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 2025-04-11.

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	12% 9% 3 8% 5% 4 4 4 5% 7% 38%	114276
	Night	12% 9% 4 9% 5% 4 4 4 5% 6% 38%	114229
Transparency	Day	44% 6% 13% 25% 11% 1	117881
	Night	44% 5% 12% 24% 12% 1	118767
Seeing	Day	15% 9% 30% 27% 14% 4	16533
	Night	15% 8% 32% 33% 11% 1	26619

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	9% 12% 5% 15% 8% 5% 5% 3 5% 6% 26%	3617
	11th to 20th	Day	6% 10% 6% 13% 7% 6% 8% 7% 9% 5% 22%	3642
	21st to end	Day	5% 16% 7% 12% 5% 4 5% 4 5% 6% 31%	4488
February	1st to 10th	Day	8% 13% 4 10% 7% 5% 5% 4 6% 7% 32%	1800
	11th to 20th	Day	7% 10% 5% 10% 7% 5% 4 4 5% 7% 36%	1754
	21st to end	Day	9% 11% 5% 11% 7% 5% 4 4 4 8% 33%	1525
March	1st to 10th	Day	11% 9% 3 6% 4 4 4 3 5% 8% 44%	4847
	11th to 20th	Day	10% 9% 3 7% 4 5% 5% 3 5% 7% 40%	4980
	21st to end	Day	20% 11% 4 10% 6% 4 4 3 5% 6% 28%	5502
April	1st to 10th	Day	17% 10% 3 8% 5% 4 4 4 5% 6% 35%	2400
	11th to 20th	Day	20% 11% 4 8% 4 5% 3 3 4 5% 33%	2291
	21st to end	Day	22% 11% 3 6% 4 4 4 4 5% 5% 32%	2470
May	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 3 5% 6% 41%	4264
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 45%	4399
	21st to end	Day	18% 7% 2 7% 5% 5% 4 4 6% 7% 35%	4824
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 8% 46%	2660
	11th to 20th	Day	13% 7% 3 7% 5% 4 4 4 7% 9% 38%	2761
	21st to end	Day	11% 5% 2 6% 5% 4 5% 4 7% 11% 41%	2850
July	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 39%	4688
	11th to 20th	Day	6% 3 1 4 3 3 3 4 5% 7% 61%	4440
	21st to end	Day	8% 4 1 5% 3 4 4 4 7% 9% 50%	4764
August	1st to 10th	Day	12% 5% 2 6% 4 4 5% 3 6% 8% 45%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 9% 44%	4041
	21st to end	Day	12% 5% 2 6% 4 4 4 3 6% 8% 46%	4358
September	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 3 5% 8% 49%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 5% 7% 46%	2237
	21st to end	Day	15% 5% 2 6% 4 3 4 3 6% 7% 45%	2009
October	1st to 10th	Day	18% 7% 2 5% 5% 4 4 3 5% 7% 41%	1900
	11th to 20th	Day	16% 10% 2 7% 5% 3 4 3 5% 5% 40%	1900
	21st to end	Day	17% 10% 4 7% 4 4 3 3 4 6% 38%	2066
November	1st to 10th	Day	13% 12% 3 9% 4 5% 3 3 4 6% 37%	1615
	11th to 20th	Day	12% 14% 6% 11% 6% 4 4 3 5% 5% 29%	1600
	21st to end	Day	13% 12% 4 7% 6% 4 4 4 5% 8% 32%	1600
December	1st to 10th	Day	8% 11% 5% 14% 10% 7% 6% 4 5% 6% 24%	2610
	11th to 20th	Day	10% 17% 6% 11% 5% 4 4 3 5% 6% 29%	3008
	21st to end	Day	20% 19% 7% 19% 5% 4 3 2 3 4 14%	3982
January	1st to 10th	Night	8% 10% 5% 15% 8% 5% 4 3 4 5% 32%	6348
	11th to 20th	Night	7% 10% 4 11% 7% 7% 10% 8% 8% 6% 22%	6337
	21st to end	Night	7% 17% 6% 9% 5% 4 4 4 5% 6% 32%	6490
February	1st to 10th	Night	7% 12% 4 9% 6% 5% 4 3 5% 7% 37%	2600
	11th to 20th	Night	6% 10% 4 7% 5% 7% 5% 4 6% 7% 40%	2533
	21st to end	Night	11% 11% 4 8% 5% 4 5% 3 6% 5% 38%	2104
March	1st to 10th	Night	10% 7% 3 6% 4 3 4 3 5% 6% 49%	5039
	11th to 20th	Night	10% 7% 3 7% 4 4 5% 4 7% 9% 39%	4980
	21st to end	Night	25% 10% 3 10% 4 4 4 3 4 5% 28%	5427
April	1st to 10th	Night	16% 10% 3 6% 4 4 4 4 5% 6% 38%	2000
	11th to 20th	Night	20% 10% 3 7% 3 3 4 3 5% 5% 38%	1840
	21st to end	Night	20% 9% 3 5% 4 4 4 4 4 6% 35%	1710
May	1st to 10th	Night	13% 8% 3 8% 4 4 4 3 5% 6% 42%	2641
	11th to 20th	Night	15% 5% 1 4 3 3 3 4 5% 7% 49%	2506
	21st to end	Night	15% 7% 2 6% 4 3 4 5% 6% 6% 41%	2743
June	1st to 10th	Night	11% 4 2 5% 4 4 5% 4 6% 8% 49%	1520
	11th to 20th	Night	8% 5% 2 7% 4 3 5% 4 7% 8% 46%	1413
	21st to end	Night	9% 4 1 5% 4 3 4 4 6% 8% 52%	1330
July	1st to 10th	Night	12% 4 1 4 4 4 4 4 7% 9% 47%	2277
	11th to 20th	Night	5% 3 1 4 2 3 3 4 6% 9% 60%	2530
	21st to end	Night	6% 3 1 5% 4 4 4 4 5% 7% 58%	2796
August	1st to 10th	Night	13% 5% 2 6% 4 3 3 3 5% 6% 51%	2836
	11th to 20th	Night	14% 4 2 5% 3 4 3 3 5% 7% 51%	2798
	21st to end	Night	10% 5% 3 6% 4 4 4 4 6% 7% 47%	3290
September	1st to 10th	Night	9% 4 1 5% 4 3 3 4 6% 8% 53%	1900
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 7% 54%	1940
	21st to end	Night	14% 4 2 5% 3 3 4 2 5% 5% 52%	2152
October	1st to 10th	Night	14% 6% 2 5% 3 4 4 3 4 6% 50%	2280
	11th to 20th	Night	14% 10% 3 5% 4 4 3 2 5% 6% 42%	2280
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 5% 6% 37%	2531
November	1st to 10th	Night	14% 9% 2 7% 4 4 4 4 5% 6% 39%	2683
	11th to 20th	Night	14% 12% 4 10% 5% 4 4 3 4 6% 34%	2800
	21st to end	Night	13% 10% 3 8% 5% 4 4 4 5% 8% 35%	2800
December	1st to 10th	Night	7% 9% 4 15% 7% 7% 5% 5% 6% 7% 29%	4580
	11th to 20th	Night	10% 15% 6% 14% 6% 5% 5% 4 5% 5% 26%	5199
	21st to end	Night	20% 17% 8% 16% 5% 3 3 2 3 4 18%	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	59% 5% 9% 18% 9%	3618
	11th to 20th	Day	58% 5% 10% 18% 9%	3642
	21st to end	Day	53% 6% 15% 17% 8%	4488
February	1st to 10th	Day	51% 6% 13% 21% 8% 2	1800
	11th to 20th	Day	48% 6% 11% 26% 9% 1	1763
	21st to end	Day	50% 5% 13% 21% 10% 1	1525
March	1st to 10th	Day	40% 5% 18% 25% 10% 3	4847
	11th to 20th	Day	42% 4 14% 30% 9% 1	4980
	21st to end	Day	58% 6% 9% 20% 8% 1	5504
April	1st to 10th	Day	49% 6% 11% 20% 13% 1	2400
	11th to 20th	Day	54% 5% 8% 19% 12% 1	2291
	21st to end	Day	54% 5% 11% 21% 9%	2470
May	1st to 10th	Day	42% 6% 11% 24% 15% 1	4302
	11th to 20th	Day	38% 6% 11% 28% 14% 2	4403
	21st to end	Day	46% 7% 12% 25% 9% 1	4837
June	1st to 10th	Day	34% 9% 13% 31% 12% 1	2660
	11th to 20th	Day	41% 9% 13% 22% 14% 1	2761
	21st to end	Day	35% 9% 17% 28% 9% 2	2850
July	1st to 10th	Day	38% 9% 17% 23% 12% 1	4616
	11th to 20th	Day	22% 6% 16% 37% 16% 2	4365
	21st to end	Day	29% 7% 16% 30% 16% 1	4614
August	1st to 10th	Day	37% 6% 12% 32% 12% 2	4090
	11th to 20th	Day	36% 7% 13% 34% 10%	4080
	21st to end	Day	35% 7% 17% 27% 13% 1	4357
September	1st to 10th	Day	33% 6% 14% 34% 11%	2280
	11th to 20th	Day	35% 8% 17% 32% 8%	2237
	21st to end	Day	38% 6% 12% 34% 10% 1	2009
October	1st to 10th	Day	32% 4 9% 34% 21%	3160
	11th to 20th	Day	39% 4 12% 28% 15% 1	3140
	21st to end	Day	44% 4 14% 28% 9%	3380
November	1st to 10th	Day	48% 6% 12% 21% 11% 2	1615
	11th to 20th	Day	57% 5% 13% 16% 9%	1600
	21st to end	Day	49% 7% 15% 21% 7% 2	1600
December	1st to 10th	Day	59% 6% 16% 14% 4 1	2610
	11th to 20th	Day	56% 5% 13% 20% 7%	3005
	21st to end	Day	76% 5% 7% 8% 4	3982
January	1st to 10th	Night	54% 5% 11% 20% 10%	6348
	11th to 20th	Night	57% 6% 11% 15% 11%	6337
	21st to end	Night	52% 5% 13% 24% 6%	6490
February	1st to 10th	Night	48% 4 14% 23% 11% 1	2600
	11th to 20th	Night	42% 6% 15% 25% 10% 1	2534
	21st to end	Night	46% 6% 12% 22% 12% 1	2104
March	1st to 10th	Night	36% 5% 14% 25% 17% 3	5039
	11th to 20th	Night	39% 5% 15% 27% 14%	4980
	21st to end	Night	59% 4 10% 18% 9% 1	5427
April	1st to 10th	Night	46% 5% 9% 23% 15% 1	2000
	11th to 20th	Night	48% 5% 8% 21% 15% 2	1840
	21st to end	Night	48% 7% 9% 21% 13% 2	1710
May	1st to 10th	Night	42% 6% 11% 22% 18% 2	2668
	11th to 20th	Night	34% 6% 11% 26% 20% 4	2507
	21st to end	Night	41% 7% 12% 24% 14% 1	2751
June	1st to 10th	Night	32% 6% 17% 28% 15% 2	1520
	11th to 20th	Night	33% 7% 15% 25% 18% 2	1413
	21st to end	Night	28% 7% 15% 31% 16% 3	1330
July	1st to 10th	Night	32% 7% 20% 26% 14% 1	2241
	11th to 20th	Night	19% 7% 15% 39% 19% 2	2482
	21st to end	Night	24% 6% 15% 34% 18% 3	2707
August	1st to 10th	Night	33% 6% 11% 31% 14% 5%	2826
	11th to 20th	Night	33% 6% 14% 32% 15% 1	2825
	21st to end	Night	35% 8% 16% 29% 11% 2	3290
September	1st to 10th	Night	28% 6% 15% 34% 16% 1	1900
	11th to 20th	Night	29% 7% 16% 32% 17%	1940
	21st to end	Night	33% 5% 13% 32% 15% 1	2152
October	1st to 10th	Night	23% 4 6% 36% 26% 4	3800
	11th to 20th	Night	34% 4 13% 26% 21% 2	3779
	21st to end	Night	43% 4 12% 27% 12%	4167
November	1st to 10th	Night	44% 6% 12% 25% 12% 1	2683
	11th to 20th	Night	53% 5% 12% 20% 10% 1	2800
	21st to end	Night	47% 6% 17% 22% 6% 1	2800
December	1st to 10th	Night	53% 6% 17% 19% 4	4580
	11th to 20th	Night	60% 5% 10% 18% 5% 1	5201
	21st to end	Night	71% 4 8% 13% 4	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	13% 11% 31% 30% 12% 4	300
	11th to 20th	Day	13% 8% 38% 34% 6% 1	300
	21st to end	Day	15% 10% 37% 25% 12% 2	330
February	1st to 10th	Day	16% 12% 42% 25% 5% 1	300
	11th to 20th	Day	15% 13% 40% 24% 6% 2	300
	21st to end	Day	14% 12% 41% 29% 4	249
March	1st to 10th	Day	17% 13% 38% 23% 8% 1	300
	11th to 20th	Day	17% 9% 39% 24% 9% 2	300
	21st to end	Day	25% 12% 29% 24% 7% 3	345
April	1st to 10th	Day	22% 14% 31% 25% 7%	500
	11th to 20th	Day	25% 13% 28% 23% 10% 1	489
	21st to end	Day	25% 11% 31% 21% 9% 3	650
May	1st to 10th	Day	20% 13% 30% 26% 10% 1	653
	11th to 20th	Day	17% 11% 30% 27% 12% 3	675
	21st to end	Day	17% 9% 28% 28% 14% 4	734
June	1st to 10th	Day	10% 7% 28% 30% 20% 5%	680
	11th to 20th	Day	12% 8% 37% 30% 11% 2	680
	21st to end	Day	13% 7% 30% 29% 15% 7%	680
July	1st to 10th	Day	9% 5% 21% 31% 23% 11%	671
	11th to 20th	Day	9% 5% 26% 29% 21% 10%	668
	21st to end	Day	6% 5% 22% 36% 24% 8%	739
August	1st to 10th	Day	10% 5% 23% 32% 24% 6%	676
	11th to 20th	Day	14% 4 21% 31% 25% 6%	665
	21st to end	Day	11% 6% 25% 31% 22% 5%	652
September	1st to 10th	Day	12% 6% 28% 31% 19% 4	490
	11th to 20th	Day	12% 4 27% 32% 20% 5%	447
	21st to end	Day	17% 13% 28% 23% 12% 8%	300
October	1st to 10th	Day	18% 10% 28% 24% 14% 6%	300
	11th to 20th	Day	14% 17% 37% 24% 7% 2	300
	21st to end	Day	21% 14% 41% 16% 6% 1	330
November	1st to 10th	Day	18% 13% 36% 23% 9% 1	300
	11th to 20th	Day	17% 19% 39% 22% 4	300
	21st to end	Day	19% 15% 42% 20% 5%	300
December	1st to 10th	Day	16% 10% 39% 24% 11%	300
	11th to 20th	Day	20% 13% 37% 26% 4	300
	21st to end	Day	33% 8% 34% 22% 3 1	330
January	1st to 10th	Night	11% 11% 36% 32% 9% 1	1000
	11th to 20th	Night	16% 7% 38% 29% 8% 1	1000
	21st to end	Night	16% 10% 37% 29% 8%	1100
February	1st to 10th	Night	13% 10% 38% 32% 7% 1	1000
	11th to 20th	Night	10% 11% 42% 28% 7% 1	979
	21st to end	Night	15% 9% 35% 31% 8% 1	784
March	1st to 10th	Night	14% 11% 37% 29% 8% 1	659
	11th to 20th	Night	16% 10% 39% 28% 7% 1	600
	21st to end	Night	23% 9% 28% 28% 10% 2	660
April	1st to 10th	Night	20% 11% 34% 30% 6%	600
	11th to 20th	Night	23% 11% 31% 27% 8% 1	565
	21st to end	Night	23% 9% 31% 27% 9% 2	570
May	1st to 10th	Night	16% 11% 34% 31% 8%	570
	11th to 20th	Night	17% 6% 32% 33% 11% 1	561
	21st to end	Night	15% 6% 25% 38% 15%	617
June	1st to 10th	Night	13% 5% 27% 41% 14% 1	570
	11th to 20th	Night	11% 4 31% 43% 11%	570
	21st to end	Night	9% 3 24% 44% 18% 1	570
July	1st to 10th	Night	6% 2 19% 45% 26% 2	555
	11th to 20th	Night	7% 4 22% 46% 20% 2	552
	21st to end	Night	5% 2 21% 46% 23% 3	624
August	1st to 10th	Night	10% 3 17% 49% 20% 2	567
	11th to 20th	Night	13% 2 18% 37% 26% 3	564
	21st to end	Night	9% 2 24% 39% 23% 3	627
September	1st to 10th	Night	10% 3 24% 42% 18% 3	570
	11th to 20th	Night	11% 4 29% 35% 19% 2	570
	21st to end	Night	16% 6% 29% 31% 16% 1	632
October	1st to 10th	Night	18% 6% 31% 31% 13% 1	760
	11th to 20th	Night	18% 13% 34% 28% 7% 1	760
	21st to end	Night	22% 9% 31% 30% 7%	836
November	1st to 10th	Night	18% 9% 32% 31% 9%	927
	11th to 20th	Night	19% 12% 35% 26% 7% 1	1000
	21st to end	Night	18% 12% 36% 26% 8%	1000
December	1st to 10th	Night	14% 10% 35% 33% 7% 1	1000
	11th to 20th	Night	18% 9% 38% 28% 7% 1	1000
	21st to end	Night	25% 9% 32% 28% 5% 1	1100

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