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 2024-04-03.

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	9% 5% 2 3 3 2 3 3 4 6% 60%	78996
	Night	11% 6% 2 4 4 2 4 4 4 6% 54%	81168
Transparency	Day	28% 3 6% 19% 25% 20%	78714
	Night	32% 3 7% 17% 22% 19%	80971
Seeing	Day	12% 10% 23% 31% 19% 6%	14554
	Night	12% 5% 13% 32% 30% 9%	24066

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	21% 10% 2 5% 4 2 3 4 3 6% 40%	2672
	11th to 20th	Day	18% 8% 3 7% 7% 4 5% 5% 4 6% 34%	2860
	21st to end	Day	22% 9% 4 6% 5% 3 5% 4 5% 7% 30%	3146
February	1st to 10th	Day	19% 8% 3 3 4 3 3 4 4 6% 44%	1710
	11th to 20th	Day	15% 6% 3 4 5% 3 4 4 6% 6% 44%	1784
	21st to end	Day	17% 8% 2 5% 4 3 3 3 4 5% 46%	1570
March	1st to 10th	Day	17% 10% 3 7% 5% 3 5% 5% 5% 8% 32%	2021
	11th to 20th	Day	18% 7% 3 5% 5% 4 5% 5% 5% 8% 37%	2090
	21st to end	Day	15% 7% 3 5% 5% 3 5% 4 5% 6% 42%	2351
April	1st to 10th	Day	16% 5% 3 5% 5% 3 4 4 4 7% 44%	2185
	11th to 20th	Day	13% 5% 2 4 3 2 3 4 5% 5% 55%	2129
	21st to end	Day	7% 4 2 3 3 3 3 3 4 8% 59%	2160
May	1st to 10th	Day	6% 5% 2 3 3 2 4 4 4 7% 61%	2178
	11th to 20th	Day	6% 4 2 2 3 1 3 4 3 6% 66%	2371
	21st to end	Day	6% 4 2 3 3 2 4 4 4 7% 61%	2723
June	1st to 10th	Day	5% 4 2 3 2 2 3 3 3 7% 65%	2520
	11th to 20th	Day	2 1 1 1 2 1 2 2 3 5% 80%	2520
	21st to end	Day	2 2 1 1 1 1 2 2 3 3 83%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 2 4 85%	2520
	11th to 20th	Day	1 1 1 1 2 1 2 2 3 3 83%	2378
	21st to end	Day	1 1 1 1 2 3 3 4 84%	2546
August	1st to 10th	Day	1 1 3 2 2 2 2 3 4 79%	2328
	11th to 20th	Day	1 1 1 1 2 2 4 86%	2152
	21st to end	Day	1 1 2 1 2 1 2 4 86%	2376
September	1st to 10th	Day	1 1 1 1 1 1 2 1 2 3 86%	2160
	11th to 20th	Day	1 1 1 2 3 2 3 4 4 6% 74%	2160
	21st to end	Day	3 1 1 1 2 2 2 2 3 4 78%	1926
October	1st to 10th	Day	4 3 1 3 3 1 3 3 3 6% 70%	1800
	11th to 20th	Day	5% 3 1 2 3 1 3 3 4 7% 68%	1800
	21st to end	Day	6% 4 2 3 3 2 3 3 4 5% 64%	1980
November	1st to 10th	Day	9% 5% 3 4 4 3 5% 4 5% 7% 53%	1802
	11th to 20th	Day	9% 5% 2 4 5% 3 5% 3 5% 6% 54%	1710
	21st to end	Day	13% 5% 2 3 3 2 3 3 5% 7% 54%	1606
December	1st to 10th	Day	12% 7% 4 5% 5% 3 4 4 5% 6% 47%	1520
	11th to 20th	Day	29% 9% 3 6% 4 2 3 4 4 5% 31%	1928
	21st to end	Day	15% 12% 4 7% 5% 2 4 5% 5% 7% 34%	2794
January	1st to 10th	Night	14% 11% 3 5% 4 3 5% 5% 5% 5% 41%	4273
	11th to 20th	Night	20% 9% 3 5% 6% 3 6% 5% 5% 6% 32%	4110
	21st to end	Night	22% 13% 3 6% 4 3 4 4 5% 5% 31%	4521
February	1st to 10th	Night	17% 7% 3 4 4 3 3 3 4 6% 47%	2470
	11th to 20th	Night	14% 7% 3 4 4 3 4 5% 5% 8% 43%	2283
	21st to end	Night	14% 8% 2 5% 4 2 3 4 4 6% 47%	1884
March	1st to 10th	Night	18% 11% 4 5% 4 2 5% 4 4 5% 37%	2157
	11th to 20th	Night	17% 8% 3 5% 4 3 3 2 4 8% 42%	2090
	21st to end	Night	14% 8% 3 4 4 3 4 4 5% 6% 43%	2245
April	1st to 10th	Night	13% 8% 2 4 5% 3 5% 5% 5% 7% 44%	1820
	11th to 20th	Night	10% 6% 2 4 5% 2 3 3 4 6% 53%	1779
	21st to end	Night	6% 5% 3 3 3 2 4 4 5% 9% 56%	1800
May	1st to 10th	Night	4 5% 3 5% 3 2 4 3 5% 8% 58%	1778
	11th to 20th	Night	5% 5% 2 3 3 3 4 4 4 9% 58%	1519
	21st to end	Night	4 3 1 4 4 3 5% 4 6% 9% 57%	1552
June	1st to 10th	Night	4 4 2 3 3 2 3 3 4 8% 65%	1440
	11th to 20th	Night	1 1 2 2 3 1 3 4 3 6% 74%	1440
	21st to end	Night	2 1 1 2 2 1 3 2 2 4 81%	1440
July	1st to 10th	Night	1 1 1 2 1 3 3 2 4 82%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 5% 82%	1576
	21st to end	Night	1 1 1 3 2 3 3 5% 6% 75%	1773
August	1st to 10th	Night	1 2 1 2 4 2 4 3 3 5% 72%	1611
	11th to 20th	Night	1 1 1 1 1 3 4 4 6% 78%	1748
	21st to end	Night	1 1 1 2 1 3 3 4 5% 79%	1980
September	1st to 10th	Night	1 2 2 2 1 2 2 2 5% 82%	1800
	11th to 20th	Night	4 3 1 3 2 1 4 3 4 5% 72%	1800
	21st to end	Night	2 3 1 2 2 1 2 2 3 5% 77%	2029
October	1st to 10th	Night	4 3 1 3 2 1 3 3 3 6% 70%	2160
	11th to 20th	Night	5% 4 2 3 3 2 3 3 4 6% 66%	2160
	21st to end	Night	6% 3 1 3 3 2 3 3 4 5% 67%	2376
November	1st to 10th	Night	10% 5% 2 4 3 1 3 3 4 7% 57%	2277
	11th to 20th	Night	8% 6% 2 4 4 2 4 5% 6% 8% 52%	2470
	21st to end	Night	10% 5% 1 4 4 2 4 4 4 6% 56%	2569
December	1st to 10th	Night	14% 7% 2 4 5% 2 4 3 4 6% 48%	2660
	11th to 20th	Night	26% 10% 3 4 5% 2 4 4 4 7% 31%	3276
	21st to end	Night	17% 10% 3 8% 6% 4 5% 5% 4 6% 32%	4862

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	47% 3 8% 28% 11% 3	2672
	11th to 20th	Day	52% 2 10% 16% 12% 8%	2860
	21st to end	Day	55% 3 7% 17% 14% 6%	3146
February	1st to 10th	Day	41% 4 5% 20% 20% 11%	1710
	11th to 20th	Day	40% 4 7% 18% 23% 8%	1793
	21st to end	Day	42% 3 6% 22% 20% 7%	1570
March	1st to 10th	Day	51% 5% 6% 17% 16% 4	2021
	11th to 20th	Day	46% 4 7% 20% 18% 6%	2090
	21st to end	Day	43% 5% 11% 20% 17% 4	2351
April	1st to 10th	Day	40% 4 11% 19% 21% 6%	2185
	11th to 20th	Day	31% 5% 12% 23% 23% 7%	2132
	21st to end	Day	25% 3 10% 30% 25% 6%	2160
May	1st to 10th	Day	22% 4 8% 26% 32% 7%	2178
	11th to 20th	Day	21% 2 6% 30% 32% 10%	2374
	21st to end	Day	24% 2 10% 31% 26% 6%	2724
June	1st to 10th	Day	21% 3 8% 26% 29% 12%	2520
	11th to 20th	Day	10% 2 7% 22% 36% 24%	2520
	21st to end	Day	9% 2 2 13% 32% 42%	2520
July	1st to 10th	Day	6% 1 1 8% 27% 56%	2451
	11th to 20th	Day	8% 2 3 10% 25% 51%	2306
	21st to end	Day	6% 1 1 8% 31% 52%	2405
August	1st to 10th	Day	12% 1 2 11% 29% 46%	2312
	11th to 20th	Day	5% 1 1 11% 34% 46%	2152
	21st to end	Day	6% 1 1 12% 36% 45%	2376
September	1st to 10th	Day	8% 1 1 9% 31% 51%	2160
	11th to 20th	Day	14% 2 3 22% 36% 23%	2160
	21st to end	Day	12% 2 2 22% 38% 24%	1926
October	1st to 10th	Day	17% 2 6% 17% 30% 27%	1800
	11th to 20th	Day	18% 3 7% 20% 24% 28%	1800
	21st to end	Day	23% 3 6% 18% 26% 24%	1980
November	1st to 10th	Day	31% 4 5% 21% 24% 15%	1802
	11th to 20th	Day	31% 4 9% 20% 24% 12%	1710
	21st to end	Day	31% 2 8% 17% 30% 12%	1606
December	1st to 10th	Day	39% 4 8% 19% 18% 13%	1520
	11th to 20th	Day	56% 3 6% 14% 15% 6%	1928
	21st to end	Day	50% 3 9% 21% 13% 4	2794
January	1st to 10th	Night	43% 4 14% 22% 13% 5%	4273
	11th to 20th	Night	52% 3 11% 15% 10% 9%	4110
	21st to end	Night	55% 3 7% 15% 11% 9%	4521
February	1st to 10th	Night	39% 3 6% 19% 20% 14%	2470
	11th to 20th	Night	39% 4 7% 17% 22% 11%	2285
	21st to end	Night	36% 5% 7% 26% 18% 7%	1884
March	1st to 10th	Night	50% 4 8% 17% 14% 7%	2157
	11th to 20th	Night	42% 3 6% 20% 20% 9%	2090
	21st to end	Night	40% 5% 10% 19% 18% 7%	2245
April	1st to 10th	Night	37% 5% 11% 19% 21% 7%	1820
	11th to 20th	Night	33% 3 8% 25% 21% 10%	1779
	21st to end	Night	25% 5% 8% 29% 26% 8%	1800
May	1st to 10th	Night	24% 4 7% 23% 29% 14%	1778
	11th to 20th	Night	24% 1 7% 23% 34% 10%	1519
	21st to end	Night	24% 2 10% 27% 29% 8%	1552
June	1st to 10th	Night	20% 3 9% 24% 28% 16%	1440
	11th to 20th	Night	12% 3 6% 14% 31% 34%	1440
	21st to end	Night	11% 1 2 14% 30% 43%	1440
July	1st to 10th	Night	7% 2 1 5% 29% 56%	1400
	11th to 20th	Night	7% 2 3 6% 29% 52%	1525
	21st to end	Night	11% 2 1 8% 29% 49%	1674
August	1st to 10th	Night	16% 2 1 12% 26% 44%	1602
	11th to 20th	Night	9% 2 1 9% 32% 47%	1748
	21st to end	Night	9% 1 1 8% 37% 42%	1980
September	1st to 10th	Night	9% 2 1 7% 24% 58%	1800
	11th to 20th	Night	16% 2 2 19% 34% 28%	1800
	21st to end	Night	12% 2 2 20% 33% 31%	2029
October	1st to 10th	Night	17% 3 5% 18% 24% 33%	2160
	11th to 20th	Night	21% 3 4 18% 25% 29%	2160
	21st to end	Night	21% 2 4 18% 29% 27%	2376
November	1st to 10th	Night	28% 4 7% 19% 22% 20%	2277
	11th to 20th	Night	29% 4 7% 21% 27% 12%	2470
	21st to end	Night	30% 3 7% 18% 23% 18%	2569
December	1st to 10th	Night	38% 3 6% 17% 20% 15%	2660
	11th to 20th	Night	54% 3 8% 13% 17% 6%	3276
	21st to end	Night	53% 4 10% 15% 13% 5%	4862

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	34% 11% 25% 19% 10% 1	261
	11th to 20th	Day	21% 13% 23% 27% 14% 3	370
	21st to end	Day	20% 10% 24% 26% 17% 3	407
February	1st to 10th	Day	23% 16% 23% 20% 14% 5%	370
	11th to 20th	Day	23% 16% 25% 25% 10%	423
	21st to end	Day	22% 14% 26% 27% 9% 2	382
March	1st to 10th	Day	26% 15% 28% 23% 7% 2	460
	11th to 20th	Day	25% 18% 19% 22% 13% 3	460
	21st to end	Day	22% 17% 31% 20% 9%	506
April	1st to 10th	Day	19% 16% 29% 22% 13% 1	429
	11th to 20th	Day	19% 17% 30% 20% 12% 1	412
	21st to end	Day	8% 19% 28% 27% 12% 6%	420
May	1st to 10th	Day	13% 14% 30% 29% 10% 3	423
	11th to 20th	Day	10% 15% 32% 25% 14% 5%	448
	21st to end	Day	10% 12% 25% 30% 19% 5%	485
June	1st to 10th	Day	9% 10% 27% 27% 18% 9%	450
	11th to 20th	Day	4 12% 28% 36% 15% 5%	450
	21st to end	Day	2 5% 24% 38% 22% 9%	450
July	1st to 10th	Day	1 3 18% 41% 30% 7%	446
	11th to 20th	Day	1 1 13% 42% 34% 10%	444
	21st to end	Day	1 13% 43% 29% 15%	487
August	1st to 10th	Day	2 3 20% 38% 28% 9%	447
	11th to 20th	Day	1 1 19% 40% 27% 13%	437
	21st to end	Day	1 2 15% 40% 32% 11%	495
September	1st to 10th	Day	3 3 16% 42% 29% 7%	450
	11th to 20th	Day	3 7% 23% 40% 22% 6%	450
	21st to end	Day	5% 7% 18% 41% 25% 4	450
October	1st to 10th	Day	5% 11% 23% 35% 20% 5%	450
	11th to 20th	Day	8% 8% 21% 32% 24% 7%	450
	21st to end	Day	10% 5% 26% 34% 20% 5%	495
November	1st to 10th	Day	11% 10% 20% 28% 25% 6%	398
	11th to 20th	Day	9% 8% 21% 30% 23% 10%	270
	21st to end	Day	13% 9% 23% 34% 17% 5%	221
December	1st to 10th	Day	17% 14% 16% 28% 17% 7%	180
	11th to 20th	Day	21% 16% 16% 27% 14% 7%	180
	21st to end	Day	25% 14% 20% 22% 13% 6%	198
January	1st to 10th	Night	21% 7% 14% 31% 20% 8%	760
	11th to 20th	Night	17% 6% 14% 33% 22% 9%	760
	21st to end	Night	22% 5% 12% 33% 21% 7%	836
February	1st to 10th	Night	21% 8% 12% 32% 21% 6%	760
	11th to 20th	Night	20% 8% 16% 31% 19% 6%	744
	21st to end	Night	20% 7% 18% 34% 18% 3	628
March	1st to 10th	Night	23% 8% 14% 31% 19% 4	760
	11th to 20th	Night	21% 8% 14% 27% 24% 5%	760
	21st to end	Night	18% 9% 19% 31% 20% 3	836
April	1st to 10th	Night	16% 8% 19% 32% 21% 4	728
	11th to 20th	Night	14% 10% 19% 32% 20% 6%	711
	21st to end	Night	8% 10% 23% 28% 22% 9%	720
May	1st to 10th	Night	7% 6% 22% 32% 25% 7%	698
	11th to 20th	Night	9% 3 12% 32% 36% 7%	457
	21st to end	Night	5% 6% 14% 35% 33% 7%	389
June	1st to 10th	Night	5% 5% 15% 34% 34% 7%	360
	11th to 20th	Night	1 5% 13% 38% 37% 5%	360
	21st to end	Night	3 7% 38% 45% 8%	360
July	1st to 10th	Night	1 7% 34% 49% 9%	350
	11th to 20th	Night	1 1 1 26% 54% 16%	481
	21st to end	Night	1 1 2 23% 55% 18%	591
August	1st to 10th	Night	2 1 6% 29% 50% 13%	537
	11th to 20th	Night	1 5% 35% 45% 13%	680
	21st to end	Night	1 4 28% 54% 13%	792
September	1st to 10th	Night	2 2 7% 33% 44% 13%	720
	11th to 20th	Night	4 1 9% 34% 40% 11%	720
	21st to end	Night	4 3 10% 32% 38% 14%	720
October	1st to 10th	Night	6% 3 11% 35% 34% 12%	720
	11th to 20th	Night	8% 4 9% 33% 35% 11%	720
	21st to end	Night	7% 4 11% 32% 28% 17%	792
November	1st to 10th	Night	12% 5% 11% 33% 30% 10%	740
	11th to 20th	Night	12% 3 15% 34% 27% 8%	760
	21st to end	Night	12% 4 14% 37% 24% 8%	760
December	1st to 10th	Night	18% 6% 15% 32% 23% 6%	760
	11th to 20th	Night	24% 6% 14% 30% 19% 6%	760
	21st to end	Night	22% 10% 17% 29% 19% 4	836

Page generated at 2024-03-31 03:08:56. Version ac07

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