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 2026-08-15.

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% 8% 3 8% 5% 4 4 4 5% 7% 40%	145346
	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 39%	136897
Transparency	Day	43% 6% 13% 26% 12% 1	148907
	Night	44% 5% 12% 25% 13% 1	141416
Seeing	Day	15% 10% 31% 27% 14% 4	18553
	Night	15% 8% 32% 33% 11% 1	28363

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% 13% 5% 16% 9% 6% 5% 3 5% 6% 22%	4691
	11th to 20th	Day	5% 11% 6% 13% 7% 7% 8% 7% 8% 5% 23%	4746
	21st to end	Day	4 12% 6% 10% 5% 4 7% 5% 7% 7% 34%	5812
February	1st to 10th	Day	8% 13% 4 10% 7% 5% 5% 4 6% 7% 31%	1890
	11th to 20th	Day	7% 10% 5% 10% 7% 5% 4 4 5% 7% 36%	1844
	21st to end	Day	9% 11% 5% 11% 6% 5% 4 4 5% 8% 34%	1599
March	1st to 10th	Day	11% 9% 3 6% 4 4 4 3 5% 8% 43%	4955
	11th to 20th	Day	10% 9% 3 7% 4 5% 5% 3 5% 7% 40%	5090
	21st to end	Day	20% 11% 4 10% 6% 4 4 3 5% 6% 28%	5624
April	1st to 10th	Day	17% 10% 3 8% 5% 4 4 4 5% 6% 34%	2520
	11th to 20th	Day	19% 11% 4 8% 4 5% 4 3 4 5% 33%	2535
	21st to end	Day	22% 10% 3 7% 4 4 4 4 5% 5% 33%	2730
May	1st to 10th	Day	11% 6% 1 5% 4 4 4 3 4 6% 52%	7951
	11th to 20th	Day	16% 7% 2 6% 3 4 3 3 4 6% 45%	8222
	21st to end	Day	15% 7% 2 7% 4 4 4 4 6% 7% 40%	9047
June	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 8% 45%	2940
	11th to 20th	Day	14% 6% 3 7% 5% 4 4 4 6% 9% 37%	3052
	21st to end	Day	12% 5% 2 5% 5% 4 5% 4 7% 10% 41%	3150
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 10% 42%	8736
	11th to 20th	Day	8% 3 1 4 3 3 3 3 5% 8% 59%	8280
	21st to end	Day	8% 4 2 6% 4 4 4 4 6% 9% 50%	8899
August	1st to 10th	Day	12% 5% 2 6% 3 4 5% 3 6% 8% 46%	4364
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 9% 44%	4224
	21st to end	Day	12% 5% 2 6% 4 4 4 3 6% 8% 46%	4496
September	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 3 5% 8% 48%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 5% 8% 45%	2355
	21st to end	Day	14% 5% 2 6% 4 3 4 3 5% 7% 46%	2114
October	1st to 10th	Day	17% 6% 2 5% 5% 4 4 3 5% 7% 41%	2000
	11th to 20th	Day	17% 9% 2 7% 5% 3 4 3 5% 5% 40%	2000
	21st to end	Day	17% 11% 3 7% 4 4 3 3 4 5% 38%	2175
November	1st to 10th	Day	12% 12% 3 9% 5% 5% 4 3 4 6% 36%	1697
	11th to 20th	Day	11% 14% 6% 11% 6% 4 4 3 5% 5% 31%	1680
	21st to end	Day	13% 12% 4 8% 6% 4 5% 4 5% 8% 32%	1680
December	1st to 10th	Day	8% 11% 5% 14% 10% 7% 6% 4 5% 6% 23%	2690
	11th to 20th	Day	10% 16% 6% 11% 5% 4 4 3 5% 6% 29%	3088
	21st to end	Day	20% 19% 7% 19% 5% 4 4 2 3 4 14%	4070
January	1st to 10th	Night	9% 12% 5% 14% 8% 5% 4 3 4 6% 30%	8232
	11th to 20th	Night	6% 11% 5% 12% 7% 7% 10% 8% 7% 6% 23%	8243
	21st to end	Night	6% 14% 5% 8% 5% 5% 5% 4 6% 6% 36%	8402
February	1st to 10th	Night	7% 12% 5% 9% 6% 6% 4 3 5% 7% 36%	2730
	11th to 20th	Night	6% 10% 4 7% 5% 6% 5% 4 6% 7% 40%	2663
	21st to end	Night	11% 11% 4 8% 5% 4 5% 3 6% 6% 38%	2206
March	1st to 10th	Night	10% 7% 3 6% 4 3 4 3 5% 6% 48%	5152
	11th to 20th	Night	10% 7% 3 7% 4 4 5% 4 7% 9% 39%	5090
	21st to end	Night	25% 10% 3 10% 4 4 4 3 4 5% 28%	5547
April	1st to 10th	Night	16% 10% 3 6% 4 4 4 4 5% 6% 37%	2100
	11th to 20th	Night	19% 10% 3 7% 3 3 4 3 4 6% 38%	2030
	21st to end	Night	21% 9% 3 6% 4 4 4 4 4 6% 36%	1890
May	1st to 10th	Night	9% 6% 2 6% 4 4 3 3 5% 6% 52%	4932
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 50%	4670
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 7% 40%	5139
June	1st to 10th	Night	11% 4 2 6% 4 4 5% 4 6% 7% 48%	1680
	11th to 20th	Night	9% 5% 2 7% 4 4 4 4 7% 8% 46%	1561
	21st to end	Night	10% 4 1 5% 4 3 4 4 7% 8% 50%	1470
July	1st to 10th	Night	10% 4 1 4 4 4 4 4 7% 9% 49%	4247
	11th to 20th	Night	6% 3 1 4 3 3 3 4 6% 10% 58%	4707
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 7% 51%	5191
August	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 5% 6% 51%	3016
	11th to 20th	Night	14% 4 2 5% 3 4 3 3 5% 7% 51%	2924
	21st to end	Night	10% 5% 3 6% 4 4 4 4 5% 7% 48%	3394
September	1st to 10th	Night	9% 4 1 5% 4 3 3 4 6% 8% 53%	2000
	11th to 20th	Night	10% 4 2 5% 4 3 4 3 5% 7% 53%	2042
	21st to end	Night	13% 4 2 5% 3 3 4 2 5% 5% 53%	2265
October	1st to 10th	Night	13% 5% 2 5% 3 4 4 3 4 7% 50%	2400
	11th to 20th	Night	14% 11% 3 6% 4 4 3 2 5% 6% 41%	2400
	21st to end	Night	17% 10% 3 7% 4 3 4 3 5% 6% 37%	2664
November	1st to 10th	Night	13% 10% 2 7% 5% 4 5% 4 5% 7% 39%	2821
	11th to 20th	Night	14% 12% 4 10% 5% 4 3 3 4 6% 35%	2940
	21st to end	Night	13% 10% 3 9% 5% 4 4 4 5% 8% 35%	2940
December	1st to 10th	Night	7% 9% 4 15% 7% 7% 5% 5% 6% 7% 28%	4720
	11th to 20th	Night	10% 15% 6% 14% 6% 5% 5% 4 5% 5% 26%	5339
	21st to end	Night	20% 17% 8% 16% 5% 3 3 2 3 4 18%	7150

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% 5% 9% 15% 8%	4692
	11th to 20th	Day	58% 4 10% 17% 9%	4746
	21st to end	Day	48% 5% 14% 23% 10%	5812
February	1st to 10th	Day	51% 6% 13% 21% 7% 2	1890
	11th to 20th	Day	47% 6% 11% 27% 8% 1	1853
	21st to end	Day	49% 5% 14% 21% 10% 1	1599
March	1st to 10th	Day	40% 5% 17% 25% 10% 3	4955
	11th to 20th	Day	42% 4 14% 29% 9% 1	5090
	21st to end	Day	57% 6% 9% 20% 8% 1	5626
April	1st to 10th	Day	50% 6% 11% 20% 13% 1	2520
	11th to 20th	Day	53% 5% 10% 19% 12% 2	2535
	21st to end	Day	53% 5% 10% 22% 10%	2730
May	1st to 10th	Day	33% 5% 11% 29% 21% 1	7989
	11th to 20th	Day	41% 5% 8% 25% 18% 5%	8226
	21st to end	Day	42% 5% 10% 30% 11% 1	9061
June	1st to 10th	Day	34% 9% 13% 31% 12% 1	2940
	11th to 20th	Day	42% 8% 13% 22% 13% 1	3052
	21st to end	Day	35% 8% 17% 28% 9% 2	3150
July	1st to 10th	Day	36% 8% 17% 31% 8%	8664
	11th to 20th	Day	23% 5% 14% 39% 17% 1	8205
	21st to end	Day	30% 6% 15% 32% 16% 1	8704
August	1st to 10th	Day	36% 6% 13% 31% 12% 2	4350
	11th to 20th	Day	36% 7% 13% 33% 10%	4263
	21st to end	Day	35% 7% 17% 28% 13% 1	4495
September	1st to 10th	Day	34% 7% 15% 34% 11%	2400
	11th to 20th	Day	36% 8% 17% 31% 8%	2355
	21st to end	Day	37% 6% 11% 34% 11% 1	2114
October	1st to 10th	Day	32% 4 9% 34% 21%	3260
	11th to 20th	Day	39% 4 12% 28% 15% 1	3240
	21st to end	Day	45% 4 14% 28% 9%	3489
November	1st to 10th	Day	49% 6% 12% 20% 12% 2	1697
	11th to 20th	Day	56% 5% 13% 17% 9%	1680
	21st to end	Day	50% 7% 15% 20% 7% 2	1680
December	1st to 10th	Day	59% 6% 16% 14% 4 1	2690
	11th to 20th	Day	56% 5% 13% 20% 7%	3085
	21st to end	Day	76% 5% 7% 8% 4	4070
January	1st to 10th	Night	55% 5% 14% 17% 8%	8233
	11th to 20th	Night	58% 6% 10% 16% 10%	8243
	21st to end	Night	47% 5% 11% 26% 10%	8402
February	1st to 10th	Night	48% 4 14% 23% 10% 1	2730
	11th to 20th	Night	42% 6% 15% 25% 11% 1	2664
	21st to end	Night	46% 6% 13% 23% 11% 1	2206
March	1st to 10th	Night	37% 5% 14% 25% 17% 3	5152
	11th to 20th	Night	39% 5% 15% 26% 14%	5090
	21st to end	Night	59% 4 10% 18% 9% 1	5547
April	1st to 10th	Night	47% 5% 9% 23% 15% 1	2100
	11th to 20th	Night	47% 4 9% 22% 15% 2	2030
	21st to end	Night	49% 6% 8% 21% 14% 2	1890
May	1st to 10th	Night	32% 5% 10% 31% 20% 2	4959
	11th to 20th	Night	37% 4 9% 21% 22% 8%	4671
	21st to end	Night	43% 5% 9% 28% 13% 1	5149
June	1st to 10th	Night	33% 6% 16% 28% 15% 2	1680
	11th to 20th	Night	34% 7% 14% 25% 17% 3	1561
	21st to end	Night	30% 7% 14% 31% 15% 3	1470
July	1st to 10th	Night	30% 7% 19% 30% 13% 1	4213
	11th to 20th	Night	21% 6% 14% 38% 20% 1	4662
	21st to end	Night	31% 6% 14% 32% 15% 1	5075
August	1st to 10th	Night	33% 6% 12% 31% 14% 4	3006
	11th to 20th	Night	34% 6% 14% 32% 14% 1	2951
	21st to end	Night	34% 8% 16% 29% 11% 2	3394
September	1st to 10th	Night	29% 6% 15% 34% 16% 1	2000
	11th to 20th	Night	29% 7% 17% 31% 16%	2042
	21st to end	Night	32% 5% 13% 33% 15% 1	2265
October	1st to 10th	Night	23% 4 6% 36% 26% 4	3920
	11th to 20th	Night	34% 4 12% 25% 21% 2	3899
	21st to end	Night	43% 4 12% 27% 12%	4300
November	1st to 10th	Night	44% 6% 12% 25% 12% 2	2821
	11th to 20th	Night	52% 5% 12% 20% 10% 1	2940
	21st to end	Night	48% 6% 17% 21% 6% 1	2940
December	1st to 10th	Night	53% 6% 17% 19% 4	4720
	11th to 20th	Night	60% 5% 10% 18% 5% 1	5341
	21st to end	Night	71% 4 9% 12% 4	7150

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% 3	330
	11th to 20th	Day	13% 10% 38% 33% 6% 1	330
	21st to end	Day	13% 10% 38% 25% 12% 2	363
February	1st to 10th	Day	16% 13% 41% 24% 5% 1	330
	11th to 20th	Day	14% 13% 39% 25% 7% 2	330
	21st to end	Day	13% 13% 41% 28% 4	273
March	1st to 10th	Day	18% 13% 38% 23% 8% 1	330
	11th to 20th	Day	18% 10% 38% 24% 8% 2	330
	21st to end	Day	24% 13% 31% 22% 7% 3	379
April	1st to 10th	Day	22% 15% 31% 25% 6%	540
	11th to 20th	Day	24% 14% 29% 23% 9% 1	573
	21st to end	Day	25% 11% 31% 21% 9% 3	750
May	1st to 10th	Day	18% 13% 29% 26% 12% 2	753
	11th to 20th	Day	19% 12% 29% 26% 11% 3	775
	21st to end	Day	18% 8% 28% 29% 14% 4	844
June	1st to 10th	Day	10% 8% 29% 30% 19% 5%	780
	11th to 20th	Day	14% 10% 36% 29% 11% 1	780
	21st to end	Day	14% 7% 32% 28% 13% 6%	780
July	1st to 10th	Day	9% 4 20% 32% 24% 11%	771
	11th to 20th	Day	8% 5% 26% 31% 21% 9%	768
	21st to end	Day	6% 5% 24% 36% 22% 7%	849
August	1st to 10th	Day	10% 5% 24% 32% 23% 6%	776
	11th to 20th	Day	14% 4 22% 30% 25% 5%	736
	21st to end	Day	11% 6% 24% 31% 22% 5%	702
September	1st to 10th	Day	12% 7% 28% 29% 19% 5%	530
	11th to 20th	Day	12% 4 27% 32% 20% 5%	485
	21st to end	Day	15% 12% 28% 22% 15% 8%	330
October	1st to 10th	Day	17% 11% 28% 24% 14% 6%	330
	11th to 20th	Day	15% 17% 37% 22% 7% 2	330
	21st to end	Day	22% 13% 40% 17% 7% 2	363
November	1st to 10th	Day	17% 17% 36% 21% 8% 1	330
	11th to 20th	Day	17% 19% 38% 22% 4	330
	21st to end	Day	19% 15% 40% 20% 5%	330
December	1st to 10th	Day	16% 11% 38% 24% 11%	330
	11th to 20th	Day	20% 14% 36% 25% 4	330
	21st to end	Day	32% 8% 36% 21% 2 1	363
January	1st to 10th	Night	12% 10% 36% 32% 9% 1	1050
	11th to 20th	Night	16% 8% 39% 28% 8% 1	1050
	21st to end	Night	15% 10% 38% 29% 8%	1155
February	1st to 10th	Night	12% 10% 39% 31% 7%	1050
	11th to 20th	Night	10% 11% 43% 28% 7% 1	1029
	21st to end	Night	15% 10% 36% 31% 8% 1	822
March	1st to 10th	Night	15% 11% 37% 29% 8% 1	692
	11th to 20th	Night	17% 10% 39% 27% 6%	630
	21st to end	Night	23% 9% 29% 28% 9% 2	693
April	1st to 10th	Night	20% 11% 34% 29% 5%	630
	11th to 20th	Night	22% 12% 31% 27% 7% 1	623
	21st to end	Night	24% 9% 30% 27% 8% 2	630
May	1st to 10th	Night	15% 10% 34% 31% 9%	630
	11th to 20th	Night	17% 7% 32% 32% 11% 1	621
	21st to end	Night	15% 6% 25% 39% 14%	683
June	1st to 10th	Night	13% 6% 27% 41% 13% 1	630
	11th to 20th	Night	12% 4 32% 42% 10%	630
	21st to end	Night	10% 4 25% 44% 17% 1	630
July	1st to 10th	Night	6% 2 19% 45% 26% 2	615
	11th to 20th	Night	7% 4 21% 46% 19% 2	612
	21st to end	Night	6% 2 22% 46% 21% 2	690
August	1st to 10th	Night	10% 3 17% 49% 20% 1	627
	11th to 20th	Night	14% 2 18% 38% 26% 2	606
	21st to end	Night	9% 2 24% 39% 24% 3	660
September	1st to 10th	Night	10% 3 25% 41% 18% 3	600
	11th to 20th	Night	11% 4 29% 35% 20% 2	600
	21st to end	Night	16% 6% 28% 33% 16% 2	665
October	1st to 10th	Night	18% 7% 30% 31% 13% 1	800
	11th to 20th	Night	19% 13% 34% 28% 6% 1	800
	21st to end	Night	23% 9% 31% 30% 6%	880
November	1st to 10th	Night	17% 10% 33% 31% 8%	975
	11th to 20th	Night	19% 12% 34% 26% 7% 1	1050
	21st to end	Night	18% 12% 37% 26% 7%	1050
December	1st to 10th	Night	14% 10% 35% 32% 7% 1	1050
	11th to 20th	Night	17% 10% 38% 28% 6%	1050
	21st to end	Night	25% 9% 33% 27% 5% 1	1155

Page generated at 2026-08-12 02:37:20. Version ac07

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