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 2024-07-05.

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	4 3 1 4 4 4 4 4 6% 7% 59%	89705
	Night	4 4 1 4 3 3 4 3 5% 7% 61%	89693
Transparency	Day	22% 4 7% 17% 25% 24%	89455
	Night	21% 4 8% 17% 26% 24%	89465
Seeing	Day	7% 11% 25% 30% 21% 6%	14164
	Night	6% 7% 24% 38% 22% 2	24329

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	3 3 1 2 1 2 3 3 3 6% 73%	3167
	11th to 20th	Day	2 2 1 2 2 3 3 3 4 5% 74%	3170
	21st to end	Day	2 4 2 4 4 4 4 4 5% 7% 62%	3487
February	1st to 10th	Day	2 4 1 4 3 4 4 3 5% 6% 63%	1900
	11th to 20th	Day	3 4 2 4 3 4 4 3 4 6% 64%	1849
	21st to end	Day	2 2 1 3 3 3 3 3 5% 8% 68%	1570
March	1st to 10th	Day	2 4 2 6% 4 4 4 4 6% 8% 55%	3130
	11th to 20th	Day	6% 6% 2 4 3 3 3 3 5% 7% 58%	3391
	21st to end	Day	1 3 1 4 4 4 4 4 6% 7% 62%	4124
April	1st to 10th	Day	3 2 1 3 3 3 3 4 6% 7% 65%	2280
	11th to 20th	Day	2 4 2 4 3 3 3 4 5% 6% 64%	2251
	21st to end	Day	3 3 1 3 2 2 2 3 4 6% 71%	2280
May	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	3800
	11th to 20th	Day	3 3 1 3 2 3 3 3 5% 7% 66%	3765
	21st to end	Day	2 1 1 2 2 2 3 3 5% 6% 74%	4419
June	1st to 10th	Day	2 2 1 3 3 4 4 5% 8% 7% 60%	2470
	11th to 20th	Day	3 2 1 4 4 3 5% 5% 8% 8% 57%	2470
	21st to end	Day	5% 4 2 4 4 5% 5% 5% 7% 9% 49%	2470
July	1st to 10th	Day	8% 4 2 7% 6% 7% 9% 7% 8% 8% 34%	2215
	11th to 20th	Day	6% 4 1 4 6% 6% 7% 7% 8% 9% 43%	2160
	21st to end	Day	5% 5% 2 5% 7% 7% 7% 6% 8% 10% 38%	2349
August	1st to 10th	Day	5% 3 2 5% 5% 6% 7% 5% 9% 11% 42%	2149
	11th to 20th	Day	7% 5% 2 9% 7% 7% 7% 6% 8% 9% 31%	2123
	21st to end	Day	8% 5% 2 7% 5% 7% 7% 5% 7% 9% 38%	2376
September	1st to 10th	Day	11% 5% 3 6% 6% 6% 5% 4 7% 8% 40%	2160
	11th to 20th	Day	10% 6% 3 6% 5% 6% 6% 5% 7% 7% 38%	2060
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 8% 49%	1980
October	1st to 10th	Day	5% 3 1 5% 4 3 4 4 5% 7% 59%	1980
	11th to 20th	Day	7% 3 1 5% 3 3 4 3 4 5% 62%	1846
	21st to end	Day	5% 3 1 3 2 2 2 2 4 4 73%	1980
November	1st to 10th	Day	4 3 1 2 2 2 3 3 4 5% 70%	1803
	11th to 20th	Day	2 4 2 4 3 2 3 3 4 5% 67%	1710
	21st to end	Day	5% 5% 2 4 3 2 2 3 4 6% 64%	1710
December	1st to 10th	Day	7% 4 1 4 3 3 4 3 5% 7% 60%	1710
	11th to 20th	Day	2 3 1 4 4 4 5% 4 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	2 3 1 3 2 3 3 3 4 5% 70%	3788
	11th to 20th	Night	3 3 1 3 2 2 2 3 5% 7% 71%	3800
	21st to end	Night	3 4 2 5% 4 4 5% 4 6% 7% 56%	4180
February	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 7% 64%	2280
	11th to 20th	Night	4 4 1 4 4 4 5% 3 5% 7% 60%	2218
	21st to end	Night	3 4 1 2 2 3 4 4 5% 7% 66%	1884
March	1st to 10th	Night	3 3 2 4 4 3 4 4 6% 8% 58%	3734
	11th to 20th	Night	5% 11% 3 5% 3 3 4 3 5% 5% 53%	3568
	21st to end	Night	2 3 1 3 3 4 4 4 6% 7% 64%	3518
April	1st to 10th	Night	5% 3 1 4 3 3 3 4 4 6% 63%	1900
	11th to 20th	Night	2 4 2 4 3 3 4 3 5% 6% 64%	1877
	21st to end	Night	2 3 1 3 2 3 3 3 4 5% 73%	1900
May	1st to 10th	Night	2 3 1 3 2 3 2 2 3 4 75%	3170
	11th to 20th	Night	4 4 1 3 2 4 3 3 4 5% 66%	3140
	21st to end	Night	2 2 1 3 3 3 4 3 5% 6% 68%	3160
June	1st to 10th	Night	2 2 1 4 4 3 5% 4 8% 10% 57%	1710
	11th to 20th	Night	4 3 1 4 5% 4 5% 5% 8% 9% 54%	1710
	21st to end	Night	5% 3 1 4 3 4 5% 5% 7% 10% 53%	1710
July	1st to 10th	Night	4 4 2 5% 5% 4 5% 5% 8% 10% 48%	1832
	11th to 20th	Night	3 3 2 3 4 4 6% 5% 7% 10% 54%	1800
	21st to end	Night	3 3 1 4 4 4 6% 5% 8% 11% 50%	1969
August	1st to 10th	Night	3 3 1 3 3 4 4 4 8% 15% 51%	1790
	11th to 20th	Night	3 3 2 4 4 5% 6% 6% 9% 15% 44%	1778
	21st to end	Night	5% 4 2 6% 4 5% 5% 4 7% 10% 47%	1980
September	1st to 10th	Night	7% 5% 1 4 4 4 5% 5% 6% 9% 49%	1800
	11th to 20th	Night	7% 5% 1 5% 4 5% 5% 5% 9% 10% 43%	1898
	21st to end	Night	7% 4 2 4 3 4 5% 5% 6% 8% 52%	1980
October	1st to 10th	Night	5% 3 1 4 4 4 4 4 5% 8% 59%	1980
	11th to 20th	Night	7% 3 2 4 3 2 4 3 5% 6% 61%	2110
	21st to end	Night	3 3 1 3 3 2 3 2 4 5% 72%	2376
November	1st to 10th	Night	5% 3 1 3 2 2 2 2 4 5% 71%	2253
	11th to 20th	Night	3 4 2 3 3 3 3 2 4 6% 68%	2470
	21st to end	Night	4 4 1 4 3 3 2 2 5% 7% 64%	2470
December	1st to 10th	Night	6% 3 1 4 3 3 4 4 5% 8% 59%	2470
	11th to 20th	Night	2 4 2 5% 4 3 3 3 5% 8% 60%	3036
	21st to end	Night	3 3 1 2 2 2 3 2 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	15% 2 2 13% 28% 40%	3168
	11th to 20th	Day	14% 2 3 15% 41% 25%	3170
	21st to end	Day	22% 2 6% 19% 35% 16%	3487
February	1st to 10th	Day	20% 3 4 16% 25% 32%	1900
	11th to 20th	Day	22% 2 6% 15% 26% 29%	1859
	21st to end	Day	16% 3 3 13% 32% 33%	1570
March	1st to 10th	Day	24% 4 3 12% 25% 33%	3130
	11th to 20th	Day	26% 3 5% 17% 26% 23%	3391
	21st to end	Day	20% 3 3 9% 34% 30%	4124
April	1st to 10th	Day	18% 3 4 13% 29% 33%	2280
	11th to 20th	Day	20% 3 4 11% 30% 32%	2254
	21st to end	Day	16% 2 3 12% 35% 31%	2280
May	1st to 10th	Day	12% 2 3 11% 23% 49%	3800
	11th to 20th	Day	17% 3 6% 12% 24% 37%	3768
	21st to end	Day	12% 3 5% 14% 19% 47%	4419
June	1st to 10th	Day	17% 5% 11% 23% 26% 17%	2470
	11th to 20th	Day	21% 5% 6% 19% 26% 23%	2470
	21st to end	Day	26% 6% 10% 24% 21% 13%	2470
July	1st to 10th	Day	40% 7% 15% 21% 13% 3	2156
	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	17% 3 2 10% 35% 35%	3788
	11th to 20th	Night	15% 2 3 17% 35% 29%	3800
	21st to end	Night	27% 2 5% 17% 36% 14%	4180
February	1st to 10th	Night	19% 2 4 15% 26% 33%	2280
	11th to 20th	Night	24% 2 7% 12% 26% 29%	2218
	21st to end	Night	19% 3 3 10% 28% 37%	1884
March	1st to 10th	Night	21% 4 2 12% 23% 37%	3734
	11th to 20th	Night	30% 6% 5% 12% 25% 22%	3568
	21st to end	Night	20% 2 4 10% 35% 30%	3518
April	1st to 10th	Night	22% 3 3 12% 29% 30%	1900
	11th to 20th	Night	21% 3 6% 13% 26% 31%	1877
	21st to end	Night	14% 3 5% 15% 27% 35%	1900
May	1st to 10th	Night	13% 4 2 9% 22% 50%	3170
	11th to 20th	Night	20% 3 7% 14% 21% 36%	3141
	21st to end	Night	17% 3 7% 14% 20% 40%	3161
June	1st to 10th	Night	19% 5% 12% 27% 23% 14%	1710
	11th to 20th	Night	22% 5% 8% 20% 26% 19%	1710
	21st to end	Night	24% 4 14% 24% 22% 13%	1710
July	1st to 10th	Night	27% 6% 20% 32% 13% 3	1782
	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% 20% 37% 25% 11%	270
	21st to end	Day	5% 29% 30% 27% 7% 2	297
February	1st to 10th	Day	9% 19% 34% 24% 14%	270
	11th to 20th	Day	12% 20% 38% 21% 8%	270
	21st to end	Day	9% 28% 40% 19% 4	225
March	1st to 10th	Day	8% 20% 45% 19% 8%	270
	11th to 20th	Day	9% 25% 40% 19% 5% 2	400
	21st to end	Day	8% 27% 43% 17% 4	506
April	1st to 10th	Day	6% 21% 44% 20% 7% 1	460
	11th to 20th	Day	8% 23% 45% 19% 5%	452
	21st to end	Day	7% 21% 41% 28% 3	460
May	1st to 10th	Day	5% 19% 40% 28% 8% 1	463
	11th to 20th	Day	5% 12% 35% 38% 9% 1	488
	21st to end	Day	4 7% 36% 41% 12% 1	690
June	1st to 10th	Day	2 3 19% 47% 25% 4	680
	11th to 20th	Day	2 3 17% 49% 26% 3	680
	21st to end	Day	5% 3 12% 34% 39% 7%	680
July	1st to 10th	Day	9% 1 6% 29% 39% 16%	469
	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	7% 10% 32% 34% 15% 1	760
	21st to end	Night	6% 12% 33% 35% 13% 1	836
February	1st to 10th	Night	6% 14% 30% 33% 15% 2	760
	11th to 20th	Night	7% 10% 36% 38% 9%	744
	21st to end	Night	8% 15% 40% 34% 4	628
March	1st to 10th	Night	6% 13% 33% 35% 11% 1	760
	11th to 20th	Night	7% 13% 33% 31% 15% 1	760
	21st to end	Night	4 12% 41% 34% 7% 1	655
April	1st to 10th	Night	5% 12% 33% 40% 10%	570
	11th to 20th	Night	6% 13% 40% 35% 7%	563
	21st to end	Night	2 12% 36% 40% 9% 1	570
May	1st to 10th	Night	4 8% 36% 41% 11% 1	570
	11th to 20th	Night	6% 6% 29% 46% 12% 1	561
	21st to end	Night	4 6% 28% 47% 15%	617
June	1st to 10th	Night	2 2 19% 54% 21% 1	570
	11th to 20th	Night	4 2 18% 53% 23% 1	570
	21st to end	Night	4 2 11% 50% 31% 1	570
July	1st to 10th	Night	5% 1 7% 38% 46% 3	539
	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 2024-07-02 05:41:26. Version ac07

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