Ballreich Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ballreich Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 31%	132275
	Night	14% 11% 4 9% 5% 4 4 4 5% 6% 32%	130776
Transparency	Day	50% 4 11% 21% 12% 1	135887
	Night	50% 4 11% 21% 13% 1	135234
Seeing	Day	19% 10% 27% 23% 14% 6%	17489
	Night	19% 9% 28% 29% 13% 1	27316

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	15% 19% 7% 18% 7% 5% 4 3 5% 5% 12%	4691
	11th to 20th	Day	12% 17% 7% 16% 9% 6% 7% 3 3 3 17%	5123
	21st to end	Day	18% 16% 8% 14% 7% 6% 6% 3 4 4 14%	5780
February	1st to 10th	Day	15% 16% 6% 13% 6% 5% 4 4 4 5% 21%	1890
	11th to 20th	Day	14% 14% 6% 13% 7% 6% 6% 4 4 5% 23%	2008
	21st to end	Day	15% 14% 5% 12% 5% 4 4 3 5% 6% 25%	1730
March	1st to 10th	Day	17% 13% 6% 14% 5% 4 4 3 4 4 26%	4739
	11th to 20th	Day	11% 12% 4 9% 4 5% 4 4 5% 7% 37%	5090
	21st to end	Day	18% 14% 5% 11% 6% 5% 4 3 4 5% 25%	5586
April	1st to 10th	Day	15% 12% 4 9% 6% 5% 5% 4 6% 6% 28%	2401
	11th to 20th	Day	15% 11% 3 7% 5% 5% 5% 4 5% 7% 33%	2656
	21st to end	Day	18% 10% 3 6% 5% 4 4 4 5% 7% 34%	2730
May	1st to 10th	Day	21% 10% 3 8% 5% 4 4 4 5% 6% 28%	5992
	11th to 20th	Day	16% 7% 3 7% 6% 5% 5% 5% 6% 8% 33%	5991
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	6576
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 4 4 6% 8% 39%	2630
	11th to 20th	Day	11% 6% 3 8% 5% 5% 5% 4 7% 9% 39%	2800
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 6% 7% 9% 38%	2800
July	1st to 10th	Day	8% 6% 3 7% 6% 5% 5% 4 7% 11% 39%	6360
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 11% 40%	6158
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 10% 39%	6424
August	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 5% 7% 10% 40%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 12% 37%	4174
	21st to end	Day	8% 5% 2 6% 4 6% 5% 4 7% 9% 43%	4611
September	1st to 10th	Day	13% 6% 2 4 4 5% 4 4 5% 7% 46%	2234
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 47%	2200
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 6% 41%	2200
October	1st to 10th	Day	12% 7% 3 8% 5% 3 4 3 6% 6% 43%	2145
	11th to 20th	Day	15% 8% 3 7% 5% 4 5% 4 5% 6% 39%	2000
	21st to end	Day	17% 10% 4 8% 5% 4 5% 4 5% 7% 30%	2090
November	1st to 10th	Day	12% 9% 4 6% 5% 4 4 4 6% 8% 40%	1852
	11th to 20th	Day	17% 13% 3 7% 5% 4 4 3 4 6% 33%	1890
	21st to end	Day	17% 15% 5% 9% 5% 4 4 3 5% 6% 25%	1890
December	1st to 10th	Day	15% 15% 7% 15% 7% 4 4 3 4 5% 21%	2870
	11th to 20th	Day	18% 19% 8% 11% 5% 4 3 3 4 5% 20%	3463
	21st to end	Day	22% 22% 7% 13% 6% 5% 3 3 3 4 14%	4267
January	1st to 10th	Night	15% 18% 7% 17% 6% 5% 4 3 4 4 17%	8232
	11th to 20th	Night	9% 18% 8% 17% 7% 5% 5% 4 4 4 17%	7825
	21st to end	Night	17% 15% 6% 11% 5% 5% 5% 4 5% 6% 22%	8401
February	1st to 10th	Night	18% 14% 5% 12% 7% 6% 4 4 4 5% 23%	2730
	11th to 20th	Night	15% 14% 5% 10% 7% 5% 5% 4 4 5% 26%	2495
	21st to end	Night	17% 15% 5% 9% 5% 5% 4 3 4 6% 26%	2076
March	1st to 10th	Night	17% 13% 4 8% 5% 4 3 3 3 5% 35%	5320
	11th to 20th	Night	10% 8% 2 6% 5% 4 5% 5% 6% 7% 41%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 4 6% 36%	5588
April	1st to 10th	Night	16% 12% 4 8% 6% 4 4 4 5% 5% 33%	2237
	11th to 20th	Night	15% 8% 2 8% 5% 4 4 4 6% 6% 36%	1911
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 5% 7% 37%	1890
May	1st to 10th	Night	20% 12% 3 9% 6% 5% 4 4 5% 5% 27%	4123
	11th to 20th	Night	15% 8% 3 7% 4 4 5% 4 5% 7% 37%	4122
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 8% 42%	4520
June	1st to 10th	Night	10% 7% 3 5% 4 4 4 4 6% 7% 45%	1765
	11th to 20th	Night	9% 5% 2 5% 4 5% 5% 5% 8% 9% 43%	1600
	21st to end	Night	8% 4 3 5% 5% 4 5% 5% 7% 10% 45%	1600
July	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	3618
	11th to 20th	Night	9% 5% 2 6% 4 5% 5% 5% 7% 10% 43%	3793
	21st to end	Night	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	4427
August	1st to 10th	Night	10% 6% 3 7% 4 5% 5% 4 7% 9% 42%	2915
	11th to 20th	Night	9% 6% 3 8% 5% 4 4 3 5% 8% 45%	2875
	21st to end	Night	6% 4 2 7% 4 5% 5% 4 6% 9% 48%	3283
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 5% 7% 50%	2145
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 4 7% 56%	2200
	21st to end	Night	13% 7% 3 7% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 5% 7% 47%	2255
	11th to 20th	Night	17% 9% 2 7% 5% 3 4 3 5% 7% 39%	2400
	21st to end	Night	20% 8% 3 7% 4 4 4 3 5% 7% 35%	2749
November	1st to 10th	Night	13% 9% 3 7% 4 3 4 4 5% 6% 43%	2666
	11th to 20th	Night	15% 12% 4 8% 5% 4 4 3 6% 6% 34%	2730
	21st to end	Night	19% 15% 4 9% 5% 4 4 3 5% 5% 26%	2730
December	1st to 10th	Night	13% 14% 5% 13% 7% 6% 5% 4 5% 5% 24%	4380
	11th to 20th	Night	18% 17% 7% 11% 5% 4 4 3 4 5% 22%	4952
	21st to end	Night	24% 19% 7% 10% 5% 4 4 3 5% 5% 13%	6933

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	73% 3 12% 10% 2	4692
	11th to 20th	Day	73% 2 7% 7% 10%	5123
	21st to end	Day	75% 2 7% 11% 5%	5780
February	1st to 10th	Day	66% 4 10% 14% 6%	1890
	11th to 20th	Day	63% 4 9% 16% 7% 1	2018
	21st to end	Day	59% 4 14% 16% 6% 2	1730
March	1st to 10th	Day	62% 3 8% 20% 7%	4739
	11th to 20th	Day	47% 4 14% 26% 9% 1	5090
	21st to end	Day	61% 4 10% 16% 8% 1	5586
April	1st to 10th	Day	55% 6% 11% 17% 11% 1	2401
	11th to 20th	Day	49% 6% 12% 19% 12% 2	2656
	21st to end	Day	49% 5% 11% 22% 11% 2	2730
May	1st to 10th	Day	55% 5% 10% 17% 12% 1	6027
	11th to 20th	Day	48% 6% 13% 21% 11% 1	5995
	21st to end	Day	44% 5% 10% 22% 15% 4	6590
June	1st to 10th	Day	41% 6% 11% 25% 14% 3	2630
	11th to 20th	Day	41% 6% 13% 22% 15% 2	2800
	21st to end	Day	39% 7% 15% 21% 15% 4	2800
July	1st to 10th	Day	39% 5% 10% 27% 17% 2	6291
	11th to 20th	Day	37% 4 16% 26% 16% 1	6083
	21st to end	Day	40% 4 13% 22% 19% 2	6240
August	1st to 10th	Day	35% 5% 18% 27% 14% 1	4221
	11th to 20th	Day	37% 6% 13% 32% 11% 1	4213
	21st to end	Day	36% 5% 13% 31% 12% 2	4610
September	1st to 10th	Day	36% 5% 12% 26% 17% 3	2234
	11th to 20th	Day	33% 6% 11% 29% 19% 2	2200
	21st to end	Day	44% 4 11% 22% 19%	2200
October	1st to 10th	Day	29% 4 13% 31% 22% 1	3496
	11th to 20th	Day	39% 4 10% 26% 20% 1	3240
	21st to end	Day	43% 4 11% 23% 18% 1	3353
November	1st to 10th	Day	43% 4 11% 26% 15% 1	1852
	11th to 20th	Day	52% 4 11% 20% 11% 2	1890
	21st to end	Day	60% 4 11% 18% 7%	1890
December	1st to 10th	Day	65% 3 14% 13% 4	2870
	11th to 20th	Day	66% 3 10% 16% 5%	3460
	21st to end	Day	77% 3 7% 9% 4	4267
January	1st to 10th	Night	71% 3 8% 15% 4	8233
	11th to 20th	Night	71% 2 8% 10% 8% 1	7825
	21st to end	Night	64% 3 12% 14% 8%	8401
February	1st to 10th	Night	63% 4 11% 14% 7% 1	2730
	11th to 20th	Night	59% 4 9% 19% 8% 1	2496
	21st to end	Night	59% 4 11% 16% 9% 2	2076
March	1st to 10th	Night	53% 3 11% 24% 9% 1	5320
	11th to 20th	Night	39% 5% 15% 28% 13% 1	5090
	21st to end	Night	50% 3 9% 22% 14% 1	5588
April	1st to 10th	Night	52% 4 12% 18% 13% 2	2237
	11th to 20th	Night	46% 5% 11% 18% 17% 2	1911
	21st to end	Night	47% 4 10% 23% 13% 2	1890
May	1st to 10th	Night	59% 4 9% 14% 13% 2	4150
	11th to 20th	Night	45% 6% 12% 22% 14% 1	4123
	21st to end	Night	40% 5% 12% 25% 16% 2	4528
June	1st to 10th	Night	36% 6% 11% 26% 17% 3	1765
	11th to 20th	Night	35% 7% 14% 25% 16% 3	1600
	21st to end	Night	33% 7% 15% 26% 17% 3	1600
July	1st to 10th	Night	36% 4 13% 26% 18% 4	3580
	11th to 20th	Night	35% 4 17% 27% 15% 2	3748
	21st to end	Night	35% 4 14% 22% 22% 3	4297
August	1st to 10th	Night	38% 5% 13% 26% 15% 2	2904
	11th to 20th	Night	37% 5% 11% 30% 16% 1	2902
	21st to end	Night	33% 5% 13% 29% 17% 2	3283
September	1st to 10th	Night	33% 6% 12% 27% 17% 5%	2145
	11th to 20th	Night	29% 5% 9% 29% 26% 2	2200
	21st to end	Night	40% 5% 11% 23% 19% 2	2200
October	1st to 10th	Night	27% 3 8% 31% 30% 1	3684
	11th to 20th	Night	38% 3 10% 28% 21% 1	3899
	21st to end	Night	37% 3 9% 31% 17% 3	4436
November	1st to 10th	Night	42% 4 11% 25% 17% 2	2666
	11th to 20th	Night	51% 4 10% 22% 12% 2	2730
	21st to end	Night	60% 3 12% 18% 7% 1	2730
December	1st to 10th	Night	62% 3 11% 17% 6% 1	4380
	11th to 20th	Night	64% 3 12% 14% 6% 1	4954
	21st to end	Night	72% 4 10% 10% 4	6933

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% 21% 30% 20% 5% 1	330
	11th to 20th	Day	26% 20% 38% 11% 3 1	330
	21st to end	Day	28% 17% 29% 21% 5%	363
February	1st to 10th	Day	26% 15% 40% 15% 3 1	330
	11th to 20th	Day	25% 17% 33% 21% 4	330
	21st to end	Day	22% 21% 42% 12% 3	273
March	1st to 10th	Day	23% 16% 38% 17% 6%	398
	11th to 20th	Day	24% 15% 34% 20% 5% 1	540
	21st to end	Day	30% 14% 32% 19% 5% 1	594
April	1st to 10th	Day	21% 15% 34% 20% 9% 1	540
	11th to 20th	Day	22% 16% 31% 18% 10% 3	532
	21st to end	Day	23% 11% 31% 23% 10% 1	540
May	1st to 10th	Day	26% 10% 33% 20% 9% 2	543
	11th to 20th	Day	21% 7% 28% 25% 12% 6%	567
	21st to end	Day	14% 8% 25% 27% 18% 9%	617
June	1st to 10th	Day	15% 5% 28% 30% 18% 5%	560
	11th to 20th	Day	13% 5% 28% 29% 19% 6%	730
	21st to end	Day	14% 6% 26% 28% 20% 6%	730
July	1st to 10th	Day	13% 3 19% 31% 21% 13%	721
	11th to 20th	Day	9% 3 23% 30% 23% 13%	633
	21st to end	Day	10% 5% 18% 26% 28% 13%	576
August	1st to 10th	Day	11% 3 14% 29% 26% 17%	526
	11th to 20th	Day	12% 2 15% 33% 24% 13%	519
	21st to end	Day	9% 4 17% 30% 25% 15%	583
September	1st to 10th	Day	15% 5% 15% 26% 24% 15%	530
	11th to 20th	Day	7% 2 15% 24% 32% 19%	530
	21st to end	Day	19% 9% 20% 24% 18% 9%	530
October	1st to 10th	Day	12% 8% 23% 26% 22% 9%	530
	11th to 20th	Day	17% 14% 31% 25% 11% 3	530
	21st to end	Day	23% 13% 36% 20% 6% 1	473
November	1st to 10th	Day	17% 13% 36% 19% 11% 3	330
	11th to 20th	Day	27% 18% 31% 15% 9% 1	330
	21st to end	Day	27% 15% 36% 19% 2	330
December	1st to 10th	Day	23% 18% 32% 18% 8% 1	278
	11th to 20th	Day	24% 20% 35% 18% 3	330
	21st to end	Day	33% 15% 32% 15% 4	363
January	1st to 10th	Night	25% 15% 35% 20% 4	1050
	11th to 20th	Night	27% 15% 38% 16% 4	894
	21st to end	Night	22% 16% 36% 22% 4	924
February	1st to 10th	Night	26% 13% 33% 26% 3	840
	11th to 20th	Night	23% 12% 38% 25% 2	823
	21st to end	Night	26% 14% 40% 18% 3	692
March	1st to 10th	Night	22% 11% 35% 26% 6%	840
	11th to 20th	Night	19% 17% 35% 23% 6% 1	840
	21st to end	Night	24% 11% 32% 26% 8%	924
April	1st to 10th	Night	23% 12% 35% 24% 5%	767
	11th to 20th	Night	19% 12% 35% 24% 9% 1	623
	21st to end	Night	21% 6% 34% 30% 8% 1	630
May	1st to 10th	Night	23% 7% 29% 32% 8% 1	630
	11th to 20th	Night	20% 7% 26% 33% 12% 1	621
	21st to end	Night	13% 2 22% 42% 18% 3	683
June	1st to 10th	Night	12% 3 25% 42% 19%	600
	11th to 20th	Night	11% 3 21% 44% 20% 1	600
	21st to end	Night	11% 3 23% 39% 24% 2	600
July	1st to 10th	Night	9% 2 15% 39% 31% 3	585
	11th to 20th	Night	10% 3 18% 40% 26% 4	582
	21st to end	Night	9% 2 14% 43% 30% 2	657
August	1st to 10th	Night	8% 2 12% 45% 30% 4	597
	11th to 20th	Night	10% 3 12% 40% 31% 5%	594
	21st to end	Night	8% 3 12% 38% 35% 4	660
September	1st to 10th	Night	13% 3 15% 34% 29% 5%	745
	11th to 20th	Night	9% 3 13% 35% 35% 5%	800
	21st to end	Night	15% 7% 20% 35% 22% 2	800
October	1st to 10th	Night	14% 7% 22% 34% 21% 3	800
	11th to 20th	Night	22% 10% 28% 30% 9% 1	800
	21st to end	Night	23% 11% 31% 28% 7% 1	880
November	1st to 10th	Night	17% 10% 33% 29% 10% 1	820
	11th to 20th	Night	21% 14% 34% 23% 7% 2	840
	21st to end	Night	28% 10% 34% 22% 5%	840
December	1st to 10th	Night	26% 12% 35% 22% 5%	840
	11th to 20th	Night	25% 12% 38% 21% 4	840
	21st to end	Night	30% 13% 33% 20% 3	1055

Page generated at 2026-05-29 01:53:26. Version ac07

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