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 2024-11-12.

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%	101054
	Night	11% 7% 2 6% 4 4 4 3 5% 6% 48%	98189
Transparency	Day	38% 5% 12% 26% 17% 3	104705
	Night	36% 4 11% 26% 19% 4	102678
Seeing	Day	15% 7% 26% 29% 18% 5%	15648
	Night	15% 8% 30% 33% 14% 1	25471

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% 9% 3 8% 6% 4 4 3 5% 5% 38%	2847
	11th to 20th	Day	11% 11% 4 9% 5% 6% 4 3 4 5% 38%	2850
	21st to end	Day	18% 15% 4 7% 3 3 3 2 3 5% 37%	3135
February	1st to 10th	Day	15% 13% 4 7% 5% 4 3 3 4 6% 38%	1710
	11th to 20th	Day	13% 10% 4 7% 4 3 4 3 5% 5% 42%	1664
	21st to end	Day	14% 9% 3 7% 4 4 4 3 5% 6% 41%	1635
March	1st to 10th	Day	14% 10% 3 6% 3 3 3 3 4 4 47%	3436
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 5% 7% 50%	3480
	21st to end	Day	17% 12% 3 7% 5% 4 4 3 5% 6% 35%	3814
April	1st to 10th	Day	14% 8% 2 7% 4 4 3 3 4 6% 44%	2215
	11th to 20th	Day	15% 9% 2 6% 4 3 4 3 5% 7% 41%	2254
	21st to end	Day	20% 9% 3 6% 5% 4 4 3 6% 7% 35%	2351
May	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 5% 6% 36%	4082
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 39%	4082
	21st to end	Day	13% 6% 2 7% 5% 4 5% 4 5% 8% 41%	4604
June	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 7% 10% 48%	2660
	11th to 20th	Day	7% 5% 2 6% 4 4 4 4 6% 9% 48%	2660
	21st to end	Day	6% 3 2 5% 3 4 4 5% 6% 10% 53%	2660
July	1st to 10th	Day	11% 7% 2 7% 5% 5% 4 4 5% 8% 43%	4126
	11th to 20th	Day	4 4 2 6% 4 4 5% 3 6% 9% 52%	4120
	21st to end	Day	6% 4 2 5% 5% 5% 6% 6% 9% 12% 41%	4465
August	1st to 10th	Day	4 4 2 5% 4 4 5% 4 6% 9% 54%	4104
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	4041
	21st to end	Day	6% 3 2 4 3 4 4 4 5% 9% 57%	4194
September	1st to 10th	Day	12% 5% 2 5% 4 3 3 2 5% 7% 53%	2280
	11th to 20th	Day	10% 4 2 5% 4 4 4 3 5% 7% 52%	2147
	21st to end	Day	8% 5% 2 5% 3 4 3 4 5% 7% 54%	2052
October	1st to 10th	Day	11% 4 2 5% 3 3 3 2 4 6% 55%	1900
	11th to 20th	Day	13% 6% 2 4 3 3 2 3 3 5% 56%	1900
	21st to end	Day	17% 8% 2 6% 4 3 2 2 4 5% 48%	2090
November	1st to 10th	Day	11% 6% 2 5% 3 2 3 3 4 6% 54%	1958
	11th to 20th	Day	16% 7% 2 4 3 3 3 2 4 6% 50%	1687
	21st to end	Day	17% 8% 2 6% 3 3 2 3 4 5% 47%	1520
December	1st to 10th	Day	12% 9% 3 7% 4 3 3 3 4 7% 45%	1520
	11th to 20th	Day	12% 11% 4 8% 5% 4 4 3 4 4 43%	1918
	21st to end	Day	17% 15% 5% 8% 4 4 4 3 5% 6% 28%	2893
January	1st to 10th	Night	11% 9% 3 12% 6% 4 3 3 4 5% 41%	4108
	11th to 20th	Night	12% 8% 3 10% 6% 8% 5% 4 4 4 36%	4120
	21st to end	Night	19% 13% 4 7% 3 3 3 2 4 5% 36%	4532
February	1st to 10th	Night	13% 11% 4 7% 4 4 4 4 5% 6% 38%	2470
	11th to 20th	Night	12% 9% 3 7% 4 3 3 4 4 6% 44%	2403
	21st to end	Night	13% 9% 3 7% 4 3 3 3 4 5% 46%	1835
March	1st to 10th	Night	15% 8% 3 6% 4 3 3 3 4 5% 47%	3419
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 54%	3480
	21st to end	Night	15% 8% 3 8% 5% 4 4 3 5% 6% 37%	3817
April	1st to 10th	Night	10% 7% 3 5% 4 3 3 3 6% 7% 50%	1963
	11th to 20th	Night	14% 8% 2 6% 3 3 3 3 5% 7% 46%	1875
	21st to end	Night	16% 6% 3 7% 3 4 3 3 5% 5% 43%	1825
May	1st to 10th	Night	14% 8% 2 6% 4 4 4 3 6% 7% 40%	2823
	11th to 20th	Night	13% 9% 3 6% 4 3 4 3 6% 8% 42%	2823
	21st to end	Night	12% 6% 1 4 3 3 3 3 5% 7% 52%	2945
June	1st to 10th	Night	7% 3 1 4 3 3 4 4 8% 9% 54%	1520
	11th to 20th	Night	3 4 2 4 5% 5% 5% 4 8% 10% 51%	1520
	21st to end	Night	4 3 1 3 3 4 4 3 7% 8% 61%	1520
July	1st to 10th	Night	11% 7% 2 6% 4 4 3 3 6% 8% 47%	2838
	11th to 20th	Night	3 3 1 4 3 3 4 3 6% 10% 59%	2850
	21st to end	Night	6% 4 2 4 4 4 5% 4 8% 12% 49%	3098
August	1st to 10th	Night	3 2 1 2 3 3 4 4 5% 9% 64%	2836
	11th to 20th	Night	10% 5% 2 6% 4 3 4 3 5% 7% 50%	2798
	21st to end	Night	3 3 1 4 4 4 4 4 6% 8% 59%	3462
September	1st to 10th	Night	10% 4 1 5% 3 3 3 2 3 6% 58%	1900
	11th to 20th	Night	9% 4 2 4 4 4 4 4 6% 6% 54%	2028
	21st to end	Night	7% 5% 2 5% 3 3 4 3 4 7% 58%	2107
October	1st to 10th	Night	8% 4 1 4 3 3 3 3 4 6% 60%	2280
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 5% 56%	2280
	21st to end	Night	15% 6% 2 5% 3 2 4 2 4 5% 52%	2508
November	1st to 10th	Night	11% 5% 2 4 3 3 3 2 3 5% 60%	2340
	11th to 20th	Night	11% 6% 2 6% 4 3 3 2 4 6% 53%	2511
	21st to end	Night	16% 8% 2 4 3 3 3 3 4 4 50%	2660
December	1st to 10th	Night	13% 8% 3 7% 4 3 4 3 4 6% 45%	2660
	11th to 20th	Night	13% 7% 3 7% 4 3 4 3 5% 6% 45%	3279
	21st to end	Night	18% 18% 5% 9% 4 3 3 2 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	47% 4 8% 27% 13% 1	2848
	11th to 20th	Day	49% 5% 10% 24% 12%	2850
	21st to end	Day	51% 3 10% 19% 16% 1	3135
February	1st to 10th	Day	49% 4 10% 23% 11% 2	1710
	11th to 20th	Day	45% 3 13% 21% 15% 3	1673
	21st to end	Day	44% 4 9% 23% 14% 5%	1635
March	1st to 10th	Day	41% 3 8% 23% 21% 3	3436
	11th to 20th	Day	35% 5% 12% 27% 17% 4	3480
	21st to end	Day	49% 6% 17% 20% 6% 1	3814
April	1st to 10th	Day	41% 4 11% 24% 17% 2	2215
	11th to 20th	Day	43% 4 11% 25% 15% 1	2254
	21st to end	Day	50% 4 11% 24% 11% 1	2351
May	1st to 10th	Day	47% 7% 10% 21% 13% 1	4120
	11th to 20th	Day	42% 8% 13% 24% 13%	4086
	21st to end	Day	39% 7% 12% 28% 12% 1	4617
June	1st to 10th	Day	30% 6% 17% 29% 16% 2	2660
	11th to 20th	Day	31% 7% 14% 30% 15% 2	2660
	21st to end	Day	26% 5% 16% 32% 16% 4	2660
July	1st to 10th	Day	40% 4 12% 23% 17% 3	4062
	11th to 20th	Day	28% 5% 13% 36% 17% 2	4051
	21st to end	Day	32% 5% 22% 31% 11%	4325
August	1st to 10th	Day	27% 4 17% 29% 22% 1	4090
	11th to 20th	Day	37% 5% 17% 26% 14% 1	4080
	21st to end	Day	25% 5% 14% 40% 14% 4	4193
September	1st to 10th	Day	32% 4 12% 26% 19% 7%	2280
	11th to 20th	Day	33% 4 10% 26% 26% 2	2147
	21st to end	Day	30% 5% 10% 28% 23% 4	2052
October	1st to 10th	Day	19% 2 6% 20% 39% 13%	3160
	11th to 20th	Day	21% 2 4 24% 39% 10%	3140
	21st to end	Day	35% 2 8% 27% 26% 3	3428
November	1st to 10th	Day	32% 3 11% 28% 21% 5%	1958
	11th to 20th	Day	37% 2 9% 25% 23% 4	1687
	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	46% 4 13% 24% 13% 1	4108
	11th to 20th	Night	51% 5% 9% 22% 11% 1	4120
	21st to end	Night	52% 3 11% 21% 11% 1	4532
February	1st to 10th	Night	46% 5% 10% 23% 15% 2	2470
	11th to 20th	Night	42% 3 11% 24% 17% 3	2404
	21st to end	Night	42% 3 7% 23% 19% 6%	1835
March	1st to 10th	Night	40% 4 9% 21% 21% 5%	3419
	11th to 20th	Night	31% 4 9% 25% 27% 3	3480
	21st to end	Night	46% 7% 14% 24% 9% 1	3817
April	1st to 10th	Night	34% 4 11% 23% 23% 5%	1963
	11th to 20th	Night	38% 3 8% 27% 19% 5%	1875
	21st to end	Night	44% 4 10% 26% 15% 1	1825
May	1st to 10th	Night	43% 7% 14% 19% 15% 2	2850
	11th to 20th	Night	39% 7% 13% 29% 11% 1	2824
	21st to end	Night	31% 7% 11% 28% 22% 1	2953
June	1st to 10th	Night	24% 7% 18% 30% 18% 4	1520
	11th to 20th	Night	26% 7% 15% 30% 20% 3	1520
	21st to end	Night	21% 4 14% 32% 23% 5%	1520
July	1st to 10th	Night	36% 5% 12% 25% 19% 3	2793
	11th to 20th	Night	21% 4 12% 39% 22% 3	2796
	21st to end	Night	28% 4 20% 33% 14% 1	2999
August	1st to 10th	Night	18% 4 20% 31% 23% 4	2826
	11th to 20th	Night	34% 3 11% 31% 19% 2	2825
	21st to end	Night	23% 4 13% 38% 18% 4	3462
September	1st to 10th	Night	28% 3 10% 29% 21% 8%	1900
	11th to 20th	Night	30% 5% 9% 25% 26% 5%	2028
	21st to end	Night	28% 4 7% 27% 29% 5%	2107
October	1st to 10th	Night	17% 2 6% 21% 42% 13%	3800
	11th to 20th	Night	23% 2 5% 24% 29% 16%	3779
	21st to end	Night	31% 3 7% 31% 24% 4	4120
November	1st to 10th	Night	29% 3 11% 28% 23% 6%	2340
	11th to 20th	Night	34% 3 7% 26% 25% 4	2511
	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	29% 12% 38% 20% 1	270
	21st to end	Day	18% 11% 43% 23% 4	297
February	1st to 10th	Day	24% 15% 37% 19% 4	270
	11th to 20th	Day	16% 21% 39% 18% 4 1	270
	21st to end	Day	24% 21% 39% 14% 2	225
March	1st to 10th	Day	20% 17% 40% 19% 4	270
	11th to 20th	Day	27% 9% 40% 18% 7%	270
	21st to end	Day	33% 15% 33% 14% 4 1	297
April	1st to 10th	Day	19% 14% 37% 23% 8% 1	395
	11th to 20th	Day	22% 13% 34% 24% 7%	452
	21st to end	Day	26% 9% 32% 24% 8% 1	531
May	1st to 10th	Day	20% 10% 36% 25% 8% 2	653
	11th to 20th	Day	21% 6% 27% 28% 14% 4	675
	21st to end	Day	18% 5% 25% 28% 16% 8%	734
June	1st to 10th	Day	8% 2 17% 36% 27% 10%	680
	11th to 20th	Day	9% 2 23% 37% 24% 6%	680
	21st to end	Day	6% 3 30% 36% 21% 4	680
July	1st to 10th	Day	13% 3 11% 29% 30% 14%	671
	11th to 20th	Day	4 1 17% 35% 32% 11%	668
	21st to end	Day	9% 1 12% 32% 35% 11%	739
August	1st to 10th	Day	8% 2 11% 41% 31% 7%	676
	11th to 20th	Day	9% 2 15% 35% 32% 7%	665
	21st to end	Day	9% 9% 29% 38% 14%	553
September	1st to 10th	Day	12% 2 12% 30% 35% 8%	490
	11th to 20th	Day	8% 1 13% 37% 31% 10%	357
	21st to end	Day	11% 3 24% 35% 19% 8%	300
October	1st to 10th	Day	12% 6% 25% 30% 18% 9%	300
	11th to 20th	Day	13% 13% 37% 29% 7% 1	300
	21st to end	Day	25% 12% 35% 22% 6% 1	330
November	1st to 10th	Day	18% 8% 34% 25% 11% 4	300
	11th to 20th	Day	18% 16% 29% 26% 11% 1	273
	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	950
	21st to end	Night	17% 13% 38% 29% 3	1045
February	1st to 10th	Night	19% 13% 41% 23% 4	950
	11th to 20th	Night	18% 14% 38% 27% 3	929
	21st to end	Night	21% 13% 40% 22% 3	579
March	1st to 10th	Night	16% 11% 43% 26% 5%	570
	11th to 20th	Night	22% 11% 31% 31% 6%	570
	21st to end	Night	24% 11% 35% 26% 3	627
April	1st to 10th	Night	14% 11% 37% 32% 6%	570
	11th to 20th	Night	19% 11% 33% 29% 8%	563
	21st to end	Night	20% 6% 37% 29% 8%	570
May	1st to 10th	Night	15% 6% 34% 36% 8% 1	570
	11th to 20th	Night	17% 3 27% 37% 15% 1	561
	21st to end	Night	16% 4 25% 34% 20% 2	617
June	1st to 10th	Night	8% 2 15% 44% 29% 2	570
	11th to 20th	Night	4 2 23% 49% 21% 2	570
	21st to end	Night	5% 2 24% 50% 18% 1	570
July	1st to 10th	Night	10% 1 9% 41% 34% 5%	555
	11th to 20th	Night	5% 1 13% 40% 38% 3	552
	21st to end	Night	8% 1 12% 40% 37% 3	624
August	1st to 10th	Night	6% 1 9% 48% 33% 2	567
	11th to 20th	Night	7% 2 11% 47% 29% 3	564
	21st to end	Night	5% 1 11% 39% 40% 4	627
September	1st to 10th	Night	12% 1 13% 44% 28% 3	570
	11th to 20th	Night	11% 2 14% 38% 31% 5%	570
	21st to end	Night	11% 2 22% 39% 22% 4	587
October	1st to 10th	Night	11% 3 25% 36% 23% 3	760
	11th to 20th	Night	15% 6% 38% 32% 10%	760
	21st to end	Night	19% 10% 32% 29% 9% 1	836
November	1st to 10th	Night	13% 7% 36% 32% 11% 1	780
	11th to 20th	Night	16% 11% 35% 28% 9%	893
	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 2024-11-08 21:45:38. Version ac07

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