Tatum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tatum 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	5% 4 1 4 3 3 3 3 5% 6% 64%	124282
	Night	4 3 1 3 2 3 3 3 5% 7% 65%	123324
Transparency	Day	22% 3 6% 18% 28% 23%	124048
	Night	20% 3 6% 18% 28% 25%	123146
Seeing	Day	6% 6% 22% 32% 25% 9%	15941
	Night	6% 5% 31% 40% 17% 1	25965

By Month

Percentage Cloud Cover SW (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	6% 9% 3 6% 3 3 3 2 3 5% 58%	3731
	11th to 20th	Day	2 2 1 2 2 2 2 3 4 6% 75%	4376
	21st to end	Day	4 6% 2 6% 4 4 4 3 5% 6% 56%	4994
February	1st to 10th	Day	4 5% 2 4 3 3 3 3 5% 5% 65%	2000
	11th to 20th	Day	5% 5% 2 5% 3 2 3 3 4 6% 60%	1949
	21st to end	Day	3 4 2 3 3 2 3 3 4 6% 65%	1650
March	1st to 10th	Day	2 4 1 4 2 3 3 3 4 6% 67%	4500
	11th to 20th	Day	9% 4 1 3 3 3 3 3 5% 6% 59%	4690
	21st to end	Day	2 4 1 4 3 4 4 4 6% 7% 62%	5969
April	1st to 10th	Day	5% 3 1 2 2 3 3 3 4 5% 68%	2400
	11th to 20th	Day	5% 4 1 4 2 2 3 3 4 5% 67%	2371
	21st to end	Day	3 3 1 3 2 2 2 3 4 6% 72%	2400
May	1st to 10th	Day	6% 3 1 3 3 2 3 2 4 5% 67%	5476
	11th to 20th	Day	6% 3 1 3 2 2 3 2 4 5% 67%	5862
	21st to end	Day	7% 3 1 3 2 2 3 3 4 7% 64%	6446
June	1st to 10th	Day	4 3 1 4 4 3 4 3 5% 8% 62%	2600
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 8% 68%	2736
	21st to end	Day	4 3 2 4 4 4 4 4 6% 8% 57%	2800
July	1st to 10th	Day	8% 4 2 5% 5% 5% 5% 4 7% 9% 46%	4501
	11th to 20th	Day	4 2 1 3 4 4 4 3 5% 8% 62%	4200
	21st to end	Day	5% 3 1 5% 4 4 5% 4 6% 9% 53%	4118
August	1st to 10th	Day	2 2 3 2 3 3 3 5% 9% 68%	3784
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 61%	3726
	21st to end	Day	6% 4 1 4 4 4 4 4 6% 8% 56%	4180
September	1st to 10th	Day	9% 4 1 5% 4 3 4 4 5% 8% 52%	2280
	11th to 20th	Day	10% 5% 2 6% 4 4 3 3 5% 6% 51%	2145
	21st to end	Day	8% 3 1 4 2 2 2 2 4 5% 66%	2090
October	1st to 10th	Day	4 2 1 2 2 1 1 2 3 5% 77%	3480
	11th to 20th	Day	7% 3 1 4 2 2 2 2 3 5% 68%	3256
	21st to end	Day	3 2 1 2 1 1 2 2 4 5% 77%	3390
November	1st to 10th	Day	5% 3 1 3 2 2 3 2 3 6% 70%	1763
	11th to 20th	Day	2 4 2 4 2 2 2 2 4 6% 70%	1800
	21st to end	Day	7% 4 2 3 2 2 3 3 4 6% 64%	1800
December	1st to 10th	Day	7% 7% 3 6% 3 4 4 2 4 5% 56%	2940
	11th to 20th	Day	2 3 1 4 3 4 3 4 4 6% 65%	3391
	21st to end	Day	3 3 2 3 2 2 2 2 3 4 75%	4488
January	1st to 10th	Night	5% 6% 2 5% 3 3 3 3 5% 6% 60%	6222
	11th to 20th	Night	2 3 1 2 2 2 2 2 5% 7% 73%	5591
	21st to end	Night	4 4 2 5% 3 3 3 3 6% 7% 60%	5984
February	1st to 10th	Night	4 4 2 4 2 2 3 3 4 6% 66%	2400
	11th to 20th	Night	5% 4 2 5% 3 3 3 3 5% 7% 60%	2338
	21st to end	Night	4 4 1 4 3 3 4 4 5% 6% 63%	1980
March	1st to 10th	Night	3 4 2 3 2 2 3 2 4 5% 71%	5374
	11th to 20th	Night	7% 4 2 4 2 2 3 3 4 6% 62%	5274
	21st to end	Night	2 3 1 2 2 2 3 3 5% 7% 68%	4984
April	1st to 10th	Night	4 3 2 3 2 2 3 3 4 5% 68%	2000
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 7% 63%	1977
	21st to end	Night	3 3 1 2 2 3 3 3 5% 6% 70%	2000
May	1st to 10th	Night	7% 4 1 3 2 2 2 2 4 6% 68%	4504
	11th to 20th	Night	6% 3 1 4 3 3 3 2 5% 6% 65%	4053
	21st to end	Night	5% 3 1 2 2 2 3 3 3 6% 70%	4451
June	1st to 10th	Night	2 3 1 4 3 3 4 5% 7% 10% 59%	1800
	11th to 20th	Night	2 1 3 2 3 3 5% 6% 9% 67%	1656
	21st to end	Night	3 3 1 4 2 3 4 4 7% 10% 58%	1600
July	1st to 10th	Night	4 4 1 4 4 4 5% 4 7% 10% 53%	2580
	11th to 20th	Night	2 2 1 4 4 4 4 4 6% 11% 59%	2762
	21st to end	Night	5% 3 1 4 3 3 4 4 7% 13% 52%	3445
August	1st to 10th	Night	2 2 1 3 3 3 4 3 7% 10% 63%	3155
	11th to 20th	Night	3 2 1 3 3 3 4 3 6% 10% 62%	3113
	21st to end	Night	4 3 1 4 4 3 5% 4 7% 11% 55%	3487
September	1st to 10th	Night	7% 5% 2 4 3 3 4 4 6% 8% 55%	1900
	11th to 20th	Night	8% 4 1 4 3 3 4 3 6% 7% 56%	2033
	21st to end	Night	8% 3 1 3 2 3 3 2 4 5% 66%	2090
October	1st to 10th	Night	4 2 1 2 1 2 2 2 2 3 79%	3480
	11th to 20th	Night	12% 3 1 2 1 2 1 2 2 3 72%	3666
	21st to end	Night	5% 3 1 2 1 2 2 1 3 6% 75%	4134
November	1st to 10th	Night	5% 3 1 2 1 2 3 2 4 5% 72%	2535
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 7% 69%	2600
	21st to end	Night	7% 3 1 3 3 3 3 3 5% 7% 63%	2600
December	1st to 10th	Night	8% 7% 2 5% 3 3 3 2 4 6% 59%	4250
	11th to 20th	Night	3 3 1 3 2 2 3 3 5% 6% 69%	4816
	21st to end	Night	3 3 1 4 3 2 2 2 3 5% 72%	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	31% 2 3 15% 31% 17%	3732
	11th to 20th	Day	13% 1 5% 22% 36% 23%	4376
	21st to end	Day	30% 3 4 18% 25% 20%	4994
February	1st to 10th	Day	21% 2 5% 17% 27% 27%	2000
	11th to 20th	Day	26% 2 4 19% 25% 23%	1959
	21st to end	Day	22% 4 2 11% 31% 31%	1650
March	1st to 10th	Day	20% 1 1 7% 31% 39%	4500
	11th to 20th	Day	27% 3 3 10% 29% 29%	4690
	21st to end	Day	22% 3 4 14% 27% 31%	5969
April	1st to 10th	Day	19% 3 4 13% 31% 30%	2400
	11th to 20th	Day	20% 3 4 16% 24% 33%	2374
	21st to end	Day	16% 2 5% 19% 31% 27%	2400
May	1st to 10th	Day	21% 2 6% 18% 22% 32%	5476
	11th to 20th	Day	20% 3 4 12% 22% 39%	5866
	21st to end	Day	21% 3 8% 15% 19% 34%	6447
June	1st to 10th	Day	21% 5% 11% 25% 24% 15%	2600
	11th to 20th	Day	16% 3 7% 17% 28% 29%	2736
	21st to end	Day	24% 5% 9% 22% 24% 16%	2800
July	1st to 10th	Day	34% 4 12% 24% 17% 9%	4432
	11th to 20th	Day	21% 3 12% 30% 28% 6%	4125
	21st to end	Day	26% 4 11% 28% 27% 4	3988
August	1st to 10th	Day	14% 3 9% 29% 33% 11%	3772
	11th to 20th	Day	20% 4 11% 20% 34% 11%	3762
	21st to end	Day	26% 4 11% 27% 26% 5%	4179
September	1st to 10th	Day	30% 6% 11% 17% 22% 13%	2280
	11th to 20th	Day	33% 5% 8% 20% 25% 10%	2145
	21st to end	Day	21% 3 5% 20% 33% 18%	2090
October	1st to 10th	Day	13% 2 5% 12% 30% 38%	3480
	11th to 20th	Day	22% 1 8% 13% 27% 29%	3256
	21st to end	Day	12% 1 1 19% 33% 32%	3390
November	1st to 10th	Day	18% 2 5% 16% 33% 25%	1763
	11th to 20th	Day	18% 2 4 19% 36% 21%	1800
	21st to end	Day	22% 3 6% 16% 30% 24%	1800
December	1st to 10th	Day	33% 2 3 18% 24% 19%	2940
	11th to 20th	Day	22% 2 5% 15% 31% 25%	3389
	21st to end	Day	16% 2 2 20% 40% 21%	4488
January	1st to 10th	Night	25% 3 3 18% 36% 15%	6222
	11th to 20th	Night	13% 2 6% 20% 35% 24%	5591
	21st to end	Night	24% 3 5% 21% 26% 22%	5984
February	1st to 10th	Night	20% 3 5% 16% 26% 31%	2400
	11th to 20th	Night	25% 3 5% 17% 26% 24%	2338
	21st to end	Night	22% 3 3 11% 25% 36%	1980
March	1st to 10th	Night	17% 2 2 8% 28% 43%	5374
	11th to 20th	Night	24% 2 4 12% 20% 38%	5274
	21st to end	Night	16% 3 6% 14% 26% 35%	4984
April	1st to 10th	Night	20% 3 5% 13% 27% 33%	2000
	11th to 20th	Night	21% 4 5% 15% 23% 32%	1977
	21st to end	Night	16% 3 6% 21% 27% 28%	2000
May	1st to 10th	Night	20% 2 7% 22% 16% 33%	4501
	11th to 20th	Night	21% 3 6% 11% 25% 33%	4054
	21st to end	Night	18% 3 9% 15% 14% 41%	4452
June	1st to 10th	Night	19% 5% 13% 27% 21% 15%	1800
	11th to 20th	Night	13% 4 9% 19% 27% 27%	1656
	21st to end	Night	19% 6% 14% 22% 24% 15%	1600
July	1st to 10th	Night	26% 3 16% 33% 17% 4	2540
	11th to 20th	Night	20% 3 15% 26% 31% 5%	2714
	21st to end	Night	24% 3 12% 31% 26% 3	3336
August	1st to 10th	Night	17% 3 10% 29% 31% 10%	3144
	11th to 20th	Night	18% 4 11% 26% 33% 8%	3143
	21st to end	Night	23% 4 14% 28% 25% 5%	3487
September	1st to 10th	Night	28% 6% 12% 18% 25% 12%	1900
	11th to 20th	Night	27% 5% 12% 21% 23% 11%	2033
	21st to end	Night	22% 3 6% 18% 32% 19%	2090
October	1st to 10th	Night	13% 2 5% 12% 23% 44%	3480
	11th to 20th	Night	21% 1 6% 14% 25% 33%	3666
	21st to end	Night	14% 1 4 15% 33% 32%	4134
November	1st to 10th	Night	16% 2 4 19% 33% 26%	2535
	11th to 20th	Night	16% 3 4 16% 36% 24%	2600
	21st to end	Night	22% 2 4 17% 31% 23%	2600
December	1st to 10th	Night	29% 1 3 17% 33% 16%	4250
	11th to 20th	Night	17% 2 3 13% 38% 26%	4817
	21st to end	Night	18% 2 2 17% 37% 24%	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	10% 7% 42% 28% 13% 1	300
	11th to 20th	Day	8% 11% 37% 30% 13% 2	300
	21st to end	Day	5% 11% 32% 35% 15% 2	330
February	1st to 10th	Day	7% 9% 35% 36% 11% 1	300
	11th to 20th	Day	8% 11% 40% 30% 11% 1	300
	21st to end	Day	6% 16% 40% 33% 5%	249
March	1st to 10th	Day	7% 10% 37% 37% 8% 1	300
	11th to 20th	Day	12% 13% 26% 34% 11% 3	345
	21st to end	Day	6% 14% 38% 32% 9% 2	550
April	1st to 10th	Day	9% 12% 35% 32% 11% 1	500
	11th to 20th	Day	7% 12% 43% 26% 10% 1	492
	21st to end	Day	5% 11% 36% 35% 12% 1	500
May	1st to 10th	Day	4 10% 33% 40% 11% 2	513
	11th to 20th	Day	7% 6% 27% 35% 20% 4	725
	21st to end	Day	5% 5% 28% 40% 20% 2	789
June	1st to 10th	Day	5% 3 16% 42% 28% 6%	730
	11th to 20th	Day	3 1 18% 46% 26% 5%	730
	21st to end	Day	5% 1 10% 36% 38% 10%	730
July	1st to 10th	Day	6% 1 6% 29% 41% 17%	697
	11th to 20th	Day	3 1 3 26% 47% 20%	622
	21st to end	Day	3 1 2 20% 46% 28%	531
August	1st to 10th	Day	3 1 5% 19% 45% 26%	487
	11th to 20th	Day	5% 1 6% 21% 45% 22%	477
	21st to end	Day	7% 1 4 22% 40% 27%	539
September	1st to 10th	Day	9% 4 27% 40% 20%	490
	11th to 20th	Day	5% 1 6% 26% 46% 17%	355
	21st to end	Day	12% 1 7% 22% 43% 15%	300
October	1st to 10th	Day	11% 2 14% 30% 33% 9%	300
	11th to 20th	Day	9% 2 22% 31% 24% 11%	300
	21st to end	Day	9% 5% 25% 26% 24% 11%	330
November	1st to 10th	Day	11% 7% 26% 31% 23% 3	300
	11th to 20th	Day	3 12% 23% 33% 23% 7%	300
	21st to end	Day	8% 13% 31% 27% 16% 4	300
December	1st to 10th	Day	9% 10% 31% 31% 18% 1	300
	11th to 20th	Day	5% 9% 33% 39% 13% 1	300
	21st to end	Day	7% 8% 42% 33% 7% 2	330
January	1st to 10th	Night	6% 7% 35% 39% 11% 1	981
	11th to 20th	Night	7% 6% 38% 35% 12% 2	800
	21st to end	Night	6% 7% 38% 40% 9%	880
February	1st to 10th	Night	6% 6% 45% 33% 9% 1	800
	11th to 20th	Night	7% 8% 46% 32% 6% 1	784
	21st to end	Night	6% 9% 52% 30% 3	660
March	1st to 10th	Night	7% 9% 44% 35% 5%	800
	11th to 20th	Night	10% 10% 37% 35% 7% 1	782
	21st to end	Night	4 11% 48% 32% 5%	660
April	1st to 10th	Night	6% 9% 46% 34% 5%	600
	11th to 20th	Night	6% 9% 50% 32% 2	593
	21st to end	Night	4 9% 47% 35% 5%	600
May	1st to 10th	Night	6% 8% 43% 39% 4	600
	11th to 20th	Night	7% 6% 35% 42% 10% 1	591
	21st to end	Night	6% 4 40% 41% 9%	650
June	1st to 10th	Night	3 2 35% 51% 9%	600
	11th to 20th	Night	2 1 36% 53% 9%	600
	21st to end	Night	3 2 25% 48% 22%	600
July	1st to 10th	Night	6% 1 12% 48% 31% 2	570
	11th to 20th	Night	3 5% 49% 41% 2	552
	21st to end	Night	3 1 5% 44% 45% 2	624
August	1st to 10th	Night	2 1 5% 40% 50% 2	567
	11th to 20th	Night	4 1 6% 41% 47% 1	564
	21st to end	Night	5% 1 6% 40% 46% 2	627
September	1st to 10th	Night	7% 2 8% 46% 35% 3	570
	11th to 20th	Night	9% 1 12% 47% 29% 1	570
	21st to end	Night	9% 1 14% 47% 28% 1	570
October	1st to 10th	Night	7% 3 21% 48% 22%	570
	11th to 20th	Night	10% 3 25% 44% 17% 2	687
	21st to end	Night	8% 6% 25% 37% 22% 2	838
November	1st to 10th	Night	7% 4 25% 44% 19% 1	975
	11th to 20th	Night	3 5% 29% 44% 18% 1	1000
	21st to end	Night	8% 7% 30% 38% 16% 1	1000
December	1st to 10th	Night	9% 5% 34% 39% 13% 1	1000
	11th to 20th	Night	7% 6% 39% 38% 10%	1000
	21st to end	Night	7% 9% 39% 33% 10% 1	1100

Page generated at 2025-07-02 23:30:13. Version ac07

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