SFA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SFA Observatory clear sky chart from 2005-11-06 to 2024-11-03.

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	13% 7% 2 6% 4 4 4 4 6% 7% 43%	101064
	Night	13% 6% 2 5% 4 3 4 3 5% 7% 48%	98007
Transparency	Day	39% 7% 11% 20% 18% 5%	100875
	Night	35% 6% 11% 20% 20% 8%	97867
Seeing	Day	14% 9% 23% 27% 21% 6%	15053
	Night	15% 9% 25% 31% 17% 2	24854

By Month

Percentage Cloud Cover SE (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	18% 10% 3 5% 3 3 3 3 4 5% 43%	2847
	11th to 20th	Day	21% 6% 2 4 2 2 2 2 4 5% 49%	2850
	21st to end	Day	24% 9% 2 4 3 2 2 2 3 6% 42%	3135
February	1st to 10th	Day	22% 9% 3 6% 4 3 3 3 4 6% 37%	1710
	11th to 20th	Day	21% 10% 3 5% 4 3 4 3 4 4 39%	1853
	21st to end	Day	21% 7% 3 6% 4 4 3 2 3 4 42%	1727
March	1st to 10th	Day	20% 13% 3 8% 5% 4 4 3 4 5% 32%	3436
	11th to 20th	Day	20% 8% 2 6% 4 4 5% 4 5% 7% 34%	3480
	21st to end	Day	17% 10% 3 6% 5% 4 4 3 5% 5% 38%	3814
April	1st to 10th	Day	14% 7% 3 7% 6% 4 3 3 5% 6% 42%	2120
	11th to 20th	Day	15% 7% 3 6% 4 4 4 5% 5% 7% 39%	2254
	21st to end	Day	12% 6% 3 7% 5% 4 4 4 5% 7% 44%	2280
May	1st to 10th	Day	19% 8% 3 8% 5% 4 5% 4 6% 7% 31%	3851
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 4 5% 44%	4082
	21st to end	Day	12% 6% 2 8% 6% 6% 5% 5% 8% 10% 33%	4477
June	1st to 10th	Day	7% 5% 2 6% 6% 5% 6% 5% 8% 9% 40%	2470
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 11% 44%	2470
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 6% 8% 10% 40%	2470
July	1st to 10th	Day	8% 6% 2 6% 6% 6% 6% 5% 8% 11% 37%	4120
	11th to 20th	Day	7% 6% 3 6% 5% 5% 6% 5% 7% 10% 42%	4120
	21st to end	Day	15% 9% 3 8% 5% 5% 5% 4 7% 8% 32%	4465
August	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 7% 68%	4104
	11th to 20th	Day	3 3 1 4 3 4 4 4 8% 11% 56%	3861
	21st to end	Day	8% 4 2 6% 5% 6% 7% 6% 8% 9% 39%	4180
September	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 5% 6% 8% 44%	2194
	11th to 20th	Day	12% 5% 2 7% 5% 4 6% 4 6% 7% 42%	2090
	21st to end	Day	7% 4 2 5% 4 4 4 4 6% 6% 53%	2090
October	1st to 10th	Day	7% 4 1 4 2 3 4 3 5% 6% 61%	1918
	11th to 20th	Day	12% 6% 2 4 3 2 4 3 4 6% 54%	1900
	21st to end	Day	15% 5% 2 5% 3 3 3 2 3 5% 54%	2090
November	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 5% 5% 50%	1878
	11th to 20th	Day	11% 6% 2 6% 4 3 2 3 5% 5% 52%	1900
	21st to end	Day	18% 7% 2 6% 4 3 3 2 4 5% 44%	1822
December	1st to 10th	Day	17% 7% 2 6% 4 3 3 3 5% 6% 44%	1710
	11th to 20th	Day	18% 8% 3 7% 4 4 4 3 4 5% 41%	2161
	21st to end	Day	17% 9% 4 8% 4 3 4 3 4 5% 41%	3135
January	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 3 5% 50%	4108
	11th to 20th	Night	19% 7% 2 5% 2 2 2 2 3 4 52%	4120
	21st to end	Night	23% 11% 2 5% 3 2 2 2 4 4 41%	4532
February	1st to 10th	Night	17% 7% 3 5% 4 3 4 3 5% 6% 44%	2470
	11th to 20th	Night	21% 8% 2 6% 4 3 2 3 3 5% 44%	2229
	21st to end	Night	22% 7% 2 5% 3 2 4 3 4 5% 44%	1727
March	1st to 10th	Night	23% 11% 2 6% 4 3 3 3 5% 6% 35%	3419
	11th to 20th	Night	19% 8% 3 7% 4 4 4 3 5% 6% 36%	3480
	21st to end	Night	14% 7% 3 7% 4 4 4 4 4 6% 44%	3817
April	1st to 10th	Night	12% 5% 3 5% 4 4 5% 4 5% 7% 45%	2057
	11th to 20th	Night	14% 5% 3 7% 4 4 4 3 4 7% 45%	1875
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 6% 51%	1900
May	1st to 10th	Night	15% 9% 3 7% 4 4 4 3 5% 9% 36%	3042
	11th to 20th	Night	14% 8% 2 6% 4 4 3 3 5% 6% 45%	2823
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 7% 10% 41%	3090
June	1st to 10th	Night	5% 4 2 4 3 3 5% 5% 6% 11% 52%	1710
	11th to 20th	Night	2 3 1 3 3 3 4 4 6% 11% 61%	1710
	21st to end	Night	6% 4 2 5% 3 5% 4 4 7% 10% 50%	1710
July	1st to 10th	Night	8% 4 1 5% 4 5% 5% 5% 8% 11% 43%	2850
	11th to 20th	Night	3 4 1 5% 5% 4 5% 4 8% 11% 51%	2850
	21st to end	Night	13% 7% 3 6% 4 4 4 4 5% 10% 39%	3098
August	1st to 10th	Night	2 1 2 1 2 3 2 5% 8% 73%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 4 7% 12% 59%	2969
	21st to end	Night	6% 4 1 4 5% 5% 6% 6% 8% 10% 46%	3476
September	1st to 10th	Night	7% 5% 2 6% 4 4 4 3 4 8% 51%	1981
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	2090
	21st to end	Night	7% 4 1 5% 3 3 4 3 5% 7% 59%	2090
October	1st to 10th	Night	5% 3 1 4 3 3 4 3 4 7% 63%	2242
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 4 5% 60%	2280
	21st to end	Night	15% 4 1 4 2 3 2 2 3 4 59%	2508
November	1st to 10th	Night	13% 4 1 4 3 3 3 3 4 6% 56%	2245
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 53%	2280
	21st to end	Night	15% 7% 2 5% 4 3 4 3 5% 4 46%	2355
December	1st to 10th	Night	17% 8% 2 4 3 4 4 3 5% 6% 45%	2470
	11th to 20th	Night	16% 8% 3 6% 4 3 3 2 3 5% 46%	3036
	21st to end	Night	16% 8% 2 5% 4 3 3 3 4 4 48%	4532

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	43% 5% 6% 17% 25% 5%	2848
	11th to 20th	Day	38% 3 6% 23% 25% 5%	2850
	21st to end	Day	46% 2 5% 17% 20% 10%	3135
February	1st to 10th	Day	50% 5% 6% 13% 17% 10%	1710
	11th to 20th	Day	49% 4 6% 14% 19% 8%	1863
	21st to end	Day	47% 5% 6% 13% 18% 11%	1727
March	1st to 10th	Day	54% 6% 8% 12% 17% 5%	3436
	11th to 20th	Day	46% 7% 8% 18% 16% 5%	3480
	21st to end	Day	47% 5% 9% 16% 21% 3	3814
April	1st to 10th	Day	43% 6% 8% 18% 17% 8%	2120
	11th to 20th	Day	42% 7% 10% 16% 17% 8%	2254
	21st to end	Day	38% 8% 9% 17% 24% 4	2280
May	1st to 10th	Day	49% 9% 12% 17% 10% 3	3886
	11th to 20th	Day	40% 7% 10% 19% 24% 1	4086
	21st to end	Day	41% 12% 15% 20% 10% 1	4489
June	1st to 10th	Day	36% 10% 14% 24% 14% 1	2470
	11th to 20th	Day	31% 10% 15% 26% 15% 3	2470
	21st to end	Day	34% 11% 15% 25% 13% 2	2470
July	1st to 10th	Day	38% 9% 23% 24% 5%	4056
	11th to 20th	Day	35% 8% 16% 28% 13%	4051
	21st to end	Day	48% 8% 14% 22% 8%	4325
August	1st to 10th	Day	15% 6% 15% 32% 31% 1	4090
	11th to 20th	Day	21% 6% 21% 41% 10% 1	3900
	21st to end	Day	38% 9% 18% 19% 16% 1	4179
September	1st to 10th	Day	36% 9% 14% 18% 15% 9%	2194
	11th to 20th	Day	38% 8% 13% 16% 17% 7%	2090
	21st to end	Day	29% 6% 11% 20% 21% 13%	2090
October	1st to 10th	Day	24% 6% 10% 19% 24% 17%	1918
	11th to 20th	Day	31% 4 7% 17% 24% 16%	1900
	21st to end	Day	33% 4 5% 15% 28% 14%	2090
November	1st to 10th	Day	35% 5% 9% 17% 24% 10%	1878
	11th to 20th	Day	35% 4 7% 18% 25% 11%	1900
	21st to end	Day	43% 5% 6% 17% 22% 7%	1822
December	1st to 10th	Day	41% 5% 10% 17% 19% 8%	1710
	11th to 20th	Day	44% 4 6% 15% 22% 8%	2159
	21st to end	Day	46% 4 5% 13% 24% 7%	3135
January	1st to 10th	Night	36% 4 7% 17% 28% 7%	4108
	11th to 20th	Night	38% 4 5% 17% 27% 10%	4120
	21st to end	Night	47% 4 6% 16% 19% 10%	4532
February	1st to 10th	Night	43% 4 7% 15% 18% 13%	2470
	11th to 20th	Night	45% 4 6% 15% 20% 10%	2229
	21st to end	Night	44% 4 8% 12% 18% 14%	1727
March	1st to 10th	Night	50% 5% 7% 14% 17% 7%	3419
	11th to 20th	Night	47% 8% 8% 13% 18% 6%	3480
	21st to end	Night	40% 7% 10% 15% 23% 6%	3817
April	1st to 10th	Night	37% 7% 10% 18% 17% 11%	2057
	11th to 20th	Night	39% 5% 11% 18% 15% 12%	1875
	21st to end	Night	35% 5% 10% 18% 25% 7%	1900
May	1st to 10th	Night	43% 10% 12% 18% 13% 4	3072
	11th to 20th	Night	39% 7% 11% 18% 22% 4	2824
	21st to end	Night	35% 10% 17% 23% 12% 1	3099
June	1st to 10th	Night	25% 9% 18% 28% 18% 1	1710
	11th to 20th	Night	17% 7% 18% 34% 19% 4	1710
	21st to end	Night	27% 9% 16% 26% 20% 2	1710
July	1st to 10th	Night	32% 10% 28% 24% 5%	2805
	11th to 20th	Night	25% 9% 23% 30% 13% 1	2796
	21st to end	Night	41% 8% 19% 24% 8% 1	2999
August	1st to 10th	Night	10% 5% 17% 33% 32% 3	2826
	11th to 20th	Night	16% 7% 22% 45% 9% 2	2996
	21st to end	Night	31% 10% 18% 23% 17% 2	3476
September	1st to 10th	Night	30% 7% 16% 21% 16% 10%	1981
	11th to 20th	Night	32% 8% 14% 19% 18% 8%	2090
	21st to end	Night	25% 6% 11% 21% 25% 12%	2090
October	1st to 10th	Night	22% 5% 9% 23% 22% 19%	2242
	11th to 20th	Night	28% 4 6% 16% 26% 19%	2280
	21st to end	Night	30% 5% 6% 16% 27% 18%	2508
November	1st to 10th	Night	30% 5% 9% 20% 24% 12%	2245
	11th to 20th	Night	31% 6% 9% 18% 23% 14%	2280
	21st to end	Night	40% 5% 7% 15% 22% 11%	2355
December	1st to 10th	Night	39% 5% 10% 17% 19% 9%	2470
	11th to 20th	Night	43% 4 6% 12% 22% 13%	3037
	21st to end	Night	40% 5% 7% 14% 28% 8%	4532

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	27% 17% 37% 15% 4	270
	11th to 20th	Day	30% 16% 34% 15% 4	270
	21st to end	Day	27% 21% 35% 15% 2	297
February	1st to 10th	Day	29% 22% 31% 16% 2	270
	11th to 20th	Day	36% 21% 32% 12%	270
	21st to end	Day	30% 24% 38% 7%	225
March	1st to 10th	Day	29% 21% 31% 17% 2	270
	11th to 20th	Day	23% 26% 36% 12% 3	270
	21st to end	Day	21% 19% 40% 16% 3	297
April	1st to 10th	Day	25% 16% 42% 15% 3	300
	11th to 20th	Day	24% 19% 26% 22% 8% 1	452
	21st to end	Day	17% 15% 37% 22% 7% 2	460
May	1st to 10th	Day	16% 11% 40% 26% 6%	510
	11th to 20th	Day	15% 10% 27% 32% 15% 2	675
	21st to end	Day	11% 10% 28% 31% 14% 4	734
June	1st to 10th	Day	8% 2 19% 38% 26% 7%	680
	11th to 20th	Day	6% 2 21% 41% 24% 6%	680
	21st to end	Day	7% 3 15% 35% 29% 11%	680
July	1st to 10th	Day	7% 1 13% 31% 36% 11%	671
	11th to 20th	Day	4 1 8% 31% 44% 11%	668
	21st to end	Day	7% 1 8% 35% 38% 12%	739
August	1st to 10th	Day	3 6% 30% 48% 12%	676
	11th to 20th	Day	6% 1 4 31% 46% 13%	563
	21st to end	Day	11% 2 8% 22% 37% 20%	539
September	1st to 10th	Day	6% 2 11% 30% 38% 13%	404
	11th to 20th	Day	8% 2 14% 33% 33% 10%	300
	21st to end	Day	11% 3 12% 36% 30% 9%	300
October	1st to 10th	Day	6% 4 24% 42% 21% 5%	300
	11th to 20th	Day	11% 7% 34% 31% 13% 5%	300
	21st to end	Day	24% 10% 28% 23% 12% 3	330
November	1st to 10th	Day	15% 7% 32% 30% 14% 1	276
	11th to 20th	Day	15% 14% 37% 26% 7% 1	270
	21st to end	Day	24% 15% 35% 19% 6% 1	270
December	1st to 10th	Day	17% 15% 36% 21% 8% 3	270
	11th to 20th	Day	26% 14% 35% 19% 6%	270
	21st to end	Day	25% 20% 36% 14% 4 1	297
January	1st to 10th	Night	23% 14% 32% 25% 5%	950
	11th to 20th	Night	25% 16% 28% 25% 7%	950
	21st to end	Night	24% 14% 33% 25% 4	1045
February	1st to 10th	Night	23% 15% 33% 25% 3	950
	11th to 20th	Night	28% 15% 32% 21% 3	754
	21st to end	Night	26% 17% 35% 20% 2	471
March	1st to 10th	Night	25% 14% 34% 23% 5%	570
	11th to 20th	Night	18% 17% 34% 26% 5% 1	570
	21st to end	Night	16% 13% 41% 23% 6%	627
April	1st to 10th	Night	17% 14% 40% 22% 6%	570
	11th to 20th	Night	18% 13% 37% 21% 10%	563
	21st to end	Night	16% 11% 39% 24% 10%	570
May	1st to 10th	Night	12% 11% 41% 27% 9%	570
	11th to 20th	Night	14% 8% 30% 34% 14% 1	561
	21st to end	Night	10% 5% 30% 34% 18% 3	617
June	1st to 10th	Night	7% 2 15% 42% 32% 3	570
	11th to 20th	Night	2 2 19% 50% 25% 2	570
	21st to end	Night	7% 1 15% 42% 30% 6%	570
July	1st to 10th	Night	6% 1 13% 39% 35% 6%	555
	11th to 20th	Night	3 1 9% 39% 43% 5%	552
	21st to end	Night	5% 1 6% 44% 36% 7%	624
August	1st to 10th	Night	3 1 5% 45% 42% 6%	567
	11th to 20th	Night	5% 1 6% 44% 38% 7%	564
	21st to end	Night	8% 2 6% 35% 43% 7%	627
September	1st to 10th	Night	9% 1 8% 36% 40% 5%	570
	11th to 20th	Night	12% 2 11% 29% 41% 5%	570
	21st to end	Night	7% 3 12% 36% 35% 7%	570
October	1st to 10th	Night	6% 2 16% 41% 30% 5%	722
	11th to 20th	Night	13% 6% 23% 39% 17% 3	760
	21st to end	Night	16% 7% 26% 31% 18% 2	836
November	1st to 10th	Night	13% 7% 26% 36% 18% 1	749
	11th to 20th	Night	12% 10% 29% 38% 10% 1	760
	21st to end	Night	19% 14% 27% 27% 13%	835
December	1st to 10th	Night	21% 13% 29% 29% 8%	950
	11th to 20th	Night	23% 11% 32% 29% 5%	950
	21st to end	Night	22% 13% 30% 27% 6%	1045

Page generated at 2024-10-31 02:39:24. Version ac07

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