Ballantyne Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ballantyne Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 6% 2 6% 4 4 4 4 6% 8% 43%	131338
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	129848
Transparency	Day	25% 38% 7% 13% 12% 4	134984
	Night	25% 37% 7% 12% 14% 5%	134272
Seeing	Day	16% 7% 22% 25% 20% 10%	17044
	Night	16% 6% 23% 32% 21% 3	27195

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	14% 7% 2 5% 3 3 3 2 3 5% 52%	5258
	11th to 20th	Day	11% 5% 2 5% 3 3 3 2 3 3 60%	5280
	21st to end	Day	19% 8% 3 7% 3 4 4 3 4 4 40%	5780
February	1st to 10th	Day	15% 7% 3 7% 3 3 5% 3 6% 5% 43%	1890
	11th to 20th	Day	21% 7% 2 5% 3 3 3 3 4 5% 45%	2010
	21st to end	Day	18% 8% 3 6% 5% 4 4 3 4 6% 40%	1798
March	1st to 10th	Day	20% 7% 3 6% 4 3 3 3 5% 6% 41%	4957
	11th to 20th	Day	9% 6% 2 5% 4 4 3 2 4 6% 54%	4980
	21st to end	Day	16% 6% 2 6% 4 3 3 3 4 5% 47%	5465
April	1st to 10th	Day	11% 4 2 4 3 4 4 4 6% 7% 50%	2203
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 4 5% 6% 49%	2374
	21st to end	Day	12% 5% 2 4 4 4 4 3 5% 7% 49%	2530
May	1st to 10th	Day	10% 7% 3 6% 5% 5% 6% 5% 7% 11% 35%	5862
	11th to 20th	Day	18% 7% 3 8% 5% 5% 5% 4 7% 9% 30%	5861
	21st to end	Day	12% 5% 2 6% 5% 5% 4 4 6% 8% 44%	6433
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 6% 7% 11% 39%	2600
	11th to 20th	Day	8% 6% 2 5% 5% 4 5% 4 8% 11% 42%	2600
	21st to end	Day	6% 4 2 6% 5% 5% 7% 5% 9% 11% 41%	2600
July	1st to 10th	Day	9% 6% 3 8% 6% 5% 7% 5% 8% 9% 35%	5900
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 6% 9% 13% 34%	5900
	21st to end	Day	8% 7% 3 8% 6% 6% 7% 5% 9% 12% 30%	6424
August	1st to 10th	Day	21% 7% 3 7% 5% 5% 6% 5% 7% 10% 25%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 4 8% 10% 36%	4174
	21st to end	Day	6% 4 1 4 4 5% 5% 4 7% 9% 51%	4355
September	1st to 10th	Day	8% 5% 2 5% 5% 5% 5% 5% 6% 9% 45%	2244
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 41%	2200
	21st to end	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	2200
October	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 6% 53%	2068
	11th to 20th	Day	13% 4 2 4 4 3 3 2 4 4 58%	2000
	21st to end	Day	13% 4 1 5% 3 3 2 2 4 5% 57%	2200
November	1st to 10th	Day	13% 4 1 5% 3 3 2 3 4 5% 55%	2057
	11th to 20th	Day	12% 6% 2 3 3 3 3 3 5% 6% 55%	1963
	21st to end	Day	14% 5% 2 5% 3 4 3 3 5% 6% 51%	1890
December	1st to 10th	Day	14% 7% 2 5% 3 3 2 2 4 5% 52%	3020
	11th to 20th	Day	17% 8% 3 6% 4 4 3 3 5% 6% 42%	3463
	21st to end	Day	20% 6% 2 4 3 3 3 3 4 5% 46%	4565
January	1st to 10th	Night	13% 7% 2 5% 3 3 3 2 3 4 56%	7635
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 4 5% 56%	7680
	21st to end	Night	18% 9% 2 6% 3 3 3 3 4 5% 44%	8401
February	1st to 10th	Night	15% 7% 3 6% 4 4 3 3 5% 5% 46%	2730
	11th to 20th	Night	21% 7% 2 5% 4 3 4 3 5% 5% 42%	2494
	21st to end	Night	16% 8% 2 5% 4 3 3 3 5% 6% 45%	2008
March	1st to 10th	Night	19% 8% 2 6% 3 3 3 3 4 5% 44%	4941
	11th to 20th	Night	11% 4 2 4 3 3 4 4 5% 7% 54%	4980
	21st to end	Night	15% 4 2 5% 4 4 4 3 5% 6% 48%	5467
April	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 6% 7% 48%	2195
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 5% 6% 52%	1975
	21st to end	Night	10% 5% 2 6% 4 4 4 5% 5% 6% 50%	1886
May	1st to 10th	Night	13% 7% 2 7% 4 4 5% 3 5% 9% 41%	4033
	11th to 20th	Night	18% 8% 3 6% 5% 4 4 4 6% 9% 32%	4032
	21st to end	Night	14% 5% 2 6% 4 4 5% 4 7% 9% 40%	4421
June	1st to 10th	Night	10% 7% 2 8% 6% 6% 5% 6% 6% 10% 35%	1800
	11th to 20th	Night	11% 5% 2 6% 6% 5% 5% 5% 8% 12% 35%	1800
	21st to end	Night	7% 6% 2 5% 6% 6% 6% 6% 8% 11% 37%	1800
July	1st to 10th	Night	10% 7% 3 9% 7% 6% 6% 5% 8% 10% 28%	4058
	11th to 20th	Night	8% 6% 3 8% 7% 7% 7% 7% 10% 13% 26%	4057
	21st to end	Night	10% 9% 3 9% 7% 6% 7% 5% 8% 11% 23%	4427
August	1st to 10th	Night	22% 9% 3 8% 4 5% 5% 4 8% 9% 24%	2915
	11th to 20th	Night	9% 7% 2 8% 6% 5% 6% 5% 8% 10% 33%	2875
	21st to end	Night	5% 4 2 6% 5% 5% 4 4 7% 9% 49%	3510
September	1st to 10th	Night	9% 5% 2 6% 4 5% 6% 4 5% 9% 45%	2151
	11th to 20th	Night	12% 7% 2 7% 4 4 6% 4 6% 7% 40%	2200
	21st to end	Night	18% 6% 2 7% 4 5% 4 4 5% 6% 39%	2200
October	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 3 6% 51%	2330
	11th to 20th	Night	13% 6% 3 4 3 3 2 3 4 6% 54%	2400
	21st to end	Night	12% 6% 2 5% 3 3 3 2 4 5% 57%	2640
November	1st to 10th	Night	14% 6% 2 5% 3 3 2 3 4 6% 52%	2461
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 6% 53%	2651
	21st to end	Night	13% 5% 2 4 3 4 4 3 5% 6% 50%	2730
December	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 51%	4380
	11th to 20th	Night	18% 8% 2 6% 3 3 3 3 4 6% 43%	4952
	21st to end	Night	19% 5% 2 5% 3 4 3 3 4 5% 47%	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	30% 20% 7% 17% 22% 5%	5259
	11th to 20th	Day	23% 21% 2 9% 33% 12%	5280
	21st to end	Day	39% 22% 2 12% 16% 9%	5780
February	1st to 10th	Day	17% 59% 3 11% 8% 2	1890
	11th to 20th	Day	20% 58% 3 8% 9% 2	2020
	21st to end	Day	20% 59% 3 4 11% 3	1798
March	1st to 10th	Day	37% 29% 5% 8% 12% 10%	4957
	11th to 20th	Day	20% 29% 3 9% 23% 16%	4980
	21st to end	Day	29% 28% 3 16% 18% 5%	5465
April	1st to 10th	Day	15% 62% 3 8% 9% 2	2203
	11th to 20th	Day	16% 63% 3 8% 7% 2	2374
	21st to end	Day	14% 62% 5% 10% 8% 1	2530
May	1st to 10th	Day	30% 33% 13% 16% 7% 1	5897
	11th to 20th	Day	38% 31% 8% 14% 7%	5865
	21st to end	Day	28% 30% 9% 14% 15% 3	6447
June	1st to 10th	Day	14% 64% 6% 10% 5% 1	2600
	11th to 20th	Day	11% 64% 9% 10% 7%	2600
	21st to end	Day	13% 62% 7% 10% 7%	2600
July	1st to 10th	Day	30% 31% 13% 19% 6%	5835
	11th to 20th	Day	26% 33% 26% 13% 1	5831
	21st to end	Day	34% 31% 11% 18% 4 1	6240
August	1st to 10th	Day	39% 38% 12% 9% 1	4221
	11th to 20th	Day	23% 40% 12% 15% 8% 1	4213
	21st to end	Day	13% 38% 9% 26% 13% 2	4355
September	1st to 10th	Day	15% 59% 6% 10% 9% 1	2244
	11th to 20th	Day	17% 58% 6% 9% 8% 1	2200
	21st to end	Day	22% 59% 6% 8% 5% 1	2200
October	1st to 10th	Day	18% 36% 4 15% 19% 8%	3367
	11th to 20th	Day	9% 35% 3 11% 20% 22%	3240
	21st to end	Day	15% 35% 2 18% 16% 14%	3538
November	1st to 10th	Day	18% 54% 3 9% 11% 4	2057
	11th to 20th	Day	15% 54% 3 12% 13% 3	1963
	21st to end	Day	16% 54% 5% 11% 12% 3	1890
December	1st to 10th	Day	24% 34% 3 11% 21% 7%	3020
	11th to 20th	Day	32% 31% 5% 11% 17% 4	3460
	21st to end	Day	31% 26% 6% 21% 13% 1	4565
January	1st to 10th	Night	27% 20% 7% 17% 24% 6%	7636
	11th to 20th	Night	25% 20% 3 11% 31% 10%	7680
	21st to end	Night	36% 22% 2 13% 18% 8%	8401
February	1st to 10th	Night	16% 59% 4 10% 9% 2	2730
	11th to 20th	Night	22% 58% 3 6% 9% 2	2495
	21st to end	Night	16% 59% 3 8% 11% 4	2008
March	1st to 10th	Night	34% 29% 5% 7% 19% 5%	4941
	11th to 20th	Night	19% 29% 4 9% 23% 17%	4980
	21st to end	Night	26% 29% 4 17% 17% 7%	5467
April	1st to 10th	Night	15% 61% 3 8% 9% 3	2195
	11th to 20th	Night	14% 63% 3 9% 7% 3	1975
	21st to end	Night	13% 63% 5% 11% 6% 1	1886
May	1st to 10th	Night	29% 32% 12% 16% 10% 1	4060
	11th to 20th	Night	35% 31% 11% 15% 8%	4033
	21st to end	Night	27% 31% 10% 19% 11% 2	4429
June	1st to 10th	Night	16% 64% 6% 9% 4 1	1800
	11th to 20th	Night	13% 65% 8% 8% 5%	1800
	21st to end	Night	14% 64% 8% 7% 6% 1	1800
July	1st to 10th	Night	36% 32% 14% 12% 5%	4015
	11th to 20th	Night	35% 37% 19% 8% 1	4006
	21st to end	Night	42% 30% 11% 13% 3	4297
August	1st to 10th	Night	42% 38% 11% 8% 1	2904
	11th to 20th	Night	25% 41% 17% 11% 5%	2902
	21st to end	Night	13% 38% 7% 21% 17% 3	3510
September	1st to 10th	Night	14% 59% 8% 9% 9% 1	2151
	11th to 20th	Night	18% 59% 6% 8% 8% 1	2200
	21st to end	Night	23% 59% 6% 9% 4 1	2200
October	1st to 10th	Night	23% 37% 6% 14% 16% 4	3811
	11th to 20th	Night	11% 35% 3 12% 19% 20%	3899
	21st to end	Night	16% 35% 3 14% 18% 14%	4252
November	1st to 10th	Night	20% 53% 3 8% 12% 3	2461
	11th to 20th	Night	16% 54% 3 8% 14% 6%	2651
	21st to end	Night	16% 54% 3 12% 12% 3	2730
December	1st to 10th	Night	24% 34% 3 13% 20% 6%	4380
	11th to 20th	Night	32% 32% 6% 10% 13% 7%	4954
	21st to end	Night	30% 26% 8% 18% 17% 1	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	22% 12% 43% 18% 4	330
	11th to 20th	Day	21% 19% 32% 22% 5% 1	330
	21st to end	Day	21% 18% 38% 17% 5% 1	363
February	1st to 10th	Day	17% 15% 40% 20% 8% 1	330
	11th to 20th	Day	36% 10% 39% 13% 2	330
	21st to end	Day	26% 15% 34% 19% 6% 1	341
March	1st to 10th	Day	22% 10% 35% 24% 8% 2	508
	11th to 20th	Day	19% 10% 40% 23% 7% 1	500
	21st to end	Day	20% 11% 35% 25% 7% 1	550
April	1st to 10th	Day	18% 13% 32% 22% 12% 2	500
	11th to 20th	Day	14% 10% 34% 24% 15% 4	492
	21st to end	Day	16% 7% 24% 31% 18% 5%	500
May	1st to 10th	Day	11% 9% 34% 25% 17% 5%	503
	11th to 20th	Day	18% 4 18% 28% 20% 11%	527
	21st to end	Day	13% 4 16% 24% 26% 17%	573
June	1st to 10th	Day	11% 4 12% 32% 29% 13%	530
	11th to 20th	Day	12% 3 13% 26% 27% 19%	530
	21st to end	Day	8% 1 9% 31% 32% 19%	530
July	1st to 10th	Day	9% 1 6% 21% 36% 26%	525
	11th to 20th	Day	9% 1 6% 23% 37% 25%	524
	21st to end	Day	6% 2 5% 24% 39% 23%	576
August	1st to 10th	Day	17% 2 8% 25% 27% 21%	526
	11th to 20th	Day	13% 3 6% 23% 31% 24%	519
	21st to end	Day	13% 2 7% 24% 31% 24%	583
September	1st to 10th	Day	10% 2 9% 28% 32% 19%	530
	11th to 20th	Day	15% 4 11% 22% 31% 18%	530
	21st to end	Day	19% 4 17% 25% 26% 9%	530
October	1st to 10th	Day	16% 3 17% 26% 25% 13%	530
	11th to 20th	Day	13% 5% 22% 28% 24% 9%	530
	21st to end	Day	15% 4 23% 29% 21% 8%	583
November	1st to 10th	Day	17% 5% 25% 30% 18% 5%	535
	11th to 20th	Day	16% 7% 35% 27% 11% 3	403
	21st to end	Day	13% 11% 33% 29% 12% 3	330
December	1st to 10th	Day	25% 11% 36% 20% 8%	330
	11th to 20th	Day	20% 9% 32% 27% 10% 2	330
	21st to end	Day	23% 11% 34% 22% 8% 2	363
January	1st to 10th	Night	20% 10% 35% 29% 6%	840
	11th to 20th	Night	20% 10% 34% 31% 5%	840
	21st to end	Night	19% 7% 39% 31% 4	924
February	1st to 10th	Night	18% 8% 36% 33% 4	840
	11th to 20th	Night	26% 9% 39% 25% 2	823
	21st to end	Night	20% 11% 38% 27% 5%	692
March	1st to 10th	Night	18% 9% 33% 35% 5%	809
	11th to 20th	Night	18% 9% 37% 27% 8% 1	800
	21st to end	Night	19% 9% 32% 33% 7%	880
April	1st to 10th	Night	15% 12% 34% 30% 10%	800
	11th to 20th	Night	14% 9% 29% 33% 15% 1	791
	21st to end	Night	12% 4 27% 40% 16%	686
May	1st to 10th	Night	13% 6% 26% 40% 15%	600
	11th to 20th	Night	16% 4 20% 36% 24% 1	591
	21st to end	Night	15% 3 11% 36% 33% 3	650
June	1st to 10th	Night	13% 2 12% 40% 28% 5%	600
	11th to 20th	Night	13% 2 12% 29% 38% 7%	600
	21st to end	Night	8% 2 11% 36% 39% 6%	600
July	1st to 10th	Night	9% 2 6% 31% 42% 9%	585
	11th to 20th	Night	10% 2 4 26% 48% 9%	582
	21st to end	Night	7% 2 6% 30% 46% 8%	657
August	1st to 10th	Night	14% 3 6% 28% 41% 8%	597
	11th to 20th	Night	11% 3 5% 28% 45% 7%	594
	21st to end	Night	10% 2 6% 27% 47% 8%	830
September	1st to 10th	Night	11% 2 7% 30% 44% 6%	800
	11th to 20th	Night	15% 3 8% 27% 40% 7%	800
	21st to end	Night	20% 3 13% 33% 27% 4	800
October	1st to 10th	Night	15% 3 16% 31% 30% 6%	800
	11th to 20th	Night	16% 4 22% 34% 22% 3	800
	21st to end	Night	14% 5% 23% 33% 22% 3	880
November	1st to 10th	Night	17% 4 21% 37% 19% 3	820
	11th to 20th	Night	14% 6% 31% 36% 12% 1	840
	21st to end	Night	15% 5% 28% 38% 13% 1	840
December	1st to 10th	Night	22% 7% 28% 36% 7%	840
	11th to 20th	Night	20% 5% 26% 38% 11%	840
	21st to end	Night	23% 7% 25% 35% 9%	924

Page generated at 2026-02-28 01:40:25. Version ac07

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