Frankfort
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Frankfort clear sky chart from 2005-11-06 to 2025-12-03.

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	14% 8% 3 7% 4 4 5% 4 6% 7% 37%	127575
	Night	15% 9% 3 7% 5% 4 4 3 5% 7% 38%	123155
Transparency	Day	45% 4 10% 24% 15% 2	131242
	Night	45% 3 10% 23% 16% 2	127557
Seeing	Day	17% 9% 24% 24% 17% 8%	16644
	Night	18% 7% 24% 31% 17% 2	26106

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% 15% 9% 7% 4 4 4 3 5% 6% 28%	4056
	11th to 20th	Day	14% 13% 5% 14% 4 4 3 2 4 5% 31%	4060
	21st to end	Day	21% 10% 7% 7% 5% 5% 3 3 5% 5% 30%	4466
February	1st to 10th	Day	18% 12% 4 9% 4 4 3 3 4 5% 33%	1970
	11th to 20th	Day	19% 10% 4 9% 5% 4 4 3 5% 5% 33%	1948
	21st to end	Day	17% 12% 4 7% 4 4 4 3 6% 6% 32%	1650
March	1st to 10th	Day	20% 12% 4 6% 4 4 4 3 4 4 36%	4480
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 6% 8% 42%	4976
	21st to end	Day	16% 8% 3 8% 5% 5% 4 3 5% 8% 37%	5465
April	1st to 10th	Day	15% 8% 3 7% 4 4 4 4 5% 8% 39%	2381
	11th to 20th	Day	17% 8% 2 6% 4 4 5% 3 5% 6% 39%	2385
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 6% 38%	2600
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 6% 4 7% 8% 26%	5862
	11th to 20th	Day	18% 8% 3 8% 5% 4 4 4 6% 7% 34%	5861
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 10% 32%	6433
June	1st to 10th	Day	11% 5% 2 7% 5% 5% 5% 4 5% 7% 43%	2600
	11th to 20th	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 9% 42%	2732
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	2800
July	1st to 10th	Day	11% 5% 2 7% 4 5% 5% 4 7% 9% 40%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 11% 35%	6020
	21st to end	Day	12% 8% 3 9% 6% 7% 6% 5% 7% 9% 28%	6424
August	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 11% 46%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	4083
	21st to end	Day	5% 3 2 5% 3 4 4 4 6% 8% 56%	4312
September	1st to 10th	Day	14% 5% 2 6% 3 5% 5% 4 5% 8% 44%	2288
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 49%	2200
	21st to end	Day	15% 5% 2 6% 4 4 4 3 4 6% 47%	2200
October	1st to 10th	Day	10% 4 1 5% 3 4 5% 2 5% 8% 53%	2200
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 4 7% 50%	2068
	21st to end	Day	19% 5% 3 6% 3 4 5% 3 4 6% 43%	2042
November	1st to 10th	Day	13% 5% 2 4 4 3 4 3 2 6% 53%	1852
	11th to 20th	Day	16% 9% 3 7% 4 4 3 3 4 6% 42%	1890
	21st to end	Day	19% 11% 3 6% 4 3 3 3 5% 6% 37%	1890
December	1st to 10th	Day	18% 10% 3 8% 4 3 3 2 4 4 40%	2948
	11th to 20th	Day	19% 13% 5% 9% 5% 4 5% 4 4 5% 29%	3373
	21st to end	Day	21% 17% 5% 10% 5% 4 4 3 4 6% 22%	4466
January	1st to 10th	Night	16% 15% 6% 13% 5% 4 4 3 5% 5% 24%	5889
	11th to 20th	Night	17% 14% 5% 11% 6% 5% 4 2 3 3 29%	5900
	21st to end	Night	20% 10% 4 7% 4 3 4 3 4 5% 36%	6490
February	1st to 10th	Night	21% 12% 4 8% 5% 4 4 3 5% 5% 28%	2428
	11th to 20th	Night	21% 10% 4 9% 4 4 4 4 4 5% 32%	2338
	21st to end	Night	21% 12% 3 8% 5% 3 4 3 3 6% 31%	1980
March	1st to 10th	Night	19% 13% 2 5% 4 4 4 3 3 5% 36%	5373
	11th to 20th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 46%	4983
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 4 5% 6% 34%	5467
April	1st to 10th	Night	14% 7% 2 5% 5% 5% 5% 5% 6% 7% 40%	2035
	11th to 20th	Night	15% 7% 3 6% 4 4 4 3 5% 6% 43%	1962
	21st to end	Night	16% 7% 3 6% 5% 4 4 4 6% 7% 39%	1800
May	1st to 10th	Night	19% 9% 4 8% 5% 5% 5% 4 5% 8% 27%	4033
	11th to 20th	Night	16% 7% 3 7% 6% 4 5% 5% 7% 8% 31%	4032
	21st to end	Night	12% 6% 3 7% 5% 4 5% 4 7% 10% 37%	4421
June	1st to 10th	Night	9% 4 2 6% 3 4 4 3 6% 10% 48%	1800
	11th to 20th	Night	7% 5% 2 5% 5% 4 3 5% 7% 10% 49%	1660
	21st to end	Night	6% 4 1 5% 3 5% 6% 4 8% 12% 47%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 3 6% 8% 48%	3618
	11th to 20th	Night	8% 5% 2 7% 5% 5% 5% 4 7% 10% 43%	3925
	21st to end	Night	10% 7% 3 9% 5% 6% 5% 5% 7% 9% 34%	4427
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 10% 49%	2915
	11th to 20th	Night	6% 6% 2 6% 5% 5% 5% 4 6% 9% 44%	2963
	21st to end	Night	4 3 1 3 3 3 4 3 5% 9% 63%	3586
September	1st to 10th	Night	10% 5% 2 4 4 5% 4 3 5% 7% 51%	2090
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 4 8% 53%	2200
	21st to end	Night	13% 6% 3 5% 4 3 3 2 5% 5% 52%	2200
October	1st to 10th	Night	9% 4 1 5% 4 3 4 2 5% 6% 58%	2200
	11th to 20th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2330
	21st to end	Night	18% 7% 2 6% 4 3 3 2 4 5% 45%	2795
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 5% 5% 50%	2666
	11th to 20th	Night	14% 9% 3 5% 4 4 3 3 5% 7% 44%	2730
	21st to end	Night	17% 10% 3 6% 5% 3 4 3 5% 6% 38%	2730
December	1st to 10th	Night	19% 10% 3 8% 4 3 3 2 4 4 39%	4277
	11th to 20th	Night	21% 11% 5% 7% 4 3 3 3 4 6% 34%	4822
	21st to end	Night	21% 15% 6% 10% 6% 5% 5% 4 4 5% 20%	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	59% 2 9% 22% 9% 1	4057
	11th to 20th	Day	58% 2 6% 19% 13% 2	4060
	21st to end	Day	57% 2 8% 22% 8% 3	4466
February	1st to 10th	Day	54% 3 8% 23% 11% 2	1970
	11th to 20th	Day	55% 3 7% 21% 12% 3	1958
	21st to end	Day	53% 3 8% 22% 11% 3	1650
March	1st to 10th	Day	53% 2 5% 18% 15% 6%	4480
	11th to 20th	Day	42% 2 9% 26% 19% 2	4976
	21st to end	Day	47% 3 11% 25% 12% 1	5465
April	1st to 10th	Day	44% 3 12% 24% 14% 2	2381
	11th to 20th	Day	46% 3 8% 24% 16% 3	2385
	21st to end	Day	47% 4 10% 24% 12% 2	2600
May	1st to 10th	Day	54% 5% 12% 18% 10% 1	5897
	11th to 20th	Day	51% 4 12% 23% 11%	5865
	21st to end	Day	45% 5% 15% 19% 15% 1	6447
June	1st to 10th	Day	40% 5% 10% 26% 18% 2	2600
	11th to 20th	Day	36% 7% 14% 24% 17% 2	2732
	21st to end	Day	37% 6% 14% 22% 19% 4	2800
July	1st to 10th	Day	40% 4 12% 23% 20% 1	6291
	11th to 20th	Day	40% 5% 15% 27% 10% 1	5948
	21st to end	Day	52% 5% 13% 18% 11% 1	6240
August	1st to 10th	Day	31% 6% 14% 36% 13% 1	4221
	11th to 20th	Day	36% 4 11% 32% 14% 2	4122
	21st to end	Day	26% 5% 11% 35% 19% 3	4311
September	1st to 10th	Day	39% 5% 11% 22% 19% 5%	2288
	11th to 20th	Day	34% 4 9% 26% 24% 3	2200
	21st to end	Day	38% 4 8% 24% 23% 3	2200
October	1st to 10th	Day	21% 3 5% 29% 39% 3	3590
	11th to 20th	Day	26% 3 8% 29% 31% 3	3347
	21st to end	Day	33% 2 8% 29% 22% 6%	3279
November	1st to 10th	Day	36% 2 8% 28% 23% 4	1852
	11th to 20th	Day	44% 2 8% 23% 18% 4	1890
	21st to end	Day	49% 3 8% 23% 17% 1	1890
December	1st to 10th	Day	50% 2 8% 24% 16% 1	2948
	11th to 20th	Day	60% 2 8% 18% 10% 1	3370
	21st to end	Day	65% 3 10% 15% 6% 1	4466
January	1st to 10th	Night	63% 2 8% 17% 9% 1	5889
	11th to 20th	Night	62% 2 5% 17% 10% 3	5900
	21st to end	Night	52% 2 7% 23% 13% 2	6490
February	1st to 10th	Night	57% 3 9% 20% 9% 1	2428
	11th to 20th	Night	55% 3 7% 21% 13% 1	2339
	21st to end	Night	57% 2 9% 16% 13% 3	1980
March	1st to 10th	Night	53% 2 5% 23% 14% 4	5373
	11th to 20th	Night	39% 3 9% 27% 20% 2	4983
	21st to end	Night	50% 3 8% 24% 13% 1	5467
April	1st to 10th	Night	43% 4 10% 23% 17% 3	2035
	11th to 20th	Night	43% 3 8% 21% 19% 6%	1962
	21st to end	Night	45% 4 11% 24% 13% 3	1800
May	1st to 10th	Night	56% 4 11% 19% 9% 1	4060
	11th to 20th	Night	49% 5% 13% 23% 9% 1	4033
	21st to end	Night	41% 6% 15% 23% 13% 2	4429
June	1st to 10th	Night	32% 4 13% 29% 18% 5%	1800
	11th to 20th	Night	31% 6% 15% 24% 21% 4	1660
	21st to end	Night	29% 6% 18% 24% 17% 6%	1600
July	1st to 10th	Night	35% 5% 11% 26% 21% 2	3580
	11th to 20th	Night	37% 7% 16% 28% 12% 2	3877
	21st to end	Night	45% 6% 14% 20% 13% 1	4297
August	1st to 10th	Night	29% 5% 15% 36% 13% 2	2904
	11th to 20th	Night	35% 6% 11% 31% 16% 2	2990
	21st to end	Night	19% 4 14% 35% 23% 4	3586
September	1st to 10th	Night	33% 4 12% 23% 21% 6%	2090
	11th to 20th	Night	31% 4 10% 25% 26% 4	2200
	21st to end	Night	36% 4 7% 24% 25% 5%	2200
October	1st to 10th	Night	22% 3 6% 22% 45% 3	3590
	11th to 20th	Night	26% 2 7% 26% 35% 5%	3790
	21st to end	Night	33% 2 6% 22% 30% 7%	4508
November	1st to 10th	Night	36% 3 7% 26% 23% 4	2666
	11th to 20th	Night	41% 3 7% 23% 21% 5%	2730
	21st to end	Night	48% 2 9% 24% 15% 2	2730
December	1st to 10th	Night	51% 2 10% 23% 14%	4277
	11th to 20th	Night	53% 2 10% 21% 12% 1	4824
	21st to end	Night	67% 3 8% 15% 6%	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	24% 20% 32% 17% 6% 1	300
	11th to 20th	Day	28% 22% 31% 16% 4	300
	21st to end	Day	24% 18% 41% 14% 3	330
February	1st to 10th	Day	24% 16% 41% 14% 4	300
	11th to 20th	Day	32% 22% 32% 10% 3 1	300
	21st to end	Day	30% 18% 37% 12% 3	249
March	1st to 10th	Day	25% 20% 37% 13% 5%	300
	11th to 20th	Day	22% 15% 38% 18% 7%	496
	21st to end	Day	27% 14% 33% 18% 7%	550
April	1st to 10th	Day	22% 15% 30% 23% 10% 1	500
	11th to 20th	Day	21% 15% 29% 20% 12% 4	492
	21st to end	Day	20% 11% 32% 26% 8% 4	500
May	1st to 10th	Day	20% 11% 30% 24% 10% 5%	503
	11th to 20th	Day	19% 8% 25% 26% 15% 8%	527
	21st to end	Day	14% 5% 23% 27% 19% 12%	573
June	1st to 10th	Day	13% 4 21% 26% 24% 12%	530
	11th to 20th	Day	8% 4 18% 34% 25% 11%	662
	21st to end	Day	10% 4 20% 30% 22% 13%	730
July	1st to 10th	Day	14% 3 13% 25% 24% 22%	721
	11th to 20th	Day	8% 3 17% 27% 27% 18%	576
	21st to end	Day	12% 3 11% 29% 29% 17%	576
August	1st to 10th	Day	12% 3 14% 25% 27% 20%	526
	11th to 20th	Day	10% 3 12% 33% 28% 14%	519
	21st to end	Day	8% 3 9% 27% 32% 21%	583
September	1st to 10th	Day	14% 3 12% 27% 28% 16%	530
	11th to 20th	Day	10% 2 10% 26% 34% 19%	530
	21st to end	Day	17% 6% 17% 25% 26% 9%	530
October	1st to 10th	Day	12% 3 16% 34% 25% 10%	530
	11th to 20th	Day	13% 11% 27% 30% 15% 5%	530
	21st to end	Day	22% 10% 36% 20% 11% 1	425
November	1st to 10th	Day	18% 8% 31% 29% 10% 3	330
	11th to 20th	Day	25% 14% 30% 22% 8% 1	330
	21st to end	Day	27% 10% 32% 24% 8%	330
December	1st to 10th	Day	22% 20% 31% 20% 7% 1	306
	11th to 20th	Day	21% 15% 38% 19% 7%	300
	21st to end	Day	29% 18% 31% 17% 5%	330
January	1st to 10th	Night	24% 15% 32% 25% 4	800
	11th to 20th	Night	29% 14% 32% 22% 3	800
	21st to end	Night	24% 11% 38% 25% 2	880
February	1st to 10th	Night	29% 11% 33% 25% 3	800
	11th to 20th	Night	28% 11% 36% 22% 2	783
	21st to end	Night	28% 12% 38% 20% 2	660
March	1st to 10th	Night	24% 11% 32% 27% 5%	800
	11th to 20th	Night	24% 10% 35% 25% 6%	800
	21st to end	Night	22% 12% 29% 29% 7%	880
April	1st to 10th	Night	17% 10% 33% 31% 9%	635
	11th to 20th	Night	17% 8% 33% 29% 11% 1	593
	21st to end	Night	17% 6% 34% 33% 9% 1	600
May	1st to 10th	Night	20% 5% 27% 32% 14% 1	600
	11th to 20th	Night	16% 6% 17% 39% 19% 3	591
	21st to end	Night	10% 3 17% 37% 27% 6%	650
June	1st to 10th	Night	10% 2 15% 43% 27% 3	600
	11th to 20th	Night	9% 2 12% 42% 32% 4	600
	21st to end	Night	6% 2 14% 45% 30% 4	600
July	1st to 10th	Night	12% 2 9% 29% 38% 10%	585
	11th to 20th	Night	8% 2 9% 36% 39% 6%	582
	21st to end	Night	12% 2 8% 36% 36% 6%	657
August	1st to 10th	Night	10% 1 7% 34% 43% 5%	597
	11th to 20th	Night	7% 3 7% 39% 39% 6%	594
	21st to end	Night	7% 1 5% 37% 42% 7%	660
September	1st to 10th	Night	12% 1 9% 36% 37% 5%	690
	11th to 20th	Night	11% 3 10% 32% 39% 6%	800
	21st to end	Night	14% 4 15% 36% 29% 3	800
October	1st to 10th	Night	11% 3 17% 35% 29% 5%	800
	11th to 20th	Night	15% 6% 24% 36% 17% 2	800
	21st to end	Night	21% 6% 30% 29% 12% 1	880
November	1st to 10th	Night	15% 7% 26% 34% 16% 2	820
	11th to 20th	Night	18% 10% 32% 30% 10% 1	840
	21st to end	Night	22% 11% 26% 30% 10% 1	840
December	1st to 10th	Night	28% 9% 29% 28% 6%	809
	11th to 20th	Night	27% 9% 32% 26% 7% 1	800
	21st to end	Night	30% 12% 28% 26% 4	880

Page generated at 2025-11-30 01:00:50. Version ac07

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