McDonald Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the McDonald Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	5% 3 2 4 4 3 5% 5% 6% 7% 57%	74730
	Night	4 3 2 3 4 2 4 4 5% 8% 60%	75084
Transparency	Day	24% 4 8% 19% 25% 20%	74480
	Night	22% 4 9% 19% 26% 21%	74856
Seeing	Day	7% 11% 24% 30% 22% 6%	13497
	Night	6% 7% 24% 38% 23% 2	23748

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	5% 4 2 3 3 2 4 4 4 7% 63%	1897
	11th to 20th	Day	3 3 1 3 3 3 3 4 4 7% 66%	1845
	21st to end	Day	3 4 2 4 4 2 3 5% 4 6% 64%	1980
February	1st to 10th	Day	2 3 1 3 4 3 5% 4 5% 7% 64%	1800
	11th to 20th	Day	3 4 2 3 3 3 4 4 4 5% 64%	1749
	21st to end	Day	2 2 1 3 3 2 2 4 4 8% 69%	1480
March	1st to 10th	Day	3 4 2 5% 4 4 5% 5% 7% 8% 54%	1800
	11th to 20th	Day	4 5% 2 4 4 2 5% 5% 5% 6% 58%	1923
	21st to end	Day	2 3 2 3 4 3 3 3 5% 8% 65%	2348
April	1st to 10th	Day	3 2 1 3 3 2 4 4 5% 7% 65%	2160
	11th to 20th	Day	2 4 2 3 3 3 3 4 4 6% 66%	2131
	21st to end	Day	3 3 1 3 2 1 3 3 4 6% 71%	2160
May	1st to 10th	Day	4 3 1 2 3 2 4 4 5% 7% 66%	2160
	11th to 20th	Day	4 4 2 3 3 3 5% 5% 5% 8% 58%	2125
	21st to end	Day	4 2 1 2 3 2 4 5% 7% 8% 61%	2489
June	1st to 10th	Day	2 1 1 2 4 3 4 6% 8% 8% 61%	2340
	11th to 20th	Day	3 2 1 4 5% 2 5% 6% 8% 9% 56%	2340
	21st to end	Day	6% 4 2 4 4 3 5% 6% 7% 10% 50%	2340
July	1st to 10th	Day	8% 4 2 6% 6% 5% 9% 9% 8% 8% 35%	2166
	11th to 20th	Day	6% 3 2 4 6% 4 7% 8% 8% 10% 43%	2160
	21st to end	Day	5% 4 2 5% 7% 5% 7% 7% 8% 11% 38%	2349
August	1st to 10th	Day	5% 3 2 4 6% 4 7% 7% 9% 12% 43%	2149
	11th to 20th	Day	7% 4 2 8% 8% 5% 8% 8% 8% 10% 31%	2123
	21st to end	Day	8% 5% 2 7% 6% 5% 7% 6% 7% 9% 38%	2376
September	1st to 10th	Day	11% 4 3 5% 7% 4 6% 5% 7% 8% 41%	2160
	11th to 20th	Day	10% 6% 3 5% 6% 4 7% 6% 7% 8% 38%	2060
	21st to end	Day	9% 5% 2 5% 5% 3 4 5% 6% 8% 49%	1980
October	1st to 10th	Day	5% 3 2 4 4 2 3 5% 5% 8% 59%	1980
	11th to 20th	Day	7% 3 1 4 4 3 4 4 4 5% 62%	1846
	21st to end	Day	5% 3 1 2 3 2 2 2 4 4 73%	1980
November	1st to 10th	Day	4 3 1 2 2 1 3 3 4 5% 70%	1803
	11th to 20th	Day	2 4 2 3 4 2 3 4 4 5% 67%	1710
	21st to end	Day	5% 5% 2 4 4 1 2 3 4 6% 64%	1710
December	1st to 10th	Day	7% 4 2 3 3 3 4 4 4 7% 60%	1710
	11th to 20th	Day	2 2 1 3 4 3 5% 5% 6% 8% 60%	2161
	21st to end	Day	3 3 1 2 2 2 3 3 5% 7% 70%	3240
January	1st to 10th	Night	4 4 2 4 4 2 3 4 4 6% 62%	2268
	11th to 20th	Night	5% 4 2 4 3 2 3 4 4 7% 63%	2220
	21st to end	Night	3 4 2 3 3 3 4 4 4 7% 62%	2376
February	1st to 10th	Night	4 3 1 4 3 2 3 4 5% 7% 64%	2160
	11th to 20th	Night	4 4 2 3 4 3 5% 5% 5% 8% 59%	2098
	21st to end	Night	3 3 1 2 3 2 4 5% 5% 6% 66%	1776
March	1st to 10th	Night	4 3 1 3 3 2 4 5% 6% 9% 58%	2160
	11th to 20th	Night	4 5% 2 4 4 3 5% 4 5% 5% 59%	2032
	21st to end	Night	3 3 1 2 3 2 3 4 5% 6% 67%	2007
April	1st to 10th	Night	5% 3 1 4 4 3 4 4 4 6% 63%	1800
	11th to 20th	Night	3 4 2 4 3 2 4 4 5% 6% 64%	1777
	21st to end	Night	2 2 2 2 2 2 3 3 4 5% 73%	1800
May	1st to 10th	Night	3 2 2 3 4 3 3 4 5% 6% 67%	1800
	11th to 20th	Night	6% 4 2 3 3 3 5% 4 5% 6% 59%	1770
	21st to end	Night	4 3 2 4 4 3 5% 6% 6% 8% 57%	1779
June	1st to 10th	Night	2 2 2 3 5% 3 5% 5% 7% 10% 57%	1620
	11th to 20th	Night	4 2 2 3 5% 3 5% 6% 8% 10% 52%	1620
	21st to end	Night	5% 3 2 4 3 3 5% 6% 7% 10% 53%	1620
July	1st to 10th	Night	4 3 2 4 5% 3 5% 7% 8% 10% 48%	1787
	11th to 20th	Night	4 2 2 3 4 3 5% 6% 7% 11% 54%	1800
	21st to end	Night	3 2 2 4 5% 3 6% 6% 8% 11% 51%	1969
August	1st to 10th	Night	3 2 1 3 4 3 4 5% 8% 15% 51%	1790
	11th to 20th	Night	3 3 2 3 5% 3 7% 7% 9% 15% 44%	1778
	21st to end	Night	5% 4 2 5% 5% 3 5% 6% 7% 11% 47%	1980
September	1st to 10th	Night	7% 4 2 3 5% 3 5% 7% 6% 9% 50%	1800
	11th to 20th	Night	7% 5% 2 5% 5% 4 5% 6% 8% 10% 44%	1898
	21st to end	Night	7% 4 2 3 4 3 5% 6% 6% 8% 52%	1980
October	1st to 10th	Night	5% 3 1 3 5% 3 4 5% 5% 8% 59%	1980
	11th to 20th	Night	7% 2 2 4 3 2 3 4 5% 6% 61%	2110
	21st to end	Night	3 3 1 2 3 2 3 3 3 5% 72%	2376
November	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 6% 71%	2253
	11th to 20th	Night	3 4 2 2 3 2 3 3 4 6% 68%	2470
	21st to end	Night	4 4 1 4 3 3 2 3 5% 7% 64%	2470
December	1st to 10th	Night	6% 3 1 3 4 2 4 4 5% 8% 59%	2470
	11th to 20th	Night	2 4 2 5% 5% 3 3 3 5% 8% 60%	3036
	21st to end	Night	3 2 1 2 2 2 3 3 4 5% 73%	4454

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	21% 2 4 18% 31% 24%	1898
	11th to 20th	Day	19% 3 4 17% 34% 23%	1845
	21st to end	Day	21% 3 5% 14% 33% 24%	1980
February	1st to 10th	Day	19% 3 5% 16% 25% 32%	1800
	11th to 20th	Day	23% 2 6% 15% 26% 28%	1759
	21st to end	Day	16% 3 3 13% 31% 35%	1480
March	1st to 10th	Day	25% 3 5% 18% 27% 23%	1800
	11th to 20th	Day	26% 3 5% 14% 23% 30%	1923
	21st to end	Day	18% 2 4 12% 32% 31%	2348
April	1st to 10th	Day	18% 3 4 13% 31% 30%	2160
	11th to 20th	Day	19% 3 4 11% 31% 33%	2134
	21st to end	Day	16% 3 3 12% 35% 31%	2160
May	1st to 10th	Day	18% 3 3 11% 27% 37%	2160
	11th to 20th	Day	24% 4 6% 18% 25% 23%	2128
	21st to end	Day	18% 5% 9% 20% 20% 28%	2489
June	1st to 10th	Day	17% 5% 11% 23% 26% 18%	2340
	11th to 20th	Day	21% 5% 7% 19% 26% 22%	2340
	21st to end	Day	26% 6% 10% 23% 21% 14%	2340
July	1st to 10th	Day	41% 7% 15% 20% 13% 4	2107
	11th to 20th	Day	29% 7% 13% 26% 20% 4	2097
	21st to end	Day	33% 6% 19% 30% 11% 1	2219
August	1st to 10th	Day	28% 6% 16% 32% 16% 2	2136
	11th to 20th	Day	41% 8% 17% 23% 9% 2	2123
	21st to end	Day	38% 6% 18% 19% 14% 4	2376
September	1st to 10th	Day	38% 6% 14% 17% 16% 9%	2160
	11th to 20th	Day	39% 6% 12% 18% 19% 7%	2060
	21st to end	Day	31% 4 7% 23% 24% 11%	1980
October	1st to 10th	Day	23% 4 9% 24% 22% 18%	1980
	11th to 20th	Day	25% 2 5% 19% 24% 25%	1846
	21st to end	Day	16% 2 2 17% 35% 28%	1980
November	1st to 10th	Day	17% 2 6% 20% 30% 25%	1803
	11th to 20th	Day	20% 2 6% 21% 29% 22%	1710
	21st to end	Day	23% 2 5% 19% 31% 21%	1710
December	1st to 10th	Day	25% 2 6% 21% 29% 17%	1710
	11th to 20th	Day	22% 2 5% 20% 29% 22%	2159
	21st to end	Day	15% 2 3 15% 32% 33%	3240
January	1st to 10th	Night	25% 2 3 13% 32% 26%	2268
	11th to 20th	Night	22% 2 3 16% 32% 24%	2220
	21st to end	Night	22% 3 4 15% 35% 21%	2376
February	1st to 10th	Night	19% 2 4 15% 26% 33%	2160
	11th to 20th	Night	25% 2 7% 12% 26% 28%	2098
	21st to end	Night	19% 3 3 10% 27% 38%	1776
March	1st to 10th	Night	22% 3 4 18% 27% 25%	2160
	11th to 20th	Night	27% 3 5% 12% 25% 28%	2032
	21st to end	Night	18% 3 4 13% 30% 32%	2007
April	1st to 10th	Night	23% 3 4 13% 31% 27%	1800
	11th to 20th	Night	21% 3 6% 13% 26% 31%	1777
	21st to end	Night	14% 3 5% 15% 28% 34%	1800
May	1st to 10th	Night	19% 3 4 13% 27% 35%	1800
	11th to 20th	Night	25% 4 7% 19% 22% 23%	1771
	21st to end	Night	24% 4 11% 21% 18% 22%	1780
June	1st to 10th	Night	20% 5% 12% 27% 23% 14%	1620
	11th to 20th	Night	23% 5% 9% 21% 25% 17%	1620
	21st to end	Night	24% 4 14% 22% 21% 13%	1620
July	1st to 10th	Night	27% 6% 20% 31% 13% 3	1737
	11th to 20th	Night	21% 6% 16% 30% 23% 4	1740
	21st to end	Night	22% 6% 22% 35% 13% 1	1859
August	1st to 10th	Night	20% 6% 19% 35% 19% 2	1779
	11th to 20th	Night	25% 7% 25% 29% 12% 2	1778
	21st to end	Night	29% 8% 19% 23% 15% 5%	1980
September	1st to 10th	Night	29% 6% 18% 21% 16% 10%	1800
	11th to 20th	Night	31% 8% 18% 19% 17% 7%	1898
	21st to end	Night	28% 4 13% 23% 21% 11%	1980
October	1st to 10th	Night	23% 5% 14% 21% 19% 18%	1980
	11th to 20th	Night	24% 2 8% 19% 24% 24%	2110
	21st to end	Night	17% 3 4 14% 31% 32%	2376
November	1st to 10th	Night	17% 3 4 16% 36% 25%	2253
	11th to 20th	Night	18% 3 4 21% 29% 25%	2470
	21st to end	Night	22% 2 5% 18% 31% 22%	2470
December	1st to 10th	Night	24% 3 6% 20% 27% 20%	2470
	11th to 20th	Night	24% 2 3 19% 28% 24%	3037
	21st to end	Night	16% 1 3 17% 37% 27%	4454

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	9% 21% 31% 29% 9%	270
	11th to 20th	Day	7% 22% 34% 25% 11%	254
	21st to end	Day	6% 30% 30% 26% 7% 2	264
February	1st to 10th	Day	7% 16% 35% 25% 16%	240
	11th to 20th	Day	13% 22% 38% 19% 9%	240
	21st to end	Day	7% 28% 40% 20% 5%	198
March	1st to 10th	Day	8% 18% 45% 19% 9%	240
	11th to 20th	Day	10% 24% 39% 19% 6% 2	363
	21st to end	Day	7% 26% 44% 19% 5%	462
April	1st to 10th	Day	5% 20% 45% 21% 7% 1	420
	11th to 20th	Day	7% 24% 44% 19% 5%	412
	21st to end	Day	7% 22% 40% 28% 3	420
May	1st to 10th	Day	6% 19% 39% 27% 8% 1	423
	11th to 20th	Day	5% 10% 35% 38% 9% 2	448
	21st to end	Day	4 6% 35% 40% 12% 2	637
June	1st to 10th	Day	2 3 17% 47% 26% 4	630
	11th to 20th	Day	2 2 15% 50% 28% 3	630
	21st to end	Day	6% 2 11% 35% 39% 8%	630
July	1st to 10th	Day	10% 1 6% 29% 38% 17%	452
	11th to 20th	Day	5% 2 29% 47% 16%	444
	21st to end	Day	4 5% 32% 45% 15%	487
August	1st to 10th	Day	4 1 4 23% 47% 21%	447
	11th to 20th	Day	7% 3 8% 28% 40% 15%	437
	21st to end	Day	10% 2 4 26% 41% 17%	495
September	1st to 10th	Day	10% 2 8% 35% 35% 10%	450
	11th to 20th	Day	11% 2 9% 35% 33% 11%	350
	21st to end	Day	10% 2 10% 36% 31% 10%	270
October	1st to 10th	Day	12% 3 16% 34% 30% 5%	270
	11th to 20th	Day	8% 6% 24% 40% 18% 4	270
	21st to end	Day	7% 13% 27% 26% 19% 7%	297
November	1st to 10th	Day	9% 9% 28% 34% 16% 4	270
	11th to 20th	Day	6% 12% 27% 33% 17% 6%	270
	21st to end	Day	13% 20% 30% 21% 12% 3	270
December	1st to 10th	Day	11% 14% 27% 31% 13% 3	270
	11th to 20th	Day	8% 20% 36% 28% 8%	270
	21st to end	Day	7% 22% 36% 27% 8%	297
January	1st to 10th	Night	6% 10% 34% 35% 13% 2	760
	11th to 20th	Night	8% 10% 32% 34% 15% 1	740
	21st to end	Night	6% 12% 32% 35% 13% 1	792
February	1st to 10th	Night	6% 13% 29% 34% 16% 2	720
	11th to 20th	Night	7% 10% 36% 37% 10%	704
	21st to end	Night	7% 14% 40% 34% 4	592
March	1st to 10th	Night	6% 12% 33% 36% 12% 1	720
	11th to 20th	Night	7% 13% 32% 32% 15% 1	720
	21st to end	Night	5% 11% 39% 36% 8% 1	621
April	1st to 10th	Night	6% 11% 32% 41% 10%	540
	11th to 20th	Night	6% 13% 39% 35% 7%	533
	21st to end	Night	2 12% 36% 40% 9% 1	540
May	1st to 10th	Night	4 8% 35% 41% 12% 1	540
	11th to 20th	Night	6% 6% 28% 47% 12% 1	531
	21st to end	Night	4 5% 27% 47% 16% 1	584
June	1st to 10th	Night	2 2 19% 54% 22% 1	540
	11th to 20th	Night	4 1 17% 53% 23% 1	540
	21st to end	Night	4 2 11% 51% 31% 1	540
July	1st to 10th	Night	5% 1 6% 38% 47% 3	525
	11th to 20th	Night	4 4 41% 47% 4	522
	21st to end	Night	3 4 42% 47% 4	591
August	1st to 10th	Night	3 1 2 29% 59% 7%	537
	11th to 20th	Night	4 1 3 27% 57% 9%	534
	21st to end	Night	6% 1 4 30% 54% 5%	594
September	1st to 10th	Night	8% 2 4 30% 50% 6%	540
	11th to 20th	Night	10% 1 6% 39% 41% 4	540
	21st to end	Night	8% 1 9% 39% 37% 5%	540
October	1st to 10th	Night	6% 2 13% 45% 31% 3	540
	11th to 20th	Night	8% 5% 19% 41% 25% 3	670
	21st to end	Night	4 6% 20% 37% 25% 7%	792
November	1st to 10th	Night	6% 3 22% 38% 26% 5%	773
	11th to 20th	Night	4 7% 23% 42% 22% 2	950
	21st to end	Night	8% 11% 26% 36% 17% 3	950
December	1st to 10th	Night	8% 8% 28% 36% 17% 2	950
	11th to 20th	Night	6% 11% 35% 36% 12% 1	950
	21st to end	Night	5% 14% 32% 36% 11% 2	993

Page generated at 2024-01-12 15:17:47. Version ac07

Copyright 2024 Attilla Danko