th to 20th	Night	19% 8% 3 8% 5% 4 4 3 5% 7% 34%	4032
	21st to end	Night	21% 8% 3 7% 4 4 4 3 6% 8% 32%	3969
June	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 3 6% 8% 42%	1600
	11th to 20th	Night	12% 7% 2 7% 4 5% 4 4 7% 9% 39%	1600
	21st to end	Night	12% 6% 2 5% 4 5% 5% 4 7% 11% 41%	1600
July	1st to 10th	Night	11% 8% 3 8% 4 4 4 4 6% 8% 40%	3690
	11th to 20th	Night	12% 7% 3 8% 5% 5% 6% 5% 6% 9% 33%	4057
	21st to end	Night	10% 8% 3 7% 6% 4 5% 4 6% 9% 39%	4427
August	1st to 10th	Night	13% 9% 3 7% 5% 5% 5% 4 7% 8% 33%	2915
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 4 5% 6% 42%	2875
	21st to end	Night	8% 5% 3 6% 5% 4 6% 4 7% 9% 43%	3453
September	1st to 10th	Night	13% 5% 2 5% 3 4 3 5% 5% 8% 48%	2000
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 5% 7% 52%	2134
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	2110
October	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	2280
	11th to 20th	Night	17% 7% 3 7% 5% 4 4 4 5% 6% 38%	2280
	21st to end	Night	25% 8% 2 5% 3 4 3 3 4 6% 36%	2508
November	1st to 10th	Night	13% 9% 3 6% 4 4 3 3 4 6% 45%	2381
	11th to 20th	Night	19% 14% 3 7% 5% 4 4 3 5% 5% 31%	2760
	21st to end	Night	20% 15% 4 7% 4 3 4 3 5% 6% 29%	2800
December	1st to 10th	Night	17% 17% 10% 15% 4 3 3 3 4 5% 18%	4580
	11th to 20th	Night	22% 18% 6% 10% 4 3 3 3 3 4 23%	5205
	21st to end	Night	21% 19% 5% 11% 6% 5% 4 3 4 5% 17%	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	77% 2 8% 9% 4	3988
	11th to 20th	Day	80% 1 5% 9% 3 2	4060
	21st to end	Day	72% 2 8% 12% 6%	4466
February	1st to 10th	Day	69% 3 8% 14% 6%	1800
	11th to 20th	Day	65% 3 9% 16% 6%	1762
	21st to end	Day	63% 3 10% 15% 7% 2	1653
March	1st to 10th	Day	55% 2 10% 24% 8% 1	4935
	11th to 20th	Day	50% 3 11% 26% 9% 1	4980
	21st to end	Day	56% 3 7% 21% 12% 1	5465
April	1st to 10th	Day	55% 3 9% 20% 11% 2	2325
	11th to 20th	Day	51% 4 11% 17% 15% 2	2374
	21st to end	Day	52% 3 9% 24% 11% 2	2595
May	1st to 10th	Day	55% 5% 10% 19% 8% 2	5897
	11th to 20th	Day	52% 5% 10% 23% 9% 1	5865
	21st to end	Day	49% 5% 13% 21% 10% 2	6899
June	1st to 10th	Day	46% 5% 14% 24% 11% 1	2800
	11th to 20th	Day	46% 5% 14% 23% 12% 1	2800
	21st to end	Day	42% 5% 13% 26% 13% 1	2800
July	1st to 10th	Day	40% 6% 10% 27% 15% 2	6168
	11th to 20th	Day	44% 7% 14% 24% 11%	5831
	21st to end	Day	42% 6% 9% 24% 19% 1	6240
August	1st to 10th	Day	49% 7% 13% 23% 8%	4221
	11th to 20th	Day	42% 5% 11% 29% 12%	4213
	21st to end	Day	35% 7% 13% 29% 14% 2	4411
September	1st to 10th	Day	39% 4 10% 24% 20% 3	2400
	11th to 20th	Day	37% 4 12% 30% 17% 1	2218
	21st to end	Day	49% 3 10% 24% 14%	2068
October	1st to 10th	Day	35% 3 9% 33% 19% 1	3160
	11th to 20th	Day	44% 4 11% 23% 13% 5%	3140
	21st to end	Day	37% 3 10% 29% 20% 1	3428
November	1st to 10th	Day	44% 3 12% 22% 17% 3	1912
	11th to 20th	Day	54% 4 8% 20% 12% 2	1637
	21st to end	Day	62% 3 11% 18% 6% 1	1600
December	1st to 10th	Day	70% 4 13% 10% 4	2600
	11th to 20th	Day	67% 3 12% 15% 4	2986
	21st to end	Day	72% 3 8% 13% 4	3960
January	1st to 10th	Night	76% 2 7% 11% 4	5953
	11th to 20th	Night	75% 1 8% 11% 3 1	5900
	21st to end	Night	70% 2 7% 13% 6% 1	6490
February	1st to 10th	Night	64% 2 9% 18% 6%	2600
	11th to 20th	Night	64% 2 9% 17% 7%	2535
	21st to end	Night	63% 2 9% 17% 9% 1	1970
March	1st to 10th	Night	56% 2 9% 24% 9% 1	4918
	11th to 20th	Night	45% 2 12% 29% 8% 3	4980
	21st to end	Night	52% 3 9% 20% 15% 1	5467
April	1st to 10th	Night	54% 3 9% 19% 13% 2	2073
	11th to 20th	Night	46% 3 12% 21% 15% 3	1975
	21st to end	Night	47% 4 8% 26% 11% 3	1821
May	1st to 10th	Night	60% 5% 8% 14% 9% 3	4060
	11th to 20th	Night	50% 4 9% 26% 10% 1	4033
	21st to end	Night	50% 4 12% 23% 9% 3	3976
June	1st to 10th	Night	40% 5% 11% 26% 15% 2	1600
	11th to 20th	Night	41% 6% 14% 26% 12% 1	1600
	21st to end	Night	38% 6% 14% 24% 16% 2	1600
July	1st to 10th	Night	42% 5% 10% 24% 17% 3	3650
	11th to 20th	Night	44% 8% 16% 20% 11% 1	4006
	21st to end	Night	41% 6% 11% 19% 22% 2	4297
August	1st to 10th	Night	46% 7% 14% 23% 10%	2904
	11th to 20th	Night	41% 5% 10% 27% 16% 1	2902
	21st to end	Night	35% 6% 14% 31% 12% 2	3453
September	1st to 10th	Night	35% 4 10% 25% 21% 4	2000
	11th to 20th	Night	31% 4 11% 29% 23% 1	2134
	21st to end	Night	45% 3 9% 24% 17% 1	2110
October	1st to 10th	Night	37% 3 8% 29% 22% 2	3800
	11th to 20th	Night	38% 4 10% 26% 15% 8%	3779
	21st to end	Night	37% 2 8% 29% 22% 2	4120
November	1st to 10th	Night	42% 3 11% 23% 18% 4	2381
	11th to 20th	Night	55% 3 8% 20% 12% 3	2760
	21st to end	Night	58% 2 10% 22% 8% 1	2800
December	1st to 10th	Night	69% 2 11% 15% 3	4580
	11th to 20th	Night	67% 2 14% 14% 3	5208
	21st to end	Night	70% 3 10% 14% 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	35% 25% 23% 11% 5% 1	285
	11th to 20th	Day	40% 24% 29% 5% 2 1	300
	21st to end	Day	36% 23% 30% 8% 2 2	330
February	1st to 10th	Day	35% 23% 32% 9% 1	300
	11th to 20th	Day	42% 25% 27% 6% 1	300
	21st to end	Day	36% 23% 32% 9%	307
March	1st to 10th	Day	31% 20% 27% 18% 4	500
	11th to 20th	Day	35% 20% 28% 12% 5% 1	500
	21st to end	Day	32% 16% 30% 16% 4 1	550
April	1st to 10th	Day	34% 17% 26% 14% 7% 1	500
	11th to 20th	Day	26% 19% 27% 15% 9% 4	492
	21st to end	Day	28% 14% 32% 16% 8% 2	500
May	1st to 10th	Day	30% 12% 30% 18% 9% 2	503
	11th to 20th	Day	24% 13% 26% 22% 9% 6%	527
	21st to end	Day	22% 8% 25% 23% 15% 7%	573
June	1st to 10th	Day	18% 9% 27% 27% 14% 4	530
	11th to 20th	Day	19% 8% 26% 26% 17% 4	530
	21st to end	Day	15% 8% 27% 28% 18% 4	530
July	1st to 10th	Day	16% 4 17% 30% 21% 12%	525
	11th to 20th	Day	16% 6% 22% 31% 17% 8%	524
	21st to end	Day	15% 4 23% 32% 18% 8%	576
August	1st to 10th	Day	16% 5% 19% 30% 22% 9%	526
	11th to 20th	Day	19% 3 15% 29% 23% 11%	519
	21st to end	Day	15% 3 22% 27% 22% 12%	583
September	1st to 10th	Day	19% 6% 18% 27% 21% 10%	530
	11th to 20th	Day	13% 3 15% 31% 26% 12%	522
	21st to end	Day	28% 9% 21% 21% 16% 5%	490
October	1st to 10th	Day	20% 8% 26% 22% 18% 6%	490
	11th to 20th	Day	23% 14% 28% 22% 10% 3	490
	21st to end	Day	31% 12% 29% 17% 10% 1	539
November	1st to 10th	Day	21% 12% 30% 23% 10% 3	450
	11th to 20th	Day	31% 17% 32% 14% 5% 1	217
	21st to end	Day	33% 19% 35% 11% 3 1	200
December	1st to 10th	Day	32% 25% 30% 10% 3 1	200
	11th to 20th	Day	36% 21% 29% 13% 2	200
	21st to end	Day	38% 19% 25% 13% 4	220
January	1st to 10th	Night	33% 19% 28% 16% 4	800
	11th to 20th	Night	34% 17% 31% 17% 3	800
	21st to end	Night	33% 18% 26% 20% 2	880
February	1st to 10th	Night	32% 16% 26% 24% 2	800
	11th to 20th	Night	32% 19% 32% 16% 1	783
	21st to end	Night	30% 16% 33% 18% 2	660
March	1st to 10th	Night	26% 18% 29% 23% 5%	800
	11th to 20th	Night	29% 16% 28% 21% 6% 1	800
	21st to end	Night	27% 14% 32% 22% 5% 1	880
April	1st to 10th	Night	30% 13% 30% 21% 6%	800
	11th to 20th	Night	24% 11% 29% 26% 9% 1	791
	21st to end	Night	20% 10% 30% 33% 7%	621
May	1st to 10th	Night	27% 7% 22% 34% 11% 1	600
	11th to 20th	Night	24% 7% 22% 35% 11%	591
	21st to end	Night	14% 2 23% 33% 25% 3	457
June	1st to 10th	Night	14% 4 22% 43% 17% 1	400
	11th to 20th	Night	15% 3 21% 43% 18% 1	400
	21st to end	Night	13% 3 25% 38% 21% 1	400
July	1st to 10th	Night	10% 3 17% 44% 23% 4	428
	11th to 20th	Night	12% 3 16% 36% 29% 4	582
	21st to end	Night	14% 2 12% 41% 29% 2	657
August	1st to 10th	Night	11% 2 12% 43% 29% 2	597
	11th to 20th	Night	15% 2 10% 39% 31% 4	594
	21st to end	Night	13% 3 14% 37% 29% 4	799
September	1st to 10th	Night	16% 3 16% 36% 26% 4	800
	11th to 20th	Night	12% 3 13% 40% 28% 4	793
	21st to end	Night	22% 6% 21% 29% 20% 3	760
October	1st to 10th	Night	19% 6% 21% 31% 20% 2	760
	11th to 20th	Night	20% 9% 28% 31% 11% 1	760
	21st to end	Night	29% 10% 23% 29% 8% 1	836
November	1st to 10th	Night	20% 9% 26% 34% 10% 1	780
	11th to 20th	Night	29% 13% 25% 24% 8% 1	800
	21st to end	Night	31% 13% 28% 22% 6%	800
December	1st to 10th	Night	33% 14% 29% 18% 6%	800
	11th to 20th	Night	31% 18% 27% 21% 4	800
	21st to end	Night	33% 15% 26% 25% 2	880

Page generated at 2025-09-16 01:49:14. Version ac07

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