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 2025-04-26.

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	7% 8% 3 7% 5% 4 4 4 5% 7% 47%	117550
	Night	8% 9% 3 7% 5% 5% 5% 4 6% 7% 43%	119851
Transparency	Day	36% 6% 11% 21% 16% 9%	117293
	Night	39% 7% 13% 22% 14% 5%	119697
Seeing	Day	11% 15% 28% 26% 16% 4	15040
	Night	12% 9% 21% 36% 20% 2	24291

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	12% 16% 6% 12% 8% 6% 5% 4 6% 6% 18%	3639
	11th to 20th	Day	9% 14% 4 9% 4 4 5% 5% 6% 6% 33%	3640
	21st to end	Day	9% 10% 3 7% 5% 5% 4 4 5% 8% 39%	4065
February	1st to 10th	Day	9% 14% 5% 11% 6% 6% 5% 5% 6% 8% 25%	1800
	11th to 20th	Day	10% 13% 4 7% 7% 5% 5% 4 8% 9% 28%	1824
	21st to end	Day	8% 14% 4 12% 7% 5% 6% 5% 5% 7% 27%	1650
March	1st to 10th	Day	6% 12% 5% 9% 6% 5% 6% 5% 7% 8% 30%	4518
	11th to 20th	Day	6% 12% 4 9% 6% 6% 5% 4 6% 7% 36%	4824
	21st to end	Day	7% 13% 5% 11% 8% 7% 7% 6% 7% 7% 22%	5875
April	1st to 10th	Day	8% 13% 4 9% 8% 6% 6% 4 7% 8% 28%	2400
	11th to 20th	Day	8% 11% 4 8% 5% 5% 5% 3 6% 7% 37%	2369
	21st to end	Day	8% 10% 3 7% 5% 5% 5% 5% 5% 7% 40%	2432
May	1st to 10th	Day	10% 10% 3 7% 5% 5% 5% 4 5% 6% 41%	4406
	11th to 20th	Day	6% 5% 2 4 4 3 3 3 4 6% 60%	4377
	21st to end	Day	8% 10% 2 7% 5% 4 5% 4 7% 7% 42%	4813
June	1st to 10th	Day	11% 8% 3 8% 5% 5% 4 4 6% 7% 40%	2660
	11th to 20th	Day	8% 6% 2 6% 4 3 5% 4 6% 7% 49%	2660
	21st to end	Day	4 3 1 4 3 3 3 3 5% 6% 64%	2660
July	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 83%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 81%	4420
	21st to end	Day	1 1 1 2 2 2 2 2 4 4 80%	4790
August	1st to 10th	Day	3 3 1 4 3 2 3 3 5% 5% 70%	4400
	11th to 20th	Day	2 1 1 2 1 2 2 2 3 6% 77%	3913
	21st to end	Day	3 2 1 3 3 2 2 2 3 5% 72%	4180
September	1st to 10th	Day	3 2 1 3 3 3 2 2 3 6% 73%	2280
	11th to 20th	Day	5% 3 2 5% 4 3 4 4 5% 7% 58%	2256
	21st to end	Day	7% 5% 2 6% 4 3 4 3 5% 7% 56%	2090
October	1st to 10th	Day	6% 5% 2 4 3 3 3 2 4 6% 61%	3180
	11th to 20th	Day	5% 5% 2 6% 5% 5% 5% 4 7% 9% 48%	3161
	21st to end	Day	9% 8% 3 10% 6% 6% 6% 5% 7% 7% 34%	3147
November	1st to 10th	Day	11% 10% 4 9% 5% 6% 5% 5% 7% 7% 30%	1764
	11th to 20th	Day	11% 13% 4 10% 7% 6% 6% 5% 6% 8% 23%	1685
	21st to end	Day	9% 11% 4 10% 6% 6% 5% 5% 6% 8% 29%	1600
December	1st to 10th	Day	6% 8% 2 6% 5% 5% 5% 3 5% 7% 48%	2620
	11th to 20th	Day	11% 12% 5% 11% 7% 7% 6% 6% 8% 8% 19%	3028
	21st to end	Day	11% 14% 5% 12% 7% 7% 6% 5% 7% 8% 19%	4004
January	1st to 10th	Night	12% 15% 6% 14% 8% 6% 6% 4 6% 6% 16%	6306
	11th to 20th	Night	12% 13% 4 8% 5% 5% 4 4 5% 5% 36%	6320
	21st to end	Night	8% 10% 3 10% 6% 5% 5% 4 5% 5% 38%	6891
February	1st to 10th	Night	10% 15% 4 10% 7% 6% 5% 4 7% 7% 26%	2600
	11th to 20th	Night	8% 13% 5% 10% 6% 6% 5% 3 6% 7% 31%	2462
	21st to end	Night	8% 13% 5% 10% 8% 5% 6% 4 6% 6% 28%	1980
March	1st to 10th	Night	6% 9% 3 7% 5% 5% 6% 5% 9% 11% 33%	5376
	11th to 20th	Night	6% 9% 3 9% 6% 5% 5% 4 7% 7% 39%	5159
	21st to end	Night	7% 11% 4 11% 8% 7% 7% 5% 6% 7% 26%	5099
April	1st to 10th	Night	9% 12% 4 7% 5% 5% 5% 4 6% 7% 35%	2000
	11th to 20th	Night	8% 10% 4 8% 5% 5% 5% 3 5% 6% 41%	1979
	21st to end	Night	8% 8% 3 7% 5% 4 5% 4 6% 7% 42%	1864
May	1st to 10th	Night	9% 11% 3 7% 5% 4 4 3 5% 6% 43%	2551
	11th to 20th	Night	6% 3 2 4 3 3 2 3 4 5% 65%	2516
	21st to end	Night	8% 9% 2 6% 4 3 4 3 5% 7% 49%	2762
June	1st to 10th	Night	10% 6% 2 5% 5% 4 4 3 6% 8% 48%	1520
	11th to 20th	Night	7% 7% 3 7% 3 2 4 2 4 6% 55%	1520
	21st to end	Night	4 3 1 4 3 4 3 3 4 6% 65%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 3 3 84%	2540
	11th to 20th	Night	1 1 3 2 2 2 2 3 5% 80%	2540
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 80%	2761
August	1st to 10th	Night	2 1 1 3 4 2 4 4 5% 7% 68%	2529
	11th to 20th	Night	1 2 2 2 2 2 3 3 6% 77%	2924
	21st to end	Night	2 2 1 2 2 2 3 2 4 5% 75%	3498
September	1st to 10th	Night	2 2 1 2 2 2 3 2 4 5% 74%	1900
	11th to 20th	Night	3 3 2 4 3 3 3 3 5% 7% 62%	1924
	21st to end	Night	6% 5% 1 4 3 4 3 3 5% 6% 59%	2090
October	1st to 10th	Night	4 4 2 5% 3 2 2 2 4 5% 66%	3797
	11th to 20th	Night	6% 6% 2 5% 3 4 4 3 5% 8% 53%	3777
	21st to end	Night	8% 7% 3 8% 6% 6% 5% 3 6% 7% 42%	4410
November	1st to 10th	Night	13% 11% 4 9% 5% 5% 5% 4 6% 7% 32%	2535
	11th to 20th	Night	14% 9% 4 10% 6% 6% 5% 4 6% 8% 28%	2713
	21st to end	Night	10% 12% 3 7% 5% 6% 5% 5% 7% 9% 31%	2800
December	1st to 10th	Night	6% 8% 3 6% 5% 4 4 4 5% 7% 49%	4560
	11th to 20th	Night	11% 14% 5% 11% 8% 8% 7% 5% 7% 8% 16%	5176
	21st to end	Night	13% 14% 4 11% 6% 6% 6% 4 8% 7% 21%	6952

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	64% 9% 11% 14% 2	3639
	11th to 20th	Day	49% 7% 13% 20% 9% 1	3640
	21st to end	Day	43% 8% 11% 21% 14% 4	4065
February	1st to 10th	Day	55% 9% 14% 17% 5%	1800
	11th to 20th	Day	51% 9% 15% 15% 8% 1	1833
	21st to end	Day	56% 11% 12% 13% 7% 1	1650
March	1st to 10th	Day	46% 7% 12% 21% 11% 2	4518
	11th to 20th	Day	45% 6% 9% 28% 11%	4824
	21st to end	Day	56% 10% 14% 15% 5%	5875
April	1st to 10th	Day	52% 9% 13% 16% 8% 1	2400
	11th to 20th	Day	46% 8% 13% 18% 13% 2	2372
	21st to end	Day	43% 9% 13% 22% 12% 2	2432
May	1st to 10th	Day	40% 8% 12% 26% 12% 1	4406
	11th to 20th	Day	26% 5% 10% 30% 25% 4	4380
	21st to end	Day	38% 9% 20% 24% 9% 1	4814
June	1st to 10th	Day	43% 8% 11% 25% 12% 1	2660
	11th to 20th	Day	33% 7% 12% 27% 18% 3	2660
	21st to end	Day	21% 5% 11% 29% 27% 8%	2660
July	1st to 10th	Day	9% 2 5% 12% 23% 49%	4351
	11th to 20th	Day	9% 3 5% 7% 23% 53%	4345
	21st to end	Day	10% 2 4 17% 38% 29%	4639
August	1st to 10th	Day	17% 3 5% 14% 34% 27%	4384
	11th to 20th	Day	12% 2 5% 32% 32% 17%	3952
	21st to end	Day	17% 2 8% 27% 28% 18%	4179
September	1st to 10th	Day	15% 3 7% 24% 32% 19%	2280
	11th to 20th	Day	25% 6% 11% 22% 24% 12%	2256
	21st to end	Day	30% 6% 11% 24% 23% 8%	2090
October	1st to 10th	Day	26% 4 9% 23% 26% 12%	3180
	11th to 20th	Day	32% 5% 19% 25% 17% 2	3161
	21st to end	Day	47% 6% 9% 25% 13% 1	3147
November	1st to 10th	Day	51% 9% 13% 18% 9% 1	1764
	11th to 20th	Day	57% 9% 13% 16% 5%	1685
	21st to end	Day	51% 8% 14% 16% 12% 1	1600
December	1st to 10th	Day	36% 6% 14% 30% 13% 1	2620
	11th to 20th	Day	57% 9% 19% 12% 3	3028
	21st to end	Day	58% 8% 16% 15% 3	4004
January	1st to 10th	Night	64% 9% 13% 11% 2	6306
	11th to 20th	Night	48% 7% 13% 23% 8% 1	6320
	21st to end	Night	45% 8% 12% 18% 15% 2	6891
February	1st to 10th	Night	56% 9% 14% 14% 7%	2600
	11th to 20th	Night	53% 8% 13% 18% 7% 1	2464
	21st to end	Night	55% 9% 11% 16% 7% 1	1980
March	1st to 10th	Night	38% 9% 13% 25% 13% 2	5376
	11th to 20th	Night	41% 7% 13% 30% 9%	5159
	21st to end	Night	52% 12% 15% 17% 4	5099
April	1st to 10th	Night	46% 8% 17% 22% 6% 1	2000
	11th to 20th	Night	43% 9% 14% 22% 10% 2	1979
	21st to end	Night	39% 10% 15% 19% 14% 2	1864
May	1st to 10th	Night	40% 10% 14% 25% 12%	2551
	11th to 20th	Night	22% 6% 12% 39% 21% 1	2516
	21st to end	Night	34% 9% 26% 24% 7%	2762
June	1st to 10th	Night	35% 8% 14% 28% 14% 1	1520
	11th to 20th	Night	32% 6% 12% 32% 14% 3	1520
	21st to end	Night	21% 5% 12% 30% 26% 7%	1520
July	1st to 10th	Night	8% 3 6% 13% 25% 44%	2500
	11th to 20th	Night	10% 3 5% 12% 22% 49%	2492
	21st to end	Night	10% 2 4 18% 38% 29%	2674
August	1st to 10th	Night	16% 3 6% 17% 38% 20%	2521
	11th to 20th	Night	10% 3 6% 41% 29% 11%	2951
	21st to end	Night	12% 3 10% 31% 29% 15%	3498
September	1st to 10th	Night	14% 3 8% 27% 32% 16%	1900
	11th to 20th	Night	21% 6% 10% 25% 24% 14%	1924
	21st to end	Night	26% 6% 10% 26% 23% 9%	2090
October	1st to 10th	Night	23% 4 8% 27% 31% 7%	3797
	11th to 20th	Night	30% 5% 21% 20% 21% 2	3777
	21st to end	Night	40% 5% 11% 29% 14% 2	4410
November	1st to 10th	Night	50% 8% 12% 18% 11% 1	2535
	11th to 20th	Night	53% 7% 13% 18% 8%	2713
	21st to end	Night	47% 9% 14% 16% 14% 1	2800
December	1st to 10th	Night	34% 7% 12% 36% 10% 1	4560
	11th to 20th	Night	61% 9% 15% 12% 3	5176
	21st to end	Night	56% 9% 16% 14% 5%	6952

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	24% 18% 26% 21% 11% 1	200
	11th to 20th	Day	18% 14% 29% 27% 10% 3	200
	21st to end	Day	14% 12% 33% 29% 13%	242
February	1st to 10th	Day	21% 21% 29% 21% 7% 1	400
	11th to 20th	Day	19% 22% 29% 21% 9%	430
	21st to end	Day	17% 22% 29% 25% 7%	414
March	1st to 10th	Day	16% 22% 33% 23% 6%	500
	11th to 20th	Day	15% 23% 26% 24% 10% 1	500
	21st to end	Day	19% 22% 29% 22% 8%	550
April	1st to 10th	Day	18% 21% 28% 22% 8% 3	500
	11th to 20th	Day	13% 25% 31% 17% 11% 3	492
	21st to end	Day	12% 19% 34% 19% 14% 2	481
May	1st to 10th	Day	13% 20% 28% 22% 14% 3	463
	11th to 20th	Day	18% 16% 27% 23% 13% 3	488
	21st to end	Day	14% 14% 22% 28% 18% 5%	529
June	1st to 10th	Day	13% 14% 26% 23% 18% 6%	490
	11th to 20th	Day	12% 18% 31% 24% 12% 2	490
	21st to end	Day	3 13% 27% 32% 19% 6%	490
July	1st to 10th	Day	3 7% 32% 35% 20% 3	486
	11th to 20th	Day	3 9% 33% 31% 20% 5%	484
	21st to end	Day	2 9% 28% 31% 25% 5%	531
August	1st to 10th	Day	3 6% 30% 32% 22% 7%	487
	11th to 20th	Day	2 6% 23% 32% 26% 10%	477
	21st to end	Day	5% 10% 26% 29% 24% 6%	539
September	1st to 10th	Day	3 7% 28% 28% 28% 6%	490
	11th to 20th	Day	7% 9% 23% 32% 23% 5%	490
	21st to end	Day	10% 10% 24% 30% 20% 5%	490
October	1st to 10th	Day	12% 12% 24% 27% 19% 5%	490
	11th to 20th	Day	9% 14% 24% 28% 19% 6%	490
	21st to end	Day	16% 15% 27% 23% 15% 5%	364
November	1st to 10th	Day	12% 18% 28% 29% 12%	300
	11th to 20th	Day	18% 16% 24% 27% 14% 2	243
	21st to end	Day	14% 16% 24% 33% 13% 1	200
December	1st to 10th	Day	12% 18% 26% 26% 13% 7%	200
	11th to 20th	Day	21% 18% 36% 22% 5%	200
	21st to end	Day	13% 25% 31% 20% 10% 1	220
January	1st to 10th	Night	19% 14% 26% 29% 12% 1	800
	11th to 20th	Night	21% 11% 20% 30% 16% 2	800
	21st to end	Night	17% 9% 22% 36% 15% 1	880
February	1st to 10th	Night	19% 11% 25% 32% 12%	800
	11th to 20th	Night	14% 12% 26% 32% 14% 1	784
	21st to end	Night	16% 15% 27% 33% 8%	660
March	1st to 10th	Night	15% 12% 26% 35% 12% 1	800
	11th to 20th	Night	11% 12% 25% 35% 16% 1	800
	21st to end	Night	13% 14% 28% 31% 13% 1	880
April	1st to 10th	Night	15% 11% 25% 32% 17% 2	800
	11th to 20th	Night	13% 11% 28% 29% 17% 3	791
	21st to end	Night	12% 12% 24% 34% 17% 1	692
May	1st to 10th	Night	9% 9% 23% 36% 22% 1	391
	11th to 20th	Night	12% 6% 15% 34% 31% 2	374
	21st to end	Night	12% 7% 19% 34% 24% 4	411
June	1st to 10th	Night	11% 6% 19% 39% 23% 2	380
	11th to 20th	Night	9% 11% 23% 37% 19% 1	380
	21st to end	Night	6% 2 16% 46% 27% 2	380
July	1st to 10th	Night	2 1 15% 46% 34% 2	370
	11th to 20th	Night	1 1 16% 54% 28% 1	368
	21st to end	Night	2 13% 47% 36% 2	416
August	1st to 10th	Night	2 1 8% 49% 38% 2	378
	11th to 20th	Night	3 1 8% 36% 43% 8%	624
	21st to end	Night	2 4 16% 42% 33% 3	836
September	1st to 10th	Night	3 3 14% 41% 36% 3	760
	11th to 20th	Night	5% 3 15% 38% 32% 7%	760
	21st to end	Night	8% 5% 13% 39% 30% 6%	760
October	1st to 10th	Night	10% 4 17% 39% 28% 2	760
	11th to 20th	Night	10% 6% 18% 39% 21% 5%	760
	21st to end	Night	12% 10% 21% 36% 17% 4	836
November	1st to 10th	Night	18% 9% 21% 39% 12% 1	780
	11th to 20th	Night	19% 12% 20% 33% 14% 2	800
	21st to end	Night	16% 11% 21% 35% 15% 3	800
December	1st to 10th	Night	16% 10% 21% 35% 16% 2	800
	11th to 20th	Night	18% 14% 24% 32% 12%	800
	21st to end	Night	22% 16% 28% 26% 8%	880

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