Burg Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Burg Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	131712
	Night	14% 10% 3 8% 5% 4 5% 4 5% 7% 36%	130463
Transparency	Day	31% 37% 6% 15% 10% 1	135399
	Night	31% 36% 6% 15% 10% 1	134846
Seeing	Day	21% 13% 26% 21% 13% 6%	16736
	Night	20% 10% 24% 28% 16% 2	26883

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	17% 17% 5% 11% 5% 4 4 3 4 6% 23%	5258
	11th to 20th	Day	12% 15% 6% 14% 6% 4 3 2 3 4 29%	5280
	21st to end	Day	18% 13% 5% 10% 8% 6% 5% 4 5% 5% 23%	5780
February	1st to 10th	Day	20% 13% 5% 9% 5% 4 3 3 4 6% 29%	1911
	11th to 20th	Day	20% 12% 4 9% 5% 4 5% 3 5% 5% 28%	2048
	21st to end	Day	20% 13% 4 7% 3 4 5% 3 4 6% 32%	1730
March	1st to 10th	Day	17% 14% 4 9% 4 4 4 3 4 6% 31%	4808
	11th to 20th	Day	12% 7% 3 7% 5% 5% 4 4 5% 7% 41%	5090
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	5586
April	1st to 10th	Day	15% 9% 4 7% 5% 4 4 4 4 7% 38%	2341
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 5% 7% 39%	2575
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 35%	2600
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 5% 7% 7% 28%	5862
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	5861
	21st to end	Day	14% 7% 3 7% 5% 5% 6% 4 7% 9% 33%	6433
June	1st to 10th	Day	10% 6% 2 5% 5% 6% 5% 5% 7% 9% 39%	2600
	11th to 20th	Day	9% 5% 3 7% 6% 5% 5% 5% 7% 8% 40%	2600
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	2600
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	5900
	11th to 20th	Day	8% 7% 3 8% 6% 5% 6% 5% 8% 10% 33%	5900
	21st to end	Day	10% 8% 3 9% 6% 6% 6% 5% 7% 9% 31%	6424
August	1st to 10th	Day	7% 5% 3 6% 5% 5% 6% 6% 8% 11% 38%	4234
	11th to 20th	Day	7% 6% 2 7% 5% 6% 6% 5% 8% 10% 37%	4174
	21st to end	Day	5% 4 2 4 3 4 4 4 6% 9% 55%	4507
September	1st to 10th	Day	13% 6% 2 5% 3 3 4 4 5% 7% 48%	2200
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 3 5% 7% 50%	2200
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 6% 43%	2200
October	1st to 10th	Day	12% 5% 2 4 4 3 3 4 6% 8% 49%	2160
	11th to 20th	Day	13% 6% 2 6% 4 3 3 3 6% 6% 48%	2000
	21st to end	Day	19% 7% 3 7% 4 4 3 3 4 6% 41%	2170
November	1st to 10th	Day	12% 6% 3 5% 4 4 4 4 4 6% 49%	1852
	11th to 20th	Day	14% 11% 3 7% 4 3 5% 4 5% 6% 38%	1890
	21st to end	Day	16% 11% 3 8% 4 4 5% 3 4 6% 36%	1890
December	1st to 10th	Day	18% 13% 4 8% 4 4 4 3 4 5% 32%	3020
	11th to 20th	Day	22% 13% 5% 8% 4 5% 5% 4 5% 4 26%	3463
	21st to end	Day	19% 16% 6% 12% 5% 5% 4 3 4 5% 21%	4565
January	1st to 10th	Night	14% 14% 5% 14% 7% 5% 5% 4 5% 5% 20%	7635
	11th to 20th	Night	15% 13% 5% 14% 7% 5% 5% 3 5% 5% 24%	7680
	21st to end	Night	17% 12% 4 10% 6% 5% 4 3 4 5% 31%	8401
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 3 4 6% 29%	2704
	11th to 20th	Night	21% 11% 3 9% 5% 4 4 3 4 4 30%	2458
	21st to end	Night	20% 15% 4 8% 4 3 3 3 4 5% 30%	2076
March	1st to 10th	Night	17% 12% 3 7% 4 4 4 3 4 5% 36%	5263
	11th to 20th	Night	12% 7% 3 6% 4 3 4 3 6% 9% 43%	5090
	21st to end	Night	15% 10% 4 9% 6% 5% 5% 4 5% 6% 32%	5588
April	1st to 10th	Night	15% 9% 3 7% 5% 4 4 4 6% 6% 38%	2294
	11th to 20th	Night	13% 6% 3 6% 4 4 5% 5% 6% 7% 40%	1905
	21st to end	Night	16% 8% 3 6% 4 3 5% 4 6% 7% 38%	1800
May	1st to 10th	Night	19% 11% 3 8% 6% 4 4 4 6% 6% 28%	4033
	11th to 20th	Night	15% 7% 3 6% 5% 4 6% 4 6% 9% 33%	4032
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	4421
June	1st to 10th	Night	7% 5% 2 5% 5% 4 5% 4 7% 10% 46%	1800
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 4 7% 11% 45%	1800
	21st to end	Night	8% 5% 1 5% 5% 4 5% 5% 7% 11% 45%	1800
July	1st to 10th	Night	7% 6% 3 7% 5% 4 4 4 6% 9% 44%	4058
	11th to 20th	Night	8% 6% 3 8% 5% 5% 6% 4 7% 10% 37%	4057
	21st to end	Night	8% 8% 3 8% 5% 5% 6% 5% 7% 9% 36%	4427
August	1st to 10th	Night	6% 6% 3 7% 5% 5% 6% 5% 7% 9% 41%	2915
	11th to 20th	Night	8% 6% 2 8% 5% 5% 5% 4 7% 7% 43%	2875
	21st to end	Night	4 3 1 4 4 4 4 5% 6% 9% 57%	3369
September	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 51%	2185
	11th to 20th	Night	9% 5% 2 5% 3 4 5% 4 4 6% 54%	2200
	21st to end	Night	14% 6% 3 6% 4 4 4 3 4 6% 45%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 53%	2240
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 8% 46%	2400
	21st to end	Night	19% 8% 2 6% 3 3 4 4 4 6% 42%	2666
November	1st to 10th	Night	13% 7% 2 5% 4 4 3 3 5% 6% 49%	2666
	11th to 20th	Night	15% 9% 3 6% 5% 4 4 3 4 6% 40%	2730
	21st to end	Night	19% 11% 4 7% 5% 3 3 3 4 6% 35%	2730
December	1st to 10th	Night	17% 11% 4 9% 4 4 4 3 4 5% 35%	4380
	11th to 20th	Night	17% 13% 6% 11% 5% 4 3 3 4 5% 29%	4952
	21st to end	Night	19% 16% 5% 11% 6% 5% 5% 4 5% 6% 20%	6633

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	52% 21% 9% 14% 5%	5259
	11th to 20th	Day	47% 21% 5% 14% 12% 2	5280
	21st to end	Day	51% 22% 5% 16% 5% 1	5780
February	1st to 10th	Day	24% 59% 4 8% 5%	1911
	11th to 20th	Day	26% 57% 2 8% 6%	2058
	21st to end	Day	20% 58% 4 11% 5% 1	1730
March	1st to 10th	Day	43% 28% 3 13% 9% 4	4808
	11th to 20th	Day	27% 28% 6% 22% 16% 2	5090
	21st to end	Day	33% 28% 6% 19% 12% 2	5586
April	1st to 10th	Day	22% 59% 4 10% 4	2341
	11th to 20th	Day	18% 60% 5% 10% 5% 1	2575
	21st to end	Day	19% 62% 5% 9% 5% 1	2600
May	1st to 10th	Day	38% 30% 9% 14% 8% 1	5897
	11th to 20th	Day	38% 29% 7% 19% 7%	5865
	21st to end	Day	35% 30% 8% 16% 10% 2	6447
June	1st to 10th	Day	16% 62% 5% 8% 8%	2600
	11th to 20th	Day	17% 63% 6% 8% 6%	2600
	21st to end	Day	15% 62% 5% 10% 6% 1	2600
July	1st to 10th	Day	29% 28% 8% 21% 12% 1	5835
	11th to 20th	Day	32% 30% 11% 19% 8%	5831
	21st to end	Day	37% 29% 9% 13% 11% 1	6240
August	1st to 10th	Day	23% 39% 10% 22% 6%	4221
	11th to 20th	Day	23% 39% 7% 20% 10%	4213
	21st to end	Day	14% 37% 10% 22% 16% 1	4507
September	1st to 10th	Day	14% 58% 6% 9% 10% 3	2200
	11th to 20th	Day	14% 57% 5% 12% 11% 1	2200
	21st to end	Day	19% 57% 4 12% 9%	2200
October	1st to 10th	Day	13% 35% 3 21% 27% 1	3524
	11th to 20th	Day	13% 36% 5% 22% 21% 3	3240
	21st to end	Day	18% 36% 8% 19% 18% 2	3484
November	1st to 10th	Day	21% 54% 5% 13% 7%	1852
	11th to 20th	Day	20% 54% 4 10% 11% 2	1890
	21st to end	Day	27% 54% 5% 11% 4	1890
December	1st to 10th	Day	33% 36% 4 17% 10%	3020
	11th to 20th	Day	45% 31% 5% 14% 5%	3460
	21st to end	Day	52% 26% 6% 12% 4	4565
January	1st to 10th	Night	53% 21% 7% 13% 5%	7636
	11th to 20th	Night	48% 21% 4 14% 9% 2	7680
	21st to end	Night	45% 21% 7% 18% 8% 1	8401
February	1st to 10th	Night	24% 59% 4 8% 5% 1	2704
	11th to 20th	Night	26% 57% 3 9% 5%	2459
	21st to end	Night	24% 58% 3 8% 6% 1	2076
March	1st to 10th	Night	39% 27% 3 21% 9% 2	5263
	11th to 20th	Night	25% 28% 5% 24% 18% 1	5090
	21st to end	Night	39% 28% 4 17% 10% 1	5588
April	1st to 10th	Night	21% 60% 4 8% 6%	2294
	11th to 20th	Night	17% 61% 4 11% 5% 1	1905
	21st to end	Night	17% 62% 4 10% 7% 1	1800
May	1st to 10th	Night	40% 31% 8% 13% 7% 1	4060
	11th to 20th	Night	35% 31% 10% 18% 6%	4033
	21st to end	Night	34% 29% 10% 17% 8% 2	4429
June	1st to 10th	Night	15% 63% 4 9% 8% 1	1800
	11th to 20th	Night	14% 63% 5% 7% 9% 1	1800
	21st to end	Night	13% 62% 6% 10% 7%	1800
July	1st to 10th	Night	27% 30% 8% 19% 14% 2	4015
	11th to 20th	Night	31% 31% 12% 18% 8%	4006
	21st to end	Night	34% 29% 8% 16% 10% 3	4297
August	1st to 10th	Night	24% 39% 10% 21% 5% 1	2904
	11th to 20th	Night	26% 39% 5% 18% 12%	2902
	21st to end	Night	12% 37% 11% 23% 16% 1	3369
September	1st to 10th	Night	12% 57% 7% 11% 10% 3	2185
	11th to 20th	Night	13% 57% 4 12% 13% 1	2200
	21st to end	Night	17% 57% 5% 12% 10%	2200
October	1st to 10th	Night	13% 36% 4 18% 28% 2	3656
	11th to 20th	Night	13% 36% 5% 18% 23% 5%	3899
	21st to end	Night	17% 35% 5% 16% 22% 5%	4302
November	1st to 10th	Night	21% 53% 5% 12% 8% 1	2666
	11th to 20th	Night	21% 54% 3 11% 10% 2	2730
	21st to end	Night	26% 54% 4 11% 4	2730
December	1st to 10th	Night	34% 35% 8% 16% 7% 1	4380
	11th to 20th	Night	40% 31% 9% 14% 5%	4954
	21st to end	Night	52% 26% 6% 12% 4	6633

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	30% 20% 34% 12% 3 1	330
	11th to 20th	Day	36% 26% 29% 8%	330
	21st to end	Day	32% 26% 28% 12% 2	363
February	1st to 10th	Day	31% 23% 31% 12% 4	330
	11th to 20th	Day	42% 23% 29% 4 2 1	330
	21st to end	Day	34% 20% 38% 7% 1	273
March	1st to 10th	Day	32% 22% 27% 15% 3 1	428
	11th to 20th	Day	28% 20% 33% 15% 4	540
	21st to end	Day	32% 18% 29% 16% 4	594
April	1st to 10th	Day	28% 20% 26% 16% 8% 1	540
	11th to 20th	Day	26% 16% 30% 16% 9% 3	516
	21st to end	Day	25% 16% 32% 20% 4 3	500
May	1st to 10th	Day	25% 17% 31% 15% 9% 2	503
	11th to 20th	Day	22% 12% 28% 23% 11% 5%	527
	21st to end	Day	17% 10% 25% 25% 15% 7%	573
June	1st to 10th	Day	13% 8% 24% 31% 17% 6%	530
	11th to 20th	Day	12% 8% 24% 32% 17% 7%	530
	21st to end	Day	12% 6% 29% 25% 19% 9%	530
July	1st to 10th	Day	14% 5% 16% 24% 22% 19%	525
	11th to 20th	Day	7% 7% 21% 31% 22% 12%	524
	21st to end	Day	14% 3 18% 31% 22% 13%	576
August	1st to 10th	Day	10% 4 19% 28% 25% 13%	526
	11th to 20th	Day	12% 6% 18% 31% 22% 11%	519
	21st to end	Day	8% 4 17% 31% 26% 14%	583
September	1st to 10th	Day	15% 5% 16% 27% 26% 12%	530
	11th to 20th	Day	10% 4 15% 27% 30% 15%	530
	21st to end	Day	20% 7% 21% 25% 22% 4	530
October	1st to 10th	Day	14% 6% 21% 29% 22% 8%	530
	11th to 20th	Day	14% 11% 33% 23% 13% 5%	530
	21st to end	Day	22% 15% 31% 22% 9% 1	553
November	1st to 10th	Day	20% 14% 35% 22% 8% 2	330
	11th to 20th	Day	30% 15% 30% 18% 7% 1	330
	21st to end	Day	27% 22% 31% 16% 5% 1	330
December	1st to 10th	Day	28% 20% 35% 11% 5%	330
	11th to 20th	Day	28% 20% 34% 13% 4 1	330
	21st to end	Day	33% 18% 31% 13% 4 1	363
January	1st to 10th	Night	28% 17% 31% 20% 4	840
	11th to 20th	Night	31% 17% 31% 18% 3	840
	21st to end	Night	25% 19% 31% 22% 2	924
February	1st to 10th	Night	29% 15% 29% 25% 2	840
	11th to 20th	Night	31% 18% 30% 19% 2	823
	21st to end	Night	29% 19% 29% 21% 1	692
March	1st to 10th	Night	25% 15% 29% 26% 5%	840
	11th to 20th	Night	25% 17% 30% 20% 7% 1	840
	21st to end	Night	22% 14% 32% 25% 7%	924
April	1st to 10th	Night	22% 14% 31% 25% 7%	824
	11th to 20th	Night	18% 10% 33% 26% 11% 1	611
	21st to end	Night	20% 7% 36% 29% 8%	600
May	1st to 10th	Night	20% 10% 28% 29% 12% 2	600
	11th to 20th	Night	19% 5% 25% 32% 17% 1	591
	21st to end	Night	12% 4 19% 33% 27% 4	650
June	1st to 10th	Night	10% 3 21% 40% 24% 3	600
	11th to 20th	Night	8% 4 16% 38% 29% 4	600
	21st to end	Night	8% 5% 20% 33% 31% 5%	600
July	1st to 10th	Night	12% 2 12% 33% 32% 10%	585
	11th to 20th	Night	7% 1 13% 37% 37% 5%	582
	21st to end	Night	11% 3 12% 35% 34% 5%	657
August	1st to 10th	Night	9% 3 10% 36% 38% 4	597
	11th to 20th	Night	7% 4 11% 36% 34% 7%	594
	21st to end	Night	8% 1 13% 33% 37% 8%	660
September	1st to 10th	Night	12% 2 13% 34% 33% 6%	785
	11th to 20th	Night	12% 3 12% 30% 35% 8%	800
	21st to end	Night	17% 6% 17% 32% 25% 3	800
October	1st to 10th	Night	13% 5% 19% 33% 26% 4	800
	11th to 20th	Night	16% 9% 26% 30% 17% 1	800
	21st to end	Night	24% 10% 25% 28% 13% 1	880
November	1st to 10th	Night	17% 10% 25% 31% 17% 1	820
	11th to 20th	Night	20% 17% 28% 25% 10% 1	840
	21st to end	Night	25% 12% 23% 30% 9% 1	840
December	1st to 10th	Night	29% 16% 25% 25% 5%	840
	11th to 20th	Night	23% 13% 31% 26% 5% 1	840
	21st to end	Night	29% 15% 27% 25% 4	924

Page generated at 2026-04-14 02:44:25. Version ac07

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