Tellus Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tellus Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	101148
	Night	13% 7% 2 6% 4 4 4 4 6% 7% 43%	98344
Transparency	Day	40% 6% 11% 20% 19% 5%	104966
	Night	38% 6% 10% 20% 20% 6%	102651
Seeing	Day	17% 10% 23% 24% 18% 9%	14872
	Night	16% 10% 23% 28% 20% 4	24894

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	17% 12% 3 6% 3 4 4 3 5% 6% 37%	2836
	11th to 20th	Day	15% 8% 4 7% 5% 3 3 3 3 4 45%	2840
	21st to end	Day	22% 12% 3 6% 4 3 3 2 3 5% 36%	3293
February	1st to 10th	Day	19% 8% 2 6% 5% 3 4 3 5% 6% 38%	1900
	11th to 20th	Day	22% 8% 3 6% 4 4 3 3 4 5% 39%	1848
	21st to end	Day	22% 7% 3 7% 4 4 3 3 4 5% 38%	1570
March	1st to 10th	Day	23% 9% 3 7% 4 3 3 3 5% 6% 32%	3170
	11th to 20th	Day	14% 7% 3 5% 4 4 3 4 5% 6% 46%	3480
	21st to end	Day	16% 7% 3 8% 5% 4 4 4 5% 7% 38%	3815
April	1st to 10th	Day	12% 6% 2 6% 4 3 5% 4 6% 7% 43%	2197
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 3 5% 6% 46%	2254
	21st to end	Day	11% 5% 1 6% 5% 4 4 5% 5% 8% 47%	2403
May	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 4 6% 9% 41%	4082
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	4081
	21st to end	Day	8% 6% 2 7% 4 5% 5% 5% 7% 9% 43%	4476
June	1st to 10th	Day	10% 6% 3 8% 5% 6% 7% 5% 7% 10% 34%	2470
	11th to 20th	Day	5% 4 1 6% 5% 5% 6% 5% 9% 11% 43%	2470
	21st to end	Day	7% 6% 2 7% 5% 6% 7% 5% 8% 9% 39%	2470
July	1st to 10th	Day	7% 6% 3 8% 7% 7% 8% 7% 10% 12% 27%	4120
	11th to 20th	Day	7% 5% 2 8% 6% 7% 7% 6% 8% 10% 34%	4120
	21st to end	Day	12% 9% 4 11% 8% 8% 7% 6% 8% 9% 18%	4466
August	1st to 10th	Day	5% 3 2 6% 5% 5% 6% 6% 10% 12% 41%	4089
	11th to 20th	Day	7% 5% 2 5% 5% 5% 6% 6% 8% 11% 41%	3729
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 6% 7% 49%	4180
September	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 39%	2128
	11th to 20th	Day	13% 7% 2 5% 5% 5% 5% 4 5% 7% 41%	2090
	21st to end	Day	12% 6% 1 6% 4 4 4 4 6% 6% 46%	2090
October	1st to 10th	Day	10% 5% 2 5% 4 3 4 3 5% 5% 55%	2090
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 4 7% 55%	2012
	21st to end	Day	13% 6% 2 4 4 3 3 3 4 4 54%	2057
November	1st to 10th	Day	12% 6% 2 6% 4 2 4 2 4 5% 54%	1762
	11th to 20th	Day	13% 6% 2 6% 3 3 3 3 5% 6% 50%	1800
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 7% 46%	1773
December	1st to 10th	Day	22% 7% 2 6% 4 4 4 3 5% 7% 37%	1710
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 3 5% 5% 41%	2153
	21st to end	Day	20% 8% 3 6% 5% 3 4 4 5% 7% 34%	3124
January	1st to 10th	Night	18% 10% 2 6% 3 3 4 3 6% 6% 38%	4109
	11th to 20th	Night	15% 10% 4 6% 3 3 3 3 5% 6% 43%	4120
	21st to end	Night	22% 14% 3 6% 3 3 2 2 3 4 38%	4361
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 2 5% 6% 43%	2280
	11th to 20th	Night	22% 7% 3 6% 4 4 4 3 4 4 41%	2218
	21st to end	Night	22% 7% 2 5% 4 4 4 4 4 7% 37%	1884
March	1st to 10th	Night	22% 10% 3 6% 4 4 4 3 5% 6% 33%	3682
	11th to 20th	Night	12% 5% 2 5% 3 4 4 3 6% 8% 49%	3480
	21st to end	Night	16% 7% 2 7% 5% 4 4 3 5% 5% 41%	3817
April	1st to 10th	Night	12% 4 2 5% 4 4 5% 5% 5% 8% 47%	1997
	11th to 20th	Night	13% 5% 2 5% 3 3 4 4 6% 7% 48%	1875
	21st to end	Night	9% 6% 2 5% 4 3 4 3 5% 7% 51%	1775
May	1st to 10th	Night	11% 8% 3 6% 5% 4 4 4 6% 8% 40%	2813
	11th to 20th	Night	16% 7% 2 6% 4 4 4 4 6% 8% 37%	2812
	21st to end	Night	6% 4 2 6% 4 5% 5% 5% 7% 12% 46%	3079
June	1st to 10th	Night	5% 6% 2 7% 5% 5% 6% 6% 8% 13% 36%	1710
	11th to 20th	Night	4 3 2 5% 5% 4 6% 5% 7% 11% 47%	1710
	21st to end	Night	6% 4 2 4 6% 4 5% 6% 8% 11% 43%	1710
July	1st to 10th	Night	7% 7% 3 8% 6% 6% 6% 5% 9% 11% 32%	2840
	11th to 20th	Night	7% 5% 2 6% 5% 6% 6% 5% 10% 13% 36%	2840
	21st to end	Night	11% 9% 3 11% 6% 6% 7% 6% 8% 9% 24%	3087
August	1st to 10th	Night	5% 3 2 6% 5% 4 6% 5% 8% 12% 43%	2825
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 48%	3097
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 7% 53%	3476
September	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 4 6% 8% 44%	2033
	11th to 20th	Night	12% 6% 2 7% 5% 4 5% 3 5% 8% 42%	2090
	21st to end	Night	11% 4 2 4 4 3 4 4 6% 7% 51%	2090
October	1st to 10th	Night	10% 4 2 4 3 3 3 2 5% 5% 58%	2090
	11th to 20th	Night	11% 5% 2 5% 3 3 3 2 3 6% 57%	2166
	21st to end	Night	11% 6% 2 5% 3 3 3 3 4 5% 55%	2536
November	1st to 10th	Night	11% 6% 2 4 3 3 3 3 4 5% 54%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 53%	2600
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 7% 45%	2562
December	1st to 10th	Night	19% 7% 3 7% 4 4 3 4 5% 6% 37%	2470
	11th to 20th	Night	16% 9% 2 6% 3 3 4 3 4 5% 46%	3042
	21st to end	Night	19% 9% 3 8% 4 4 4 3 5% 7% 33%	4532

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	48% 4 6% 16% 23% 3	2837
	11th to 20th	Day	43% 3 7% 21% 23% 3	2840
	21st to end	Day	51% 3 4 17% 20% 4	3293
February	1st to 10th	Day	46% 3 8% 18% 21% 4	1900
	11th to 20th	Day	48% 3 6% 18% 19% 6%	1858
	21st to end	Day	50% 3 7% 13% 20% 7%	1570
March	1st to 10th	Day	53% 4 9% 11% 14% 9%	3170
	11th to 20th	Day	39% 4 7% 18% 23% 9%	3480
	21st to end	Day	45% 4 9% 20% 19% 4	3815
April	1st to 10th	Day	39% 4 9% 22% 20% 6%	2197
	11th to 20th	Day	40% 4 6% 18% 25% 7%	2254
	21st to end	Day	33% 5% 11% 27% 20% 3	2403
May	1st to 10th	Day	38% 6% 14% 23% 17% 2	4120
	11th to 20th	Day	46% 7% 11% 23% 12% 1	4085
	21st to end	Day	35% 7% 14% 23% 20% 2	4489
June	1st to 10th	Day	42% 8% 16% 21% 11% 2	2470
	11th to 20th	Day	30% 8% 19% 28% 13% 1	2470
	21st to end	Day	36% 11% 17% 22% 12% 3	2470
July	1st to 10th	Day	41% 13% 21% 19% 5% 1	4055
	11th to 20th	Day	40% 11% 21% 20% 6% 2	4051
	21st to end	Day	58% 11% 15% 12% 5%	4324
August	1st to 10th	Day	31% 8% 16% 33% 11%	4076
	11th to 20th	Day	32% 9% 18% 24% 14% 2	3765
	21st to end	Day	31% 8% 12% 24% 21% 3	4179
September	1st to 10th	Day	39% 8% 14% 17% 17% 5%	2128
	11th to 20th	Day	41% 6% 11% 19% 20% 2	2090
	21st to end	Day	37% 5% 11% 17% 23% 7%	2090
October	1st to 10th	Day	24% 4 7% 13% 39% 13%	3480
	11th to 20th	Day	21% 2 5% 17% 29% 25%	3317
	21st to end	Day	25% 3 6% 24% 29% 12%	3371
November	1st to 10th	Day	34% 3 6% 17% 31% 9%	1762
	11th to 20th	Day	35% 3 7% 18% 28% 9%	1800
	21st to end	Day	39% 4 8% 18% 26% 6%	1773
December	1st to 10th	Day	47% 4 7% 20% 17% 5%	1710
	11th to 20th	Day	46% 2 5% 19% 22% 6%	2150
	21st to end	Day	48% 5% 7% 20% 17% 2	3124
January	1st to 10th	Night	45% 4 5% 19% 24% 3	4109
	11th to 20th	Night	42% 4 6% 23% 19% 7%	4120
	21st to end	Night	52% 2 4 14% 23% 5%	4361
February	1st to 10th	Night	42% 4 7% 20% 20% 6%	2280
	11th to 20th	Night	48% 3 5% 17% 20% 7%	2219
	21st to end	Night	47% 4 6% 17% 18% 8%	1884
March	1st to 10th	Night	52% 4 9% 14% 15% 6%	3682
	11th to 20th	Night	34% 4 8% 18% 26% 10%	3480
	21st to end	Night	44% 4 6% 21% 19% 5%	3817
April	1st to 10th	Night	35% 5% 10% 19% 25% 6%	1997
	11th to 20th	Night	34% 4 7% 22% 23% 9%	1875
	21st to end	Night	32% 4 10% 29% 20% 4	1775
May	1st to 10th	Night	40% 7% 14% 22% 16% 2	2840
	11th to 20th	Night	43% 6% 14% 24% 11%	2813
	21st to end	Night	29% 7% 16% 27% 19% 3	3087
June	1st to 10th	Night	36% 10% 18% 21% 13% 2	1710
	11th to 20th	Night	28% 7% 22% 26% 15% 1	1710
	21st to end	Night	31% 9% 20% 21% 14% 5%	1710
July	1st to 10th	Night	43% 11% 20% 19% 7% 1	2795
	11th to 20th	Night	36% 12% 21% 22% 7% 1	2786
	21st to end	Night	54% 12% 14% 14% 5% 1	2988
August	1st to 10th	Night	31% 9% 16% 34% 10% 1	2814
	11th to 20th	Night	32% 8% 14% 29% 16% 1	3127
	21st to end	Night	29% 8% 13% 20% 27% 3	3476
September	1st to 10th	Night	37% 8% 16% 18% 15% 6%	2033
	11th to 20th	Night	39% 8% 11% 21% 19% 2	2090
	21st to end	Night	32% 6% 11% 22% 21% 8%	2090
October	1st to 10th	Night	23% 4 9% 14% 39% 12%	3480
	11th to 20th	Night	19% 2 4 17% 34% 23%	3587
	21st to end	Night	23% 3 4 24% 30% 16%	4172
November	1st to 10th	Night	32% 4 9% 18% 26% 11%	2536
	11th to 20th	Night	33% 3 5% 20% 28% 11%	2600
	21st to end	Night	39% 4 6% 20% 25% 6%	2562
December	1st to 10th	Night	46% 4 8% 20% 18% 4	2470
	11th to 20th	Night	41% 3 7% 17% 27% 5%	3044
	21st to end	Night	49% 5% 6% 23% 15% 2	4532

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	28% 22% 35% 13% 2	270
	11th to 20th	Day	29% 22% 29% 15% 4 1	270
	21st to end	Day	24% 26% 34% 12% 3 1	297
February	1st to 10th	Day	29% 18% 36% 15% 2	270
	11th to 20th	Day	38% 18% 32% 11% 1	270
	21st to end	Day	33% 22% 34% 9% 2	225
March	1st to 10th	Day	31% 16% 32% 17% 3 1	294
	11th to 20th	Day	23% 19% 30% 20% 6%	460
	21st to end	Day	22% 18% 33% 20% 7% 1	506
April	1st to 10th	Day	23% 14% 30% 19% 12% 2	460
	11th to 20th	Day	15% 15% 34% 23% 13%	452
	21st to end	Day	16% 11% 29% 26% 15% 3	460
May	1st to 10th	Day	16% 9% 30% 27% 13% 6%	463
	11th to 20th	Day	16% 5% 20% 31% 18% 11%	487
	21st to end	Day	11% 5% 19% 33% 19% 12%	529
June	1st to 10th	Day	13% 3 20% 29% 22% 12%	490
	11th to 20th	Day	6% 4 18% 26% 25% 21%	490
	21st to end	Day	10% 3 17% 28% 25% 17%	490
July	1st to 10th	Day	11% 4 9% 25% 29% 23%	485
	11th to 20th	Day	8% 2 8% 20% 35% 26%	484
	21st to end	Day	11% 3 7% 23% 36% 21%	532
August	1st to 10th	Day	11% 3 11% 26% 27% 22%	486
	11th to 20th	Day	12% 3 11% 31% 26% 18%	479
	21st to end	Day	12% 5% 7% 27% 28% 20%	539
September	1st to 10th	Day	9% 5% 12% 28% 29% 17%	490
	11th to 20th	Day	17% 4 14% 25% 30% 10%	490
	21st to end	Day	14% 4 20% 26% 26% 10%	490
October	1st to 10th	Day	15% 4 21% 32% 20% 8%	490
	11th to 20th	Day	12% 12% 23% 28% 21% 3	490
	21st to end	Day	18% 10% 24% 28% 17% 3	506
November	1st to 10th	Day	18% 14% 29% 22% 13% 4	300
	11th to 20th	Day	19% 17% 35% 20% 7% 2	300
	21st to end	Day	14% 18% 38% 20% 9% 1	291
December	1st to 10th	Day	29% 20% 29% 16% 6% 1	270
	11th to 20th	Day	24% 19% 31% 20% 6%	270
	21st to end	Day	31% 15% 34% 15% 4	297
January	1st to 10th	Night	24% 21% 29% 20% 6%	760
	11th to 20th	Night	24% 17% 30% 22% 6%	760
	21st to end	Night	23% 18% 33% 23% 3	836
February	1st to 10th	Night	22% 14% 33% 26% 5%	760
	11th to 20th	Night	28% 18% 30% 23% 1	743
	21st to end	Night	26% 19% 36% 18% 2	628
March	1st to 10th	Night	21% 15% 33% 25% 5%	760
	11th to 20th	Night	17% 15% 34% 23% 10% 1	760
	21st to end	Night	18% 14% 32% 27% 9%	836
April	1st to 10th	Night	16% 14% 31% 27% 12%	667
	11th to 20th	Night	18% 13% 27% 29% 12% 1	563
	21st to end	Night	13% 8% 26% 35% 17% 1	570
May	1st to 10th	Night	12% 8% 23% 33% 22% 1	570
	11th to 20th	Night	15% 5% 21% 31% 23% 5%	561
	21st to end	Night	9% 3 14% 34% 34% 6%	617
June	1st to 10th	Night	9% 2 12% 39% 32% 5%	570
	11th to 20th	Night	4 3 12% 31% 36% 14%	570
	21st to end	Night	6% 2 7% 35% 41% 8%	570
July	1st to 10th	Night	9% 2 8% 24% 43% 14%	555
	11th to 20th	Night	8% 2 6% 23% 47% 15%	552
	21st to end	Night	9% 3 8% 24% 45% 11%	624
August	1st to 10th	Night	7% 2 5% 32% 41% 13%	567
	11th to 20th	Night	9% 3 9% 27% 43% 9%	564
	21st to end	Night	14% 2 10% 26% 38% 10%	627
September	1st to 10th	Night	11% 3 12% 28% 36% 11%	703
	11th to 20th	Night	16% 3 14% 29% 34% 4	760
	21st to end	Night	13% 4 16% 32% 33% 2	760
October	1st to 10th	Night	11% 5% 22% 33% 22% 7%	760
	11th to 20th	Night	13% 8% 25% 30% 22% 2	760
	21st to end	Night	13% 11% 22% 35% 17% 2	836
November	1st to 10th	Night	15% 8% 26% 29% 20% 1	780
	11th to 20th	Night	17% 12% 31% 28% 11% 1	800
	21st to end	Night	20% 14% 28% 26% 12%	789
December	1st to 10th	Night	24% 16% 29% 21% 8% 1	760
	11th to 20th	Night	26% 12% 31% 24% 8%	760
	21st to end	Night	28% 14% 26% 26% 6%	836

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