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 2024-01-14.

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% 7% 3 5% 5% 3 4 4 5% 7% 44%	74629
	Night	12% 7% 3 5% 4 3 4 4 5% 6% 48%	75128
Transparency	Day	40% 5% 12% 26% 15% 2	74352
	Night	37% 4 11% 26% 19% 3	74925
Seeing	Day	15% 7% 25% 28% 18% 5%	14460
	Night	15% 8% 30% 33% 14% 1	24439

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	13% 7% 2 6% 6% 3 3 4 5% 5% 45%	1707
	11th to 20th	Day	14% 10% 4 7% 4 3 4 5% 4 6% 40%	1647
	21st to end	Day	14% 9% 4 6% 5% 3 3 4 4 6% 42%	1782
February	1st to 10th	Day	15% 12% 4 6% 6% 3 4 3 4 6% 38%	1620
	11th to 20th	Day	14% 9% 5% 6% 5% 3 4 4 5% 6% 41%	1574
	21st to end	Day	15% 9% 3 6% 5% 3 4 4 5% 6% 40%	1542
March	1st to 10th	Day	13% 8% 4 7% 5% 3 4 4 5% 5% 42%	1980
	11th to 20th	Day	23% 8% 4 7% 4 2 4 3 4 6% 35%	1980
	21st to end	Day	22% 12% 3 6% 5% 3 4 3 4 6% 31%	2178
April	1st to 10th	Day	14% 7% 2 6% 5% 3 3 4 4 6% 44%	2098
	11th to 20th	Day	16% 9% 2 6% 5% 3 4 4 5% 7% 40%	2134
	21st to end	Day	20% 8% 3 6% 5% 3 4 4 5% 7% 34%	2227
May	1st to 10th	Day	19% 8% 2 6% 5% 4 5% 4 4 6% 37%	2340
	11th to 20th	Day	17% 7% 2 4 5% 4 4 5% 5% 8% 39%	2302
	21st to end	Day	15% 7% 4 6% 6% 4 5% 5% 5% 9% 33%	2603
June	1st to 10th	Day	7% 4 2 4 5% 3 5% 5% 7% 10% 49%	2520
	11th to 20th	Day	8% 5% 2 5% 5% 3 5% 5% 5% 9% 48%	2520
	21st to end	Day	6% 3 2 4 4 2 4 5% 5% 10% 54%	2520
July	1st to 10th	Day	9% 5% 2 6% 5% 3 5% 5% 7% 10% 44%	2346
	11th to 20th	Day	4 3 2 4 4 3 5% 5% 7% 9% 54%	2340
	21st to end	Day	8% 4 2 5% 5% 4 5% 7% 8% 10% 42%	2546
August	1st to 10th	Day	6% 5% 2 5% 5% 3 5% 6% 6% 11% 45%	2328
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 6% 6% 9% 47%	2301
	21st to end	Day	8% 4 2 3 4 3 5% 6% 5% 8% 54%	2390
September	1st to 10th	Day	13% 4 3 4 5% 2 3 3 5% 7% 51%	2160
	11th to 20th	Day	11% 4 2 5% 4 3 4 4 5% 6% 52%	2034
	21st to end	Day	8% 4 2 4 4 3 3 4 5% 7% 56%	1944
October	1st to 10th	Day	12% 4 3 4 4 2 3 4 4 7% 53%	1800
	11th to 20th	Day	14% 6% 2 4 3 2 3 3 3 5% 55%	1800
	21st to end	Day	18% 8% 2 4 4 2 2 2 3 5% 47%	1980
November	1st to 10th	Day	11% 6% 2 4 4 2 3 3 5% 6% 55%	1858
	11th to 20th	Day	16% 7% 3 4 3 2 3 3 4 5% 50%	1677
	21st to end	Day	17% 7% 3 5% 3 2 3 3 4 6% 47%	1520
December	1st to 10th	Day	12% 8% 4 5% 5% 2 3 4 4 6% 45%	1520
	11th to 20th	Day	12% 11% 4 7% 5% 2 5% 4 3 4 44%	1918
	21st to end	Day	18% 15% 5% 7% 6% 3 4 4 5% 6% 28%	2893
January	1st to 10th	Night	11% 8% 3 5% 5% 3 4 4 4 6% 48%	2458
	11th to 20th	Night	13% 7% 4 7% 5% 3 4 4 4 6% 43%	2386
	21st to end	Night	11% 8% 4 7% 4 3 5% 4 4 6% 44%	2574
February	1st to 10th	Night	13% 11% 4 7% 5% 3 4 5% 4 7% 39%	2340
	11th to 20th	Night	13% 8% 4 6% 5% 3 4 4 4 6% 43%	2273
	21st to end	Night	14% 9% 3 6% 5% 3 3 3 4 5% 45%	1730
March	1st to 10th	Night	13% 6% 3 5% 5% 3 3 4 5% 6% 48%	1980
	11th to 20th	Night	18% 8% 3 5% 4 2 3 4 4 6% 43%	1980
	21st to end	Night	20% 8% 3 7% 5% 3 4 4 5% 6% 35%	2178
April	1st to 10th	Night	10% 6% 3 5% 4 2 4 4 5% 7% 50%	1860
	11th to 20th	Night	15% 8% 2 5% 4 2 3 3 5% 7% 46%	1775
	21st to end	Night	17% 6% 3 7% 4 3 3 4 5% 5% 44%	1729
May	1st to 10th	Night	15% 6% 3 5% 5% 3 5% 4 5% 7% 41%	1620
	11th to 20th	Night	14% 6% 3 4 4 2 3 4 5% 7% 47%	1593
	21st to end	Night	13% 6% 2 5% 5% 2 4 5% 5% 9% 44%	1669
June	1st to 10th	Night	6% 2 2 3 4 2 4 5% 7% 10% 55%	1440
	11th to 20th	Night	3 4 2 4 6% 3 5% 5% 8% 10% 51%	1440
	21st to end	Night	4 2 1 3 3 3 4 4 6% 8% 63%	1440
July	1st to 10th	Night	7% 3 2 4 4 3 4 5% 7% 9% 51%	1608
	11th to 20th	Night	3 2 1 3 4 3 4 4 6% 9% 60%	1620
	21st to end	Night	7% 3 2 4 5% 3 4 4 6% 11% 51%	1772
August	1st to 10th	Night	5% 3 2 3 4 3 5% 5% 6% 9% 55%	1611
	11th to 20th	Night	6% 4 2 4 4 2 3 4 6% 8% 57%	1600
	21st to end	Night	5% 3 2 3 4 2 4 4 7% 7% 60%	1966
September	1st to 10th	Night	11% 4 2 3 4 3 4 3 3 7% 57%	1800
	11th to 20th	Night	9% 4 2 3 4 3 4 4 6% 7% 54%	1921
	21st to end	Night	7% 4 2 4 4 2 3 4 4 7% 59%	1996
October	1st to 10th	Night	9% 4 2 4 4 3 3 4 4 6% 58%	2160
	11th to 20th	Night	14% 4 2 4 4 2 3 3 4 5% 55%	2160
	21st to end	Night	15% 6% 2 5% 3 2 3 3 4 5% 52%	2376
November	1st to 10th	Night	10% 4 2 4 3 2 3 3 3 6% 60%	2220
	11th to 20th	Night	12% 6% 2 5% 4 2 3 3 4 6% 53%	2498
	21st to end	Night	16% 6% 3 4 4 2 4 4 4 4 50%	2660
December	1st to 10th	Night	13% 7% 3 6% 4 2 4 4 4 7% 45%	2660
	11th to 20th	Night	14% 7% 3 6% 5% 3 4 4 5% 6% 45%	3279
	21st to end	Night	18% 18% 6% 7% 5% 2 4 3 3 4 30%	4756

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	41% 3 9% 27% 18% 1	1708
	11th to 20th	Day	46% 5% 10% 25% 13% 1	1647
	21st to end	Day	43% 4 15% 25% 12% 1	1782
February	1st to 10th	Day	50% 4 11% 23% 10% 2	1620
	11th to 20th	Day	46% 3 13% 20% 14% 3	1583
	21st to end	Day	46% 4 9% 24% 14% 4	1542
March	1st to 10th	Day	44% 3 10% 22% 17% 4	1980
	11th to 20th	Day	52% 4 10% 21% 11% 3	1980
	21st to end	Day	55% 4 13% 18% 8% 1	2178
April	1st to 10th	Day	41% 4 12% 25% 16% 2	2098
	11th to 20th	Day	44% 4 11% 25% 14% 1	2134
	21st to end	Day	50% 4 11% 23% 11% 1	2227
May	1st to 10th	Day	50% 5% 11% 21% 12% 1	2340
	11th to 20th	Day	43% 7% 13% 25% 12%	2306
	21st to end	Day	47% 7% 14% 21% 10% 1	2603
June	1st to 10th	Day	30% 6% 17% 30% 16% 1	2520
	11th to 20th	Day	33% 8% 14% 29% 15% 2	2520
	21st to end	Day	26% 5% 16% 33% 17% 4	2520
July	1st to 10th	Day	36% 6% 15% 25% 15% 3	2282
	11th to 20th	Day	25% 7% 16% 30% 19% 3	2271
	21st to end	Day	33% 7% 14% 32% 13% 1	2405
August	1st to 10th	Day	33% 7% 19% 26% 15% 1	2314
	11th to 20th	Day	31% 8% 13% 31% 16% 2	2301
	21st to end	Day	28% 7% 13% 27% 20% 6%	2390
September	1st to 10th	Day	33% 5% 12% 27% 17% 6%	2160
	11th to 20th	Day	33% 4 10% 25% 27% 3	2034
	21st to end	Day	29% 5% 10% 29% 23% 4	1944
October	1st to 10th	Day	33% 3 11% 26% 22% 4	1800
	11th to 20th	Day	34% 3 7% 28% 23% 5%	1800
	21st to end	Day	42% 2 8% 23% 18% 5%	1980
November	1st to 10th	Day	31% 3 12% 29% 20% 5%	1858
	11th to 20th	Day	37% 2 9% 25% 23% 4	1677
	21st to end	Day	40% 3 9% 27% 19% 1	1520
December	1st to 10th	Day	40% 4 11% 31% 13% 1	1520
	11th to 20th	Day	45% 3 10% 27% 13% 2	1915
	21st to end	Day	58% 4 11% 19% 8% 1	2893
January	1st to 10th	Night	39% 3 13% 26% 17% 2	2458
	11th to 20th	Night	43% 4 11% 27% 15% 1	2386
	21st to end	Night	42% 4 12% 27% 13% 2	2574
February	1st to 10th	Night	46% 4 10% 23% 15% 2	2340
	11th to 20th	Night	43% 3 12% 23% 16% 3	2274
	21st to end	Night	43% 3 8% 23% 18% 5%	1730
March	1st to 10th	Night	39% 3 8% 24% 20% 5%	1980
	11th to 20th	Night	45% 4 9% 22% 17% 3	1980
	21st to end	Night	51% 5% 11% 21% 11% 1	2178
April	1st to 10th	Night	34% 4 11% 24% 22% 5%	1860
	11th to 20th	Night	39% 3 8% 27% 18% 4	1775
	21st to end	Night	44% 3 10% 26% 16% 1	1729
May	1st to 10th	Night	43% 6% 12% 22% 15% 1	1620
	11th to 20th	Night	37% 5% 14% 26% 15% 2	1594
	21st to end	Night	37% 7% 16% 28% 11% 1	1670
June	1st to 10th	Night	23% 7% 18% 31% 18% 3	1440
	11th to 20th	Night	27% 7% 15% 29% 20% 3	1440
	21st to end	Night	20% 5% 14% 33% 23% 5%	1440
July	1st to 10th	Night	27% 8% 13% 31% 17% 4	1563
	11th to 20th	Night	20% 6% 14% 32% 25% 4	1566
	21st to end	Night	27% 6% 14% 32% 20% 2	1673
August	1st to 10th	Night	26% 6% 17% 34% 15% 3	1601
	11th to 20th	Night	25% 5% 10% 34% 24% 2	1600
	21st to end	Night	22% 5% 13% 26% 27% 7%	1966
September	1st to 10th	Night	29% 4 10% 30% 20% 7%	1800
	11th to 20th	Night	30% 5% 9% 25% 26% 5%	1921
	21st to end	Night	27% 4 8% 28% 29% 5%	1996
October	1st to 10th	Night	28% 4 10% 27% 25% 7%	2160
	11th to 20th	Night	33% 3 6% 27% 23% 7%	2160
	21st to end	Night	37% 3 7% 24% 24% 5%	2376
November	1st to 10th	Night	28% 4 11% 28% 24% 6%	2220
	11th to 20th	Night	35% 3 7% 26% 25% 4	2498
	21st to end	Night	39% 3 8% 28% 20% 3	2660
December	1st to 10th	Night	41% 4 12% 26% 17% 1	2660
	11th to 20th	Night	41% 4 9% 29% 16% 1	3281
	21st to end	Night	59% 4 7% 18% 10% 2	4756

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% 11% 34% 27% 6%	270
	11th to 20th	Day	30% 12% 37% 19% 1	249
	21st to end	Day	17% 12% 46% 21% 4	264
February	1st to 10th	Day	24% 16% 38% 18% 4	240
	11th to 20th	Day	17% 23% 39% 16% 4 1	240
	21st to end	Day	26% 20% 37% 14% 3 1	198
March	1st to 10th	Day	20% 18% 39% 19% 4	240
	11th to 20th	Day	30% 8% 37% 18% 7%	240
	21st to end	Day	34% 15% 34% 13% 4	264
April	1st to 10th	Day	18% 13% 37% 24% 8% 1	358
	11th to 20th	Day	22% 13% 32% 25% 7%	412
	21st to end	Day	26% 8% 32% 25% 8% 1	487
May	1st to 10th	Day	20% 9% 35% 26% 8% 2	603
	11th to 20th	Day	22% 6% 28% 28% 12% 4	625
	21st to end	Day	18% 5% 24% 27% 17% 8%	679
June	1st to 10th	Day	7% 3 16% 35% 28% 11%	630
	11th to 20th	Day	9% 2 22% 36% 25% 7%	630
	21st to end	Day	7% 2 30% 35% 22% 4	630
July	1st to 10th	Day	12% 3 9% 29% 32% 15%	621
	11th to 20th	Day	4 1 15% 35% 33% 11%	618
	21st to end	Day	10% 1 10% 33% 35% 11%	684
August	1st to 10th	Day	8% 2 10% 42% 31% 7%	626
	11th to 20th	Day	7% 2 14% 34% 35% 8%	615
	21st to end	Day	10% 9% 28% 38% 14%	509
September	1st to 10th	Day	13% 2 13% 30% 34% 8%	450
	11th to 20th	Day	8% 1 12% 36% 32% 11%	324
	21st to end	Day	10% 3 23% 36% 20% 7%	270
October	1st to 10th	Day	13% 7% 27% 29% 15% 9%	270
	11th to 20th	Day	14% 14% 40% 27% 5% 1	270
	21st to end	Day	28% 10% 33% 22% 6% 1	297
November	1st to 10th	Day	16% 8% 34% 26% 12% 4	270
	11th to 20th	Day	18% 16% 29% 26% 11% 1	270
	21st to end	Day	24% 10% 33% 26% 7% 1	270
December	1st to 10th	Day	14% 12% 41% 21% 9% 3	270
	11th to 20th	Day	17% 16% 33% 28% 6% 1	270
	21st to end	Day	25% 15% 39% 20% 1	297
January	1st to 10th	Night	16% 13% 38% 26% 8%	950
	11th to 20th	Night	17% 14% 39% 25% 5% 1	918
	21st to end	Night	15% 14% 38% 29% 3	990
February	1st to 10th	Night	19% 13% 41% 22% 4	900
	11th to 20th	Night	18% 14% 38% 27% 3	879
	21st to end	Night	22% 14% 39% 22% 3	546
March	1st to 10th	Night	16% 11% 42% 25% 5%	540
	11th to 20th	Night	23% 10% 30% 30% 7%	540
	21st to end	Night	24% 11% 35% 25% 3	594
April	1st to 10th	Night	14% 11% 36% 34% 6%	540
	11th to 20th	Night	19% 10% 32% 30% 8%	533
	21st to end	Night	20% 6% 37% 29% 8%	540
May	1st to 10th	Night	16% 6% 33% 37% 8% 1	540
	11th to 20th	Night	17% 3 28% 36% 14% 1	531
	21st to end	Night	16% 4 24% 33% 21% 2	584
June	1st to 10th	Night	7% 2 14% 44% 30% 3	540
	11th to 20th	Night	4 2 22% 49% 21% 2	540
	21st to end	Night	5% 2 23% 51% 19% 1	540
July	1st to 10th	Night	9% 1 8% 42% 35% 5%	525
	11th to 20th	Night	5% 1 12% 41% 39% 3	522
	21st to end	Night	8% 1 11% 41% 36% 2	591
August	1st to 10th	Night	7% 1 8% 48% 34% 2	537
	11th to 20th	Night	7% 2 10% 47% 31% 4	534
	21st to end	Night	5% 1 10% 37% 42% 5%	594
September	1st to 10th	Night	12% 1 13% 44% 27% 3	540
	11th to 20th	Night	11% 1 15% 36% 31% 5%	540
	21st to end	Night	10% 2 22% 40% 22% 4	556
October	1st to 10th	Night	12% 3 26% 35% 22% 3	720
	11th to 20th	Night	16% 6% 38% 31% 9%	720
	21st to end	Night	19% 10% 33% 28% 10% 1	792
November	1st to 10th	Night	12% 7% 36% 32% 11% 1	740
	11th to 20th	Night	16% 11% 36% 28% 8%	888
	21st to end	Night	20% 12% 35% 27% 7%	950
December	1st to 10th	Night	17% 10% 37% 29% 6%	950
	11th to 20th	Night	21% 9% 33% 32% 5%	950
	21st to end	Night	20% 15% 35% 27% 3	1045

Page generated at 2024-01-11 03:22:37. Version ac07

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