Telescope Row
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Telescope Row clear sky chart from 2005-11-06 to 2025-07-21.

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	8% 5% 1 4 3 3 3 2 4 5% 63%	123971
	Night	9% 7% 2 4 3 3 4 3 4 6% 56%	124068
Transparency	Day	24% 3 6% 18% 26% 23%	123727
	Night	29% 3 7% 17% 23% 22%	123899
Seeing	Day	11% 10% 23% 31% 19% 6%	16309
	Night	12% 5% 13% 32% 29% 8%	25680

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	16% 8% 2 4 3 2 3 3 4 8% 48%	3842
	11th to 20th	Day	12% 6% 2 6% 4 3 4 2 3 4 54%	4100
	21st to end	Day	17% 8% 3 6% 3 4 4 3 4 6% 42%	4510
February	1st to 10th	Day	20% 8% 2 4 3 3 3 3 5% 6% 43%	1800
	11th to 20th	Day	15% 6% 2 5% 5% 4 4 3 6% 6% 43%	1881
	21st to end	Day	16% 9% 2 5% 3 4 3 3 4 5% 46%	1650
March	1st to 10th	Day	13% 13% 4 9% 6% 6% 6% 4 5% 6% 30%	4813
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 5% 7% 46%	5000
	21st to end	Day	13% 11% 3 7% 5% 5% 5% 4 5% 8% 34%	5612
April	1st to 10th	Day	15% 7% 2 6% 4 4 3 3 5% 7% 45%	2400
	11th to 20th	Day	12% 5% 1 4 3 3 3 3 5% 6% 55%	2369
	21st to end	Day	7% 5% 2 4 3 3 3 3 5% 8% 58%	2400
May	1st to 10th	Day	5% 3 1 2 1 2 2 2 3 5% 74%	5484
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 5% 78%	6028
	21st to end	Day	2 3 1 2 2 2 3 2 3 5% 76%	6902
June	1st to 10th	Day	4 4 1 4 2 2 3 3 4 6% 66%	2800
	11th to 20th	Day	1 1 1 1 2 1 2 2 3 5% 81%	2800
	21st to end	Day	2 2 1 1 1 1 2 2 2 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 91%	4560
	11th to 20th	Day	1 1 1 1 1 1 2 1 2 3 85%	4306
	21st to end	Day	1 1 2 2 2 4 86%	4456
August	1st to 10th	Day	1 1 2 1 2 3 3 4 5% 78%	4091
	11th to 20th	Day	1 1 1 2 1 2 3 89%	3766
	21st to end	Day	1 1 1 1 1 2 1 2 4 85%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 1 2 3 86%	2280
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 6% 73%	2280
	21st to end	Day	3 1 1 1 1 2 2 1 3 4 79%	2032
October	1st to 10th	Day	2 3 1 3 2 2 2 2 3 6% 75%	3180
	11th to 20th	Day	3 2 1 2 2 2 4 2 4 6% 72%	3161
	21st to end	Day	4 3 2 4 3 3 3 3 5% 6% 64%	3448
November	1st to 10th	Day	8% 6% 2 4 4 3 5% 3 5% 6% 54%	1898
	11th to 20th	Day	9% 6% 2 5% 5% 4 4 3 5% 7% 53%	1800
	21st to end	Day	14% 6% 1 4 3 2 4 3 5% 7% 52%	1690
December	1st to 10th	Day	7% 5% 2 5% 4 3 3 3 5% 8% 54%	2620
	11th to 20th	Day	24% 9% 2 5% 4 3 4 3 5% 7% 33%	3028
	21st to end	Day	20% 12% 3 8% 4 4 4 4 5% 6% 29%	4004
January	1st to 10th	Night	10% 9% 2 5% 3 3 4 4 5% 7% 47%	6108
	11th to 20th	Night	14% 7% 2 4 3 4 4 3 4 5% 51%	5880
	21st to end	Night	16% 10% 2 6% 3 3 4 3 4 5% 44%	6468
February	1st to 10th	Night	18% 8% 2 4 3 3 3 2 4 6% 46%	2600
	11th to 20th	Night	15% 8% 2 5% 3 4 4 4 5% 7% 42%	2406
	21st to end	Night	13% 9% 2 5% 3 3 3 3 4 6% 48%	1980
March	1st to 10th	Night	17% 19% 4 7% 3 3 4 2 3 4 33%	5079
	11th to 20th	Night	16% 11% 3 5% 3 3 3 2 4 5% 46%	5000
	21st to end	Night	17% 15% 3 5% 4 4 4 3 5% 6% 33%	5361
April	1st to 10th	Night	11% 9% 2 5% 5% 4 5% 3 6% 6% 45%	2000
	11th to 20th	Night	9% 7% 2 5% 5% 3 3 3 4 7% 53%	1979
	21st to end	Night	6% 7% 2 5% 3 3 4 3 5% 9% 54%	2000
May	1st to 10th	Night	3 5% 2 5% 4 3 3 2 3 5% 66%	4496
	11th to 20th	Night	2 5% 1 3 3 3 2 2 4 7% 68%	3873
	21st to end	Night	2 2 1 3 2 2 4 2 4 5% 73%	3972
June	1st to 10th	Night	3 4 2 3 3 3 3 2 5% 8% 66%	1600
	11th to 20th	Night	1 2 2 2 2 2 3 3 3 6% 76%	1600
	21st to end	Night	1 1 1 2 1 2 2 1 2 3 83%	1600
July	1st to 10th	Night	1 1 1 2 1 1 3 90%	2620
	11th to 20th	Night	1 1 1 1 1 3 2 3 5% 81%	2876
	21st to end	Night	1 2 2 7% 3 4 6% 74%	3109
August	1st to 10th	Night	1 2 1 2 2 2 3 2 4 6% 76%	2848
	11th to 20th	Night	1 1 1 4 2 3 4 84%	3081
	21st to end	Night	2 1 2 1 2 3 2 3 5% 79%	3498
September	1st to 10th	Night	1 2 2 1 1 2 2 2 5% 82%	1900
	11th to 20th	Night	4 3 1 3 2 2 4 2 4 5% 70%	1900
	21st to end	Night	2 3 1 2 2 2 2 2 3 5% 78%	2143
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 5% 76%	3800
	11th to 20th	Night	3 3 1 3 3 2 7% 3 4 6% 66%	3777
	21st to end	Night	5% 5% 1 4 3 3 3 3 4 5% 63%	4121
November	1st to 10th	Night	9% 6% 1 5% 3 2 4 3 4 7% 57%	2400
	11th to 20th	Night	8% 7% 2 4 3 3 4 4 6% 7% 51%	2600
	21st to end	Night	11% 6% 1 4 4 3 3 3 4 6% 54%	2705
December	1st to 10th	Night	8% 5% 2 4 4 3 4 3 4 6% 58%	4560
	11th to 20th	Night	23% 10% 2 5% 4 4 4 4 7% 9% 29%	5176
	21st to end	Night	21% 11% 3 9% 5% 5% 5% 4 4 5% 26%	6952

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	36% 3 15% 23% 17% 5%	3842
	11th to 20th	Day	36% 1 7% 13% 24% 19%	4100
	21st to end	Day	43% 3 5% 13% 19% 16%	4510
February	1st to 10th	Day	42% 3 5% 19% 20% 11%	1800
	11th to 20th	Day	40% 4 8% 17% 22% 8%	1890
	21st to end	Day	42% 3 6% 22% 20% 7%	1650
March	1st to 10th	Day	53% 5% 7% 15% 18% 3	4813
	11th to 20th	Day	37% 5% 12% 26% 12% 8%	5000
	21st to end	Day	47% 6% 12% 22% 11% 2	5612
April	1st to 10th	Day	39% 4 11% 21% 20% 5%	2400
	11th to 20th	Day	30% 5% 12% 24% 22% 7%	2372
	21st to end	Day	26% 3 11% 30% 25% 6%	2400
May	1st to 10th	Day	14% 3 6% 28% 34% 15%	5484
	11th to 20th	Day	12% 2 5% 34% 36% 10%	6031
	21st to end	Day	12% 2 6% 31% 41% 8%	6904
June	1st to 10th	Day	20% 2 8% 27% 31% 11%	2800
	11th to 20th	Day	9% 2 6% 21% 38% 25%	2800
	21st to end	Day	9% 1 2 12% 31% 45%	2800
July	1st to 10th	Day	4 1 1 5% 16% 75%	4491
	11th to 20th	Day	7% 2 3 6% 15% 67%	4234
	21st to end	Day	3 1 3 6% 29% 58%	4316
August	1st to 10th	Day	10% 1 2 8% 27% 52%	4076
	11th to 20th	Day	3 1 2 7% 39% 49%	3802
	21st to end	Day	6% 1 2 9% 35% 46%	4179
September	1st to 10th	Day	7% 1 1 8% 31% 51%	2280
	11th to 20th	Day	14% 2 3 22% 36% 22%	2280
	21st to end	Day	11% 2 2 21% 39% 26%	2032
October	1st to 10th	Day	13% 2 4 11% 28% 42%	3180
	11th to 20th	Day	13% 4 7% 23% 31% 22%	3161
	21st to end	Day	19% 4 6% 20% 30% 20%	3448
November	1st to 10th	Day	30% 4 6% 21% 24% 14%	1898
	11th to 20th	Day	31% 4 9% 21% 24% 11%	1800
	21st to end	Day	32% 3 8% 17% 29% 11%	1690
December	1st to 10th	Day	28% 3 5% 20% 35% 8%	2620
	11th to 20th	Day	50% 5% 8% 15% 15% 7%	3028
	21st to end	Day	55% 4 10% 17% 10% 3	4004
January	1st to 10th	Night	34% 5% 16% 22% 17% 6%	6108
	11th to 20th	Night	37% 2 8% 13% 25% 16%	5880
	21st to end	Night	42% 3 6% 12% 17% 20%	6468
February	1st to 10th	Night	40% 3 6% 18% 20% 15%	2600
	11th to 20th	Night	40% 4 7% 17% 21% 11%	2408
	21st to end	Night	36% 5% 7% 25% 19% 8%	1980
March	1st to 10th	Night	55% 3 7% 14% 17% 5%	5079
	11th to 20th	Night	41% 4 7% 23% 18% 8%	5000
	21st to end	Night	49% 7% 9% 20% 12% 4	5361
April	1st to 10th	Night	36% 5% 12% 20% 21% 6%	2000
	11th to 20th	Night	32% 3 8% 25% 21% 9%	1979
	21st to end	Night	26% 6% 7% 28% 25% 8%	2000
May	1st to 10th	Night	18% 5% 6% 22% 32% 18%	4493
	11th to 20th	Night	18% 2 5% 32% 32% 11%	3873
	21st to end	Night	13% 3 6% 28% 37% 14%	3972
June	1st to 10th	Night	19% 3 8% 25% 31% 15%	1600
	11th to 20th	Night	11% 3 5% 13% 32% 35%	1600
	21st to end	Night	10% 1 2 13% 30% 45%	1600
July	1st to 10th	Night	4 1 3 17% 75%	2580
	11th to 20th	Night	6% 1 4 5% 19% 64%	2825
	21st to end	Night	7% 1 8% 6% 28% 50%	3011
August	1st to 10th	Night	12% 1 1 7% 26% 52%	2839
	11th to 20th	Night	5% 2 3 6% 33% 52%	3111
	21st to end	Night	9% 1 5% 7% 34% 43%	3498
September	1st to 10th	Night	8% 2 1 6% 24% 59%	1900
	11th to 20th	Night	16% 3 2 19% 34% 26%	1900
	21st to end	Night	12% 2 2 19% 34% 32%	2143
October	1st to 10th	Night	12% 2 3 11% 18% 54%	3800
	11th to 20th	Night	15% 5% 8% 20% 27% 24%	3777
	21st to end	Night	23% 3 4 20% 27% 25%	4121
November	1st to 10th	Night	28% 4 7% 19% 24% 19%	2400
	11th to 20th	Night	29% 4 8% 22% 26% 11%	2600
	21st to end	Night	32% 3 7% 18% 23% 17%	2705
December	1st to 10th	Night	28% 3 5% 20% 29% 16%	4560
	11th to 20th	Night	50% 4 9% 12% 18% 7%	5176
	21st to end	Night	58% 6% 9% 13% 9% 5%	6952

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	31% 12% 25% 20% 10% 1	286
	11th to 20th	Day	20% 13% 23% 27% 14% 3	400
	21st to end	Day	20% 9% 24% 27% 18% 3	440
February	1st to 10th	Day	25% 16% 23% 20% 13% 5%	400
	11th to 20th	Day	23% 16% 27% 24% 9%	460
	21st to end	Day	21% 14% 27% 28% 9% 1	414
March	1st to 10th	Day	25% 16% 28% 22% 7% 2	500
	11th to 20th	Day	25% 17% 21% 21% 13% 3	500
	21st to end	Day	22% 17% 31% 20% 10%	550
April	1st to 10th	Day	19% 16% 29% 23% 13% 1	500
	11th to 20th	Day	17% 16% 30% 23% 12% 2	492
	21st to end	Day	8% 17% 29% 27% 12% 6%	500
May	1st to 10th	Day	13% 15% 32% 28% 11% 3	503
	11th to 20th	Day	10% 14% 31% 25% 15% 5%	528
	21st to end	Day	8% 11% 25% 31% 19% 5%	573
June	1st to 10th	Day	8% 9% 27% 29% 19% 8%	530
	11th to 20th	Day	3 12% 29% 36% 15% 5%	530
	21st to end	Day	2 5% 23% 39% 22% 9%	530
July	1st to 10th	Day	1 3 19% 40% 29% 8%	526
	11th to 20th	Day	1 1 12% 43% 34% 10%	524
	21st to end	Day	1 13% 43% 28% 14%	532
August	1st to 10th	Day	2 3 19% 39% 28% 10%	487
	11th to 20th	Day	1 3 21% 38% 26% 12%	477
	21st to end	Day	1 2 15% 39% 31% 11%	539
September	1st to 10th	Day	3 3 17% 42% 29% 7%	490
	11th to 20th	Day	3 9% 22% 39% 20% 6%	490
	21st to end	Day	5% 7% 19% 41% 24% 4	490
October	1st to 10th	Day	5% 11% 22% 35% 22% 6%	490
	11th to 20th	Day	8% 9% 22% 32% 24% 7%	490
	21st to end	Day	9% 7% 26% 34% 19% 5%	539
November	1st to 10th	Day	11% 11% 22% 28% 24% 5%	434
	11th to 20th	Day	11% 9% 24% 27% 20% 9%	300
	21st to end	Day	15% 9% 23% 33% 16% 4	245
December	1st to 10th	Day	16% 13% 18% 30% 18% 7%	200
	11th to 20th	Day	21% 15% 18% 25% 16% 7%	200
	21st to end	Day	26% 13% 22% 21% 12% 5%	220
January	1st to 10th	Night	20% 8% 15% 31% 20% 8%	800
	11th to 20th	Night	16% 6% 15% 34% 21% 9%	800
	21st to end	Night	21% 5% 13% 34% 21% 6%	880
February	1st to 10th	Night	22% 8% 13% 31% 20% 6%	800
	11th to 20th	Night	20% 8% 17% 30% 19% 6%	784
	21st to end	Night	19% 6% 20% 35% 17% 3	660
March	1st to 10th	Night	24% 8% 15% 31% 19% 4	800
	11th to 20th	Night	23% 8% 15% 27% 23% 5%	800
	21st to end	Night	18% 9% 20% 30% 20% 3	880
April	1st to 10th	Night	16% 9% 20% 32% 21% 3	800
	11th to 20th	Night	13% 9% 19% 34% 20% 5%	791
	21st to end	Night	9% 10% 24% 28% 22% 8%	800
May	1st to 10th	Night	7% 6% 24% 33% 24% 7%	775
	11th to 20th	Night	8% 3 13% 33% 35% 7%	508
	21st to end	Night	5% 6% 14% 36% 33% 6%	433
June	1st to 10th	Night	5% 4 15% 36% 34% 7%	400
	11th to 20th	Night	1 5% 14% 40% 36% 4	400
	21st to end	Night	2 1 8% 37% 46% 7%	400
July	1st to 10th	Night	1 7% 35% 49% 8%	390
	11th to 20th	Night	1 1 1 25% 56% 16%	537
	21st to end	Night	1 1 2 24% 55% 17%	624
August	1st to 10th	Night	2 1 5% 29% 51% 13%	567
	11th to 20th	Night	1 7% 37% 43% 12%	719
	21st to end	Night	1 5% 28% 53% 13%	836
September	1st to 10th	Night	2 1 7% 34% 44% 12%	760
	11th to 20th	Night	4 2 10% 34% 40% 10%	760
	21st to end	Night	3 3 9% 34% 37% 13%	760
October	1st to 10th	Night	5% 3 11% 36% 35% 11%	760
	11th to 20th	Night	8% 4 9% 34% 34% 11%	760
	21st to end	Night	7% 4 12% 33% 27% 17%	836
November	1st to 10th	Night	11% 5% 11% 33% 29% 10%	780
	11th to 20th	Night	12% 4 18% 33% 26% 8%	800
	21st to end	Night	13% 5% 15% 36% 23% 8%	800
December	1st to 10th	Night	18% 5% 16% 32% 23% 7%	800
	11th to 20th	Night	24% 6% 14% 31% 19% 6%	800
	21st to end	Night	22% 10% 19% 28% 18% 4	880

Page generated at 2025-07-17 22:10:04. Version ac07

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