Spray
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spray clear sky chart from 2005-11-06 to 2026-08-30.

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	6% 7% 2 6% 4 4 4 4 5% 7% 51%	153099
	Night	7% 8% 3 7% 5% 4 4 4 6% 7% 46%	146414
Transparency	Day	33% 5% 10% 22% 19% 9%	147863
	Night	36% 6% 12% 24% 16% 5%	142651
Seeing	Day	11% 15% 28% 27% 16% 4	16810
	Night	12% 9% 21% 36% 20% 2	25824

By Month

Percentage Cloud Cover SW (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	11% 15% 5% 11% 8% 7% 6% 5% 7% 7% 19%	4722
	11th to 20th	Day	7% 11% 3 7% 3 3 4 4 6% 7% 44%	4740
	21st to end	Day	8% 9% 3 7% 5% 5% 4 4 6% 9% 39%	5262
February	1st to 10th	Day	8% 14% 5% 11% 6% 6% 5% 5% 6% 9% 26%	1890
	11th to 20th	Day	10% 13% 4 7% 7% 6% 5% 4 8% 9% 28%	1918
	21st to end	Day	8% 13% 4 12% 7% 5% 6% 5% 5% 7% 28%	1730
March	1st to 10th	Day	6% 12% 5% 10% 6% 5% 6% 5% 8% 9% 30%	4618
	11th to 20th	Day	6% 12% 4 9% 6% 6% 5% 4 6% 7% 36%	4930
	21st to end	Day	7% 13% 5% 11% 8% 7% 7% 6% 7% 7% 23%	6005
April	1st to 10th	Day	8% 12% 4 9% 8% 6% 6% 4 7% 9% 29%	2520
	11th to 20th	Day	8% 11% 4 8% 5% 5% 5% 4 6% 7% 37%	2489
	21st to end	Day	8% 10% 3 7% 5% 4 5% 4 5% 6% 42%	2620
May	1st to 10th	Day	7% 6% 2 5% 3 4 4 3 5% 7% 55%	8155
	11th to 20th	Day	6% 6% 2 5% 4 4 5% 4 6% 8% 51%	8160
	21st to end	Day	6% 6% 2 5% 4 4 4 4 6% 8% 53%	8992
June	1st to 10th	Day	10% 8% 3 7% 5% 5% 4 4 6% 7% 43%	2940
	11th to 20th	Day	7% 6% 2 6% 4 3 5% 4 6% 7% 51%	2940
	21st to end	Day	4 3 1 4 3 3 3 4 5% 6% 63%	2940
July	1st to 10th	Day	2 2 1 2 1 2 2 3 3 5% 78%	8219
	11th to 20th	Day	1 1 1 3 3 2 2 2 3 4 78%	8220
	21st to end	Day	1 1 1 2 2 2 3 2 3 4 79%	8922
August	1st to 10th	Day	3 3 1 4 3 3 3 3 5% 6% 67%	6350
	11th to 20th	Day	3 1 1 2 1 2 2 2 3 5% 78%	5757
	21st to end	Day	4 3 2 4 3 3 3 3 4 6% 65%	6093
September	1st to 10th	Day	3 2 1 3 3 3 2 3 4 6% 71%	2400
	11th to 20th	Day	5% 3 2 5% 4 3 4 4 5% 7% 59%	2375
	21st to end	Day	7% 5% 2 6% 4 3 4 3 5% 7% 56%	2200
October	1st to 10th	Day	6% 5% 2 4 3 3 3 2 5% 6% 61%	3280
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 4 6% 9% 48%	3261
	21st to end	Day	9% 8% 3 10% 6% 6% 6% 5% 7% 7% 34%	3248
November	1st to 10th	Day	11% 10% 4 9% 6% 6% 5% 5% 6% 7% 31%	1854
	11th to 20th	Day	11% 13% 4 10% 7% 6% 6% 5% 6% 8% 24%	1769
	21st to end	Day	9% 11% 4 10% 6% 6% 5% 5% 6% 9% 29%	1680
December	1st to 10th	Day	6% 8% 2 6% 4 5% 5% 3 5% 7% 47%	2700
	11th to 20th	Day	11% 12% 5% 11% 7% 7% 6% 6% 8% 8% 19%	3108
	21st to end	Day	11% 14% 5% 12% 7% 7% 6% 5% 7% 8% 20%	4092
January	1st to 10th	Night	11% 14% 6% 13% 8% 7% 6% 5% 7% 7% 18%	8172
	11th to 20th	Night	9% 10% 3 6% 4 4 4 3 5% 6% 47%	8220
	21st to end	Night	7% 9% 3 9% 7% 6% 5% 4 6% 7% 37%	8920
February	1st to 10th	Night	9% 14% 4 10% 7% 6% 5% 4 7% 7% 28%	2730
	11th to 20th	Night	8% 12% 5% 10% 6% 6% 5% 4 6% 7% 31%	2588
	21st to end	Night	8% 13% 5% 10% 8% 5% 6% 4 6% 6% 28%	2076
March	1st to 10th	Night	6% 9% 3 7% 5% 5% 6% 5% 8% 11% 34%	5496
	11th to 20th	Night	5% 9% 3 9% 6% 5% 5% 4 7% 8% 39%	5273
	21st to end	Night	7% 11% 4 11% 8% 7% 7% 5% 6% 7% 26%	5211
April	1st to 10th	Night	9% 11% 4 6% 5% 5% 5% 5% 7% 8% 36%	2100
	11th to 20th	Night	8% 10% 4 8% 5% 5% 5% 3 6% 6% 40%	2079
	21st to end	Night	7% 8% 3 7% 5% 4 4 4 6% 7% 44%	2000
May	1st to 10th	Night	5% 6% 2 4 3 4 3 2 6% 7% 58%	4748
	11th to 20th	Night	6% 6% 2 4 4 3 3 3 4 6% 58%	4700
	21st to end	Night	8% 6% 2 4 3 3 3 3 5% 6% 58%	5181
June	1st to 10th	Night	9% 5% 2 5% 4 3 4 3 6% 7% 50%	1680
	11th to 20th	Night	7% 6% 3 7% 3 3 4 2 4 6% 55%	1680
	21st to end	Night	4 3 2 4 3 4 3 3 4 5% 65%	1680
July	1st to 10th	Night	1 1 2 2 2 2 2 4 5% 79%	4740
	11th to 20th	Night	1 1 1 3 2 3 4 3 3 4 75%	4740
	21st to end	Night	2 1 1 2 2 2 2 2 4 6% 76%	5153
August	1st to 10th	Night	3 2 1 3 3 3 4 4 5% 8% 65%	3662
	11th to 20th	Night	2 2 1 2 2 2 2 3 4 5% 76%	4314
	21st to end	Night	3 3 1 3 3 3 3 2 5% 6% 68%	5105
September	1st to 10th	Night	2 2 1 3 3 2 3 2 4 6% 72%	2000
	11th to 20th	Night	3 4 2 4 3 3 3 3 5% 7% 63%	2025
	21st to end	Night	6% 5% 1 4 3 4 3 3 5% 6% 60%	2200
October	1st to 10th	Night	4 4 2 5% 3 3 2 2 4 5% 65%	3917
	11th to 20th	Night	6% 6% 2 5% 3 4 4 3 5% 8% 53%	3897
	21st to end	Night	8% 7% 3 8% 6% 6% 5% 3 6% 7% 43%	4551
November	1st to 10th	Night	13% 11% 4 9% 5% 5% 5% 4 6% 7% 32%	2665
	11th to 20th	Night	14% 10% 4 10% 6% 6% 5% 4 6% 8% 28%	2849
	21st to end	Night	10% 12% 3 7% 5% 6% 6% 5% 7% 9% 31%	2940
December	1st to 10th	Night	6% 8% 3 6% 5% 4 4 4 5% 7% 48%	4700
	11th to 20th	Night	11% 14% 5% 11% 8% 8% 7% 5% 7% 8% 17%	5316
	21st to end	Night	13% 14% 4 11% 6% 6% 6% 4 8% 7% 21%	7106

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	61% 8% 14% 15% 2	4723
	11th to 20th	Day	38% 6% 11% 27% 16% 3	4740
	21st to end	Day	41% 7% 12% 24% 14% 3	5262
February	1st to 10th	Day	53% 9% 14% 18% 6%	1890
	11th to 20th	Day	51% 9% 15% 16% 8% 1	1927
	21st to end	Day	55% 10% 12% 14% 7% 1	1730
March	1st to 10th	Day	46% 7% 13% 21% 10% 2	4618
	11th to 20th	Day	45% 6% 9% 28% 11%	4930
	21st to end	Day	55% 10% 14% 15% 5%	6005
April	1st to 10th	Day	51% 9% 14% 18% 8% 1	2520
	11th to 20th	Day	46% 7% 14% 19% 13% 2	2492
	21st to end	Day	42% 9% 12% 23% 13% 2	2620
May	1st to 10th	Day	28% 5% 9% 31% 25% 3	8155
	11th to 20th	Day	30% 4 12% 30% 21% 3	8163
	21st to end	Day	29% 6% 17% 29% 18% 2	8994
June	1st to 10th	Day	40% 8% 11% 25% 15% 1	2940
	11th to 20th	Day	31% 6% 11% 27% 20% 4	2940
	21st to end	Day	21% 4 11% 29% 27% 8%	2940
July	1st to 10th	Day	12% 2 4 20% 30% 32%	8151
	11th to 20th	Day	13% 2 3 10% 28% 44%	8145
	21st to end	Day	11% 2 4 22% 40% 22%	8723
August	1st to 10th	Day	17% 3 5% 14% 34% 28%	4664
	11th to 20th	Day	12% 2 5% 32% 32% 17%	4204
	21st to end	Day	17% 2 8% 27% 28% 18%	4420
September	1st to 10th	Day	16% 3 8% 24% 31% 18%	2400
	11th to 20th	Day	25% 6% 11% 23% 25% 11%	2375
	21st to end	Day	29% 5% 10% 24% 23% 8%	2200
October	1st to 10th	Day	26% 4 10% 24% 26% 11%	3280
	11th to 20th	Day	32% 5% 18% 26% 16% 2	3261
	21st to end	Day	46% 6% 9% 25% 13% 1	3248
November	1st to 10th	Day	50% 8% 14% 19% 8% 1	1854
	11th to 20th	Day	56% 9% 13% 16% 5%	1769
	21st to end	Day	50% 8% 15% 16% 11%	1680
December	1st to 10th	Day	37% 6% 14% 29% 12% 1	2700
	11th to 20th	Day	57% 9% 19% 12% 3	3108
	21st to end	Day	58% 8% 16% 15% 3	4092
January	1st to 10th	Night	61% 8% 16% 13% 2	8172
	11th to 20th	Night	38% 6% 11% 29% 14% 2	8220
	21st to end	Night	44% 7% 12% 22% 14% 1	8920
February	1st to 10th	Night	54% 9% 15% 15% 7%	2730
	11th to 20th	Night	52% 8% 13% 19% 7% 1	2590
	21st to end	Night	55% 9% 10% 16% 8% 2	2076
March	1st to 10th	Night	38% 9% 13% 25% 13% 2	5496
	11th to 20th	Night	40% 7% 14% 29% 9% 1	5273
	21st to end	Night	52% 12% 14% 17% 5%	5211
April	1st to 10th	Night	45% 8% 16% 24% 6% 1	2100
	11th to 20th	Night	44% 9% 14% 22% 10% 2	2079
	21st to end	Night	38% 10% 15% 21% 15% 1	2000
May	1st to 10th	Night	25% 6% 11% 31% 26% 1	4746
	11th to 20th	Night	27% 5% 15% 37% 17% 1	4700
	21st to end	Night	27% 6% 21% 31% 14% 1	5182
June	1st to 10th	Night	33% 7% 14% 28% 16% 2	1680
	11th to 20th	Night	32% 6% 11% 31% 17% 4	1680
	21st to end	Night	22% 4 12% 29% 26% 7%	1680
July	1st to 10th	Night	9% 2 5% 25% 31% 27%	4700
	11th to 20th	Night	13% 2 5% 12% 26% 41%	4692
	21st to end	Night	12% 1 3 28% 39% 17%	5040
August	1st to 10th	Night	16% 3 6% 17% 38% 20%	2681
	11th to 20th	Night	11% 3 6% 40% 29% 11%	3139
	21st to end	Night	13% 3 10% 31% 29% 15%	3698
September	1st to 10th	Night	15% 3 9% 26% 31% 15%	2000
	11th to 20th	Night	21% 6% 10% 25% 25% 13%	2025
	21st to end	Night	25% 6% 10% 26% 24% 10%	2200
October	1st to 10th	Night	23% 4 8% 27% 31% 7%	3917
	11th to 20th	Night	31% 5% 21% 20% 21% 2	3897
	21st to end	Night	39% 5% 11% 30% 14% 2	4551
November	1st to 10th	Night	49% 8% 13% 18% 10% 1	2665
	11th to 20th	Night	53% 7% 14% 18% 8%	2849
	21st to end	Night	47% 8% 15% 16% 13% 1	2940
December	1st to 10th	Night	34% 7% 13% 35% 10% 1	4700
	11th to 20th	Night	60% 9% 15% 12% 3	5316
	21st to end	Night	56% 9% 16% 14% 5%	7106

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	23% 20% 25% 21% 10% 1	220
	11th to 20th	Day	16% 13% 26% 30% 12% 3	220
	21st to end	Day	12% 12% 33% 30% 12%	265
February	1st to 10th	Day	20% 20% 29% 22% 8% 1	430
	11th to 20th	Day	19% 21% 30% 20% 10% 1	464
	21st to end	Day	19% 22% 28% 24% 6%	446
March	1st to 10th	Day	16% 22% 32% 23% 6%	540
	11th to 20th	Day	16% 22% 27% 24% 9% 1	540
	21st to end	Day	19% 22% 29% 22% 8%	594
April	1st to 10th	Day	17% 19% 28% 23% 9% 3	540
	11th to 20th	Day	13% 25% 32% 17% 10% 2	532
	21st to end	Day	12% 17% 35% 21% 13% 2	540
May	1st to 10th	Day	12% 19% 28% 24% 15% 3	543
	11th to 20th	Day	18% 17% 28% 22% 12% 3	568
	21st to end	Day	12% 13% 23% 28% 18% 6%	617
June	1st to 10th	Day	12% 15% 26% 25% 17% 6%	570
	11th to 20th	Day	10% 17% 32% 26% 12% 2	570
	21st to end	Day	3 12% 28% 32% 20% 5%	570
July	1st to 10th	Day	3 7% 30% 35% 21% 3	566
	11th to 20th	Day	3 9% 34% 30% 20% 5%	564
	21st to end	Day	1 9% 28% 31% 25% 5%	619
August	1st to 10th	Day	3 6% 29% 32% 22% 7%	567
	11th to 20th	Day	2 6% 24% 32% 25% 10%	557
	21st to end	Day	5% 10% 25% 29% 25% 6%	620
September	1st to 10th	Day	3 6% 26% 29% 29% 7%	530
	11th to 20th	Day	7% 9% 22% 32% 23% 6%	530
	21st to end	Day	10% 9% 24% 30% 21% 5%	530
October	1st to 10th	Day	11% 12% 26% 28% 18% 5%	530
	11th to 20th	Day	10% 15% 24% 29% 18% 5%	530
	21st to end	Day	15% 14% 28% 23% 16% 4	399
November	1st to 10th	Day	12% 18% 29% 29% 12%	330
	11th to 20th	Day	17% 16% 25% 27% 13% 1	267
	21st to end	Day	13% 16% 25% 34% 12% 1	220
December	1st to 10th	Day	12% 20% 26% 25% 12% 6%	220
	11th to 20th	Day	20% 17% 35% 23% 5%	220
	21st to end	Day	14% 24% 30% 20% 10% 1	242
January	1st to 10th	Night	19% 14% 26% 29% 12% 1	840
	11th to 20th	Night	20% 10% 20% 31% 17% 2	840
	21st to end	Night	16% 8% 22% 37% 15% 1	924
February	1st to 10th	Night	18% 11% 25% 33% 12%	840
	11th to 20th	Night	14% 11% 26% 33% 14% 1	824
	21st to end	Night	16% 16% 28% 32% 8%	692
March	1st to 10th	Night	15% 13% 26% 35% 11% 1	840
	11th to 20th	Night	11% 12% 27% 34% 15% 1	840
	21st to end	Night	13% 14% 29% 31% 12% 1	924
April	1st to 10th	Night	14% 11% 24% 33% 17% 2	840
	11th to 20th	Night	12% 11% 28% 31% 16% 2	831
	21st to end	Night	11% 11% 25% 35% 16% 1	740
May	1st to 10th	Night	8% 8% 22% 39% 21% 1	432
	11th to 20th	Night	11% 7% 17% 35% 29% 2	414
	21st to end	Night	11% 7% 19% 36% 24% 3	455
June	1st to 10th	Night	10% 5% 20% 40% 23% 2	420
	11th to 20th	Night	9% 10% 22% 41% 18% 1	420
	21st to end	Night	6% 2 16% 46% 27% 2	420
July	1st to 10th	Night	1 1 14% 47% 35% 2	410
	11th to 20th	Night	1 1 16% 54% 27% 1	408
	21st to end	Night	2 13% 48% 35% 2	460
August	1st to 10th	Night	2 8% 51% 35% 2	418
	11th to 20th	Night	3 2 8% 39% 40% 8%	692
	21st to end	Night	3 4 16% 43% 31% 3	916
September	1st to 10th	Night	3 3 14% 41% 36% 3	800
	11th to 20th	Night	5% 3 15% 38% 32% 7%	800
	21st to end	Night	8% 5% 13% 38% 30% 6%	800
October	1st to 10th	Night	10% 4 19% 38% 28% 2	800
	11th to 20th	Night	11% 7% 19% 40% 20% 5%	800
	21st to end	Night	12% 10% 21% 37% 16% 4	880
November	1st to 10th	Night	18% 10% 22% 39% 12%	820
	11th to 20th	Night	19% 12% 21% 34% 13% 2	840
	21st to end	Night	15% 11% 21% 35% 15% 3	840
December	1st to 10th	Night	15% 11% 22% 34% 16% 1	840
	11th to 20th	Night	18% 14% 24% 33% 11%	840
	21st to end	Night	21% 16% 29% 26% 8%	924

Page generated at 2026-08-27 01:07:15. Version ac07

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