Lowell Discovery Telescope
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lowell Discovery Telescope clear sky chart from 2005-11-06 to 2025-11-03.

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 4 2 5% 3 4 4 3 5% 6% 61%	135786
	Night	2 4 2 5% 3 3 3 3 5% 6% 64%	131789
Transparency	Day	24% 3 5% 17% 30% 21%	130618
	Night	21% 3 5% 17% 31% 22%	127959
Seeing	Day	13% 18% 28% 25% 12% 4	16206
	Night	8% 15% 24% 30% 20% 4	26328

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	1 3 3 7% 4 4 5% 3 4 5% 61%	4098
	11th to 20th	Day	1 4 2 5% 3 3 4 2 3 4 70%	4100
	21st to end	Day	3 8% 4 7% 3 2 3 3 4 6% 56%	4510
February	1st to 10th	Day	2 8% 3 5% 4 2 3 3 5% 6% 58%	1805
	11th to 20th	Day	2 8% 4 4 3 3 3 3 6% 7% 58%	1950
	21st to end	Day	2 8% 3 7% 5% 4 3 2 4 6% 56%	1680
March	1st to 10th	Day	2 9% 3 4 3 3 3 3 4 5% 61%	4954
	11th to 20th	Day	2 13% 4 7% 4 5% 5% 4 6% 6% 44%	5000
	21st to end	Day	2 5% 2 5% 3 3 4 3 5% 7% 61%	5486
April	1st to 10th	Day	2 4 2 5% 4 3 4 4 5% 6% 62%	2200
	11th to 20th	Day	1 5% 2 4 3 2 2 3 4 6% 68%	2332
	21st to end	Day	2 5% 2 3 3 3 3 3 3 6% 67%	2564
May	1st to 10th	Day	4 5% 2 5% 3 3 3 3 4 5% 64%	5880
	11th to 20th	Day	2 3 1 4 3 3 4 3 5% 6% 66%	5841
	21st to end	Day	1 2 1 2 2 2 2 2 4 5% 76%	6423
June	1st to 10th	Day	1 2 1 3 2 2 3 3 4 7% 72%	2600
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 4 83%	2600
	21st to end	Day	2 2 1 4 3 3 4 4 4 5% 67%	2600
July	1st to 10th	Day	2 2 1 4 4 4 4 3 4 5% 67%	5879
	11th to 20th	Day	4 3 2 5% 5% 6% 8% 7% 9% 10% 42%	5880
	21st to end	Day	3 3 1 5% 5% 5% 6% 5% 7% 8% 53%	6401
August	1st to 10th	Day	3 3 2 6% 6% 6% 6% 5% 6% 8% 49%	5779
	11th to 20th	Day	2 3 1 5% 4 5% 6% 5% 7% 9% 52%	5798
	21st to end	Day	3 4 2 7% 5% 6% 6% 5% 7% 8% 46%	6077
September	1st to 10th	Day	5% 5% 2 6% 6% 6% 5% 5% 6% 6% 46%	2200
	11th to 20th	Day	4 2 1 4 3 3 4 3 4 5% 66%	2200
	21st to end	Day	3 3 1 5% 2 3 4 3 4 6% 66%	2200
October	1st to 10th	Day	3 4 2 4 3 3 4 3 4 4 67%	3473
	11th to 20th	Day	5% 4 1 4 2 2 2 2 3 6% 70%	3250
	21st to end	Day	1 2 1 2 2 2 3 2 4 5% 75%	3548
November	1st to 10th	Day	2 3 2 3 2 2 3 3 4 6% 70%	1979
	11th to 20th	Day	1 3 1 3 2 3 3 3 4 5% 71%	1830
	21st to end	Day	3 7% 2 3 2 2 3 2 5% 6% 65%	1800
December	1st to 10th	Day	3 4 1 3 2 2 3 2 4 5% 71%	2950
	11th to 20th	Day	2 6% 3 5% 3 3 3 4 8% 9% 53%	3409
	21st to end	Day	3 7% 3 7% 3 3 4 3 5% 7% 56%	4510
January	1st to 10th	Night	1 4 2 6% 4 3 4 3 5% 6% 62%	5867
	11th to 20th	Night	1 4 1 5% 2 2 2 2 4 6% 71%	5880
	21st to end	Night	3 7% 3 9% 3 4 4 3 4 5% 54%	6468
February	1st to 10th	Night	1 7% 3 5% 3 3 3 3 4 5% 64%	2590
	11th to 20th	Night	2 7% 3 5% 3 3 3 4 4 6% 60%	2338
	21st to end	Night	3 8% 2 6% 3 4 4 3 5% 6% 57%	1950
March	1st to 10th	Night	2 6% 4 8% 3 3 3 3 5% 6% 57%	4940
	11th to 20th	Night	2 10% 4 8% 4 5% 4 3 5% 7% 48%	5000
	21st to end	Night	2 6% 2 4 3 3 4 3 6% 7% 60%	5489
April	1st to 10th	Night	1 5% 2 4 3 3 3 3 4 6% 65%	2200
	11th to 20th	Night	1 4 1 4 3 3 3 3 4 5% 70%	2014
	21st to end	Night	1 2 1 2 3 3 2 2 4 6% 75%	1832
May	1st to 10th	Night	2 4 2 4 3 3 4 2 4 4 69%	4100
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 5% 79%	4074
	21st to end	Night	1 1 1 2 1 2 2 2 4 5% 79%	4474
June	1st to 10th	Night	2 3 1 2 2 2 3 3 5% 5% 73%	1800
	11th to 20th	Night	1 1 2 1 1 2 1 2 3 86%	1800
	21st to end	Night	2 2 1 3 3 3 3 2 4 4 73%	1800
July	1st to 10th	Night	1 2 1 3 3 3 4 3 5% 6% 69%	4100
	11th to 20th	Night	2 3 1 6% 5% 6% 7% 6% 9% 10% 45%	4100
	21st to end	Night	3 3 1 4 4 5% 5% 5% 8% 9% 53%	4472
August	1st to 10th	Night	2 3 1 6% 5% 6% 6% 4 7% 10% 50%	4033
	11th to 20th	Night	2 2 1 3 3 3 5% 4 7% 10% 61%	4051
	21st to end	Night	3 4 2 6% 4 5% 5% 5% 7% 9% 49%	4896
September	1st to 10th	Night	3 4 2 5% 4 4 5% 3 7% 8% 57%	2200
	11th to 20th	Night	2 2 1 2 2 2 2 2 4 4 76%	2200
	21st to end	Night	2 2 1 4 2 3 3 2 4 5% 72%	2200
October	1st to 10th	Night	2 3 1 2 2 2 3 3 3 4 75%	3723
	11th to 20th	Night	2 3 1 3 1 2 2 2 4 7% 72%	3898
	21st to end	Night	2 2 1 3 2 2 3 2 4 5% 73%	4252
November	1st to 10th	Night	3 4 1 3 2 2 3 3 4 5% 70%	2364
	11th to 20th	Night	1 2 1 3 2 2 3 3 4 5% 73%	2565
	21st to end	Night	3 6% 2 5% 3 3 3 2 4 5% 63%	2600
December	1st to 10th	Night	2 4 2 4 2 2 3 3 5% 6% 68%	4240
	11th to 20th	Night	2 5% 2 6% 3 3 3 3 5% 7% 61%	4811
	21st to end	Night	2 6% 3 6% 3 4 3 2 4 4 63%	6468

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	25% 2 3 23% 33% 15%	4099
	11th to 20th	Day	20% 2 6% 19% 22% 31%	4100
	21st to end	Day	29% 2 3 12% 30% 24%	4510
February	1st to 10th	Day	27% 2 7% 16% 28% 20%	1805
	11th to 20th	Day	26% 4 4 22% 26% 18%	1960
	21st to end	Day	31% 2 3 16% 27% 22%	1680
March	1st to 10th	Day	25% 3 2 16% 34% 19%	4954
	11th to 20th	Day	39% 3 3 20% 25% 10%	5000
	21st to end	Day	25% 2 4 29% 32% 8%	5486
April	1st to 10th	Day	23% 3 4 20% 37% 14%	2200
	11th to 20th	Day	19% 2 2 17% 39% 21%	2335
	21st to end	Day	19% 2 4 17% 42% 16%	2564
May	1st to 10th	Day	21% 3 5% 15% 40% 16%	5880
	11th to 20th	Day	19% 2 2 11% 41% 24%	5844
	21st to end	Day	12% 2 2 8% 33% 43%	6424
June	1st to 10th	Day	14% 2 2 14% 35% 32%	2600
	11th to 20th	Day	8% 2 1 7% 30% 52%	2600
	21st to end	Day	20% 2 3 9% 30% 36%	2600
July	1st to 10th	Day	19% 4 5% 10% 19% 43%	5816
	11th to 20th	Day	30% 8% 11% 25% 18% 9%	5811
	21st to end	Day	27% 5% 15% 17% 21% 14%	6217
August	1st to 10th	Day	41% 7% 18% 14% 12% 9%	4208
	11th to 20th	Day	30% 6% 13% 20% 20% 11%	4197
	21st to end	Day	32% 6% 12% 19% 23% 8%	4380
September	1st to 10th	Day	35% 5% 8% 14% 21% 17%	2200
	11th to 20th	Day	21% 2 3 15% 36% 22%	2200
	21st to end	Day	21% 3 3 15% 34% 24%	2200
October	1st to 10th	Day	20% 3 3 11% 24% 38%	3473
	11th to 20th	Day	16% 3 3 11% 47% 20%	3250
	21st to end	Day	12% 3 3 12% 39% 32%	3548
November	1st to 10th	Day	17% 2 3 23% 34% 20%	1979
	11th to 20th	Day	15% 3 5% 17% 37% 24%	1830
	21st to end	Day	23% 2 4 18% 34% 20%	1800
December	1st to 10th	Day	17% 2 4 20% 37% 20%	2950
	11th to 20th	Day	26% 2 4 19% 38% 11%	3408
	21st to end	Day	30% 2 4 24% 29% 11%	4510
January	1st to 10th	Night	24% 2 3 17% 39% 15%	5867
	11th to 20th	Night	17% 2 5% 20% 30% 27%	5880
	21st to end	Night	31% 3 3 11% 28% 24%	6468
February	1st to 10th	Night	24% 2 6% 18% 27% 23%	2590
	11th to 20th	Night	25% 2 5% 23% 25% 19%	2339
	21st to end	Night	28% 3 2 14% 29% 23%	1950
March	1st to 10th	Night	25% 5% 2 15% 34% 18%	4940
	11th to 20th	Night	33% 4 2 21% 29% 12%	5000
	21st to end	Night	24% 2 4 31% 31% 8%	5489
April	1st to 10th	Night	21% 2 5% 19% 36% 17%	2200
	11th to 20th	Night	16% 2 3 16% 37% 26%	2014
	21st to end	Night	13% 2 4 21% 43% 17%	1832
May	1st to 10th	Night	19% 3 4 18% 38% 18%	4097
	11th to 20th	Night	11% 1 2 14% 43% 28%	4075
	21st to end	Night	8% 2 2 8% 37% 43%	4474
June	1st to 10th	Night	14% 2 1 12% 35% 36%	1800
	11th to 20th	Night	8% 1 1 6% 30% 54%	1800
	21st to end	Night	16% 3 3 9% 26% 44%	1800
July	1st to 10th	Night	15% 4 5% 11% 21% 44%	4055
	11th to 20th	Night	27% 6% 14% 27% 16% 10%	4046
	21st to end	Night	24% 5% 18% 19% 18% 16%	4344
August	1st to 10th	Night	35% 7% 23% 13% 12% 11%	2928
	11th to 20th	Night	19% 4 18% 26% 21% 12%	2928
	21st to end	Night	23% 5% 13% 22% 28% 9%	3522
September	1st to 10th	Night	24% 5% 11% 17% 26% 17%	2200
	11th to 20th	Night	13% 3 5% 19% 33% 28%	2200
	21st to end	Night	15% 4 4 15% 34% 28%	2200
October	1st to 10th	Night	13% 2 3 13% 27% 41%	3723
	11th to 20th	Night	12% 2 3 16% 43% 23%	3898
	21st to end	Night	12% 4 3 11% 38% 33%	4252
November	1st to 10th	Night	18% 3 3 21% 35% 20%	2364
	11th to 20th	Night	15% 2 5% 17% 37% 25%	2565
	21st to end	Night	25% 3 3 16% 31% 23%	2600
December	1st to 10th	Night	18% 2 3 19% 34% 23%	4240
	11th to 20th	Night	24% 2 3 15% 38% 17%	4811
	21st to end	Night	26% 2 4 24% 27% 16%	6468

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	18% 26% 31% 17% 8% 1	300
	11th to 20th	Day	16% 25% 30% 21% 7% 2	300
	21st to end	Day	15% 28% 28% 18% 10% 2	330
February	1st to 10th	Day	22% 24% 28% 18% 6% 1	305
	11th to 20th	Day	25% 27% 29% 15% 3	495
	21st to end	Day	27% 28% 28% 14% 3	414
March	1st to 10th	Day	26% 29% 26% 15% 4	500
	11th to 20th	Day	22% 24% 26% 19% 6% 3	500
	21st to end	Day	25% 29% 28% 13% 4 1	550
April	1st to 10th	Day	14% 27% 32% 17% 7% 2	500
	11th to 20th	Day	20% 32% 30% 12% 5% 2	492
	21st to end	Day	16% 28% 28% 18% 7% 3	500
May	1st to 10th	Day	11% 26% 38% 19% 5% 1	503
	11th to 20th	Day	11% 20% 33% 23% 9% 3	528
	21st to end	Day	11% 22% 34% 22% 8% 2	573
June	1st to 10th	Day	4 16% 37% 31% 10% 2	530
	11th to 20th	Day	6% 18% 42% 21% 10% 3	530
	21st to end	Day	4 13% 34% 37% 9% 3	530
July	1st to 10th	Day	5% 5% 32% 38% 16% 4	526
	11th to 20th	Day	6% 3 22% 37% 24% 8%	524
	21st to end	Day	7% 5% 16% 37% 26% 9%	575
August	1st to 10th	Day	6% 4 22% 40% 20% 8%	527
	11th to 20th	Day	6% 6% 17% 44% 20% 7%	517
	21st to end	Day	5% 5% 19% 36% 26% 9%	583
September	1st to 10th	Day	6% 7% 22% 36% 21% 8%	530
	11th to 20th	Day	6% 13% 33% 31% 12% 6%	530
	21st to end	Day	10% 11% 28% 32% 15% 4	530
October	1st to 10th	Day	10% 21% 25% 26% 13% 4	455
	11th to 20th	Day	12% 17% 27% 24% 15% 5%	330
	21st to end	Day	15% 17% 28% 24% 13% 4	363
November	1st to 10th	Day	16% 21% 25% 21% 12% 5%	306
	11th to 20th	Day	14% 19% 26% 24% 13% 5%	300
	21st to end	Day	21% 18% 29% 20% 11% 2	300
December	1st to 10th	Day	14% 24% 26% 23% 11% 2	300
	11th to 20th	Day	13% 20% 32% 25% 7% 3	300
	21st to end	Day	22% 23% 29% 18% 6% 2	330
January	1st to 10th	Night	10% 19% 26% 28% 13% 4	800
	11th to 20th	Night	10% 20% 31% 23% 13% 2	800
	21st to end	Night	11% 25% 26% 25% 10% 2	880
February	1st to 10th	Night	12% 19% 28% 28% 11% 2	800
	11th to 20th	Night	10% 22% 32% 25% 9% 1	784
	21st to end	Night	13% 24% 27% 24% 9% 2	660
March	1st to 10th	Night	15% 25% 28% 22% 8% 1	800
	11th to 20th	Night	11% 17% 28% 26% 14% 3	800
	21st to end	Night	10% 22% 29% 27% 11% 1	880
April	1st to 10th	Night	8% 24% 28% 27% 12% 1	800
	11th to 20th	Night	10% 20% 32% 29% 8% 1	630
	21st to end	Night	7% 21% 28% 28% 13% 3	600
May	1st to 10th	Night	5% 18% 33% 34% 11% 1	600
	11th to 20th	Night	6% 13% 27% 31% 20% 3	591
	21st to end	Night	6% 16% 31% 31% 15% 2	650
June	1st to 10th	Night	3 10% 32% 38% 16% 1	600
	11th to 20th	Night	2 10% 32% 37% 17% 1	600
	21st to end	Night	3 5% 24% 41% 24% 3	600
July	1st to 10th	Night	2 1 17% 44% 33% 3	585
	11th to 20th	Night	4 1 11% 37% 40% 7%	582
	21st to end	Night	5% 3 9% 29% 41% 12%	657
August	1st to 10th	Night	4 2 10% 32% 40% 12%	597
	11th to 20th	Night	4 2 7% 36% 41% 10%	594
	21st to end	Night	4 2 8% 29% 46% 12%	690
September	1st to 10th	Night	4 4 11% 29% 44% 8%	800
	11th to 20th	Night	3 5% 19% 33% 36% 5%	800
	21st to end	Night	4 10% 16% 31% 31% 7%	800
October	1st to 10th	Night	6% 14% 22% 32% 23% 3	800
	11th to 20th	Night	7% 11% 24% 35% 20% 4	800
	21st to end	Night	5% 17% 22% 29% 21% 7%	880
November	1st to 10th	Night	8% 16% 24% 28% 18% 5%	788
	11th to 20th	Night	6% 17% 25% 30% 18% 5%	800
	21st to end	Night	12% 18% 25% 27% 15% 3	800
December	1st to 10th	Night	13% 16% 26% 29% 16% 1	800
	11th to 20th	Night	10% 19% 28% 28% 14% 2	800
	21st to end	Night	13% 21% 28% 28% 8% 2	880

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