Tusaire Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tusaire Observatory clear sky chart from 2005-11-06 to 2024-02-20.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 37%	78746
	Night	12% 9% 4 7% 6% 3 4 4 5% 6% 38%	80499
Transparency	Day	46% 7% 13% 23% 10% 1	78445
	Night	46% 6% 13% 23% 11% 1	80321
Seeing	Day	15% 9% 30% 28% 14% 4	14973
	Night	16% 8% 32% 33% 11% 1	25079

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	13% 13% 6% 13% 8% 4 5% 5% 5% 6% 22%	2527
	11th to 20th	Day	6% 9% 6% 13% 10% 6% 9% 8% 6% 4 23%	2538
	21st to end	Day	7% 19% 8% 10% 6% 3 5% 6% 6% 6% 24%	3135
February	1st to 10th	Day	8% 12% 5% 8% 8% 3 5% 5% 6% 7% 33%	1710
	11th to 20th	Day	8% 10% 5% 9% 8% 4 4 5% 5% 7% 36%	1655
	21st to end	Day	11% 11% 5% 10% 8% 3 5% 5% 4 7% 30%	1368
March	1st to 10th	Day	12% 12% 5% 8% 6% 3 5% 4 5% 8% 32%	1944
	11th to 20th	Day	14% 8% 5% 7% 5% 3 5% 4 5% 8% 37%	1980
	21st to end	Day	19% 11% 5% 6% 6% 4 5% 5% 5% 5% 30%	2191
April	1st to 10th	Day	17% 10% 4 7% 6% 3 4 4 4 6% 35%	2160
	11th to 20th	Day	20% 11% 4 7% 5% 4 4 4 4 5% 33%	2168
	21st to end	Day	22% 10% 4 5% 5% 3 4 4 5% 5% 32%	2340
May	1st to 10th	Day	17% 9% 3 6% 7% 4 6% 4 5% 6% 34%	2438
	11th to 20th	Day	17% 6% 3 6% 5% 3 5% 4 5% 7% 38%	2479
	21st to end	Day	14% 7% 2 7% 5% 3 5% 5% 6% 7% 38%	2725
June	1st to 10th	Day	13% 5% 2 5% 4 3 5% 5% 6% 8% 45%	2520
	11th to 20th	Day	12% 6% 3 6% 5% 3 4 5% 6% 9% 40%	2615
	21st to end	Day	10% 4 2 4 6% 3 5% 6% 7% 11% 42%	2700
July	1st to 10th	Day	7% 3 2 4 5% 3 5% 6% 7% 10% 51%	2664
	11th to 20th	Day	9% 4 2 4 5% 2 4 4 5% 8% 52%	2520
	21st to end	Day	6% 4 2 4 4 3 5% 6% 7% 9% 50%	2720
August	1st to 10th	Day	9% 4 2 6% 6% 4 5% 5% 7% 9% 42%	2328
	11th to 20th	Day	13% 5% 2 5% 4 3 5% 4 6% 8% 45%	2301
	21st to end	Day	10% 4 2 4 4 3 4 4 5% 8% 52%	2484
September	1st to 10th	Day	10% 5% 2 5% 5% 3 6% 4 5% 8% 47%	2160
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 5% 5% 8% 46%	2120
	21st to end	Day	16% 5% 2 5% 5% 3 4 4 6% 8% 43%	1904
October	1st to 10th	Day	18% 7% 3 4 6% 3 5% 4 4 7% 39%	1800
	11th to 20th	Day	16% 10% 3 6% 5% 2 4 4 5% 5% 39%	1800
	21st to end	Day	18% 10% 4 6% 5% 3 3 4 4 5% 38%	1958
November	1st to 10th	Day	13% 11% 4 8% 6% 3 4 4 4 6% 37%	1533
	11th to 20th	Day	12% 13% 7% 11% 7% 3 5% 4 5% 5% 29%	1520
	21st to end	Day	13% 11% 4 6% 6% 2 4 5% 5% 8% 33%	1520
December	1st to 10th	Day	11% 13% 6% 9% 9% 5% 5% 5% 5% 5% 27%	1520
	11th to 20th	Day	11% 14% 7% 10% 7% 4 4 4 5% 7% 27%	1918
	21st to end	Day	22% 16% 6% 13% 6% 3 4 3 3 5% 19%	2783
January	1st to 10th	Night	11% 12% 7% 13% 6% 3 4 4 4 5% 30%	4428
	11th to 20th	Night	7% 10% 5% 12% 9% 5% 9% 8% 6% 6% 23%	4431
	21st to end	Night	10% 20% 8% 10% 6% 3 4 4 4 5% 27%	4532
February	1st to 10th	Night	7% 11% 4 8% 7% 4 5% 4 5% 7% 38%	2470
	11th to 20th	Night	6% 9% 4 7% 5% 5% 6% 5% 6% 7% 40%	2397
	21st to end	Night	13% 10% 4 8% 6% 3 5% 4 5% 5% 37%	1888
March	1st to 10th	Night	12% 10% 4 7% 7% 3 5% 5% 4 6% 37%	2034
	11th to 20th	Night	13% 9% 5% 7% 5% 2 5% 4 4 6% 41%	1980
	21st to end	Night	19% 8% 3 6% 5% 3 5% 4 5% 7% 35%	2165
April	1st to 10th	Night	16% 10% 3 6% 5% 3 4 5% 5% 7% 37%	1800
	11th to 20th	Night	20% 10% 4 7% 4 3 3 4 4 6% 36%	1737
	21st to end	Night	20% 9% 4 5% 5% 4 3 5% 4 6% 35%	1620
May	1st to 10th	Night	16% 8% 3 6% 3 3 4 3 5% 7% 40%	1522
	11th to 20th	Night	15% 5% 3 4 6% 3 5% 5% 4 7% 43%	1416
	21st to end	Night	14% 6% 2 5% 5% 3 5% 5% 6% 7% 44%	1551
June	1st to 10th	Night	11% 3 2 5% 4 3 5% 4 5% 8% 49%	1440
	11th to 20th	Night	9% 5% 2 6% 4 3 4 5% 6% 9% 47%	1339
	21st to end	Night	8% 3 2 4 5% 2 4 5% 6% 9% 52%	1260
July	1st to 10th	Night	4 3 1 3 4 2 4 5% 6% 10% 58%	1291
	11th to 20th	Night	7% 4 1 4 3 2 3 5% 6% 10% 55%	1440
	21st to end	Night	4 2 2 4 4 3 4 5% 5% 8% 59%	1595
August	1st to 10th	Night	8% 4 2 5% 5% 2 3 4 7% 8% 52%	1611
	11th to 20th	Night	13% 4 2 4 3 3 4 4 5% 7% 53%	1600
	21st to end	Night	8% 4 3 4 4 3 3 4 5% 6% 56%	1872
September	1st to 10th	Night	10% 4 2 4 5% 2 3 5% 6% 8% 52%	1800
	11th to 20th	Night	10% 3 2 4 5% 2 4 4 5% 7% 54%	1837
	21st to end	Night	15% 4 2 5% 4 2 4 3 5% 6% 50%	2038
October	1st to 10th	Night	14% 6% 2 4 4 3 4 4 4 7% 48%	2160
	11th to 20th	Night	15% 10% 3 5% 4 3 3 3 5% 6% 40%	2160
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 5% 6% 37%	2397
November	1st to 10th	Night	14% 8% 3 6% 5% 3 5% 4 5% 6% 41%	2545
	11th to 20th	Night	15% 11% 5% 10% 6% 3 4 4 4 6% 33%	2660
	21st to end	Night	13% 9% 3 7% 6% 3 4 5% 5% 8% 36%	2660
December	1st to 10th	Night	10% 11% 5% 11% 8% 4 5% 4 4 7% 30%	2660
	11th to 20th	Night	11% 13% 6% 10% 8% 4 5% 5% 5% 6% 28%	3279
	21st to end	Night	22% 15% 6% 10% 7% 2 4 3 4 4 23%	4884

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% 6% 10% 17% 4	2528
	11th to 20th	Day	61% 7% 12% 16% 5%	2538
	21st to end	Day	59% 7% 13% 16% 5%	3135
February	1st to 10th	Day	50% 6% 13% 22% 8% 2	1710
	11th to 20th	Day	48% 6% 11% 25% 8% 1	1664
	21st to end	Day	53% 5% 11% 20% 11% 1	1368
March	1st to 10th	Day	52% 5% 15% 20% 8% 1	1944
	11th to 20th	Day	47% 5% 16% 24% 7% 1	1980
	21st to end	Day	55% 6% 9% 23% 7% 1	2191
April	1st to 10th	Day	49% 6% 11% 20% 13% 1	2160
	11th to 20th	Day	54% 5% 9% 19% 11% 2	2168
	21st to end	Day	54% 5% 11% 20% 9%	2340
May	1st to 10th	Day	50% 7% 11% 21% 10% 2	2438
	11th to 20th	Day	45% 7% 12% 23% 12% 1	2483
	21st to end	Day	45% 7% 11% 23% 12% 1	2725
June	1st to 10th	Day	35% 9% 14% 31% 11% 1	2520
	11th to 20th	Day	40% 9% 13% 22% 14% 1	2615
	21st to end	Day	34% 9% 18% 29% 8% 2	2700
July	1st to 10th	Day	27% 9% 15% 32% 16% 1	2592
	11th to 20th	Day	30% 7% 15% 27% 18% 3	2445
	21st to end	Day	28% 9% 17% 28% 17% 2	2569
August	1st to 10th	Day	37% 8% 14% 29% 9% 3	2314
	11th to 20th	Day	37% 7% 14% 31% 11%	2301
	21st to end	Day	30% 6% 14% 31% 16% 2	2484
September	1st to 10th	Day	35% 7% 14% 33% 12%	2160
	11th to 20th	Day	36% 8% 16% 32% 8%	2120
	21st to end	Day	40% 7% 12% 33% 8%	1904
October	1st to 10th	Day	45% 6% 11% 26% 11%	1800
	11th to 20th	Day	46% 5% 13% 24% 10% 1	1800
	21st to end	Day	49% 5% 12% 25% 9%	1958
November	1st to 10th	Day	48% 6% 12% 20% 12% 2	1533
	11th to 20th	Day	57% 5% 13% 15% 9%	1520
	21st to end	Day	48% 7% 15% 21% 6% 2	1520
December	1st to 10th	Day	57% 7% 17% 15% 5%	1520
	11th to 20th	Day	57% 6% 17% 16% 4	1915
	21st to end	Day	69% 6% 8% 10% 6%	2783
January	1st to 10th	Night	57% 6% 11% 19% 6% 1	4428
	11th to 20th	Night	59% 7% 12% 15% 6%	4431
	21st to end	Night	60% 5% 10% 19% 5% 1	4532
February	1st to 10th	Night	47% 4 13% 24% 10% 1	2470
	11th to 20th	Night	42% 7% 15% 24% 10% 2	2398
	21st to end	Night	49% 5% 11% 22% 12% 1	1888
March	1st to 10th	Night	48% 7% 15% 22% 8%	2034
	11th to 20th	Night	45% 5% 12% 23% 14% 1	1980
	21st to end	Night	49% 5% 12% 24% 8% 1	2165
April	1st to 10th	Night	46% 6% 10% 23% 15% 1	1800
	11th to 20th	Night	50% 4 8% 21% 15% 2	1737
	21st to end	Night	49% 7% 8% 21% 13% 2	1620
May	1st to 10th	Night	44% 6% 13% 22% 15% 1	1522
	11th to 20th	Night	40% 7% 14% 23% 14% 2	1417
	21st to end	Night	38% 8% 13% 23% 16% 3	1552
June	1st to 10th	Night	32% 6% 17% 28% 15% 2	1440
	11th to 20th	Night	33% 7% 14% 25% 18% 2	1339
	21st to end	Night	28% 7% 15% 31% 15% 4	1260
July	1st to 10th	Night	22% 6% 15% 36% 18% 2	1255
	11th to 20th	Night	23% 8% 14% 33% 17% 4	1392
	21st to end	Night	22% 6% 16% 32% 18% 5%	1506
August	1st to 10th	Night	29% 8% 13% 32% 14% 4	1601
	11th to 20th	Night	31% 6% 16% 30% 15% 2	1600
	21st to end	Night	28% 7% 13% 31% 17% 3	1872
September	1st to 10th	Night	29% 6% 14% 34% 16% 1	1800
	11th to 20th	Night	29% 7% 16% 32% 17%	1837
	21st to end	Night	35% 5% 14% 32% 13% 1	2038
October	1st to 10th	Night	37% 6% 10% 31% 15% 2	2160
	11th to 20th	Night	44% 5% 14% 24% 12% 1	2160
	21st to end	Night	48% 5% 12% 23% 11% 1	2397
November	1st to 10th	Night	44% 6% 12% 24% 13% 2	2545
	11th to 20th	Night	53% 5% 12% 19% 10%	2660
	21st to end	Night	46% 7% 18% 22% 6% 1	2660
December	1st to 10th	Night	55% 5% 15% 20% 6%	2660
	11th to 20th	Night	56% 7% 12% 18% 5% 2	3281
	21st to end	Night	65% 5% 11% 15% 4	4884

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	14% 10% 31% 29% 12% 3	270
	11th to 20th	Day	13% 7% 39% 34% 7% 1	270
	21st to end	Day	15% 8% 36% 25% 12% 2	297
February	1st to 10th	Day	16% 11% 43% 25% 5% 1	270
	11th to 20th	Day	16% 14% 39% 23% 6% 2	267
	21st to end	Day	15% 12% 39% 30% 4	198
March	1st to 10th	Day	18% 12% 36% 24% 10% 1	240
	11th to 20th	Day	18% 8% 37% 26% 10% 3	240
	21st to end	Day	25% 13% 27% 23% 8% 4	277
April	1st to 10th	Day	23% 14% 30% 25% 7%	420
	11th to 20th	Day	26% 11% 27% 24% 11% 1	446
	21st to end	Day	25% 11% 31% 20% 10% 3	600
May	1st to 10th	Day	20% 12% 30% 26% 10% 1	603
	11th to 20th	Day	19% 11% 30% 25% 12% 3	625
	21st to end	Day	17% 8% 27% 29% 15% 4	679
June	1st to 10th	Day	10% 6% 28% 30% 20% 5%	630
	11th to 20th	Day	11% 7% 36% 32% 12% 2	630
	21st to end	Day	11% 6% 30% 31% 15% 7%	630
July	1st to 10th	Day	8% 4 21% 32% 24% 11%	621
	11th to 20th	Day	9% 6% 26% 30% 21% 8%	618
	21st to end	Day	6% 5% 22% 36% 24% 7%	684
August	1st to 10th	Day	10% 4 22% 33% 25% 6%	626
	11th to 20th	Day	14% 4 21% 31% 25% 6%	615
	21st to end	Day	11% 6% 24% 31% 23% 5%	603
September	1st to 10th	Day	13% 5% 29% 29% 20% 4	450
	11th to 20th	Day	13% 4 28% 30% 20% 5%	410
	21st to end	Day	19% 14% 28% 22% 11% 7%	270
October	1st to 10th	Day	19% 9% 27% 23% 15% 7%	270
	11th to 20th	Day	13% 17% 35% 25% 7% 2	270
	21st to end	Day	21% 14% 41% 15% 6% 1	297
November	1st to 10th	Day	18% 13% 36% 23% 9% 1	270
	11th to 20th	Day	16% 18% 40% 22% 4	270
	21st to end	Day	20% 14% 41% 20% 6%	270
December	1st to 10th	Day	15% 10% 39% 24% 12%	270
	11th to 20th	Day	20% 13% 36% 27% 4	270
	21st to end	Day	33% 9% 34% 22% 2 1	297
January	1st to 10th	Night	11% 10% 36% 32% 9% 1	950
	11th to 20th	Night	17% 7% 38% 29% 8% 1	950
	21st to end	Night	16% 10% 36% 29% 8%	1045
February	1st to 10th	Night	11% 10% 38% 33% 8% 1	950
	11th to 20th	Night	10% 11% 42% 28% 8% 1	927
	21st to end	Night	16% 9% 34% 30% 9% 1	704
March	1st to 10th	Night	15% 10% 36% 29% 9% 1	594
	11th to 20th	Night	17% 8% 39% 28% 7% 1	540
	21st to end	Night	23% 9% 27% 29% 11% 2	594
April	1st to 10th	Night	19% 11% 35% 29% 6%	540
	11th to 20th	Night	24% 10% 30% 27% 8% 1	533
	21st to end	Night	23% 9% 30% 27% 9% 2	540
May	1st to 10th	Night	17% 10% 34% 31% 8%	540
	11th to 20th	Night	17% 6% 32% 32% 11% 1	531
	21st to end	Night	15% 6% 25% 38% 15%	584
June	1st to 10th	Night	13% 4 26% 42% 14% 1	540
	11th to 20th	Night	11% 4 30% 44% 11%	540
	21st to end	Night	9% 3 24% 44% 19% 1	540
July	1st to 10th	Night	5% 2 20% 45% 26% 2	525
	11th to 20th	Night	8% 4 22% 46% 18% 2	522
	21st to end	Night	5% 2 21% 47% 22% 2	591
August	1st to 10th	Night	9% 3 16% 50% 21% 2	537
	11th to 20th	Night	14% 2 19% 36% 27% 3	534
	21st to end	Night	9% 2 24% 39% 24% 3	594
September	1st to 10th	Night	10% 2 24% 42% 19% 3	540
	11th to 20th	Night	11% 4 29% 35% 19% 2	540
	21st to end	Night	17% 7% 30% 31% 14% 2	598
October	1st to 10th	Night	19% 6% 30% 30% 13% 1	720
	11th to 20th	Night	19% 13% 33% 28% 7% 1	720
	21st to end	Night	23% 9% 30% 31% 7%	792
November	1st to 10th	Night	18% 9% 32% 31% 9%	879
	11th to 20th	Night	19% 12% 35% 26% 7% 1	950
	21st to end	Night	18% 11% 37% 27% 8%	950
December	1st to 10th	Night	15% 10% 34% 33% 8% 1	950
	11th to 20th	Night	18% 8% 39% 28% 7% 1	950
	21st to end	Night	25% 9% 32% 28% 5% 1	1045

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