TASS H (Droege) Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the TASS H (Droege) Observatory 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	13% 9% 3 7% 5% 4 5% 4 6% 7% 36%	127046
	Night	14% 10% 3 8% 4 4 4 3 5% 6% 38%	123899
Transparency	Day	46% 4 11% 24% 14% 2	130567
	Night	46% 3 11% 23% 15% 2	128452
Seeing	Day	17% 13% 29% 24% 12% 4	17282
	Night	18% 11% 30% 29% 11% 1	27114

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	12% 11% 6% 13% 6% 6% 5% 4 6% 6% 25%	3617
	11th to 20th	Day	10% 13% 6% 12% 4 5% 3 6% 6% 4 29%	3620
	21st to end	Day	16% 17% 5% 9% 5% 3 3 3 4 6% 29%	4138
February	1st to 10th	Day	15% 14% 6% 7% 6% 6% 5% 4 4 6% 26%	1829
	11th to 20th	Day	14% 12% 4 8% 6% 4 4 3 4 6% 34%	1948
	21st to end	Day	14% 14% 5% 9% 5% 4 4 3 5% 6% 31%	1650
March	1st to 10th	Day	17% 12% 3 6% 4 3 3 3 4 5% 41%	4480
	11th to 20th	Day	9% 7% 3 6% 5% 5% 5% 4 5% 7% 43%	4689
	21st to end	Day	20% 14% 6% 12% 6% 5% 5% 3 5% 5% 20%	5967
April	1st to 10th	Day	18% 13% 3 7% 5% 4 4 4 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 6% 5% 5% 5% 4 5% 6% 35%	2374
	21st to end	Day	20% 9% 3 6% 5% 6% 4 3 5% 6% 33%	2495
May	1st to 10th	Day	17% 8% 2 6% 5% 5% 5% 4 6% 8% 35%	5862
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	6217
	21st to end	Day	14% 7% 3 7% 5% 4 4 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 4 4 7% 7% 46%	2800
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	2800
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 5% 7% 8% 39%	2800
July	1st to 10th	Day	10% 6% 2 7% 6% 5% 6% 6% 7% 9% 36%	6360
	11th to 20th	Day	9% 5% 3 6% 5% 4 5% 4 7% 10% 41%	6360
	21st to end	Day	10% 7% 2 7% 6% 5% 5% 5% 7% 9% 38%	6924
August	1st to 10th	Day	8% 5% 2 7% 4 4 5% 5% 6% 9% 44%	4464
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 10% 42%	3849
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 7% 47%	2400
	11th to 20th	Day	11% 4 1 4 3 4 4 3 5% 7% 53%	2281
	21st to end	Day	10% 7% 3 6% 4 4 4 4 6% 8% 45%	2200
October	1st to 10th	Day	12% 7% 2 6% 5% 3 4 3 5% 7% 47%	2131
	11th to 20th	Day	17% 9% 3 7% 4 5% 4 4 4 8% 36%	2000
	21st to end	Day	19% 9% 3 6% 4 4 4 3 4 6% 37%	1980
November	1st to 10th	Day	12% 9% 3 6% 4 4 4 3 5% 5% 45%	1852
	11th to 20th	Day	15% 13% 4 8% 4 3 5% 3 5% 7% 33%	1890
	21st to end	Day	17% 12% 4 10% 5% 3 3 3 4 5% 34%	1806
December	1st to 10th	Day	9% 10% 4 9% 5% 4 4 3 4 5% 43%	2626
	11th to 20th	Day	12% 15% 5% 12% 6% 5% 5% 5% 4 7% 24%	3008
	21st to end	Day	24% 23% 8% 11% 6% 5% 3 2 3 4 11%	3982
January	1st to 10th	Night	13% 13% 6% 13% 5% 4 4 3 4 4 31%	6348
	11th to 20th	Night	13% 16% 6% 12% 7% 5% 4 4 5% 4 23%	6360
	21st to end	Night	17% 14% 7% 9% 5% 3 4 3 3 5% 32%	6836
February	1st to 10th	Night	15% 14% 4 8% 6% 4 4 3 3 6% 32%	2570
	11th to 20th	Night	14% 12% 4 8% 5% 4 4 3 4 6% 37%	2338
	21st to end	Night	17% 11% 3 8% 4 3 4 4 5% 6% 36%	1980
March	1st to 10th	Night	16% 10% 4 6% 4 3 4 2 4 5% 41%	5373
	11th to 20th	Night	11% 6% 3 6% 4 4 4 4 5% 7% 47%	5311
	21st to end	Night	19% 12% 5% 9% 5% 4 4 3 5% 5% 28%	4962
April	1st to 10th	Night	16% 10% 2 7% 4 4 4 3 5% 6% 39%	2000
	11th to 20th	Night	18% 9% 3 6% 5% 3 3 3 5% 6% 38%	1975
	21st to end	Night	17% 9% 3 8% 4 4 4 3 6% 7% 36%	1905
May	1st to 10th	Night	17% 8% 2 7% 4 4 5% 4 5% 7% 36%	4033
	11th to 20th	Night	12% 6% 2 5% 3 4 4 4 7% 9% 44%	3692
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 7% 41%	3942
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 5% 9% 49%	1600
	11th to 20th	Night	9% 5% 2 5% 4 5% 4 4 7% 9% 46%	1600
	21st to end	Night	10% 6% 2 5% 5% 5% 6% 4 7% 8% 43%	1600
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 4 7% 8% 40%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 4 6% 9% 50%	3617
	21st to end	Night	9% 6% 2 7% 4 5% 5% 5% 6% 8% 43%	3947
August	1st to 10th	Night	5% 4 2 6% 5% 4 5% 4 7% 9% 49%	2675
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 50%	3200
	21st to end	Night	8% 4 1 6% 3 4 4 3 5% 9% 53%	3586
September	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 5% 8% 54%	2000
	11th to 20th	Night	9% 4 1 5% 3 3 3 3 5% 8% 56%	2097
	21st to end	Night	10% 6% 2 7% 4 4 5% 3 6% 7% 47%	2200
October	1st to 10th	Night	11% 4 2 4 4 3 3 4 5% 6% 53%	2265
	11th to 20th	Night	18% 6% 2 6% 4 3 3 2 4 6% 46%	2400
	21st to end	Night	20% 9% 2 6% 4 4 3 3 5% 6% 38%	2860
November	1st to 10th	Night	13% 8% 3 6% 3 4 4 3 5% 6% 47%	2666
	11th to 20th	Night	15% 11% 4 7% 4 3 4 3 5% 6% 37%	2730
	21st to end	Night	17% 10% 4 8% 5% 4 3 3 5% 5% 36%	2809
December	1st to 10th	Night	12% 10% 2 8% 4 4 3 3 4 7% 42%	4609
	11th to 20th	Night	18% 15% 4 9% 5% 3 4 3 4 6% 30%	5199
	21st to end	Night	24% 19% 6% 9% 5% 4 4 3 4 4 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	60% 2 12% 18% 7%	3618
	11th to 20th	Day	56% 3 9% 11% 15% 5%	3620
	21st to end	Day	58% 2 12% 19% 9%	4138
February	1st to 10th	Day	60% 3 11% 15% 9% 2	1829
	11th to 20th	Day	52% 4 9% 24% 9% 2	1958
	21st to end	Day	55% 4 11% 20% 9% 2	1650
March	1st to 10th	Day	48% 2 8% 26% 12% 3	4480
	11th to 20th	Day	41% 2 11% 34% 11% 1	4689
	21st to end	Day	67% 3 10% 13% 6% 1	5969
April	1st to 10th	Day	55% 4 10% 21% 11%	2400
	11th to 20th	Day	49% 4 9% 23% 12% 3	2374
	21st to end	Day	53% 3 11% 19% 12% 2	2495
May	1st to 10th	Day	47% 4 10% 24% 14% 1	5897
	11th to 20th	Day	43% 4 10% 25% 16% 1	6221
	21st to end	Day	43% 4 11% 26% 13% 3	6949
June	1st to 10th	Day	36% 3 12% 28% 18% 2	2800
	11th to 20th	Day	37% 5% 11% 25% 19% 3	2800
	21st to end	Day	40% 6% 12% 24% 15% 3	2800
July	1st to 10th	Day	41% 7% 13% 24% 14% 1	6291
	11th to 20th	Day	35% 7% 14% 27% 16% 1	6285
	21st to end	Day	40% 7% 15% 21% 15% 2	6728
August	1st to 10th	Day	36% 5% 16% 29% 13% 1	4449
	11th to 20th	Day	35% 7% 12% 30% 15% 1	3885
	21st to end	Day	32% 5% 12% 29% 19% 3	4311
September	1st to 10th	Day	36% 4 11% 27% 18% 4	2400
	11th to 20th	Day	30% 4 10% 30% 23% 3	2281
	21st to end	Day	37% 4 8% 28% 21% 2	2200
October	1st to 10th	Day	26% 4 7% 31% 32% 2	3469
	11th to 20th	Day	33% 3 11% 27% 24% 1	3240
	21st to end	Day	42% 3 9% 28% 14% 3	3180
November	1st to 10th	Day	41% 3 13% 25% 16% 2	1852
	11th to 20th	Day	53% 2 10% 23% 11% 1	1890
	21st to end	Day	54% 3 10% 24% 9%	1806
December	1st to 10th	Day	45% 3 17% 27% 8%	2626
	11th to 20th	Day	60% 4 11% 18% 6% 1	3005
	21st to end	Day	79% 3 5% 8% 4	3982
January	1st to 10th	Night	58% 2 9% 20% 11%	6348
	11th to 20th	Night	64% 3 9% 10% 14% 1	6360
	21st to end	Night	58% 2 11% 17% 11% 1	6836
February	1st to 10th	Night	56% 2 10% 18% 12% 1	2570
	11th to 20th	Night	50% 3 10% 23% 14% 1	2339
	21st to end	Night	50% 3 11% 20% 12% 3	1980
March	1st to 10th	Night	48% 2 11% 25% 11% 4	5373
	11th to 20th	Night	38% 3 10% 31% 14% 3	5311
	21st to end	Night	59% 3 9% 20% 9% 1	4962
April	1st to 10th	Night	47% 3 11% 25% 13% 2	2000
	11th to 20th	Night	48% 3 8% 22% 16% 4	1975
	21st to end	Night	48% 4 10% 23% 13% 3	1905
May	1st to 10th	Night	48% 4 10% 19% 15% 3	4060
	11th to 20th	Night	37% 4 15% 26% 14% 3	3693
	21st to end	Night	44% 4 9% 23% 16% 4	3950
June	1st to 10th	Night	34% 4 12% 27% 20% 3	1600
	11th to 20th	Night	34% 4 11% 28% 21% 2	1600
	21st to end	Night	38% 6% 15% 22% 17% 2	1600
July	1st to 10th	Night	39% 6% 14% 24% 14% 3	3580
	11th to 20th	Night	31% 6% 13% 30% 19% 2	3572
	21st to end	Night	35% 8% 14% 23% 18% 3	3832
August	1st to 10th	Night	30% 7% 16% 28% 18% 1	2666
	11th to 20th	Night	30% 6% 15% 30% 18% 1	3230
	21st to end	Night	29% 5% 11% 34% 17% 4	3586
September	1st to 10th	Night	31% 3 11% 29% 24% 3	2000
	11th to 20th	Night	29% 2 7% 34% 25% 2	2097
	21st to end	Night	36% 4 10% 30% 18% 2	2200
October	1st to 10th	Night	22% 3 7% 38% 29% 2	3707
	11th to 20th	Night	36% 3 6% 24% 28% 2	3899
	21st to end	Night	41% 3 8% 26% 19% 3	4610
November	1st to 10th	Night	39% 3 8% 27% 19% 3	2666
	11th to 20th	Night	49% 2 11% 24% 13% 2	2730
	21st to end	Night	52% 3 11% 23% 11%	2809
December	1st to 10th	Night	43% 3 14% 28% 10% 2	4609
	11th to 20th	Night	57% 3 12% 17% 10% 1	5201
	21st to end	Night	72% 3 8% 12% 6%	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	22% 18% 36% 20% 4 1	300
	11th to 20th	Day	23% 20% 38% 14% 3 1	300
	21st to end	Day	24% 23% 28% 20% 4 1	330
February	1st to 10th	Day	24% 21% 38% 15% 2	300
	11th to 20th	Day	27% 21% 32% 17% 3	300
	21st to end	Day	26% 24% 37% 10% 2 1	249
March	1st to 10th	Day	24% 26% 36% 13% 1	300
	11th to 20th	Day	30% 20% 32% 14% 4	315
	21st to end	Day	29% 18% 30% 17% 5% 1	550
April	1st to 10th	Day	26% 16% 32% 20% 6%	500
	11th to 20th	Day	23% 24% 23% 19% 10% 2	492
	21st to end	Day	24% 16% 29% 23% 7% 1	500
May	1st to 10th	Day	22% 15% 31% 22% 9% 2	503
	11th to 20th	Day	19% 13% 32% 23% 10% 3	675
	21st to end	Day	16% 12% 31% 22% 13% 5%	789
June	1st to 10th	Day	12% 11% 30% 28% 13% 5%	730
	11th to 20th	Day	12% 10% 32% 29% 14% 3	730
	21st to end	Day	12% 10% 32% 29% 12% 5%	730
July	1st to 10th	Day	8% 5% 25% 31% 20% 12%	721
	11th to 20th	Day	12% 7% 27% 30% 16% 8%	718
	21st to end	Day	8% 6% 28% 35% 18% 6%	794
August	1st to 10th	Day	7% 4 22% 36% 20% 10%	684
	11th to 20th	Day	12% 5% 21% 28% 24% 11%	519
	21st to end	Day	7% 5% 21% 34% 23% 10%	583
September	1st to 10th	Day	13% 8% 23% 29% 22% 7%	530
	11th to 20th	Day	9% 4 18% 34% 25% 9%	530
	21st to end	Day	12% 12% 26% 25% 18% 6%	530
October	1st to 10th	Day	14% 10% 28% 28% 15% 4	461
	11th to 20th	Day	20% 23% 33% 18% 6%	330
	21st to end	Day	30% 16% 32% 16% 5%	363
November	1st to 10th	Day	20% 19% 32% 21% 5% 2	330
	11th to 20th	Day	28% 21% 31% 14% 5% 1	330
	21st to end	Day	27% 16% 35% 19% 3	330
December	1st to 10th	Day	18% 24% 37% 15% 6%	306
	11th to 20th	Day	22% 22% 36% 15% 4 1	300
	21st to end	Day	30% 22% 34% 13% 2	330
January	1st to 10th	Night	19% 16% 38% 22% 5%	1000
	11th to 20th	Night	23% 16% 35% 21% 4 1	1000
	21st to end	Night	21% 13% 36% 26% 3	1100
February	1st to 10th	Night	23% 15% 35% 23% 4 1	970
	11th to 20th	Night	21% 15% 36% 24% 4	783
	21st to end	Night	22% 16% 38% 21% 3	660
March	1st to 10th	Night	21% 16% 34% 22% 6%	800
	11th to 20th	Night	17% 12% 37% 27% 7%	765
	21st to end	Night	24% 13% 32% 23% 6% 1	660
April	1st to 10th	Night	20% 14% 35% 24% 6% 1	600
	11th to 20th	Night	22% 15% 30% 25% 7%	593
	21st to end	Night	20% 12% 33% 27% 8%	600
May	1st to 10th	Night	18% 9% 34% 30% 8% 1	600
	11th to 20th	Night	17% 9% 29% 32% 13%	591
	21st to end	Night	14% 6% 27% 36% 16%	650
June	1st to 10th	Night	10% 6% 28% 40% 15% 1	600
	11th to 20th	Night	11% 5% 32% 41% 12% 1	600
	21st to end	Night	11% 6% 29% 36% 17% 1	600
July	1st to 10th	Night	8% 4 16% 45% 24% 3	585
	11th to 20th	Night	7% 3 20% 41% 26% 3	582
	21st to end	Night	6% 2 19% 46% 25% 2	657
August	1st to 10th	Night	5% 3 15% 46% 28% 4	597
	11th to 20th	Night	7% 3 14% 42% 30% 4	594
	21st to end	Night	8% 2 17% 43% 28% 2	660
September	1st to 10th	Night	12% 4 20% 36% 25% 3	600
	11th to 20th	Night	9% 4 20% 36% 28% 3	697
	21st to end	Night	12% 8% 22% 34% 22% 3	800
October	1st to 10th	Night	13% 6% 26% 36% 17% 2	800
	11th to 20th	Night	20% 13% 30% 27% 9% 1	800
	21st to end	Night	24% 14% 29% 23% 9%	880
November	1st to 10th	Night	18% 11% 38% 24% 9% 1	820
	11th to 20th	Night	21% 18% 31% 24% 6%	840
	21st to end	Night	21% 15% 35% 23% 5%	919
December	1st to 10th	Night	23% 14% 35% 23% 4	1011
	11th to 20th	Night	24% 14% 32% 26% 5%	1000
	21st to end	Night	31% 15% 31% 21% 2	1100

Page generated at 2025-11-30 00:00:57. Version ac07

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