Telescopes in Education
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Telescopes in Education clear sky chart from 2005-11-06 to 2024-11-30.

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	2 2 1 3 3 3 4 3 5% 7% 66%	105215
	Night	2 2 1 3 3 3 4 3 5% 7% 65%	103143
Transparency	Day	17% 6% 8% 14% 27% 28%	104979
	Night	18% 6% 7% 14% 27% 27%	102968
Seeing	Day	6% 11% 22% 31% 23% 7%	14719
	Night	4 5% 20% 41% 26% 3	24790

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	2 2 1 3 3 3 3 4 5% 6% 69%	2858
	11th to 20th	Day	2 2 1 3 2 3 3 3 5% 6% 72%	2860
	21st to end	Day	4 5% 2 5% 4 3 3 3 5% 9% 55%	3146
February	1st to 10th	Day	2 2 1 4 2 2 3 3 5% 6% 70%	1710
	11th to 20th	Day	4 4 1 3 2 2 3 3 5% 5% 67%	1824
	21st to end	Day	2 3 1 3 2 3 3 3 4 5% 71%	1727
March	1st to 10th	Day	1 2 1 3 3 3 4 3 5% 7% 68%	3444
	11th to 20th	Day	3 3 1 3 2 2 3 3 6% 7% 67%	3490
	21st to end	Day	1 3 1 3 3 4 4 4 7% 9% 61%	3825
April	1st to 10th	Day	1 1 1 2 1 1 2 3 5% 7% 74%	2162
	11th to 20th	Day	2 1 1 1 1 2 2 2 3 4 81%	2251
	21st to end	Day	1 2 1 1 1 1 2 2 3 5% 81%	2280
May	1st to 10th	Day	1 1 1 1 2 2 1 2 3 86%	3800
	11th to 20th	Day	1 1 1 2 2 2 2 4 5% 79%	4057
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4477
June	1st to 10th	Day	2 2 1 1 2 3 3 3 5% 6% 71%	2470
	11th to 20th	Day	2 2 2 2 2 2 3 4 6% 75%	2470
	21st to end	Day	4 4 1 4 5% 4 5% 4 7% 8% 54%	2470
July	1st to 10th	Day	2 2 1 3 3 5% 6% 5% 7% 9% 55%	4110
	11th to 20th	Day	2 2 1 4 4 6% 6% 5% 8% 11% 51%	4110
	21st to end	Day	4 5% 2 7% 6% 6% 7% 7% 10% 12% 35%	4454
August	1st to 10th	Day	3 3 2 6% 5% 5% 7% 5% 9% 10% 46%	4092
	11th to 20th	Day	2 3 1 5% 4 5% 5% 5% 8% 10% 51%	3789
	21st to end	Day	3 3 2 6% 5% 5% 6% 7% 9% 10% 44%	4180
September	1st to 10th	Day	5% 4 2 5% 5% 4 6% 5% 7% 7% 50%	2275
	11th to 20th	Day	4 2 1 4 4 4 5% 4 6% 8% 58%	2090
	21st to end	Day	4 2 1 3 2 3 3 3 4 6% 68%	2087
October	1st to 10th	Day	1 1 1 2 1 2 2 2 4 5% 79%	3170
	11th to 20th	Day	2 2 1 2 2 2 2 1 3 3 81%	3150
	21st to end	Day	1 1 1 1 1 1 1 3 2 88%	3438
November	1st to 10th	Day	1 1 1 3 2 2 2 2 3 6% 77%	1959
	11th to 20th	Day	2 2 1 2 2 3 2 3 4 6% 75%	2000
	21st to end	Day	4 3 1 3 3 3 4 4 4 6% 64%	1965
December	1st to 10th	Day	4 2 1 3 3 3 4 4 5% 6% 65%	1710
	11th to 20th	Day	2 4 2 5% 4 4 4 3 4 7% 60%	2169
	21st to end	Day	2 3 1 3 3 3 3 3 5% 6% 68%	3146
January	1st to 10th	Night	2 3 1 3 3 3 3 4 5% 6% 68%	4097
	11th to 20th	Night	1 2 1 2 2 2 2 2 4 5% 78%	4110
	21st to end	Night	7% 5% 2 5% 3 4 4 4 6% 7% 52%	4521
February	1st to 10th	Night	1 2 1 3 3 3 3 3 6% 6% 70%	2470
	11th to 20th	Night	3 3 1 3 2 2 3 3 4 5% 71%	2238
	21st to end	Night	1 2 3 2 3 3 2 7% 6% 70%	1727
March	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 6% 71%	3430
	11th to 20th	Night	3 2 1 3 2 2 3 3 4 6% 72%	3490
	21st to end	Night	1 2 1 3 2 3 4 4 5% 7% 66%	3828
April	1st to 10th	Night	1 2 1 1 2 2 3 3 4 6% 75%	2015
	11th to 20th	Night	1 2 1 2 2 2 2 3 4 5% 76%	1877
	21st to end	Night	1 2 1 1 2 2 2 2 4 81%	1900
May	1st to 10th	Night	1 1 1 1 1 1 2 2 2 3 87%	3170
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 5% 77%	2844
	21st to end	Night	1 1 1 1 1 1 2 2 90%	3110
June	1st to 10th	Night	3 2 1 3 2 2 2 3 4 5% 73%	1710
	11th to 20th	Night	2 2 2 2 2 2 3 4 5% 75%	1710
	21st to end	Night	3 3 1 4 5% 5% 6% 5% 8% 8% 52%	1710
July	1st to 10th	Night	2 3 1 5% 5% 6% 6% 6% 9% 10% 48%	2860
	11th to 20th	Night	2 4 1 6% 7% 7% 9% 7% 11% 13% 33%	2860
	21st to end	Night	4 5% 3 9% 7% 9% 8% 8% 11% 15% 20%	3109
August	1st to 10th	Night	3 4 2 8% 7% 7% 7% 7% 10% 12% 34%	2847
	11th to 20th	Night	3 3 2 6% 6% 6% 8% 7% 10% 13% 37%	3048
	21st to end	Night	3 4 2 7% 6% 7% 8% 7% 11% 11% 35%	3487
September	1st to 10th	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 46%	1900
	11th to 20th	Night	5% 3 1 4 3 3 5% 4 5% 7% 60%	2089
	21st to end	Night	2 2 1 3 3 3 3 3 4 4 70%	2091
October	1st to 10th	Night	1 1 1 2 2 2 2 2 3 4 80%	3800
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 5% 77%	3778
	21st to end	Night	1 1 1 1 1 1 1 2 3 4 86%	4120
November	1st to 10th	Night	1 1 1 2 1 2 2 2 3 7% 78%	2340
	11th to 20th	Night	2 2 1 2 2 3 2 2 4 5% 75%	2400
	21st to end	Night	3 3 1 3 3 3 3 2 5% 7% 66%	2425
December	1st to 10th	Night	3 3 1 3 2 3 3 3 5% 6% 66%	2470
	11th to 20th	Night	2 2 1 3 4 4 5% 4 5% 7% 62%	3041
	21st to end	Night	3 3 1 3 2 3 4 3 5% 6% 66%	4521

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	16% 5% 6% 12% 39% 22%	2859
	11th to 20th	Day	13% 6% 5% 15% 43% 18%	2860
	21st to end	Day	25% 8% 6% 21% 27% 13%	3146
February	1st to 10th	Day	17% 4 4 14% 28% 32%	1710
	11th to 20th	Day	19% 4 6% 13% 29% 28%	1833
	21st to end	Day	17% 3 4 12% 27% 37%	1727
March	1st to 10th	Day	15% 4 7% 9% 39% 25%	3444
	11th to 20th	Day	12% 6% 8% 17% 35% 22%	3490
	21st to end	Day	16% 6% 11% 14% 34% 20%	3825
April	1st to 10th	Day	11% 3 4 9% 38% 35%	2162
	11th to 20th	Day	9% 2 3 8% 29% 49%	2254
	21st to end	Day	9% 2 3 9% 31% 45%	2280
May	1st to 10th	Day	7% 1 2 5% 22% 62%	3800
	11th to 20th	Day	8% 2 6% 8% 30% 46%	4060
	21st to end	Day	5% 1 2 4 16% 72%	4477
June	1st to 10th	Day	13% 4 4 7% 28% 43%	2470
	11th to 20th	Day	11% 2 5% 6% 20% 56%	2470
	21st to end	Day	25% 7% 8% 12% 19% 28%	2470
July	1st to 10th	Day	21% 9% 14% 14% 21% 21%	4046
	11th to 20th	Day	22% 11% 15% 25% 21% 7%	4041
	21st to end	Day	33% 14% 22% 23% 9% 1	4314
August	1st to 10th	Day	28% 11% 16% 23% 18% 4	4078
	11th to 20th	Day	25% 9% 12% 22% 26% 6%	3826
	21st to end	Day	29% 12% 15% 21% 15% 8%	4179
September	1st to 10th	Day	31% 9% 10% 14% 20% 16%	2275
	11th to 20th	Day	23% 6% 8% 17% 26% 21%	2090
	21st to end	Day	17% 4 7% 17% 32% 23%	2087
October	1st to 10th	Day	9% 3 5% 10% 22% 50%	3170
	11th to 20th	Day	12% 2 3 11% 30% 42%	3150
	21st to end	Day	6% 1 3 10% 34% 46%	3438
November	1st to 10th	Day	11% 3 5% 18% 36% 28%	1959
	11th to 20th	Day	13% 4 5% 18% 33% 28%	2000
	21st to end	Day	21% 4 4 18% 29% 23%	1965
December	1st to 10th	Day	19% 4 7% 22% 28% 19%	1710
	11th to 20th	Day	26% 5% 6% 19% 29% 15%	2168
	21st to end	Day	16% 4 8% 16% 41% 15%	3146
January	1st to 10th	Night	17% 5% 7% 13% 41% 18%	4097
	11th to 20th	Night	11% 4 3 20% 47% 16%	4110
	21st to end	Night	30% 7% 7% 20% 25% 12%	4521
February	1st to 10th	Night	15% 4 6% 15% 27% 32%	2470
	11th to 20th	Night	17% 4 4 16% 30% 28%	2239
	21st to end	Night	14% 5% 6% 11% 28% 35%	1727
March	1st to 10th	Night	13% 4 5% 12% 42% 25%	3430
	11th to 20th	Night	12% 4 7% 18% 40% 20%	3490
	21st to end	Night	14% 5% 10% 19% 34% 18%	3828
April	1st to 10th	Night	12% 3 5% 13% 35% 33%	2015
	11th to 20th	Night	12% 2 4 10% 29% 43%	1877
	21st to end	Night	10% 2 4 10% 34% 41%	1900
May	1st to 10th	Night	6% 2 2 5% 30% 55%	3170
	11th to 20th	Night	9% 3 4 9% 25% 49%	2845
	21st to end	Night	5% 1 2 5% 20% 67%	3110
June	1st to 10th	Night	14% 3 4 8% 29% 43%	1710
	11th to 20th	Night	12% 3 5% 5% 20% 55%	1710
	21st to end	Night	25% 8% 10% 12% 17% 28%	1710
July	1st to 10th	Night	26% 10% 14% 14% 18% 18%	2815
	11th to 20th	Night	33% 13% 15% 22% 10% 7%	2806
	21st to end	Night	45% 14% 20% 16% 4 1	3011
August	1st to 10th	Night	36% 13% 17% 19% 11% 4	2838
	11th to 20th	Night	33% 12% 13% 17% 21% 4	3077
	21st to end	Night	35% 12% 20% 17% 11% 5%	3487
September	1st to 10th	Night	31% 12% 12% 13% 18% 14%	1900
	11th to 20th	Night	22% 6% 8% 18% 25% 20%	2089
	21st to end	Night	17% 5% 7% 16% 34% 21%	2091
October	1st to 10th	Night	10% 3 6% 9% 23% 49%	3800
	11th to 20th	Night	11% 4 4 13% 22% 45%	3778
	21st to end	Night	5% 2 3 9% 31% 50%	4120
November	1st to 10th	Night	9% 4 5% 17% 37% 28%	2340
	11th to 20th	Night	13% 3 5% 18% 33% 28%	2400
	21st to end	Night	19% 5% 6% 17% 29% 24%	2425
December	1st to 10th	Night	20% 5% 6% 21% 28% 20%	2470
	11th to 20th	Night	21% 7% 8% 19% 30% 15%	3041
	21st to end	Night	19% 4 6% 16% 35% 21%	4521

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% 15% 26% 35% 12% 1	270
	11th to 20th	Day	8% 7% 27% 39% 16% 3	270
	21st to end	Day	7% 13% 26% 38% 14% 2	297
February	1st to 10th	Day	6% 16% 24% 30% 20% 3	270
	11th to 20th	Day	15% 18% 31% 25% 11%	345
	21st to end	Day	13% 29% 33% 20% 6%	382
March	1st to 10th	Day	7% 26% 34% 25% 7% 2	460
	11th to 20th	Day	8% 22% 29% 27% 10% 3	460
	21st to end	Day	9% 28% 31% 22% 8% 2	506
April	1st to 10th	Day	5% 19% 34% 25% 13% 3	460
	11th to 20th	Day	5% 24% 33% 27% 10% 2	452
	21st to end	Day	5% 21% 34% 27% 10% 3	460
May	1st to 10th	Day	6% 19% 32% 27% 13% 3	463
	11th to 20th	Day	4 14% 31% 32% 17% 3	488
	21st to end	Day	3 12% 36% 35% 14% 2	529
June	1st to 10th	Day	4 7% 26% 38% 21% 5%	490
	11th to 20th	Day	2 6% 23% 39% 25% 5%	490
	21st to end	Day	6% 3 13% 40% 32% 7%	490
July	1st to 10th	Day	5% 2 8% 34% 42% 9%	486
	11th to 20th	Day	6% 1 5% 26% 48% 15%	484
	21st to end	Day	6% 2 6% 25% 41% 19%	531
August	1st to 10th	Day	5% 1 6% 23% 42% 24%	487
	11th to 20th	Day	4 3 6% 32% 42% 13%	477
	21st to end	Day	6% 1 6% 26% 39% 22%	539
September	1st to 10th	Day	8% 2 12% 36% 33% 9%	490
	11th to 20th	Day	4 4 13% 36% 34% 10%	490
	21st to end	Day	5% 7% 18% 36% 23% 11%	487
October	1st to 10th	Day	3 6% 19% 33% 28% 12%	300
	11th to 20th	Day	9% 7% 19% 34% 20% 10%	300
	21st to end	Day	5% 7% 21% 31% 26% 10%	330
November	1st to 10th	Day	5% 11% 27% 34% 17% 5%	300
	11th to 20th	Day	3 7% 26% 35% 26% 4	300
	21st to end	Day	7% 17% 26% 33% 14% 1	299
December	1st to 10th	Day	8% 11% 24% 33% 20% 3	270
	11th to 20th	Day	6% 14% 29% 39% 11% 1	270
	21st to end	Day	10% 14% 28% 35% 9% 3	297
January	1st to 10th	Night	5% 5% 24% 44% 19% 3	760
	11th to 20th	Night	5% 4 22% 45% 21% 3	760
	21st to end	Night	7% 7% 25% 42% 16% 2	836
February	1st to 10th	Night	3 9% 26% 39% 21% 3	760
	11th to 20th	Night	6% 10% 28% 42% 14% 1	744
	21st to end	Night	3 12% 35% 38% 11% 1	628
March	1st to 10th	Night	4 11% 31% 41% 12% 1	760
	11th to 20th	Night	4 8% 30% 37% 18% 3	760
	21st to end	Night	3 10% 28% 41% 16% 2	836
April	1st to 10th	Night	3 7% 33% 42% 14% 1	685
	11th to 20th	Night	3 10% 33% 40% 15% 1	563
	21st to end	Night	1 10% 30% 45% 12% 2	570
May	1st to 10th	Night	2 7% 29% 46% 15% 1	570
	11th to 20th	Night	2 5% 26% 47% 18% 1	561
	21st to end	Night	2 4 25% 49% 19%	617
June	1st to 10th	Night	5% 1 14% 55% 22% 2	570
	11th to 20th	Night	2 2 19% 52% 25% 1	570
	21st to end	Night	4 2 9% 44% 40% 1	570
July	1st to 10th	Night	6% 1 6% 41% 40% 5%	555
	11th to 20th	Night	5% 1 7% 33% 49% 6%	552
	21st to end	Night	5% 1 5% 29% 52% 8%	624
August	1st to 10th	Night	5% 2 6% 23% 53% 11%	567
	11th to 20th	Night	7% 1 5% 25% 54% 8%	564
	21st to end	Night	5% 1 7% 25% 53% 9%	627
September	1st to 10th	Night	7% 2 6% 36% 45% 4	570
	11th to 20th	Night	6% 1 9% 34% 44% 6%	759
	21st to end	Night	3 1 13% 42% 35% 5%	760
October	1st to 10th	Night	3 2 16% 44% 32% 4	760
	11th to 20th	Night	4 3 16% 43% 29% 4	760
	21st to end	Night	3 4 13% 41% 32% 6%	836
November	1st to 10th	Night	2 4 19% 43% 26% 6%	780
	11th to 20th	Night	3 5% 19% 44% 26% 3	800
	21st to end	Night	5% 6% 27% 39% 20% 4	800
December	1st to 10th	Night	6% 4 22% 43% 21% 3	760
	11th to 20th	Night	3 7% 25% 43% 19% 2	760
	21st to end	Night	6% 8% 25% 40% 16% 4	836

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