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 2025-07-06.

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 5% 7% 44%	120119
	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	118896
Transparency	Day	38% 6% 10% 20% 19% 7%	123771
	Night	35% 5% 10% 19% 21% 10%	123361
Seeing	Day	15% 9% 24% 27% 20% 6%	15936
	Night	16% 9% 25% 31% 17% 2	25773

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	17% 10% 4 5% 3 2 3 2 3 4 46%	4077
	11th to 20th	Day	15% 6% 2 4 2 2 3 2 5% 6% 53%	4080
	21st to end	Day	26% 11% 2 4 3 2 2 1 2 5% 42%	4488
February	1st to 10th	Day	22% 9% 3 6% 4 4 3 3 4 6% 38%	1800
	11th to 20th	Day	21% 10% 3 5% 4 3 3 3 4 4 39%	1954
	21st to end	Day	21% 8% 3 7% 4 4 3 2 3 4 41%	1815
March	1st to 10th	Day	19% 12% 3 7% 4 3 3 2 3 5% 38%	4936
	11th to 20th	Day	14% 6% 2 5% 3 3 4 3 4 6% 49%	4980
	21st to end	Day	16% 9% 2 6% 4 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	14% 7% 3 7% 6% 4 3 3 5% 6% 42%	2232
	11th to 20th	Day	15% 7% 3 6% 4 4 4 5% 5% 7% 40%	2374
	21st to end	Day	11% 6% 3 7% 5% 4 4 4 5% 6% 44%	2400
May	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 6% 7% 34%	5533
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 5% 6% 40%	5862
	21st to end	Day	10% 6% 2 7% 6% 6% 5% 5% 7% 10% 36%	6434
June	1st to 10th	Day	7% 6% 2 6% 6% 5% 6% 5% 8% 9% 40%	2600
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 5% 8% 11% 43%	2600
	21st to end	Day	6% 5% 2 6% 6% 6% 6% 6% 8% 10% 40%	2600
July	1st to 10th	Day	8% 6% 2 6% 6% 6% 6% 5% 8% 11% 37%	4186
	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 6% 3 4 4 4 5% 5% 48%	1958
	11th to 20th	Day	11% 6% 2 6% 4 3 2 3 5% 5% 53%	2000
	21st to end	Day	17% 7% 2 6% 4 3 3 2 4 5% 45%	1917
December	1st to 10th	Day	21% 5% 1 5% 4 3 3 2 4 5% 46%	2940
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 5% 40%	3391
	21st to end	Day	17% 8% 3 7% 4 3 3 2 4 4 44%	4488
January	1st to 10th	Night	16% 6% 2 5% 3 2 3 2 4 4 53%	5888
	11th to 20th	Night	14% 6% 2 5% 2 2 2 2 4 5% 55%	5900
	21st to end	Night	26% 11% 2 5% 3 2 2 2 3 4 39%	6490
February	1st to 10th	Night	18% 7% 3 6% 4 3 3 3 5% 6% 44%	2600
	11th to 20th	Night	21% 9% 3 6% 3 3 2 2 3 5% 43%	2348
	21st to end	Night	21% 7% 2 5% 3 2 4 3 4 5% 44%	1815
March	1st to 10th	Night	22% 9% 2 5% 3 3 3 3 4 6% 41%	4919
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 5% 7% 49%	4980
	21st to end	Night	17% 8% 3 7% 4 4 3 3 3 5% 43%	5467
April	1st to 10th	Night	13% 5% 3 5% 4 4 5% 4 5% 6% 45%	2165
	11th to 20th	Night	14% 5% 3 7% 4 4 4 3 4 7% 45%	1975
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	2000
May	1st to 10th	Night	14% 9% 3 9% 5% 4 5% 3 5% 8% 35%	4363
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 4 6% 7% 40%	4053
	21st to end	Night	10% 6% 2 7% 4 4 5% 4 7% 11% 41%	4443
June	1st to 10th	Night	5% 4 2 4 3 3 5% 5% 6% 11% 52%	1800
	11th to 20th	Night	3 3 1 3 3 3 4 4 6% 11% 60%	1800
	21st to end	Night	6% 4 2 5% 3 5% 4 4 7% 11% 50%	1800
July	1st to 10th	Night	8% 4 1 5% 5% 5% 5% 5% 8% 11% 42%	2895
	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	14% 5% 1 4 3 3 3 3 4 6% 54%	2340
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 54%	2400
	21st to end	Night	15% 7% 2 5% 4 3 4 3 5% 4 48%	2479
December	1st to 10th	Night	19% 7% 3 5% 3 4 3 3 4 5% 45%	4250
	11th to 20th	Night	17% 9% 2 6% 4 4 3 3 3 4 44%	4816
	21st to end	Night	14% 9% 2 5% 3 3 3 3 3 4 50%	6490

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% 3 5% 24% 21% 3	4078
	11th to 20th	Day	33% 2 4 22% 26% 11%	4080
	21st to end	Day	49% 2 3 14% 18% 13%	4488
February	1st to 10th	Day	49% 5% 6% 12% 18% 10%	1800
	11th to 20th	Day	50% 4 6% 14% 18% 8%	1964
	21st to end	Day	48% 5% 6% 13% 18% 11%	1815
March	1st to 10th	Day	50% 4 5% 10% 19% 11%	4936
	11th to 20th	Day	35% 5% 6% 14% 21% 18%	4980
	21st to end	Day	44% 4 9% 20% 17% 5%	5464
April	1st to 10th	Day	44% 6% 8% 18% 17% 8%	2232
	11th to 20th	Day	42% 7% 10% 15% 18% 8%	2374
	21st to end	Day	39% 7% 9% 18% 23% 4	2400
May	1st to 10th	Day	47% 7% 11% 22% 11% 2	5568
	11th to 20th	Day	43% 6% 13% 17% 20% 2	5866
	21st to end	Day	40% 10% 12% 21% 16% 2	6447
June	1st to 10th	Day	36% 10% 14% 24% 14% 1	2600
	11th to 20th	Day	32% 9% 15% 26% 15% 3	2600
	21st to end	Day	34% 11% 16% 26% 12% 2	2600
July	1st to 10th	Day	39% 9% 23% 24% 6%	4122
	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	15% 4 6% 12% 23% 41%	3178
	11th to 20th	Day	20% 3 5% 20% 29% 24%	3140
	21st to end	Day	28% 4 4 16% 36% 13%	3428
November	1st to 10th	Day	37% 5% 9% 17% 24% 9%	1958
	11th to 20th	Day	34% 4 7% 18% 25% 12%	2000
	21st to end	Day	41% 5% 7% 17% 23% 7%	1917
December	1st to 10th	Day	42% 3 7% 17% 25% 6%	2940
	11th to 20th	Day	46% 4 6% 13% 21% 11%	3389
	21st to end	Day	44% 3 5% 14% 28% 6%	4488
January	1st to 10th	Night	36% 3 6% 24% 26% 5%	5888
	11th to 20th	Night	33% 3 4 18% 32% 11%	5900
	21st to end	Night	51% 3 4 13% 16% 13%	6490
February	1st to 10th	Night	43% 4 7% 15% 18% 14%	2600
	11th to 20th	Night	46% 3 6% 15% 20% 9%	2348
	21st to end	Night	44% 4 7% 13% 17% 14%	1815
March	1st to 10th	Night	45% 4 5% 12% 21% 13%	4919
	11th to 20th	Night	36% 6% 6% 14% 18% 20%	4980
	21st to end	Night	44% 5% 8% 16% 20% 7%	5467
April	1st to 10th	Night	38% 7% 10% 17% 17% 11%	2165
	11th to 20th	Night	38% 5% 10% 18% 17% 12%	1975
	21st to end	Night	36% 5% 10% 18% 24% 6%	2000
May	1st to 10th	Night	46% 7% 12% 20% 12% 3	4393
	11th to 20th	Night	42% 6% 13% 16% 20% 2	4054
	21st to end	Night	36% 8% 17% 24% 12% 3	4452
June	1st to 10th	Night	25% 9% 19% 28% 18% 1	1800
	11th to 20th	Night	19% 7% 18% 33% 18% 4	1800
	21st to end	Night	27% 8% 17% 26% 19% 2	1800
July	1st to 10th	Night	33% 10% 28% 24% 5%	2850
	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	14% 4 5% 14% 20% 43%	3736
	11th to 20th	Night	17% 3 4 18% 32% 27%	3779
	21st to end	Night	25% 3 4 15% 37% 16%	4120
November	1st to 10th	Night	32% 5% 9% 19% 24% 11%	2340
	11th to 20th	Night	31% 5% 8% 17% 24% 15%	2400
	21st to end	Night	39% 5% 7% 15% 22% 11%	2479
December	1st to 10th	Night	43% 3 6% 14% 28% 5%	4250
	11th to 20th	Night	45% 3 7% 12% 22% 11%	4817
	21st to end	Night	38% 4 6% 14% 30% 7%	6490

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% 38% 14% 3	300
	11th to 20th	Day	29% 17% 35% 15% 4	300
	21st to end	Day	29% 21% 34% 14% 2	330
February	1st to 10th	Day	28% 21% 34% 16% 2	300
	11th to 20th	Day	37% 21% 31% 11%	300
	21st to end	Day	30% 24% 37% 9% 1	249
March	1st to 10th	Day	30% 21% 29% 17% 2	300
	11th to 20th	Day	24% 27% 35% 11% 2	300
	21st to end	Day	22% 19% 41% 15% 3	330
April	1st to 10th	Day	26% 16% 40% 15% 2	332
	11th to 20th	Day	24% 20% 27% 21% 8% 1	492
	21st to end	Day	16% 16% 36% 22% 8% 2	500
May	1st to 10th	Day	16% 12% 40% 26% 6%	553
	11th to 20th	Day	16% 11% 28% 30% 14% 2	725
	21st to end	Day	12% 11% 29% 31% 14% 4	789
June	1st to 10th	Day	8% 2 21% 37% 25% 6%	730
	11th to 20th	Day	6% 2 22% 40% 24% 5%	730
	21st to end	Day	7% 3 14% 36% 29% 11%	730
July	1st to 10th	Day	8% 1 13% 30% 36% 11%	697
	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% 29% 22% 12% 3	330
November	1st to 10th	Day	17% 8% 31% 30% 13% 1	300
	11th to 20th	Day	15% 14% 35% 28% 6% 1	300
	21st to end	Day	22% 17% 34% 20% 6% 1	300
December	1st to 10th	Day	19% 15% 36% 20% 7% 3	300
	11th to 20th	Day	26% 13% 36% 18% 6%	300
	21st to end	Day	26% 20% 35% 14% 4 1	330
January	1st to 10th	Night	23% 14% 32% 26% 5%	1000
	11th to 20th	Night	24% 17% 29% 24% 6%	1000
	21st to end	Night	24% 13% 33% 25% 4	1100
February	1st to 10th	Night	24% 15% 34% 24% 3	1000
	11th to 20th	Night	29% 16% 32% 21% 3	793
	21st to end	Night	25% 17% 35% 20% 3	495
March	1st to 10th	Night	25% 14% 34% 23% 5%	600
	11th to 20th	Night	18% 18% 34% 26% 4 1	600
	21st to end	Night	17% 13% 40% 24% 6%	660
April	1st to 10th	Night	18% 14% 39% 23% 6%	600
	11th to 20th	Night	18% 14% 36% 21% 10%	593
	21st to end	Night	17% 11% 38% 25% 10%	600
May	1st to 10th	Night	12% 11% 40% 28% 9%	600
	11th to 20th	Night	15% 9% 30% 32% 13% 1	591
	21st to end	Night	10% 5% 30% 35% 17% 3	650
June	1st to 10th	Night	7% 3 16% 41% 31% 3	600
	11th to 20th	Night	3 2 19% 50% 24% 2	600
	21st to end	Night	7% 1 15% 43% 30% 6%	600
July	1st to 10th	Night	6% 1 13% 39% 35% 6%	570
	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	14% 7% 25% 36% 17% 1	780
	11th to 20th	Night	12% 10% 29% 38% 10% 1	800
	21st to end	Night	19% 14% 27% 27% 12%	879
December	1st to 10th	Night	22% 13% 29% 29% 8%	1000
	11th to 20th	Night	23% 11% 32% 29% 5%	1000
	21st to end	Night	22% 13% 30% 28% 6%	1100

Page generated at 2025-07-03 00:30:18. Version ac07

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