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 2024-03-01.

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	8% 8% 4 6% 6% 3 5% 5% 5% 7% 43%	78390
	Night	9% 9% 4 7% 6% 4 5% 5% 6% 7% 39%	81034
Transparency	Day	41% 7% 12% 19% 16% 6%	78093
	Night	43% 8% 13% 18% 13% 4	80852
Seeing	Day	12% 15% 28% 26% 16% 4	13550
	Night	13% 9% 20% 35% 20% 2	22822

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% 7% 10% 9% 4 6% 5% 5% 7% 18%	2539
	11th to 20th	Day	14% 18% 6% 10% 7% 4 6% 5% 6% 6% 17%	2540
	21st to end	Day	13% 13% 4 8% 9% 5% 6% 6% 6% 9% 22%	2827
February	1st to 10th	Day	9% 13% 6% 10% 7% 5% 5% 6% 6% 9% 26%	1710
	11th to 20th	Day	10% 12% 5% 6% 7% 4 6% 5% 8% 9% 28%	1730
	21st to end	Day	9% 12% 5% 11% 8% 4 7% 6% 5% 7% 26%	1570
March	1st to 10th	Day	8% 14% 7% 8% 9% 4 5% 5% 6% 7% 25%	1801
	11th to 20th	Day	7% 13% 5% 9% 7% 5% 6% 6% 6% 7% 28%	1908
	21st to end	Day	10% 14% 7% 10% 9% 5% 6% 7% 5% 7% 20%	2334
April	1st to 10th	Day	9% 12% 5% 8% 9% 4 7% 5% 7% 8% 27%	2160
	11th to 20th	Day	9% 11% 5% 8% 5% 4 5% 5% 6% 7% 35%	2129
	21st to end	Day	9% 10% 3 7% 5% 4 5% 5% 5% 7% 40%	2245
May	1st to 10th	Day	7% 8% 4 6% 6% 4 5% 5% 5% 7% 45%	2506
	11th to 20th	Day	10% 8% 3 7% 7% 4 5% 5% 5% 7% 39%	2477
	21st to end	Day	12% 9% 3 6% 6% 3 5% 5% 6% 6% 39%	2723
June	1st to 10th	Day	12% 7% 3 6% 6% 3 5% 4 6% 8% 40%	2520
	11th to 20th	Day	8% 6% 3 5% 5% 2 4 5% 7% 7% 48%	2520
	21st to end	Day	4 3 1 3 3 2 3 4 5% 6% 65%	2520
July	1st to 10th	Day	2 2 1 2 2 2 3 4 4 5% 73%	2520
	11th to 20th	Day	2 1 1 2 3 1 3 3 4 5% 75%	2520
	21st to end	Day	1 1 1 1 2 1 2 2 3 4 80%	2741
August	1st to 10th	Day	2 2 1 2 3 2 3 3 5% 6% 70%	2507
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 5% 75%	2237
	21st to end	Day	3 3 1 2 2 2 2 4 4 5% 72%	2376
September	1st to 10th	Day	3 2 1 2 3 2 2 3 3 5% 73%	2160
	11th to 20th	Day	5% 3 2 4 5% 2 4 4 5% 7% 59%	2138
	21st to end	Day	7% 5% 2 5% 5% 2 4 4 5% 7% 54%	1980
October	1st to 10th	Day	10% 8% 4 5% 5% 3 4 4 5% 7% 47%	1800
	11th to 20th	Day	8% 6% 2 5% 6% 3 5% 5% 6% 8% 46%	1800
	21st to end	Day	11% 7% 3 8% 6% 4 6% 5% 6% 6% 39%	1815
November	1st to 10th	Day	12% 10% 4 7% 6% 5% 5% 7% 6% 8% 31%	1674
	11th to 20th	Day	11% 13% 5% 8% 9% 5% 6% 6% 6% 7% 24%	1601
	21st to end	Day	10% 11% 5% 9% 7% 4 5% 7% 6% 8% 28%	1520
December	1st to 10th	Day	10% 11% 4 7% 7% 4 7% 6% 6% 8% 30%	1520
	11th to 20th	Day	9% 13% 5% 9% 9% 4 6% 7% 8% 9% 21%	1928
	21st to end	Day	11% 15% 5% 12% 9% 5% 6% 5% 5% 7% 20%	2794
January	1st to 10th	Night	11% 15% 7% 11% 9% 5% 6% 6% 7% 7% 16%	4406
	11th to 20th	Night	17% 17% 5% 9% 8% 4 5% 5% 5% 6% 18%	4420
	21st to end	Night	12% 13% 5% 10% 9% 5% 6% 6% 7% 7% 21%	4827
February	1st to 10th	Night	10% 13% 4 9% 8% 4 6% 5% 7% 7% 27%	2470
	11th to 20th	Night	9% 12% 5% 8% 7% 5% 6% 4 6% 7% 31%	2336
	21st to end	Night	9% 12% 6% 9% 9% 4 6% 6% 6% 6% 27%	1884
March	1st to 10th	Night	11% 11% 4 7% 6% 4 6% 6% 7% 8% 29%	2165
	11th to 20th	Night	8% 9% 3 8% 8% 5% 6% 6% 8% 9% 29%	2048
	21st to end	Night	8% 12% 5% 8% 8% 4 6% 7% 6% 7% 28%	2021
April	1st to 10th	Night	10% 11% 5% 7% 5% 4 6% 6% 6% 8% 34%	1800
	11th to 20th	Night	9% 10% 5% 8% 6% 4 6% 4 5% 6% 38%	1779
	21st to end	Night	9% 8% 4 6% 6% 3 4 5% 5% 7% 42%	1715
May	1st to 10th	Night	6% 6% 3 5% 5% 4 4 4 6% 7% 50%	1451
	11th to 20th	Night	10% 5% 3 4 6% 3 5% 5% 6% 7% 47%	1416
	21st to end	Night	11% 7% 2 6% 5% 3 5% 5% 5% 9% 42%	1552
June	1st to 10th	Night	10% 5% 3 4 5% 3 5% 4 6% 8% 48%	1440
	11th to 20th	Night	8% 6% 3 6% 4 2 4 3 4 7% 54%	1440
	21st to end	Night	4 3 2 3 4 3 4 3 4 6% 65%	1440
July	1st to 10th	Night	1 2 1 2 2 2 3 4 5% 6% 72%	1440
	11th to 20th	Night	1 1 1 2 3 1 3 2 3 5% 79%	1440
	21st to end	Night	1 1 1 1 2 1 2 3 3 5% 79%	1576
August	1st to 10th	Night	3 2 1 2 3 1 3 3 5% 8% 70%	1432
	11th to 20th	Night	2 2 1 2 2 1 2 3 3 6% 77%	1658
	21st to end	Night	2 2 1 2 3 1 3 2 4 6% 75%	1980
September	1st to 10th	Night	3 2 1 2 3 2 3 2 3 5% 75%	1800
	11th to 20th	Night	4 3 2 3 3 2 3 4 5% 7% 64%	1822
	21st to end	Night	7% 5% 2 4 4 3 3 4 5% 6% 58%	1980
October	1st to 10th	Night	8% 6% 3 5% 4 2 3 3 6% 7% 52%	2157
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 5% 8% 50%	2160
	21st to end	Night	10% 8% 3 7% 7% 4 5% 4 4 6% 43%	2539
November	1st to 10th	Night	14% 10% 4 8% 7% 3 5% 5% 6% 6% 31%	2405
	11th to 20th	Night	14% 8% 4 9% 8% 4 6% 5% 6% 8% 28%	2577
	21st to end	Night	10% 10% 4 6% 5% 5% 6% 6% 7% 9% 31%	2660
December	1st to 10th	Night	10% 12% 4 7% 7% 4 6% 6% 6% 9% 28%	2660
	11th to 20th	Night	10% 12% 5% 9% 8% 5% 7% 7% 7% 8% 20%	3276
	21st to end	Night	10% 15% 5% 10% 7% 4 6% 6% 8% 8% 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	57% 9% 14% 14% 6% 1	1801
	11th to 20th	Day	55% 8% 12% 19% 6%	1908
	21st to end	Day	62% 11% 12% 11% 3	2334
April	1st to 10th	Day	54% 9% 13% 15% 8% 1	2160
	11th to 20th	Day	48% 8% 14% 17% 11% 2	2132
	21st to end	Day	43% 9% 13% 21% 12% 1	2245
May	1st to 10th	Day	38% 7% 13% 28% 13% 1	2506
	11th to 20th	Day	44% 7% 13% 24% 11% 1	2480
	21st to end	Day	45% 9% 13% 21% 10% 1	2724
June	1st to 10th	Day	43% 9% 12% 25% 11% 1	2520
	11th to 20th	Day	34% 7% 12% 27% 17% 3	2520
	21st to end	Day	21% 5% 11% 29% 26% 8%	2520
July	1st to 10th	Day	15% 4 9% 21% 29% 22%	2451
	11th to 20th	Day	13% 4 9% 12% 32% 31%	2445
	21st to end	Day	9% 3 5% 13% 41% 30%	2589
August	1st to 10th	Day	15% 4 8% 21% 34% 18%	2490
	11th to 20th	Day	14% 3 6% 22% 35% 19%	2237
	21st to end	Day	15% 3 7% 25% 35% 14%	2376
September	1st to 10th	Day	15% 4 7% 24% 32% 18%	2160
	11th to 20th	Day	25% 6% 10% 22% 26% 12%	2138
	21st to end	Day	31% 6% 10% 23% 23% 7%	1980
October	1st to 10th	Day	38% 7% 10% 21% 18% 7%	1800
	11th to 20th	Day	36% 7% 14% 23% 17% 4	1800
	21st to end	Day	45% 7% 11% 21% 15% 1	1815
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	51% 10% 13% 17% 7% 2	2165
	11th to 20th	Night	48% 11% 17% 19% 4 1	2048
	21st to end	Night	53% 11% 16% 16% 4	2021
April	1st to 10th	Night	48% 8% 16% 20% 7% 1	1800
	11th to 20th	Night	47% 8% 13% 21% 9% 2	1779
	21st to end	Night	41% 10% 15% 19% 14% 2	1715
May	1st to 10th	Night	33% 7% 16% 29% 15%	1451
	11th to 20th	Night	36% 9% 16% 26% 11% 1	1416
	21st to end	Night	41% 8% 17% 22% 11% 1	1552
June	1st to 10th	Night	35% 8% 15% 28% 14% 1	1440
	11th to 20th	Night	33% 7% 12% 32% 13% 3	1440
	21st to end	Night	21% 5% 12% 30% 25% 7%	1440
July	1st to 10th	Night	14% 6% 11% 24% 25% 21%	1400
	11th to 20th	Night	10% 4 10% 19% 30% 28%	1392
	21st to end	Night	9% 3 5% 17% 42% 24%	1488
August	1st to 10th	Night	15% 4 10% 24% 34% 14%	1424
	11th to 20th	Night	11% 4 8% 25% 34% 18%	1658
	21st to end	Night	13% 4 7% 27% 36% 13%	1980
September	1st to 10th	Night	14% 3 8% 27% 32% 16%	1800
	11th to 20th	Night	20% 6% 9% 24% 25% 15%	1822
	21st to end	Night	27% 6% 10% 25% 23% 9%	1980
October	1st to 10th	Night	32% 6% 11% 25% 21% 5%	2157
	11th to 20th	Night	33% 7% 13% 22% 21% 4	2160
	21st to end	Night	43% 7% 13% 18% 16% 3	2539
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% 21% 30% 23% 7%	382
March	1st to 10th	Day	16% 23% 33% 22% 6%	421
	11th to 20th	Day	15% 23% 27% 25% 10%	420
	21st to end	Day	20% 22% 28% 22% 8%	462
April	1st to 10th	Day	20% 21% 26% 22% 7% 3	420
	11th to 20th	Day	15% 26% 30% 17% 9% 3	412
	21st to end	Day	13% 19% 34% 19% 13% 1	420
May	1st to 10th	Day	12% 19% 28% 23% 15% 3	423
	11th to 20th	Day	19% 16% 27% 22% 13% 4	448
	21st to end	Day	14% 13% 22% 30% 17% 5%	485
June	1st to 10th	Day	12% 14% 27% 23% 18% 6%	450
	11th to 20th	Day	11% 19% 31% 25% 12% 2	450
	21st to end	Day	3 13% 26% 33% 19% 6%	450
July	1st to 10th	Day	3 8% 32% 36% 19% 3	446
	11th to 20th	Day	3 9% 34% 29% 20% 5%	444
	21st to end	Day	1 10% 29% 30% 25% 5%	487
August	1st to 10th	Day	3 6% 30% 31% 22% 7%	447
	11th to 20th	Day	2 5% 22% 32% 27% 11%	437
	21st to end	Day	5% 11% 26% 29% 24% 6%	495
September	1st to 10th	Day	3 7% 28% 28% 28% 6%	450
	11th to 20th	Day	7% 9% 23% 33% 23% 6%	450
	21st to end	Day	11% 10% 26% 29% 20% 5%	450
October	1st to 10th	Day	13% 13% 24% 27% 19% 5%	450
	11th to 20th	Day	10% 14% 24% 27% 18% 6%	450
	21st to end	Day	17% 14% 25% 22% 17% 5%	330
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	17% 14% 27% 32% 9%	628
March	1st to 10th	Night	16% 12% 25% 34% 12% 1	722
	11th to 20th	Night	12% 11% 25% 35% 16% 1	720
	21st to end	Night	14% 14% 27% 31% 14% 1	792
April	1st to 10th	Night	15% 11% 23% 32% 17% 2	720
	11th to 20th	Night	14% 12% 26% 28% 18% 3	711
	21st to end	Night	13% 12% 24% 34% 16% 1	635
May	1st to 10th	Night	9% 8% 22% 37% 23% 1	371
	11th to 20th	Night	13% 5% 15% 34% 31% 2	354
	21st to end	Night	12% 7% 20% 33% 25% 4	389
June	1st to 10th	Night	11% 6% 19% 38% 24% 3	360
	11th to 20th	Night	9% 11% 22% 37% 20% 1	360
	21st to end	Night	6% 3 16% 44% 29% 2	360
July	1st to 10th	Night	2 1 15% 45% 35% 2	350
	11th to 20th	Night	1 1 17% 52% 29% 1	348
	21st to end	Night	2 11% 47% 37% 2	394
August	1st to 10th	Night	2 1 8% 48% 39% 3	358
	11th to 20th	Night	3 1 7% 35% 45% 9%	590
	21st to end	Night	3 4 16% 42% 33% 4	792
September	1st to 10th	Night	3 3 14% 40% 36% 3	720
	11th to 20th	Night	6% 3 13% 37% 34% 8%	720
	21st to end	Night	8% 5% 12% 38% 30% 6%	720
October	1st to 10th	Night	11% 4 18% 37% 29% 2	720
	11th to 20th	Night	11% 6% 19% 38% 21% 5%	720
	21st to end	Night	13% 10% 19% 36% 18% 5%	792
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

Page generated at 2024-02-27 03:30:19. Version ac07

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