Ballard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ballard clear sky chart from 2005-11-06 to 2026-01-02.

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	12% 7% 2 6% 4 4 4 3 5% 7% 45%	126816
	Night	11% 7% 3 6% 4 4 4 3 5% 6% 48%	124782
Transparency	Day	38% 4 12% 26% 17% 3	130428
	Night	37% 4 10% 26% 19% 4	129246
Seeing	Day	15% 7% 26% 28% 17% 5%	17176
	Night	15% 8% 30% 33% 13% 1	27067

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	18% 9% 3 8% 6% 4 4 3 5% 5% 37%	4086
	11th to 20th	Day	8% 10% 4 9% 5% 6% 5% 3 4 5% 40%	4080
	21st to end	Day	15% 13% 3 6% 3 4 3 3 4 5% 41%	4488
February	1st to 10th	Day	15% 12% 4 7% 5% 4 3 2 4 6% 37%	1800
	11th to 20th	Day	14% 10% 4 7% 4 3 4 3 5% 5% 41%	1754
	21st to end	Day	13% 8% 3 7% 4 4 4 4 5% 6% 42%	1719
March	1st to 10th	Day	13% 9% 3 6% 3 3 2 2 4 4 50%	4936
	11th to 20th	Day	11% 6% 2 5% 3 3 4 3 5% 7% 54%	4980
	21st to end	Day	14% 10% 3 7% 5% 5% 5% 4 6% 7% 35%	5462
April	1st to 10th	Day	14% 8% 2 7% 4 4 3 3 4 6% 44%	2332
	11th to 20th	Day	16% 9% 2 6% 4 3 4 3 5% 7% 41%	2374
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 6% 7% 34%	2475
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 3 5% 7% 40%	5862
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 5% 8% 42%	5862
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 7% 40%	6617
June	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	2800
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 9% 47%	2800
	21st to end	Day	6% 3 2 5% 3 4 4 4 6% 10% 53%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 4 4 4 6% 8% 46%	5906
	11th to 20th	Day	7% 5% 2 6% 4 4 5% 4 6% 10% 46%	5900
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 9% 11% 42%	6422
August	1st to 10th	Day	4 4 2 5% 4 4 5% 4 6% 9% 54%	4234
	11th to 20th	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 46%	4171
	21st to end	Day	6% 4 2 4 3 4 5% 4 5% 9% 56%	4327
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 7% 52%	2400
	11th to 20th	Day	10% 4 2 6% 4 4 4 3 5% 7% 51%	2260
	21st to end	Day	9% 5% 2 5% 3 4 3 4 4 7% 54%	2160
October	1st to 10th	Day	11% 4 2 5% 3 3 3 3 5% 6% 55%	2000
	11th to 20th	Day	13% 6% 2 4 3 3 2 3 4 5% 56%	2000
	21st to end	Day	18% 8% 2 6% 3 3 2 2 3 5% 47%	2200
November	1st to 10th	Day	11% 6% 2 5% 3 2 3 3 4 6% 54%	2058
	11th to 20th	Day	15% 7% 2 4 3 3 3 2 4 6% 50%	1853
	21st to end	Day	17% 8% 2 7% 3 3 2 3 4 5% 46%	1680
December	1st to 10th	Day	8% 7% 3 6% 4 3 3 3 4 6% 53%	2690
	11th to 20th	Day	10% 8% 3 7% 4 4 4 2 4 5% 50%	3088
	21st to end	Day	18% 20% 6% 9% 4 4 3 2 4 5% 26%	4240
January	1st to 10th	Night	14% 9% 4 12% 6% 5% 3 3 3 5% 37%	5902
	11th to 20th	Night	10% 7% 3 11% 5% 7% 4 3 3 4 41%	5900
	21st to end	Night	16% 10% 4 7% 3 3 3 3 4 5% 43%	6490
February	1st to 10th	Night	13% 11% 3 7% 5% 4 4 4 5% 6% 38%	2600
	11th to 20th	Night	13% 10% 3 8% 4 3 3 4 4 6% 42%	2533
	21st to end	Night	13% 9% 2 7% 4 3 3 3 4 5% 47%	1928
March	1st to 10th	Night	15% 9% 3 6% 3 3 3 3 4 4 47%	4919
	11th to 20th	Night	9% 6% 2 4 2 3 3 3 5% 7% 55%	4980
	21st to end	Night	13% 7% 3 8% 5% 5% 5% 4 6% 7% 39%	5467
April	1st to 10th	Night	10% 7% 3 5% 4 3 3 3 6% 7% 49%	2066
	11th to 20th	Night	14% 8% 2 6% 3 3 3 2 5% 7% 46%	1975
	21st to end	Night	16% 7% 3 7% 3 4 3 3 5% 6% 42%	1921
May	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 7% 43%	4053
	11th to 20th	Night	12% 8% 3 6% 3 3 3 3 5% 8% 47%	4053
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 6% 49%	4242
June	1st to 10th	Night	7% 3 2 4 3 3 4 4 8% 9% 53%	1600
	11th to 20th	Night	4 4 2 4 5% 5% 5% 3 8% 10% 51%	1600
	21st to end	Night	4 3 1 3 3 4 4 3 7% 8% 61%	1600
July	1st to 10th	Night	8% 6% 2 6% 4 4 3 3 6% 9% 48%	4066
	11th to 20th	Night	5% 4 2 5% 4 4 5% 3 6% 10% 52%	4077
	21st to end	Night	6% 4 2 5% 4 4 5% 4 8% 13% 46%	4449
August	1st to 10th	Night	3 2 1 2 3 3 4 4 5% 9% 64%	2926
	11th to 20th	Night	10% 5% 2 6% 4 3 3 3 5% 7% 51%	2888
	21st to end	Night	3 3 1 4 4 4 4 4 6% 7% 59%	3571
September	1st to 10th	Night	11% 5% 1 5% 4 3 4 3 3 6% 57%	2000
	11th to 20th	Night	8% 4 2 4 4 4 4 4 6% 7% 53%	2135
	21st to end	Night	7% 5% 1 5% 3 3 4 3 4 7% 58%	2218
October	1st to 10th	Night	8% 4 1 4 3 3 3 3 4 6% 60%	2400
	11th to 20th	Night	13% 4 2 5% 3 3 3 2 4 5% 56%	2400
	21st to end	Night	16% 6% 2 5% 3 2 3 2 4 5% 51%	2640
November	1st to 10th	Night	10% 5% 2 4 3 3 3 2 3 5% 60%	2460
	11th to 20th	Night	11% 6% 2 6% 4 3 3 2 4 6% 53%	2762
	21st to end	Night	16% 8% 2 5% 3 3 3 3 4 4 49%	2940
December	1st to 10th	Night	8% 7% 2 6% 3 3 4 3 4 6% 54%	4720
	11th to 20th	Night	13% 8% 3 6% 4 3 3 3 5% 6% 47%	5339
	21st to end	Night	19% 19% 5% 8% 4 3 3 2 3 4 29%	6962

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	49% 3 9% 26% 12% 1	4087
	11th to 20th	Day	45% 4 8% 24% 16% 4	4080
	21st to end	Day	47% 3 9% 18% 17% 6%	4488
February	1st to 10th	Day	50% 4 10% 23% 11% 2	1800
	11th to 20th	Day	46% 3 12% 20% 15% 3	1763
	21st to end	Day	43% 4 9% 24% 15% 5%	1719
March	1st to 10th	Day	38% 3 7% 23% 23% 6%	4936
	11th to 20th	Day	31% 4 9% 23% 26% 8%	4980
	21st to end	Day	48% 5% 16% 24% 6%	5464
April	1st to 10th	Day	42% 4 11% 24% 17% 2	2332
	11th to 20th	Day	43% 4 11% 25% 15% 1	2374
	21st to end	Day	50% 4 11% 23% 10% 1	2475
May	1st to 10th	Day	44% 6% 11% 24% 15% 1	5900
	11th to 20th	Day	39% 6% 13% 25% 15% 3	5866
	21st to end	Day	44% 5% 11% 27% 13%	6631
June	1st to 10th	Day	31% 6% 16% 29% 17% 2	2800
	11th to 20th	Day	32% 7% 14% 30% 15% 2	2800
	21st to end	Day	26% 5% 17% 32% 17% 4	2800
July	1st to 10th	Day	37% 3 13% 25% 20% 2	5842
	11th to 20th	Day	34% 4 14% 32% 14% 1	5831
	21st to end	Day	33% 4 20% 34% 10%	6240
August	1st to 10th	Day	27% 4 17% 29% 21% 1	4220
	11th to 20th	Day	36% 5% 17% 27% 14% 1	4210
	21st to end	Day	26% 5% 13% 39% 14% 4	4326
September	1st to 10th	Day	32% 4 12% 26% 19% 7%	2400
	11th to 20th	Day	33% 4 10% 26% 25% 2	2260
	21st to end	Day	30% 5% 9% 28% 24% 4	2160
October	1st to 10th	Day	20% 2 6% 21% 39% 13%	3260
	11th to 20th	Day	21% 2 4 24% 39% 10%	3240
	21st to end	Day	35% 2 7% 27% 25% 3	3538
November	1st to 10th	Day	31% 3 11% 30% 20% 4	2058
	11th to 20th	Day	37% 2 9% 25% 22% 5%	1853
	21st to end	Day	41% 3 9% 28% 18% 1	1680
December	1st to 10th	Day	34% 3 11% 36% 17% 1	2690
	11th to 20th	Day	38% 3 10% 31% 16% 2	3085
	21st to end	Day	62% 3 10% 18% 6% 1	4240
January	1st to 10th	Night	51% 3 11% 22% 12% 1	5902
	11th to 20th	Night	47% 4 7% 23% 15% 3	5900
	21st to end	Night	45% 3 9% 20% 17% 6%	6490
February	1st to 10th	Night	46% 4 10% 23% 14% 2	2600
	11th to 20th	Night	44% 3 11% 23% 16% 3	2534
	21st to end	Night	41% 2 7% 24% 19% 6%	1928
March	1st to 10th	Night	41% 3 7% 20% 24% 5%	4919
	11th to 20th	Night	29% 3 7% 22% 31% 7%	4980
	21st to end	Night	43% 5% 14% 25% 11% 2	5467
April	1st to 10th	Night	35% 4 11% 23% 23% 5%	2066
	11th to 20th	Night	38% 3 8% 27% 19% 5%	1975
	21st to end	Night	44% 3 11% 26% 15% 1	1921
May	1st to 10th	Night	42% 6% 13% 23% 14% 1	4080
	11th to 20th	Night	36% 5% 13% 24% 16% 6%	4054
	21st to end	Night	37% 5% 9% 26% 21% 2	4250
June	1st to 10th	Night	25% 7% 18% 30% 18% 4	1600
	11th to 20th	Night	26% 7% 15% 29% 21% 2	1600
	21st to end	Night	21% 4 15% 32% 23% 5%	1600
July	1st to 10th	Night	33% 4 13% 25% 23% 2	4023
	11th to 20th	Night	27% 4 13% 37% 17% 2	4026
	21st to end	Night	29% 4 18% 37% 12% 1	4320
August	1st to 10th	Night	18% 4 20% 31% 22% 4	2916
	11th to 20th	Night	34% 3 11% 32% 19% 2	2915
	21st to end	Night	23% 4 13% 38% 18% 4	3571
September	1st to 10th	Night	30% 3 10% 29% 21% 8%	2000
	11th to 20th	Night	30% 5% 10% 26% 24% 4	2135
	21st to end	Night	28% 4 8% 27% 30% 5%	2218
October	1st to 10th	Night	17% 2 6% 21% 42% 13%	3920
	11th to 20th	Night	23% 2 6% 24% 29% 16%	3899
	21st to end	Night	32% 3 6% 31% 24% 4	4252
November	1st to 10th	Night	29% 3 10% 29% 23% 6%	2460
	11th to 20th	Night	34% 2 8% 26% 24% 5%	2762
	21st to end	Night	40% 3 8% 28% 19% 3	2940
December	1st to 10th	Night	33% 2 12% 35% 16% 1	4720
	11th to 20th	Night	40% 3 10% 27% 18% 2	5341
	21st to end	Night	61% 3 8% 20% 7% 1	6962

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	21% 10% 37% 26% 5%	303
	11th to 20th	Day	26% 14% 39% 20% 1	300
	21st to end	Day	18% 11% 45% 23% 4	330
February	1st to 10th	Day	25% 15% 38% 18% 3	300
	11th to 20th	Day	18% 22% 38% 17% 4 1	300
	21st to end	Day	22% 20% 40% 15% 3	249
March	1st to 10th	Day	21% 17% 39% 19% 4	300
	11th to 20th	Day	27% 10% 39% 18% 6%	300
	21st to end	Day	31% 15% 36% 14% 4 1	330
April	1st to 10th	Day	21% 15% 35% 22% 7%	432
	11th to 20th	Day	23% 14% 33% 23% 6%	492
	21st to end	Day	26% 9% 33% 24% 8% 1	575
May	1st to 10th	Day	19% 10% 35% 26% 8% 2	703
	11th to 20th	Day	21% 7% 27% 27% 13% 4	725
	21st to end	Day	19% 6% 25% 27% 16% 7%	789
June	1st to 10th	Day	8% 3 17% 35% 26% 10%	730
	11th to 20th	Day	9% 2 22% 36% 24% 6%	730
	21st to end	Day	7% 2 30% 36% 21% 4	730
July	1st to 10th	Day	12% 2 12% 30% 30% 14%	721
	11th to 20th	Day	5% 1 17% 36% 31% 10%	718
	21st to end	Day	9% 1 12% 32% 34% 11%	794
August	1st to 10th	Day	7% 2 13% 42% 29% 7%	726
	11th to 20th	Day	8% 2 14% 34% 34% 8%	715
	21st to end	Day	10% 10% 30% 36% 14%	598
September	1st to 10th	Day	12% 2 13% 32% 34% 8%	530
	11th to 20th	Day	8% 1 13% 37% 31% 9%	390
	21st to end	Day	12% 3 23% 34% 20% 8%	330
October	1st to 10th	Day	12% 6% 25% 31% 18% 8%	330
	11th to 20th	Day	13% 13% 37% 28% 8% 2	330
	21st to end	Day	26% 11% 34% 23% 5% 1	363
November	1st to 10th	Day	15% 9% 38% 24% 10% 4	330
	11th to 20th	Day	17% 15% 33% 23% 11% 1	330
	21st to end	Day	23% 12% 33% 25% 7% 1	330
December	1st to 10th	Day	13% 14% 41% 22% 7% 3	330
	11th to 20th	Day	15% 16% 39% 24% 6% 1	330
	21st to end	Day	26% 14% 37% 21% 1	363
January	1st to 10th	Night	17% 12% 38% 25% 7%	1006
	11th to 20th	Night	16% 14% 40% 25% 4 1	1000
	21st to end	Night	16% 13% 39% 29% 3	1100
February	1st to 10th	Night	19% 13% 41% 22% 4	1000
	11th to 20th	Night	19% 15% 37% 26% 3	979
	21st to end	Night	20% 13% 41% 23% 3	608
March	1st to 10th	Night	17% 11% 42% 26% 5%	600
	11th to 20th	Night	22% 11% 32% 30% 6%	600
	21st to end	Night	24% 11% 36% 25% 3	660
April	1st to 10th	Night	14% 11% 37% 32% 6%	600
	11th to 20th	Night	20% 12% 33% 28% 8%	593
	21st to end	Night	20% 6% 37% 29% 8%	600
May	1st to 10th	Night	16% 6% 34% 37% 8% 1	600
	11th to 20th	Night	17% 3 28% 36% 15% 1	591
	21st to end	Night	17% 4 25% 33% 19% 2	650
June	1st to 10th	Night	8% 2 16% 43% 28% 3	600
	11th to 20th	Night	5% 2 22% 48% 22% 2	600
	21st to end	Night	5% 2 24% 50% 19% 1	600
July	1st to 10th	Night	9% 1 9% 42% 34% 5%	585
	11th to 20th	Night	5% 1 14% 41% 37% 3	582
	21st to end	Night	8% 1 12% 40% 37% 3	657
August	1st to 10th	Night	6% 1 9% 50% 32% 2	597
	11th to 20th	Night	7% 2 10% 46% 31% 3	594
	21st to end	Night	5% 1 10% 39% 40% 4	660
September	1st to 10th	Night	12% 1 13% 44% 27% 3	600
	11th to 20th	Night	10% 2 13% 39% 31% 5%	600
	21st to end	Night	11% 2 21% 39% 23% 3	618
October	1st to 10th	Night	10% 3 25% 37% 22% 3	800
	11th to 20th	Night	15% 6% 37% 32% 10%	800
	21st to end	Night	20% 10% 33% 28% 9% 1	880
November	1st to 10th	Night	13% 7% 36% 32% 10% 1	820
	11th to 20th	Night	15% 11% 36% 28% 8%	982
	21st to end	Night	20% 12% 34% 27% 7%	1050
December	1st to 10th	Night	17% 11% 38% 29% 6%	1050
	11th to 20th	Night	20% 10% 35% 31% 4	1050
	21st to end	Night	20% 14% 35% 27% 3	1155

Page generated at 2025-12-30 00:14:08. Version ac07

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