Baskin Engineering Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Baskin Engineering Observatory clear sky chart from 2005-11-06 to 2024-03-28.

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	9% 5% 2 4 4 2 4 4 5% 8% 53%	78948
	Night	10% 6% 2 4 4 3 4 4 5% 8% 50%	81091
Transparency	Day	30% 5% 11% 21% 21% 12%	78670
	Night	34% 5% 11% 19% 19% 12%	80891
Seeing	Day	9% 5% 18% 32% 28% 8%	14613
	Night	11% 3 11% 31% 33% 10%	24131

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	15% 6% 2 6% 5% 3 4 4 5% 6% 43%	2763
	11th to 20th	Day	18% 7% 3 6% 6% 4 5% 5% 5% 7% 35%	2860
	21st to end	Day	20% 8% 3 6% 6% 3 4 5% 6% 7% 32%	3146
February	1st to 10th	Day	19% 5% 2 5% 4 2 4 3 5% 7% 45%	1710
	11th to 20th	Day	14% 5% 3 5% 5% 3 4 5% 5% 7% 45%	1770
	21st to end	Day	14% 5% 3 6% 4 2 5% 5% 4 6% 46%	1570
March	1st to 10th	Day	16% 7% 3 7% 6% 3 5% 6% 5% 8% 36%	2052
	11th to 20th	Day	15% 6% 3 6% 5% 3 5% 4 6% 8% 39%	2090
	21st to end	Day	11% 6% 2 4 5% 3 5% 5% 5% 8% 46%	2287
April	1st to 10th	Day	12% 6% 3 4 5% 3 4 5% 5% 8% 47%	2160
	11th to 20th	Day	8% 5% 2 5% 3 2 3 5% 5% 8% 54%	2129
	21st to end	Day	5% 4 2 5% 4 2 4 5% 5% 8% 54%	2160
May	1st to 10th	Day	5% 5% 2 4 4 2 4 4 5% 10% 55%	2193
	11th to 20th	Day	5% 4 2 3 4 2 3 4 5% 9% 59%	2320
	21st to end	Day	5% 8% 2 6% 5% 3 4 5% 5% 7% 51%	2723
June	1st to 10th	Day	4 5% 2 5% 4 2 4 4 6% 8% 55%	2520
	11th to 20th	Day	3 4 2 3 3 2 3 4 5% 9% 63%	2520
	21st to end	Day	2 3 2 4 4 2 3 3 4 9% 64%	2520
July	1st to 10th	Day	2 4 1 3 3 2 3 5% 5% 10% 62%	2493
	11th to 20th	Day	3 3 2 4 3 2 4 4 6% 9% 59%	2340
	21st to end	Day	1 3 1 3 3 2 4 5% 6% 10% 61%	2546
August	1st to 10th	Day	2 5% 2 4 4 2 4 5% 5% 10% 58%	2328
	11th to 20th	Day	4 2 3 3 2 3 4 6% 9% 64%	2165
	21st to end	Day	1 5% 2 5% 3 2 3 4 6% 10% 59%	2376
September	1st to 10th	Day	2 3 1 2 3 1 2 3 4 6% 72%	2160
	11th to 20th	Day	3 4 2 3 4 2 3 4 5% 10% 59%	2160
	21st to end	Day	3 3 1 2 2 1 3 3 5% 6% 72%	1914
October	1st to 10th	Day	3 2 2 3 4 2 3 4 4 7% 67%	1800
	11th to 20th	Day	6% 3 2 4 4 3 3 4 4 7% 62%	1800
	21st to end	Day	7% 4 2 4 3 2 3 4 4 6% 61%	1980
November	1st to 10th	Day	8% 4 1 4 4 3 4 4 6% 7% 55%	1845
	11th to 20th	Day	9% 5% 2 5% 4 2 4 4 5% 8% 53%	1710
	21st to end	Day	11% 3 2 3 3 2 3 4 4 8% 57%	1596
December	1st to 10th	Day	15% 5% 2 4 5% 2 4 4 5% 9% 45%	1520
	11th to 20th	Day	28% 8% 3 4 4 2 4 4 5% 6% 33%	1928
	21st to end	Day	21% 6% 2 5% 4 3 4 4 5% 8% 37%	2794
January	1st to 10th	Night	13% 7% 2 5% 4 2 3 4 5% 6% 49%	4186
	11th to 20th	Night	19% 7% 3 5% 5% 3 6% 5% 6% 7% 35%	4110
	21st to end	Night	20% 7% 3 5% 5% 3 5% 5% 6% 7% 34%	4521
February	1st to 10th	Night	17% 5% 2 4 4 2 3 4 4 6% 49%	2470
	11th to 20th	Night	13% 5% 2 4 3 3 5% 6% 6% 7% 46%	2296
	21st to end	Night	12% 6% 3 3 3 2 3 3 6% 7% 52%	1884
March	1st to 10th	Night	16% 10% 3 6% 5% 3 4 5% 5% 6% 37%	2123
	11th to 20th	Night	16% 6% 2 4 4 2 3 4 6% 9% 45%	2090
	21st to end	Night	14% 5% 3 3 5% 2 4 4 5% 7% 48%	2223
April	1st to 10th	Night	12% 5% 2 4 4 3 4 5% 6% 9% 48%	1800
	11th to 20th	Night	10% 4 2 4 4 3 3 4 6% 7% 53%	1779
	21st to end	Night	5% 4 2 4 3 2 4 4 6% 10% 57%	1800
May	1st to 10th	Night	5% 5% 2 4 4 2 3 4 6% 8% 56%	1764
	11th to 20th	Night	4 5% 2 3 4 3 5% 5% 7% 10% 53%	1572
	21st to end	Night	4 7% 2 5% 5% 2 6% 4 6% 9% 49%	1552
June	1st to 10th	Night	4 3 2 5% 5% 3 3 4 5% 9% 58%	1440
	11th to 20th	Night	1 5% 2 5% 5% 3 4 4 7% 7% 58%	1440
	21st to end	Night	2 3 2 5% 4 3 3 5% 6% 10% 58%	1440
July	1st to 10th	Night	1 5% 2 4 5% 3 3 3 6% 9% 59%	1462
	11th to 20th	Night	2 5% 3 6% 3 3 4 4 7% 10% 53%	1620
	21st to end	Night	1 6% 3 8% 7% 4 6% 6% 9% 12% 38%	1773
August	1st to 10th	Night	1 7% 4 7% 6% 4 5% 5% 8% 10% 43%	1611
	11th to 20th	Night	1 5% 2 5% 6% 3 5% 6% 6% 10% 52%	1736
	21st to end	Night	1 6% 4 7% 4 3 4 4 7% 8% 53%	1980
September	1st to 10th	Night	1 3 1 3 4 2 4 4 6% 8% 64%	1800
	11th to 20th	Night	3 5% 2 5% 3 3 4 4 6% 7% 60%	1800
	21st to end	Night	2 4 2 2 3 1 3 3 4 7% 69%	2042
October	1st to 10th	Night	4 4 2 4 4 2 2 4 5% 5% 64%	2160
	11th to 20th	Night	5% 4 2 4 3 2 3 4 6% 8% 59%	2160
	21st to end	Night	6% 4 1 4 4 2 2 3 5% 7% 62%	2376
November	1st to 10th	Night	8% 4 1 3 4 2 3 3 5% 7% 59%	2233
	11th to 20th	Night	9% 4 2 3 4 2 3 4 5% 10% 53%	2470
	21st to end	Night	7% 4 1 4 4 2 4 3 4 6% 59%	2580
December	1st to 10th	Night	15% 5% 2 3 3 2 4 3 4 7% 50%	2660
	11th to 20th	Night	29% 8% 2 4 4 2 3 3 5% 6% 34%	3276
	21st to end	Night	21% 7% 2 4 5% 3 3 4 5% 6% 39%	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	38% 7% 15% 27% 10% 4	2763
	11th to 20th	Day	46% 8% 13% 15% 11% 8%	2860
	21st to end	Day	48% 7% 11% 15% 13% 5%	3146
February	1st to 10th	Day	37% 5% 8% 20% 20% 10%	1710
	11th to 20th	Day	36% 7% 11% 19% 19% 8%	1779
	21st to end	Day	36% 7% 10% 24% 17% 7%	1570
March	1st to 10th	Day	42% 10% 10% 19% 15% 4	2052
	11th to 20th	Day	41% 7% 11% 19% 17% 5%	2090
	21st to end	Day	33% 10% 15% 21% 16% 4	2287
April	1st to 10th	Day	34% 8% 15% 20% 19% 5%	2160
	11th to 20th	Day	28% 7% 15% 26% 19% 6%	2132
	21st to end	Day	28% 7% 15% 27% 19% 4	2160
May	1st to 10th	Day	24% 6% 13% 25% 26% 5%	2193
	11th to 20th	Day	23% 6% 15% 28% 22% 7%	2323
	21st to end	Day	32% 9% 16% 22% 19% 3	2724
June	1st to 10th	Day	28% 8% 13% 23% 22% 6%	2520
	11th to 20th	Day	22% 4 13% 23% 27% 11%	2520
	21st to end	Day	22% 3 9% 18% 28% 19%	2520
July	1st to 10th	Day	22% 2 6% 18% 27% 24%	2425
	11th to 20th	Day	26% 3 9% 18% 23% 22%	2271
	21st to end	Day	23% 2 7% 18% 29% 21%	2405
August	1st to 10th	Day	27% 3 8% 18% 26% 19%	2312
	11th to 20th	Day	21% 2 8% 20% 26% 23%	2165
	21st to end	Day	27% 2 6% 21% 26% 19%	2376
September	1st to 10th	Day	18% 2 4 15% 27% 33%	2160
	11th to 20th	Day	22% 4 12% 21% 28% 13%	2160
	21st to end	Day	16% 3 8% 23% 33% 16%	1914
October	1st to 10th	Day	18% 5% 10% 19% 26% 22%	1800
	11th to 20th	Day	23% 4 11% 19% 21% 21%	1800
	21st to end	Day	26% 3 11% 23% 19% 19%	1980
November	1st to 10th	Day	28% 6% 9% 24% 23% 10%	1845
	11th to 20th	Day	29% 5% 12% 21% 24% 9%	1710
	21st to end	Day	26% 5% 11% 19% 27% 12%	1596
December	1st to 10th	Day	36% 6% 12% 19% 17% 11%	1520
	11th to 20th	Day	50% 6% 11% 14% 14% 6%	1928
	21st to end	Day	43% 6% 13% 22% 12% 4	2794
January	1st to 10th	Night	35% 6% 19% 26% 11% 4	4186
	11th to 20th	Night	46% 7% 15% 13% 10% 8%	4110
	21st to end	Night	48% 6% 13% 15% 11% 7%	4521
February	1st to 10th	Night	38% 5% 9% 17% 18% 13%	2470
	11th to 20th	Night	34% 6% 12% 19% 21% 8%	2298
	21st to end	Night	31% 5% 12% 30% 16% 6%	1884
March	1st to 10th	Night	45% 7% 13% 16% 14% 5%	2123
	11th to 20th	Night	38% 5% 9% 22% 20% 6%	2090
	21st to end	Night	35% 7% 14% 21% 17% 5%	2223
April	1st to 10th	Night	34% 7% 15% 21% 17% 6%	1800
	11th to 20th	Night	31% 6% 12% 28% 16% 8%	1779
	21st to end	Night	24% 8% 14% 30% 19% 5%	1800
May	1st to 10th	Night	26% 6% 13% 22% 23% 10%	1764
	11th to 20th	Night	29% 7% 14% 21% 21% 7%	1573
	21st to end	Night	35% 6% 16% 21% 19% 3	1552
June	1st to 10th	Night	27% 8% 11% 23% 22% 8%	1440
	11th to 20th	Night	27% 5% 9% 15% 28% 16%	1440
	21st to end	Night	26% 6% 8% 13% 25% 22%	1440
July	1st to 10th	Night	27% 4 4 12% 25% 29%	1421
	11th to 20th	Night	29% 6% 7% 15% 20% 24%	1566
	21st to end	Night	40% 5% 7% 12% 17% 19%	1674
August	1st to 10th	Night	41% 5% 6% 13% 17% 18%	1602
	11th to 20th	Night	31% 2 5% 14% 25% 24%	1736
	21st to end	Night	32% 4 6% 13% 22% 22%	1980
September	1st to 10th	Night	22% 3 5% 12% 27% 32%	1800
	11th to 20th	Night	25% 4 8% 20% 28% 14%	1800
	21st to end	Night	20% 3 6% 19% 33% 20%	2042
October	1st to 10th	Night	23% 5% 8% 16% 23% 26%	2160
	11th to 20th	Night	26% 5% 8% 20% 19% 22%	2160
	21st to end	Night	26% 4 7% 20% 22% 21%	2376
November	1st to 10th	Night	27% 5% 9% 24% 22% 13%	2233
	11th to 20th	Night	28% 5% 12% 22% 23% 10%	2470
	21st to end	Night	27% 5% 8% 21% 24% 15%	2580
December	1st to 10th	Night	35% 5% 10% 20% 18% 13%	2660
	11th to 20th	Night	51% 4 11% 14% 15% 5%	3276
	21st to end	Night	44% 7% 14% 17% 12% 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	27% 7% 17% 25% 18% 6%	313
	11th to 20th	Day	16% 4 20% 27% 27% 5%	370
	21st to end	Day	14% 5% 20% 30% 25% 6%	407
February	1st to 10th	Day	19% 6% 22% 22% 21% 9%	370
	11th to 20th	Day	14% 12% 23% 29% 19% 4	416
	21st to end	Day	11% 9% 24% 35% 18% 3	382
March	1st to 10th	Day	22% 11% 21% 30% 15% 2	460
	11th to 20th	Day	20% 7% 22% 26% 19% 6%	460
	21st to end	Day	13% 9% 28% 31% 16% 3	491
April	1st to 10th	Day	14% 7% 27% 32% 18% 4	420
	11th to 20th	Day	9% 11% 27% 32% 16% 5%	412
	21st to end	Day	6% 7% 30% 30% 21% 6%	420
May	1st to 10th	Day	8% 8% 28% 30% 21% 6%	423
	11th to 20th	Day	10% 5% 24% 32% 24% 6%	448
	21st to end	Day	10% 8% 22% 29% 25% 7%	485
June	1st to 10th	Day	6% 5% 17% 40% 25% 7%	450
	11th to 20th	Day	5% 5% 23% 36% 28% 4	450
	21st to end	Day	3 3 14% 43% 31% 6%	450
July	1st to 10th	Day	6% 12% 41% 33% 8%	446
	11th to 20th	Day	4 6% 40% 40% 10%	444
	21st to end	Day	2 1 7% 38% 41% 12%	487
August	1st to 10th	Day	5% 12% 43% 34% 6%	447
	11th to 20th	Day	3 8% 40% 40% 9%	437
	21st to end	Day	5% 1 7% 36% 40% 11%	495
September	1st to 10th	Day	2 2 10% 32% 39% 14%	450
	11th to 20th	Day	5% 1 16% 37% 31% 10%	450
	21st to end	Day	4 3 12% 31% 39% 11%	450
October	1st to 10th	Day	3 2 20% 29% 33% 13%	450
	11th to 20th	Day	6% 2 16% 28% 33% 15%	450
	21st to end	Day	9% 4 16% 27% 30% 14%	495
November	1st to 10th	Day	9% 6% 16% 23% 31% 15%	441
	11th to 20th	Day	9% 4 14% 31% 29% 13%	270
	21st to end	Day	9% 3 15% 33% 32% 9%	216
December	1st to 10th	Day	17% 7% 16% 23% 27% 9%	180
	11th to 20th	Day	16% 6% 19% 24% 25% 9%	180
	21st to end	Day	16% 12% 20% 26% 18% 8%	198
January	1st to 10th	Night	18% 6% 13% 32% 25% 6%	760
	11th to 20th	Night	16% 4 13% 33% 26% 9%	760
	21st to end	Night	16% 5% 10% 34% 29% 5%	836
February	1st to 10th	Night	20% 6% 12% 29% 26% 7%	760
	11th to 20th	Night	15% 6% 16% 33% 24% 6%	744
	21st to end	Night	14% 7% 17% 36% 23% 4	628
March	1st to 10th	Night	19% 8% 14% 33% 23% 4	760
	11th to 20th	Night	17% 4 14% 31% 27% 6%	760
	21st to end	Night	15% 4 18% 35% 24% 4	820
April	1st to 10th	Night	14% 4 16% 37% 26% 3	720
	11th to 20th	Night	11% 7% 17% 35% 23% 6%	711
	21st to end	Night	8% 4 20% 33% 25% 10%	720
May	1st to 10th	Night	9% 3 17% 33% 31% 8%	684
	11th to 20th	Night	7% 2 10% 35% 35% 11%	510
	21st to end	Night	8% 4 15% 34% 30% 8%	389
June	1st to 10th	Night	6% 2 10% 38% 36% 8%	360
	11th to 20th	Night	5% 2 9% 39% 39% 6%	360
	21st to end	Night	5% 6% 31% 48% 10%	360
July	1st to 10th	Night	6% 4 26% 52% 12%	371
	11th to 20th	Night	7% 1 21% 46% 24%	522
	21st to end	Night	6% 1 2 25% 48% 18%	591
August	1st to 10th	Night	8% 1 7% 28% 43% 13%	537
	11th to 20th	Night	6% 1 4 28% 48% 14%	668
	21st to end	Night	6% 1 4 24% 47% 18%	792
September	1st to 10th	Night	3 5% 25% 48% 18%	720
	11th to 20th	Night	5% 1 6% 30% 44% 14%	720
	21st to end	Night	5% 2 7% 25% 44% 18%	720
October	1st to 10th	Night	6% 2 8% 32% 37% 15%	720
	11th to 20th	Night	8% 2 7% 28% 40% 15%	720
	21st to end	Night	9% 2 9% 29% 34% 18%	792
November	1st to 10th	Night	10% 2 11% 30% 36% 12%	740
	11th to 20th	Night	12% 2 8% 34% 36% 8%	760
	21st to end	Night	9% 2 13% 38% 30% 8%	760
December	1st to 10th	Night	16% 4 12% 33% 28% 7%	760
	11th to 20th	Night	21% 4 13% 35% 22% 4	760
	21st to end	Night	19% 6% 18% 34% 19% 4	836

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