SRAF
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SRAF clear sky chart from 2005-11-06 to 2024-12-02.

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% 7% 2 6% 4 4 4 3 5% 6% 51%	105395
	Night	11% 8% 2 6% 4 4 4 3 5% 6% 47%	102918
Transparency	Day	32% 6% 10% 18% 19% 14%	105139
	Night	37% 6% 11% 18% 17% 11%	102763
Seeing	Day	11% 9% 22% 31% 23% 5%	14530
	Night	14% 4 18% 38% 23% 3	23843

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	16% 18% 5% 10% 7% 5% 6% 4 6% 5% 17%	2539
	11th to 20th	Day	17% 18% 5% 11% 6% 6% 6% 3 5% 6% 16%	2540
	21st to end	Day	20% 18% 5% 12% 7% 5% 5% 4 5% 6% 14%	2794
February	1st to 10th	Day	15% 14% 6% 8% 5% 5% 4 3 5% 7% 29%	1659
	11th to 20th	Day	12% 13% 3 8% 5% 4 6% 3 5% 9% 32%	1850
	21st to end	Day	11% 12% 4 8% 6% 5% 6% 4 6% 7% 31%	1570
March	1st to 10th	Day	11% 13% 4 9% 7% 6% 6% 5% 7% 7% 26%	3138
	11th to 20th	Day	9% 7% 2 4 3 3 4 3 4 5% 55%	3335
	21st to end	Day	10% 10% 3 9% 6% 5% 6% 5% 8% 9% 28%	4027
April	1st to 10th	Day	11% 10% 3 7% 5% 4 4 4 6% 8% 38%	2280
	11th to 20th	Day	8% 8% 2 7% 5% 4 4 4 6% 8% 44%	2287
	21st to end	Day	9% 7% 2 7% 4 4 4 3 6% 8% 46%	2470
May	1st to 10th	Day	8% 8% 2 6% 5% 5% 4 4 5% 6% 47%	4110
	11th to 20th	Day	5% 4 1 4 2 2 3 2 4 5% 66%	4245
	21st to end	Day	6% 5% 1 5% 4 4 5% 4 6% 8% 52%	4813
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 6% 9% 42%	2660
	11th to 20th	Day	8% 4 2 4 3 3 4 4 6% 7% 56%	2660
	21st to end	Day	3 3 2 3 2 3 3 3 5% 5% 70%	2660
July	1st to 10th	Day	1 1 2 2 2 2 2 3 3 82%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	4420
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 76%	4790
August	1st to 10th	Day	4 4 2 5% 3 4 3 3 4 7% 62%	4260
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 4 79%	4028
	21st to end	Day	2 3 1 3 2 2 2 2 3 4 76%	4188
September	1st to 10th	Day	3 2 1 2 2 2 3 2 3 5% 74%	2280
	11th to 20th	Day	6% 3 1 4 2 2 3 3 5% 7% 63%	2116
	21st to end	Day	7% 3 2 4 2 3 3 2 4 5% 66%	2090
October	1st to 10th	Day	5% 4 1 4 3 3 3 2 3 4 67%	3490
	11th to 20th	Day	8% 4 2 5% 3 3 4 3 5% 6% 55%	3021
	21st to end	Day	8% 8% 2 6% 4 4 5% 3 5% 7% 48%	3101
November	1st to 10th	Day	15% 8% 3 6% 4 4 5% 4 5% 6% 40%	1764
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 4 6% 8% 34%	1800
	21st to end	Day	12% 9% 3 9% 5% 5% 5% 5% 7% 8% 33%	1740
December	1st to 10th	Day	17% 12% 3 10% 5% 3 4 3 6% 7% 30%	1528
	11th to 20th	Day	16% 16% 4 12% 7% 5% 6% 5% 6% 6% 17%	1928
	21st to end	Day	14% 13% 4 10% 6% 5% 5% 4 6% 7% 25%	2794
January	1st to 10th	Night	17% 16% 5% 11% 7% 5% 5% 3 5% 6% 21%	4406
	11th to 20th	Night	24% 18% 4 9% 5% 6% 4 3 4 4 20%	4420
	21st to end	Night	21% 14% 4 10% 5% 4 4 3 5% 6% 24%	4862
February	1st to 10th	Night	18% 11% 3 8% 5% 4 4 4 5% 6% 32%	2519
	11th to 20th	Night	15% 11% 2 6% 5% 5% 5% 3 5% 6% 37%	2218
	21st to end	Night	13% 14% 3 7% 5% 5% 4 5% 6% 7% 30%	1884
March	1st to 10th	Night	12% 15% 3 7% 5% 4 4 4 6% 6% 34%	3736
	11th to 20th	Night	12% 10% 2 5% 3 3 3 3 4 4 52%	3632
	21st to end	Night	10% 10% 3 9% 6% 6% 6% 5% 7% 9% 30%	3622
April	1st to 10th	Night	13% 9% 2 6% 4 3 3 4 6% 7% 42%	1900
	11th to 20th	Night	11% 8% 2 6% 5% 4 5% 4 4 5% 45%	1839
	21st to end	Night	7% 6% 2 5% 5% 4 4 3 5% 6% 51%	1710
May	1st to 10th	Night	12% 6% 2 4 3 4 4 4 6% 8% 47%	2860
	11th to 20th	Night	5% 4 1 4 3 3 3 4 5% 7% 62%	2649
	21st to end	Night	7% 7% 2 6% 4 4 4 3 4 7% 52%	2762
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 9% 45%	1520
	11th to 20th	Night	7% 5% 2 4 3 4 4 3 4 6% 58%	1520
	21st to end	Night	3 3 1 3 3 3 4 3 5% 6% 67%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 4 80%	2540
	11th to 20th	Night	1 1 1 2 2 2 3 2 3 6% 78%	2540
	21st to end	Night	2 2 1 4 3 3 3 4 4 6% 68%	2761
August	1st to 10th	Night	3 3 2 6% 4 4 4 4 5% 6% 59%	2674
	11th to 20th	Night	1 2 1 2 2 2 3 2 4 5% 77%	2811
	21st to end	Night	3 3 1 3 1 2 2 2 2 4 76%	3488
September	1st to 10th	Night	3 2 1 2 2 2 2 3 3 4 75%	1900
	11th to 20th	Night	5% 4 2 4 3 2 3 3 4 6% 63%	2063
	21st to end	Night	7% 3 2 4 3 2 3 2 3 6% 67%	2090
October	1st to 10th	Night	4 3 1 3 3 3 3 3 3 4 69%	3490
	11th to 20th	Night	8% 4 1 4 3 4 3 3 4 6% 59%	3908
	21st to end	Night	11% 6% 2 5% 4 4 4 4 6% 7% 46%	4457
November	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	2535
	11th to 20th	Night	18% 9% 2 6% 4 4 4 4 6% 6% 37%	2600
	21st to end	Night	14% 9% 2 6% 4 4 5% 3 5% 6% 41%	2658
December	1st to 10th	Night	17% 11% 3 8% 4 4 4 4 6% 7% 33%	2686
	11th to 20th	Night	18% 13% 4 10% 6% 5% 4 4 5% 5% 25%	3276
	21st to end	Night	15% 12% 3 9% 5% 5% 5% 4 6% 7% 29%	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	63% 13% 14% 9% 1	2539
	11th to 20th	Day	65% 10% 12% 11% 2	2540
	21st to end	Day	67% 11% 13% 8% 1	2794
February	1st to 10th	Day	55% 10% 15% 17% 3 1	1659
	11th to 20th	Day	49% 11% 15% 19% 6%	1860
	21st to end	Day	49% 10% 15% 17% 8% 1	1570
March	1st to 10th	Day	49% 10% 16% 16% 9% 1	3138
	11th to 20th	Day	31% 7% 11% 37% 13% 1	3335
	21st to end	Day	46% 12% 16% 18% 7% 1	4027
April	1st to 10th	Day	43% 7% 16% 21% 11% 2	2280
	11th to 20th	Day	37% 7% 12% 24% 17% 3	2290
	21st to end	Day	35% 8% 12% 27% 17% 1	2470
May	1st to 10th	Day	34% 6% 11% 24% 20% 4	4110
	11th to 20th	Day	20% 5% 7% 27% 30% 12%	4248
	21st to end	Day	27% 8% 13% 33% 17% 3	4814
June	1st to 10th	Day	36% 8% 12% 25% 18% 1	2660
	11th to 20th	Day	24% 7% 9% 23% 27% 10%	2660
	21st to end	Day	17% 4 7% 18% 34% 21%	2660
July	1st to 10th	Day	10% 2 4 9% 19% 58%	4351
	11th to 20th	Day	11% 1 3 5% 21% 58%	4345
	21st to end	Day	13% 2 4 6% 31% 45%	4639
August	1st to 10th	Day	24% 2 6% 9% 26% 33%	4244
	11th to 20th	Day	11% 2 3 15% 38% 32%	4067
	21st to end	Day	15% 2 4 18% 34% 27%	4187
September	1st to 10th	Day	15% 2 5% 17% 31% 30%	2280
	11th to 20th	Day	20% 4 9% 22% 30% 13%	2116
	21st to end	Day	22% 3 7% 26% 32% 9%	2090
October	1st to 10th	Day	22% 4 7% 18% 41% 9%	3490
	11th to 20th	Day	30% 4 11% 26% 25% 4	3021
	21st to end	Day	35% 5% 11% 29% 19% 1	3101
November	1st to 10th	Day	43% 8% 15% 23% 11%	1764
	11th to 20th	Day	48% 7% 19% 21% 5%	1800
	21st to end	Day	49% 9% 19% 17% 6% 1	1740
December	1st to 10th	Day	55% 9% 19% 13% 4	1528
	11th to 20th	Day	62% 12% 15% 9% 2	1928
	21st to end	Day	54% 10% 17% 15% 5%	2794
January	1st to 10th	Night	61% 10% 16% 11% 1	4406
	11th to 20th	Night	67% 8% 12% 11% 2	4420
	21st to end	Night	59% 11% 15% 12% 3	4862
February	1st to 10th	Night	53% 8% 16% 19% 4 1	2519
	11th to 20th	Night	47% 9% 16% 20% 9% 1	2219
	21st to end	Night	51% 9% 12% 17% 10% 1	1884
March	1st to 10th	Night	46% 7% 15% 19% 12% 1	3736
	11th to 20th	Night	35% 5% 7% 28% 23% 1	3632
	21st to end	Night	44% 11% 14% 23% 7%	3622
April	1st to 10th	Night	40% 5% 12% 25% 16% 3	1900
	11th to 20th	Night	40% 6% 10% 23% 19% 3	1839
	21st to end	Night	33% 5% 11% 26% 23% 2	1710
May	1st to 10th	Night	32% 5% 10% 24% 24% 4	2860
	11th to 20th	Night	22% 4 8% 23% 37% 7%	2649
	21st to end	Night	31% 6% 12% 28% 22% 3	2762
June	1st to 10th	Night	35% 8% 12% 24% 20% 1	1520
	11th to 20th	Night	27% 5% 9% 20% 30% 9%	1520
	21st to end	Night	19% 3 6% 18% 31% 22%	1520
July	1st to 10th	Night	9% 2 4 7% 19% 59%	2500
	11th to 20th	Night	10% 1 4 6% 17% 62%	2492
	21st to end	Night	18% 2 5% 4 28% 44%	2674
August	1st to 10th	Night	25% 2 7% 8% 21% 36%	2666
	11th to 20th	Night	11% 2 5% 9% 27% 45%	2838
	21st to end	Night	16% 2 3 12% 33% 35%	3488
September	1st to 10th	Night	13% 3 5% 13% 30% 37%	1900
	11th to 20th	Night	22% 4 7% 18% 26% 23%	2063
	21st to end	Night	21% 3 5% 20% 32% 18%	2090
October	1st to 10th	Night	20% 3 6% 18% 30% 23%	3490
	11th to 20th	Night	27% 3 10% 25% 29% 6%	3908
	21st to end	Night	36% 4 10% 27% 20% 2	4457
November	1st to 10th	Night	41% 6% 12% 22% 18% 1	2535
	11th to 20th	Night	48% 6% 13% 24% 9%	2600
	21st to end	Night	44% 6% 17% 23% 9% 1	2658
December	1st to 10th	Night	49% 10% 17% 15% 7%	2686
	11th to 20th	Night	59% 7% 16% 14% 3	3276
	21st to end	Night	49% 12% 20% 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	23% 11% 19% 27% 16% 4	180
	11th to 20th	Day	18% 9% 19% 30% 17% 6%	180
	21st to end	Day	20% 5% 20% 33% 16% 7%	198
February	1st to 10th	Day	24% 9% 29% 19% 15% 4	277
	11th to 20th	Day	20% 10% 27% 27% 13% 2	455
	21st to end	Day	18% 15% 26% 28% 12% 1	382
March	1st to 10th	Day	19% 13% 31% 27% 10% 1	460
	11th to 20th	Day	15% 10% 28% 27% 16% 2	460
	21st to end	Day	15% 16% 29% 26% 13% 1	506
April	1st to 10th	Day	15% 19% 23% 22% 17% 4	460
	11th to 20th	Day	12% 17% 30% 25% 13% 4	452
	21st to end	Day	10% 16% 29% 26% 16% 3	460
May	1st to 10th	Day	8% 16% 29% 26% 18% 2	463
	11th to 20th	Day	16% 9% 25% 28% 19% 3	488
	21st to end	Day	10% 9% 16% 33% 24% 8%	529
June	1st to 10th	Day	9% 7% 22% 33% 23% 7%	490
	11th to 20th	Day	8% 12% 26% 34% 17% 3	490
	21st to end	Day	4 6% 25% 34% 26% 6%	490
July	1st to 10th	Day	3 4 18% 42% 28% 5%	486
	11th to 20th	Day	1 3 20% 43% 29% 4	484
	21st to end	Day	2 2 15% 41% 31% 8%	531
August	1st to 10th	Day	4 4 18% 40% 29% 6%	487
	11th to 20th	Day	3 1 19% 31% 36% 10%	477
	21st to end	Day	3 6% 17% 33% 32% 11%	539
September	1st to 10th	Day	5% 3 14% 30% 36% 12%	490
	11th to 20th	Day	7% 5% 14% 33% 33% 8%	490
	21st to end	Day	7% 4 20% 31% 32% 6%	490
October	1st to 10th	Day	7% 6% 19% 27% 33% 8%	490
	11th to 20th	Day	11% 7% 20% 25% 28% 8%	386
	21st to end	Day	13% 12% 23% 30% 19% 4	330
November	1st to 10th	Day	16% 8% 22% 31% 21% 2	300
	11th to 20th	Day	18% 6% 22% 30% 20% 4	300
	21st to end	Day	16% 5% 20% 34% 21% 5%	270
December	1st to 10th	Day	25% 7% 16% 27% 16% 9%	182
	11th to 20th	Day	22% 7% 26% 27% 17% 1	180
	21st to end	Day	18% 9% 23% 31% 17% 2	198
January	1st to 10th	Night	23% 7% 19% 29% 19% 3	760
	11th to 20th	Night	22% 6% 16% 31% 21% 3	760
	21st to end	Night	22% 3 15% 34% 23% 3	836
February	1st to 10th	Night	22% 7% 19% 35% 15% 2	760
	11th to 20th	Night	20% 3 21% 36% 18% 2	744
	21st to end	Night	19% 8% 21% 35% 15% 2	628
March	1st to 10th	Night	18% 7% 25% 33% 15% 2	760
	11th to 20th	Night	17% 6% 20% 37% 20% 1	760
	21st to end	Night	16% 8% 25% 37% 13% 2	836
April	1st to 10th	Night	16% 6% 20% 38% 18% 2	760
	11th to 20th	Night	13% 7% 26% 36% 16% 2	711
	21st to end	Night	10% 7% 22% 42% 17% 2	570
May	1st to 10th	Night	8% 7% 22% 41% 20% 1	570
	11th to 20th	Night	10% 4 20% 34% 29% 3	455
	21st to end	Night	11% 5% 16% 34% 31% 3	411
June	1st to 10th	Night	11% 4 17% 41% 24% 3	380
	11th to 20th	Night	9% 5% 23% 38% 24% 2	380
	21st to end	Night	3 1 17% 48% 30% 1	380
July	1st to 10th	Night	1 2 16% 46% 34% 1	370
	11th to 20th	Night	2 1 13% 58% 26%	368
	21st to end	Night	3 11% 49% 35% 1	416
August	1st to 10th	Night	3 1 11% 51% 32% 2	458
	11th to 20th	Night	1 1 11% 47% 38% 2	564
	21st to end	Night	4 2 12% 45% 33% 5%	826
September	1st to 10th	Night	4 1 9% 46% 38% 3	760
	11th to 20th	Night	6% 2 10% 39% 37% 6%	760
	21st to end	Night	8% 2 15% 38% 30% 7%	760
October	1st to 10th	Night	8% 2 15% 45% 27% 3	760
	11th to 20th	Night	11% 3 14% 39% 28% 5%	760
	21st to end	Night	14% 3 18% 40% 22% 4	836
November	1st to 10th	Night	18% 4 19% 35% 22% 2	780
	11th to 20th	Night	21% 5% 20% 36% 17% 2	800
	21st to end	Night	19% 3 17% 37% 19% 4	800
December	1st to 10th	Night	21% 4 18% 36% 18% 3	768
	11th to 20th	Night	25% 5% 16% 36% 16% 2	760
	21st to end	Night	29% 7% 19% 33% 11% 2	836

Page generated at 2024-11-29 08:43:06. Version ac07

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