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 2024-10-25.

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% 6% 48%	101434
	Night	8% 9% 3 7% 5% 5% 5% 4 6% 7% 43%	97515
Transparency	Day	36% 7% 11% 20% 17% 10%	101177
	Night	39% 8% 13% 20% 14% 6%	97361
Seeing	Day	11% 15% 28% 26% 16% 4	14500
	Night	12% 9% 21% 36% 20% 2	23561

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% 17% 6% 11% 8% 6% 6% 4 6% 6% 18%	2539
	11th to 20th	Day	13% 19% 6% 11% 5% 5% 6% 4 6% 6% 17%	2540
	21st to end	Day	13% 14% 4 10% 7% 7% 5% 4 6% 8% 21%	2827
February	1st to 10th	Day	9% 14% 5% 11% 6% 6% 5% 5% 6% 9% 25%	1710
	11th to 20th	Day	10% 13% 4 7% 7% 5% 5% 4 8% 9% 28%	1730
	21st to end	Day	8% 14% 5% 12% 7% 5% 6% 5% 5% 7% 26%	1570
March	1st to 10th	Day	6% 12% 5% 9% 7% 6% 6% 5% 7% 8% 28%	3138
	11th to 20th	Day	5% 9% 3 7% 4 5% 4 4 5% 7% 46%	3366
	21st to end	Day	8% 14% 5% 12% 8% 7% 6% 6% 6% 7% 21%	4097
April	1st to 10th	Day	8% 13% 4 9% 8% 6% 6% 4 7% 8% 28%	2280
	11th to 20th	Day	9% 12% 4 8% 5% 5% 5% 3 6% 7% 36%	2249
	21st to end	Day	9% 11% 3 7% 5% 5% 5% 5% 5% 7% 39%	2370
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	10% 8% 3 5% 4 4 4 3 5% 6% 48%	1900
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 4 6% 8% 45%	1900
	21st to end	Day	11% 8% 3 9% 5% 5% 6% 4 6% 6% 38%	1856
November	1st to 10th	Day	11% 11% 4 8% 5% 6% 5% 5% 7% 7% 30%	1674
	11th to 20th	Day	11% 13% 4 10% 7% 6% 6% 5% 6% 7% 24%	1601
	21st to end	Day	9% 12% 4 10% 6% 6% 5% 5% 6% 8% 28%	1520
December	1st to 10th	Day	9% 12% 3 8% 6% 6% 7% 4 7% 8% 30%	1520
	11th to 20th	Day	9% 13% 5% 11% 7% 6% 6% 6% 8% 9% 21%	1928
	21st to end	Day	11% 16% 5% 13% 7% 7% 5% 4 5% 7% 20%	2794
January	1st to 10th	Night	11% 16% 6% 13% 8% 7% 6% 5% 7% 6% 16%	4406
	11th to 20th	Night	17% 18% 5% 10% 6% 6% 5% 5% 5% 6% 18%	4420
	21st to end	Night	12% 14% 4 12% 7% 7% 6% 5% 7% 7% 21%	4827
February	1st to 10th	Night	10% 14% 4 10% 7% 6% 5% 4 7% 6% 27%	2470
	11th to 20th	Night	8% 13% 5% 9% 6% 6% 5% 3 6% 7% 30%	2336
	21st to end	Night	8% 14% 5% 10% 8% 6% 6% 4 6% 6% 26%	1884
March	1st to 10th	Night	6% 10% 3 8% 5% 7% 6% 5% 8% 10% 30%	3736
	11th to 20th	Night	5% 7% 2 7% 5% 4 4 4 7% 7% 47%	3597
	21st to end	Night	7% 12% 4 12% 9% 7% 7% 6% 7% 7% 22%	3555
April	1st to 10th	Night	10% 12% 4 7% 5% 5% 5% 4 6% 7% 35%	1900
	11th to 20th	Night	8% 11% 4 8% 5% 5% 5% 4 5% 6% 39%	1879
	21st to end	Night	8% 9% 4 7% 5% 4 5% 4 6% 7% 41%	1810
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	7% 6% 2 6% 4 3 3 3 5% 7% 54%	2277
	11th to 20th	Night	8% 6% 2 6% 4 5% 4 3 5% 8% 50%	2280
	21st to end	Night	10% 8% 3 8% 6% 5% 4 3 4 6% 43%	2603
November	1st to 10th	Night	13% 11% 4 9% 5% 5% 5% 4 6% 6% 31%	2405
	11th to 20th	Night	13% 9% 4 10% 7% 6% 6% 4 6% 8% 28%	2577
	21st to end	Night	10% 12% 3 7% 5% 6% 6% 5% 7% 9% 31%	2660
December	1st to 10th	Night	10% 12% 4 8% 6% 5% 5% 5% 6% 9% 28%	2660
	11th to 20th	Night	10% 13% 5% 11% 7% 7% 7% 5% 7% 8% 19%	3276
	21st to end	Night	10% 15% 5% 11% 6% 6% 6% 4 9% 7% 21%	4862

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	65% 10% 11% 12% 2	2539
	11th to 20th	Day	64% 8% 11% 11% 4 2	2540
	21st to end	Day	58% 10% 15% 12% 4 1	2827
February	1st to 10th	Day	55% 9% 14% 16% 5%	1710
	11th to 20th	Day	51% 10% 15% 15% 8% 1	1739
	21st to end	Day	57% 11% 12% 13% 6% 1	1570
March	1st to 10th	Day	48% 9% 13% 19% 10%	3138
	11th to 20th	Day	36% 8% 11% 32% 13%	3366
	21st to end	Day	59% 12% 12% 13% 4	4097
April	1st to 10th	Day	53% 9% 13% 15% 8% 1	2280
	11th to 20th	Day	47% 8% 13% 18% 12% 2	2252
	21st to end	Day	43% 9% 13% 22% 11% 1	2370
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	37% 6% 10% 21% 18% 8%	1900
	11th to 20th	Day	36% 7% 15% 22% 17% 3	1900
	21st to end	Day	45% 7% 11% 21% 15% 1	1856
November	1st to 10th	Day	51% 9% 13% 18% 9% 1	1674
	11th to 20th	Day	57% 10% 12% 16% 5%	1601
	21st to end	Day	51% 8% 14% 14% 12% 1	1520
December	1st to 10th	Day	51% 10% 16% 15% 7% 1	1520
	11th to 20th	Day	55% 10% 17% 13% 5%	1928
	21st to end	Day	60% 9% 15% 14% 2	2794
January	1st to 10th	Night	63% 12% 13% 10% 3	4406
	11th to 20th	Night	63% 9% 11% 10% 5% 2	4420
	21st to end	Night	57% 12% 16% 10% 5% 1	4827
February	1st to 10th	Night	55% 10% 15% 13% 7%	2470
	11th to 20th	Night	53% 8% 13% 18% 7% 1	2338
	21st to end	Night	57% 9% 10% 16% 7% 1	1884
March	1st to 10th	Night	42% 11% 13% 22% 11% 1	3736
	11th to 20th	Night	33% 9% 14% 35% 9% 1	3597
	21st to end	Night	54% 14% 15% 13% 4	3555
April	1st to 10th	Night	48% 8% 17% 20% 6% 1	1900
	11th to 20th	Night	45% 9% 14% 21% 10% 2	1879
	21st to end	Night	40% 10% 15% 19% 13% 2	1810
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	31% 6% 11% 25% 22% 5%	2277
	11th to 20th	Night	33% 7% 14% 21% 21% 4	2280
	21st to end	Night	43% 7% 13% 19% 16% 3	2603
November	1st to 10th	Night	51% 9% 11% 18% 11% 1	2405
	11th to 20th	Night	53% 8% 14% 18% 7%	2577
	21st to end	Night	47% 9% 14% 15% 15% 1	2660
December	1st to 10th	Night	50% 11% 16% 15% 6% 1	2660
	11th to 20th	Night	55% 11% 16% 13% 5%	3276
	21st to end	Night	54% 10% 16% 15% 5%	4862

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	25% 19% 24% 18% 12% 1	180
	11th to 20th	Day	20% 14% 28% 24% 11% 3	180
	21st to end	Day	14% 14% 33% 28% 11%	218
February	1st to 10th	Day	21% 21% 29% 20% 7% 1	370
	11th to 20th	Day	19% 23% 29% 19% 9% 1	396
	21st to end	Day	18% 22% 29% 23% 7%	382
March	1st to 10th	Day	16% 22% 33% 22% 7%	460
	11th to 20th	Day	14% 22% 26% 25% 11% 1	460
	21st to end	Day	20% 22% 29% 22% 8%	506
April	1st to 10th	Day	19% 22% 27% 22% 7% 3	460
	11th to 20th	Day	14% 26% 31% 16% 10% 3	452
	21st to end	Day	12% 19% 35% 19% 13% 1	460
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	17% 14% 25% 23% 16% 5%	345
November	1st to 10th	Day	12% 17% 29% 30% 12%	270
	11th to 20th	Day	16% 14% 24% 28% 16% 2	219
	21st to end	Day	13% 16% 26% 31% 14% 1	180
December	1st to 10th	Day	13% 19% 27% 25% 11% 6%	180
	11th to 20th	Day	21% 18% 34% 21% 5%	180
	21st to end	Day	13% 24% 29% 21% 12% 1	198
January	1st to 10th	Night	19% 14% 25% 28% 13% 1	760
	11th to 20th	Night	22% 11% 20% 28% 17% 2	760
	21st to end	Night	18% 9% 21% 36% 15% 1	836
February	1st to 10th	Night	19% 11% 25% 33% 13%	760
	11th to 20th	Night	15% 12% 25% 32% 15% 1	744
	21st to end	Night	16% 15% 27% 32% 9%	628
March	1st to 10th	Night	15% 13% 26% 34% 12% 1	760
	11th to 20th	Night	11% 11% 25% 35% 17% 1	760
	21st to end	Night	14% 13% 28% 31% 13% 1	836
April	1st to 10th	Night	15% 11% 24% 31% 17% 2	760
	11th to 20th	Night	13% 12% 28% 28% 17% 3	751
	21st to end	Night	12% 12% 24% 34% 16% 1	670
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% 20% 36% 17% 4	812
November	1st to 10th	Night	18% 9% 21% 38% 12% 1	740
	11th to 20th	Night	18% 12% 20% 34% 14% 2	760
	21st to end	Night	15% 11% 21% 35% 15% 3	760
December	1st to 10th	Night	16% 11% 22% 34% 16% 1	760
	11th to 20th	Night	18% 13% 23% 33% 13%	760
	21st to end	Night	22% 16% 28% 26% 8%	836

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