St. F.X. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. F.X. Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	17% 12% 4 10% 6% 5% 5% 4 5% 6% 26%	127143
	Night	17% 12% 4 11% 6% 5% 5% 4 5% 6% 25%	122779
Transparency	Day	56% 7% 12% 17% 7% 1	130652
	Night	57% 7% 12% 16% 7% 1	127364
Seeing	Day	25% 18% 31% 20% 6% 1	18903
	Night	25% 17% 31% 21% 5%	26367

By Month

Percentage Cloud Cover NE (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	10% 18% 9% 23% 7% 5% 5% 3 4 3 13%	3616
	11th to 20th	Day	11% 15% 11% 16% 6% 6% 4 4 4 5% 18%	3591
	21st to end	Day	12% 13% 5% 13% 7% 6% 6% 5% 6% 5% 22%	4269
February	1st to 10th	Day	17% 14% 5% 12% 8% 7% 4 3 5% 5% 21%	1800
	11th to 20th	Day	14% 16% 5% 10% 8% 7% 4 3 5% 5% 22%	1753
	21st to end	Day	10% 17% 6% 9% 7% 5% 6% 4 5% 6% 26%	1593
March	1st to 10th	Day	20% 14% 4 10% 7% 6% 4 3 4 4 24%	4630
	11th to 20th	Day	10% 18% 7% 11% 7% 5% 5% 3 5% 6% 22%	4980
	21st to end	Day	21% 19% 7% 13% 6% 4 4 3 4 4 16%	5649
April	1st to 10th	Day	19% 15% 6% 9% 5% 4 4 3 5% 5% 25%	2400
	11th to 20th	Day	20% 14% 4 7% 5% 4 3 3 4 7% 29%	2374
	21st to end	Day	22% 16% 4 8% 4 4 3 4 5% 6% 26%	2569
May	1st to 10th	Day	25% 19% 3 7% 4 4 3 3 5% 6% 21%	6343
	11th to 20th	Day	20% 12% 3 7% 4 4 4 4 6% 6% 30%	6318
	21st to end	Day	18% 11% 3 7% 5% 5% 5% 4 6% 7% 30%	6948
June	1st to 10th	Day	27% 12% 3 7% 5% 4 4 4 5% 7% 23%	2800
	11th to 20th	Day	21% 9% 2 7% 5% 4 4 3 5% 7% 33%	2800
	21st to end	Day	23% 11% 4 9% 6% 4 4 4 5% 6% 25%	2800
July	1st to 10th	Day	17% 9% 3 8% 6% 5% 6% 5% 7% 8% 26%	6360
	11th to 20th	Day	12% 6% 3 7% 6% 5% 6% 4 7% 9% 36%	6360
	21st to end	Day	12% 6% 2 7% 5% 5% 6% 4 8% 9% 37%	6927
August	1st to 10th	Day	16% 9% 3 8% 5% 6% 5% 5% 6% 7% 29%	4377
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 7% 7% 32%	4175
	21st to end	Day	11% 6% 3 6% 4 5% 6% 4 6% 7% 42%	4358
September	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 5% 7% 37%	2400
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 6% 7% 35%	2400
	21st to end	Day	18% 8% 3 6% 5% 5% 5% 4 6% 7% 36%	2106
October	1st to 10th	Day	18% 9% 2 9% 5% 5% 5% 5% 5% 7% 31%	2000
	11th to 20th	Day	20% 10% 3 10% 5% 5% 4 4 5% 8% 25%	1980
	21st to end	Day	20% 14% 4 10% 6% 5% 4 3 4 7% 22%	1989
November	1st to 10th	Day	20% 14% 5% 10% 6% 4 4 4 6% 6% 22%	1690
	11th to 20th	Day	19% 20% 6% 11% 7% 4 4 4 4 5% 17%	1600
	21st to end	Day	15% 20% 7% 13% 6% 6% 5% 4 5% 4 15%	1600
December	1st to 10th	Day	14% 15% 6% 14% 7% 7% 5% 3 4 5% 18%	2610
	11th to 20th	Day	25% 16% 5% 16% 6% 6% 4 2 4 4 13%	2996
	21st to end	Day	18% 12% 11% 19% 5% 5% 4 4 4 4 14%	3982
January	1st to 10th	Night	12% 15% 8% 21% 8% 6% 5% 3 4 5% 13%	6349
	11th to 20th	Night	14% 14% 7% 18% 7% 6% 5% 4 4 4 16%	6314
	21st to end	Night	10% 10% 6% 18% 9% 7% 7% 4 5% 5% 20%	6692
February	1st to 10th	Night	18% 13% 5% 12% 8% 6% 5% 3 5% 4 21%	2600
	11th to 20th	Night	14% 14% 5% 11% 9% 7% 6% 5% 4 5% 20%	2533
	21st to end	Night	11% 15% 5% 12% 7% 5% 5% 3 5% 5% 27%	2035
March	1st to 10th	Night	21% 15% 5% 10% 7% 5% 4 3 4 5% 21%	5192
	11th to 20th	Night	18% 15% 6% 8% 6% 5% 4 3 5% 6% 25%	4979
	21st to end	Night	25% 16% 5% 12% 5% 4 3 3 3 4 20%	5283
April	1st to 10th	Night	21% 15% 4 7% 6% 6% 5% 3 4 6% 24%	2000
	11th to 20th	Night	18% 13% 3 8% 5% 4 4 3 5% 6% 30%	1976
	21st to end	Night	22% 13% 4 7% 5% 4 5% 3 4 5% 27%	1830
May	1st to 10th	Night	26% 21% 3 6% 4 4 3 3 4 6% 19%	3615
	11th to 20th	Night	18% 14% 4 7% 4 4 5% 4 6% 6% 29%	3596
	21st to end	Night	14% 12% 4 8% 5% 5% 5% 4 6% 7% 30%	3948
June	1st to 10th	Night	26% 9% 3 8% 5% 4 5% 2 5% 6% 26%	1600
	11th to 20th	Night	21% 8% 2 6% 5% 4 4 3 5% 8% 35%	1600
	21st to end	Night	22% 9% 3 8% 6% 4 5% 4 5% 8% 28%	1600
July	1st to 10th	Night	14% 8% 4 10% 6% 7% 6% 5% 8% 9% 24%	3619
	11th to 20th	Night	12% 8% 3 6% 4 4 5% 4 6% 10% 39%	3620
	21st to end	Night	15% 7% 3 7% 5% 4 5% 4 7% 8% 36%	3948
August	1st to 10th	Night	13% 8% 3 9% 6% 6% 5% 4 6% 7% 33%	2774
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	2872
	21st to end	Night	12% 6% 2 6% 4 4 4 4 7% 9% 42%	3538
September	1st to 10th	Night	15% 7% 3 7% 5% 4 5% 4 6% 8% 38%	2000
	11th to 20th	Night	17% 8% 2 6% 4 4 3 3 6% 8% 39%	2000
	21st to end	Night	18% 8% 2 6% 4 4 4 4 5% 7% 38%	2289
October	1st to 10th	Night	19% 8% 2 7% 5% 4 4 4 6% 7% 36%	2400
	11th to 20th	Night	19% 8% 3 8% 5% 4 5% 3 5% 8% 32%	2377
	21st to end	Night	24% 13% 3 8% 6% 4 5% 3 4 5% 25%	2608
November	1st to 10th	Night	17% 13% 4 10% 6% 5% 5% 4 5% 6% 24%	2603
	11th to 20th	Night	19% 16% 5% 11% 6% 5% 5% 4 5% 6% 19%	2800
	21st to end	Night	21% 16% 5% 10% 7% 5% 5% 4 5% 6% 17%	2800
December	1st to 10th	Night	15% 12% 5% 13% 7% 7% 6% 3 5% 5% 22%	4580
	11th to 20th	Night	23% 14% 4 11% 7% 6% 5% 3 4 5% 20%	5213
	21st to end	Night	23% 14% 5% 14% 6% 5% 4 3 4 4 17%	6996

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	75% 5% 10% 6% 3	3617
	11th to 20th	Day	64% 8% 13% 9% 5%	3620
	21st to end	Day	57% 8% 9% 14% 10% 3	4269
February	1st to 10th	Day	65% 5% 11% 11% 7% 1	1800
	11th to 20th	Day	63% 7% 12% 12% 6%	1761
	21st to end	Day	58% 5% 11% 14% 9% 3	1593
March	1st to 10th	Day	63% 5% 8% 13% 11% 1	4634
	11th to 20th	Day	61% 7% 15% 12% 4 1	4980
	21st to end	Day	71% 4 8% 11% 4 1	5649
April	1st to 10th	Day	62% 6% 8% 12% 10% 3	2400
	11th to 20th	Day	57% 4 10% 17% 11% 1	2374
	21st to end	Day	60% 7% 10% 14% 8% 1	2569
May	1st to 10th	Day	62% 7% 10% 16% 5% 1	6355
	11th to 20th	Day	53% 8% 12% 20% 7%	6322
	21st to end	Day	50% 10% 12% 20% 9%	6948
June	1st to 10th	Day	60% 7% 11% 17% 5% 1	2800
	11th to 20th	Day	51% 8% 11% 20% 10% 1	2800
	21st to end	Day	60% 8% 8% 14% 9% 1	2800
July	1st to 10th	Day	54% 7% 15% 16% 8%	6290
	11th to 20th	Day	43% 8% 16% 24% 8%	6284
	21st to end	Day	43% 8% 17% 25% 7%	6730
August	1st to 10th	Day	54% 8% 12% 19% 7%	4363
	11th to 20th	Day	49% 8% 13% 23% 6%	4214
	21st to end	Day	39% 6% 12% 29% 13%	4358
September	1st to 10th	Day	45% 7% 13% 23% 11% 1	2400
	11th to 20th	Day	48% 7% 11% 22% 12% 1	2400
	21st to end	Day	46% 7% 11% 23% 12%	2106
October	1st to 10th	Day	51% 8% 13% 19% 8%	3260
	11th to 20th	Day	47% 7% 13% 21% 11%	3220
	21st to end	Day	52% 7% 16% 18% 7% 1	3259
November	1st to 10th	Day	61% 8% 12% 14% 5%	1690
	11th to 20th	Day	68% 6% 12% 9% 5%	1600
	21st to end	Day	66% 8% 14% 7% 4	1600
December	1st to 10th	Day	64% 7% 9% 16% 4	2610
	11th to 20th	Day	73% 6% 10% 5% 7%	2995
	21st to end	Day	69% 8% 12% 8% 2 1	3982
January	1st to 10th	Night	74% 7% 10% 7% 2	6349
	11th to 20th	Night	68% 7% 12% 10% 3	6360
	21st to end	Night	59% 9% 9% 13% 8% 1	6692
February	1st to 10th	Night	64% 6% 10% 11% 7% 1	2600
	11th to 20th	Night	65% 6% 11% 12% 5% 1	2536
	21st to end	Night	59% 4 10% 17% 9% 1	2035
March	1st to 10th	Night	63% 6% 6% 13% 10% 1	5201
	11th to 20th	Night	59% 5% 14% 14% 7% 1	4980
	21st to end	Night	68% 4 7% 14% 6% 1	5283
April	1st to 10th	Night	61% 6% 10% 12% 10% 2	2000
	11th to 20th	Night	54% 6% 9% 19% 11% 2	1976
	21st to end	Night	57% 6% 10% 15% 11% 1	1830
May	1st to 10th	Night	67% 6% 7% 13% 8%	3620
	11th to 20th	Night	54% 7% 11% 21% 7% 1	3597
	21st to end	Night	49% 11% 13% 23% 4	3949
June	1st to 10th	Night	58% 8% 9% 19% 7%	1600
	11th to 20th	Night	48% 7% 12% 19% 14% 1	1600
	21st to end	Night	54% 10% 10% 16% 9% 1	1600
July	1st to 10th	Night	54% 7% 15% 18% 6% 1	3581
	11th to 20th	Night	40% 7% 15% 25% 11% 1	3572
	21st to end	Night	46% 7% 16% 24% 7%	3831
August	1st to 10th	Night	49% 8% 15% 23% 5%	2763
	11th to 20th	Night	45% 10% 16% 25% 5%	2899
	21st to end	Night	35% 9% 17% 28% 11%	3537
September	1st to 10th	Night	42% 7% 13% 26% 11%	2000
	11th to 20th	Night	41% 7% 14% 27% 11% 1	2000
	21st to end	Night	45% 7% 12% 26% 10% 1	2289
October	1st to 10th	Night	49% 7% 15% 20% 10%	3920
	11th to 20th	Night	43% 7% 13% 26% 10%	3876
	21st to end	Night	50% 6% 14% 20% 8% 1	4294
November	1st to 10th	Night	59% 9% 12% 14% 6%	2603
	11th to 20th	Night	63% 8% 14% 10% 5%	2800
	21st to end	Night	64% 8% 15% 9% 3	2800
December	1st to 10th	Night	58% 7% 12% 17% 6%	4580
	11th to 20th	Night	64% 6% 10% 12% 6% 2	5215
	21st to end	Night	68% 5% 13% 11% 3	6996

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	30% 24% 33% 12% 2	400
	11th to 20th	Day	27% 25% 28% 15% 6%	400
	21st to end	Day	23% 24% 36% 15% 2	440
February	1st to 10th	Day	30% 23% 31% 14% 2 1	400
	11th to 20th	Day	27% 31% 26% 15% 2	396
	21st to end	Day	23% 21% 35% 17% 4	331
March	1st to 10th	Day	25% 23% 34% 16% 2	433
	11th to 20th	Day	21% 27% 33% 14% 4 1	500
	21st to end	Day	31% 24% 26% 15% 3 1	550
April	1st to 10th	Day	32% 28% 26% 11% 2	500
	11th to 20th	Day	31% 23% 30% 12% 3	492
	21st to end	Day	28% 21% 29% 17% 4 1	500
May	1st to 10th	Day	34% 17% 28% 15% 3 2	701
	11th to 20th	Day	24% 12% 33% 22% 8% 1	724
	21st to end	Day	24% 15% 35% 21% 5% 1	790
June	1st to 10th	Day	33% 13% 29% 18% 7% 1	730
	11th to 20th	Day	22% 14% 34% 23% 5% 1	730
	21st to end	Day	26% 13% 28% 23% 8% 1	730
July	1st to 10th	Day	19% 9% 35% 29% 7% 1	720
	11th to 20th	Day	16% 8% 34% 31% 10% 1	718
	21st to end	Day	19% 10% 29% 30% 10% 1	795
August	1st to 10th	Day	16% 7% 31% 32% 10% 3	616
	11th to 20th	Day	18% 10% 33% 26% 11% 3	519
	21st to end	Day	17% 13% 30% 26% 12% 2	583
September	1st to 10th	Day	17% 15% 34% 26% 7% 2	530
	11th to 20th	Day	25% 13% 28% 23% 8% 2	530
	21st to end	Day	22% 16% 31% 19% 9% 3	447
October	1st to 10th	Day	23% 13% 29% 28% 5% 2	420
	11th to 20th	Day	25% 20% 28% 16% 8% 2	414
	21st to end	Day	28% 20% 31% 16% 5%	429
November	1st to 10th	Day	25% 25% 29% 17% 3 1	395
	11th to 20th	Day	35% 24% 24% 13% 3 1	400
	21st to end	Day	32% 26% 26% 13% 3 1	400
December	1st to 10th	Day	30% 25% 29% 14% 3	400
	11th to 20th	Day	35% 25% 28% 11% 1	400
	21st to end	Day	23% 29% 32% 13% 3 1	440
January	1st to 10th	Night	27% 24% 33% 14% 2	900
	11th to 20th	Night	26% 23% 32% 16% 3	900
	21st to end	Night	23% 26% 32% 16% 3	923
February	1st to 10th	Night	29% 18% 31% 19% 3	800
	11th to 20th	Night	25% 28% 30% 15% 2	783
	21st to end	Night	21% 20% 34% 20% 5%	660
March	1st to 10th	Night	26% 19% 33% 20% 3	800
	11th to 20th	Night	23% 20% 30% 21% 6% 1	800
	21st to end	Night	28% 19% 30% 20% 4	800
April	1st to 10th	Night	29% 21% 30% 19% 3	600
	11th to 20th	Night	25% 19% 32% 19% 4 1	594
	21st to end	Night	30% 14% 29% 22% 5%	600
May	1st to 10th	Night	33% 14% 27% 19% 7% 1	600
	11th to 20th	Night	26% 11% 31% 24% 7% 1	592
	21st to end	Night	21% 14% 39% 21% 4	649
June	1st to 10th	Night	30% 9% 33% 23% 6%	600
	11th to 20th	Night	26% 9% 34% 27% 4 1	600
	21st to end	Night	24% 12% 30% 27% 6%	600
July	1st to 10th	Night	18% 7% 34% 34% 7% 1	586
	11th to 20th	Night	15% 6% 35% 35% 9% 1	582
	21st to end	Night	18% 7% 29% 36% 9%	656
August	1st to 10th	Night	15% 6% 29% 38% 12%	598
	11th to 20th	Night	19% 6% 28% 33% 12% 1	592
	21st to end	Night	16% 9% 28% 38% 10%	660
September	1st to 10th	Night	17% 9% 33% 32% 9% 1	600
	11th to 20th	Night	21% 13% 27% 28% 12%	600
	21st to end	Night	23% 12% 32% 26% 6% 1	740
October	1st to 10th	Night	22% 13% 31% 25% 9% 1	800
	11th to 20th	Night	22% 16% 32% 22% 7%	793
	21st to end	Night	30% 22% 29% 16% 4	889
November	1st to 10th	Night	27% 22% 30% 16% 4 1	880
	11th to 20th	Night	28% 25% 27% 14% 5%	900
	21st to end	Night	32% 23% 28% 15% 2	900
December	1st to 10th	Night	30% 23% 30% 13% 4	900
	11th to 20th	Night	33% 25% 29% 11% 2	900
	21st to end	Night	29% 25% 29% 13% 3 1	990

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