Hale Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hale Mountain Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	12% 7% 3 5% 5% 3 4 4 5% 7% 44%	74983
	Night	11% 6% 3 5% 4 2 4 4 5% 7% 50%	74807
Transparency	Day	39% 6% 10% 23% 18% 4	74707
	Night	35% 5% 10% 24% 21% 5%	74603
Seeing	Day	16% 14% 29% 25% 13% 3	14278
	Night	15% 15% 31% 26% 11% 1	24242

By Month

Percentage Cloud Cover SE (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	11% 10% 4 5% 4 2 3 4 4 6% 47%	1707
	11th to 20th	Day	12% 9% 2 6% 5% 2 3 4 4 5% 47%	1647
	21st to end	Day	14% 9% 4 4 4 2 3 3 3 5% 48%	1782
February	1st to 10th	Day	16% 9% 4 6% 7% 3 4 4 3 3 41%	1620
	11th to 20th	Day	14% 10% 4 6% 3 3 4 3 4 6% 42%	1604
	21st to end	Day	17% 11% 4 6% 4 2 4 4 4 5% 39%	1606
March	1st to 10th	Day	14% 8% 3 6% 5% 3 4 4 5% 6% 44%	1980
	11th to 20th	Day	19% 9% 3 6% 4 3 4 4 4 5% 40%	1980
	21st to end	Day	18% 9% 3 7% 7% 4 4 3 4 6% 35%	2178
April	1st to 10th	Day	12% 6% 3 6% 6% 3 4 5% 6% 6% 43%	2062
	11th to 20th	Day	16% 8% 2 6% 6% 3 4 4 5% 6% 40%	2134
	21st to end	Day	17% 10% 3 6% 6% 3 4 3 4 5% 39%	2165
May	1st to 10th	Day	16% 7% 3 7% 5% 3 5% 4 4 6% 39%	2340
	11th to 20th	Day	12% 6% 3 7% 6% 4 5% 5% 6% 7% 39%	2302
	21st to end	Day	11% 6% 3 7% 7% 4 6% 6% 6% 8% 36%	2531
June	1st to 10th	Day	9% 5% 3 5% 5% 3 5% 6% 7% 9% 43%	2340
	11th to 20th	Day	7% 4 3 5% 6% 3 6% 6% 6% 9% 45%	2340
	21st to end	Day	4 4 2 4 5% 3 5% 6% 6% 9% 52%	2340
July	1st to 10th	Day	9% 5% 2 5% 6% 3 7% 6% 8% 11% 37%	2340
	11th to 20th	Day	4 3 2 4 5% 3 6% 6% 7% 10% 50%	2340
	21st to end	Day	7% 4 2 5% 6% 5% 6% 6% 8% 11% 41%	2546
August	1st to 10th	Day	6% 5% 2 4 5% 4 6% 6% 8% 10% 44%	2328
	11th to 20th	Day	9% 5% 3 5% 5% 3 6% 6% 6% 9% 44%	2281
	21st to end	Day	5% 5% 2 5% 5% 4 5% 6% 6% 8% 50%	2376
September	1st to 10th	Day	12% 4 2 4 3 3 3 4 5% 7% 53%	2160
	11th to 20th	Day	10% 5% 2 6% 5% 3 4 5% 7% 8% 46%	1984
	21st to end	Day	7% 4 2 4 4 3 4 5% 5% 7% 54%	1967
October	1st to 10th	Day	9% 6% 2 5% 3 2 3 4 5% 6% 55%	1800
	11th to 20th	Day	13% 4 2 4 4 2 2 3 4 5% 57%	1800
	21st to end	Day	18% 7% 3 3 4 2 3 2 3 4 50%	1980
November	1st to 10th	Day	10% 6% 3 3 4 3 3 3 4 6% 57%	1858
	11th to 20th	Day	10% 7% 3 5% 4 2 4 3 4 6% 53%	1849
	21st to end	Day	15% 8% 3 5% 3 2 3 4 5% 6% 46%	1710
December	1st to 10th	Day	14% 7% 3 5% 5% 3 4 4 3 6% 46%	1710
	11th to 20th	Day	12% 11% 4 7% 5% 2 4 4 4 6% 43%	2161
	21st to end	Day	14% 15% 8% 6% 5% 3 4 4 4 6% 32%	3135
January	1st to 10th	Night	12% 6% 3 5% 4 3 3 4 5% 6% 49%	2458
	11th to 20th	Night	13% 7% 3 6% 3 2 3 3 4 5% 50%	2392
	21st to end	Night	13% 7% 3 5% 4 2 3 3 3 5% 53%	2574
February	1st to 10th	Night	15% 9% 3 6% 4 2 4 4 5% 6% 43%	2340
	11th to 20th	Night	13% 7% 4 6% 4 3 4 4 3 6% 47%	2259
	21st to end	Night	14% 9% 3 5% 5% 3 5% 4 4 5% 45%	1649
March	1st to 10th	Night	13% 7% 3 6% 5% 3 5% 4 6% 6% 44%	1980
	11th to 20th	Night	17% 9% 3 5% 4 3 3 4 4 5% 43%	1980
	21st to end	Night	15% 7% 3 5% 5% 3 4 4 5% 6% 43%	2178
April	1st to 10th	Night	12% 6% 2 5% 4 3 5% 4 5% 6% 49%	1896
	11th to 20th	Night	12% 7% 3 5% 4 3 3 4 5% 6% 48%	1775
	21st to end	Night	14% 7% 3 6% 5% 3 4 4 4 6% 44%	1792
May	1st to 10th	Night	11% 5% 3 5% 4 3 5% 5% 5% 7% 47%	1620
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 8% 46%	1593
	21st to end	Night	9% 6% 3 6% 6% 3 5% 5% 6% 10% 43%	1745
June	1st to 10th	Night	5% 4 1 4 4 2 5% 5% 6% 9% 57%	1620
	11th to 20th	Night	5% 3 1 3 5% 2 4 4 6% 10% 57%	1620
	21st to end	Night	3 1 1 2 3 2 4 5% 5% 9% 64%	1620
July	1st to 10th	Night	5% 3 2 4 4 3 5% 6% 7% 12% 48%	1620
	11th to 20th	Night	3 2 1 2 3 2 3 4 6% 12% 60%	1620
	21st to end	Night	3 3 2 4 4 2 5% 4 7% 12% 53%	1772
August	1st to 10th	Night	3 2 1 3 6% 2 5% 6% 7% 11% 54%	1611
	11th to 20th	Night	6% 4 2 4 4 3 4 6% 6% 8% 52%	1618
	21st to end	Night	4 3 1 4 4 3 4 4 6% 8% 57%	1980
September	1st to 10th	Night	7% 4 2 4 4 1 4 4 5% 7% 58%	1800
	11th to 20th	Night	9% 4 2 5% 4 2 4 4 6% 8% 51%	1971
	21st to end	Night	7% 4 2 4 4 3 4 4 5% 6% 59%	1980
October	1st to 10th	Night	6% 3 1 4 3 2 4 4 5% 7% 60%	2153
	11th to 20th	Night	13% 4 1 3 4 3 3 4 4 4 57%	2160
	21st to end	Night	17% 6% 3 4 3 2 2 3 4 5% 52%	2376
November	1st to 10th	Night	11% 6% 2 3 3 2 3 3 3 5% 60%	2220
	11th to 20th	Night	10% 6% 2 4 4 2 3 3 4 6% 56%	2327
	21st to end	Night	14% 7% 3 5% 4 2 3 2 4 5% 51%	2470
December	1st to 10th	Night	12% 9% 3 5% 4 2 4 4 4 6% 47%	2470
	11th to 20th	Night	12% 9% 3 6% 5% 3 4 4 4 4 44%	3036
	21st to end	Night	14% 13% 8% 7% 5% 2 3 3 3 4 36%	4532

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	39% 4 8% 24% 20% 4	1708
	11th to 20th	Day	39% 4 9% 25% 22% 2	1647
	21st to end	Day	39% 4 7% 28% 19% 3	1782
February	1st to 10th	Day	48% 4 9% 20% 16% 3	1620
	11th to 20th	Day	45% 3 8% 23% 18% 3	1613
	21st to end	Day	49% 4 6% 17% 17% 8%	1606
March	1st to 10th	Day	41% 4 8% 19% 19% 9%	1980
	11th to 20th	Day	49% 4 7% 21% 14% 5%	1980
	21st to end	Day	52% 5% 9% 20% 13% 2	2178
April	1st to 10th	Day	41% 6% 10% 21% 18% 4	2062
	11th to 20th	Day	45% 5% 10% 20% 15% 4	2134
	21st to end	Day	50% 5% 7% 21% 15% 2	2165
May	1st to 10th	Day	47% 6% 9% 20% 17% 1	2340
	11th to 20th	Day	44% 7% 11% 23% 13% 1	2306
	21st to end	Day	45% 8% 14% 21% 12%	2531
June	1st to 10th	Day	37% 9% 14% 26% 13% 2	2340
	11th to 20th	Day	35% 9% 14% 26% 15% 1	2340
	21st to end	Day	27% 8% 14% 29% 18% 4	2340
July	1st to 10th	Day	36% 10% 17% 23% 13% 1	2276
	11th to 20th	Day	26% 9% 15% 30% 17% 3	2271
	21st to end	Day	35% 9% 15% 29% 11% 1	2405
August	1st to 10th	Day	32% 10% 17% 25% 15% 1	2314
	11th to 20th	Day	35% 8% 12% 25% 18% 1	2281
	21st to end	Day	31% 9% 14% 25% 17% 4	2376
September	1st to 10th	Day	32% 5% 10% 26% 18% 8%	2160
	11th to 20th	Day	33% 7% 11% 22% 22% 5%	1984
	21st to end	Day	29% 6% 10% 21% 28% 6%	1967
October	1st to 10th	Day	32% 5% 10% 21% 24% 8%	1800
	11th to 20th	Day	32% 3 6% 21% 28% 10%	1800
	21st to end	Day	40% 3 8% 24% 17% 8%	1980
November	1st to 10th	Day	30% 5% 9% 26% 22% 8%	1858
	11th to 20th	Day	33% 3 7% 25% 23% 9%	1849
	21st to end	Day	40% 3 8% 20% 24% 5%	1710
December	1st to 10th	Day	39% 5% 9% 24% 20% 3	1710
	11th to 20th	Day	44% 3 6% 20% 23% 4	2159
	21st to end	Day	52% 4 8% 19% 14% 3	3135
January	1st to 10th	Night	36% 4 9% 22% 23% 5%	2458
	11th to 20th	Night	38% 3 10% 24% 22% 3	2392
	21st to end	Night	35% 3 6% 26% 23% 6%	2574
February	1st to 10th	Night	43% 4 9% 24% 16% 5%	2340
	11th to 20th	Night	40% 4 8% 25% 19% 4	2260
	21st to end	Night	43% 3 7% 17% 21% 9%	1649
March	1st to 10th	Night	41% 4 8% 17% 21% 10%	1980
	11th to 20th	Night	44% 6% 8% 19% 17% 5%	1980
	21st to end	Night	42% 7% 8% 25% 15% 3	2178
April	1st to 10th	Night	37% 5% 9% 22% 20% 7%	1896
	11th to 20th	Night	37% 5% 9% 23% 18% 8%	1775
	21st to end	Night	42% 4 9% 23% 20% 2	1792
May	1st to 10th	Night	39% 6% 11% 21% 20% 3	1620
	11th to 20th	Night	34% 8% 15% 24% 17% 2	1594
	21st to end	Night	35% 9% 17% 25% 13% 1	1746
June	1st to 10th	Night	23% 8% 15% 32% 20% 2	1620
	11th to 20th	Night	22% 6% 14% 33% 21% 3	1620
	21st to end	Night	19% 6% 15% 30% 24% 7%	1620
July	1st to 10th	Night	25% 10% 17% 27% 19% 1	1575
	11th to 20th	Night	18% 7% 14% 33% 25% 3	1566
	21st to end	Night	23% 8% 15% 36% 18% 1	1673
August	1st to 10th	Night	22% 9% 16% 35% 17% 1	1601
	11th to 20th	Night	30% 7% 13% 25% 23% 3	1618
	21st to end	Night	24% 7% 14% 27% 23% 4	1980
September	1st to 10th	Night	26% 5% 11% 29% 21% 9%	1800
	11th to 20th	Night	31% 7% 11% 22% 24% 6%	1971
	21st to end	Night	27% 5% 7% 23% 31% 7%	1980
October	1st to 10th	Night	24% 5% 12% 23% 25% 10%	2153
	11th to 20th	Night	31% 4 6% 19% 30% 11%	2160
	21st to end	Night	37% 4 7% 20% 22% 10%	2376
November	1st to 10th	Night	29% 4 8% 24% 25% 9%	2220
	11th to 20th	Night	31% 4 7% 24% 26% 9%	2327
	21st to end	Night	38% 2 7% 21% 25% 6%	2470
December	1st to 10th	Night	39% 4 9% 20% 22% 5%	2470
	11th to 20th	Night	43% 3 7% 20% 21% 6%	3037
	21st to end	Night	51% 3 10% 17% 15% 4	4532

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	26% 26% 34% 11% 3	270
	11th to 20th	Day	25% 31% 35% 7% 2	249
	21st to end	Day	22% 34% 30% 12% 3	264
February	1st to 10th	Day	29% 28% 35% 8%	240
	11th to 20th	Day	32% 32% 28% 7% 1	240
	21st to end	Day	39% 25% 28% 7% 1	198
March	1st to 10th	Day	25% 30% 30% 14%	240
	11th to 20th	Day	35% 22% 33% 10% 1	240
	21st to end	Day	34% 30% 28% 6% 2	264
April	1st to 10th	Day	23% 26% 33% 13% 4 1	322
	11th to 20th	Day	27% 21% 36% 13% 3	412
	21st to end	Day	26% 21% 36% 13% 4	425
May	1st to 10th	Day	17% 21% 41% 16% 4	603
	11th to 20th	Day	16% 15% 36% 24% 8% 1	625
	21st to end	Day	13% 10% 40% 21% 12% 4	679
June	1st to 10th	Day	10% 6% 26% 36% 19% 3	630
	11th to 20th	Day	7% 7% 30% 38% 16% 3	630
	21st to end	Day	4 6% 35% 35% 18% 3	630
July	1st to 10th	Day	10% 3 20% 36% 24% 8%	621
	11th to 20th	Day	4 3 22% 40% 26% 5%	618
	21st to end	Day	8% 1 15% 40% 29% 7%	684
August	1st to 10th	Day	8% 4 18% 44% 23% 4	626
	11th to 20th	Day	11% 4 18% 41% 24% 3	595
	21st to end	Day	6% 2 17% 36% 31% 8%	495
September	1st to 10th	Day	10% 2 16% 40% 26% 6%	450
	11th to 20th	Day	5% 1 26% 41% 22% 4	274
	21st to end	Day	11% 7% 29% 35% 17% 2	270
October	1st to 10th	Day	10% 14% 34% 27% 11% 4	270
	11th to 20th	Day	19% 17% 42% 19% 2 1	270
	21st to end	Day	32% 19% 32% 14% 2 1	297
November	1st to 10th	Day	16% 18% 43% 17% 5% 1	270
	11th to 20th	Day	20% 24% 33% 19% 4	270
	21st to end	Day	26% 27% 28% 16% 3	270
December	1st to 10th	Day	22% 26% 36% 11% 3 2	270
	11th to 20th	Day	26% 25% 37% 9% 2 1	270
	21st to end	Day	23% 31% 36% 8% 2	297
January	1st to 10th	Night	19% 22% 37% 18% 4	950
	11th to 20th	Night	21% 24% 35% 17% 3	920
	21st to end	Night	19% 24% 33% 21% 3	990
February	1st to 10th	Night	23% 23% 34% 17% 3	900
	11th to 20th	Night	20% 28% 36% 14% 2	865
	21st to end	Night	26% 24% 31% 17% 2	465
March	1st to 10th	Night	19% 21% 34% 24% 2	540
	11th to 20th	Night	24% 19% 33% 22% 3	540
	21st to end	Night	19% 24% 38% 15% 4 1	594
April	1st to 10th	Night	18% 23% 35% 20% 4	540
	11th to 20th	Night	16% 21% 37% 20% 5%	533
	21st to end	Night	19% 16% 40% 17% 7%	540
May	1st to 10th	Night	14% 13% 40% 25% 7%	540
	11th to 20th	Night	15% 12% 33% 29% 11% 1	531
	21st to end	Night	10% 10% 34% 27% 16% 2	584
June	1st to 10th	Night	6% 4 24% 40% 24% 1	540
	11th to 20th	Night	5% 2 36% 40% 16% 2	540
	21st to end	Night	3 2 28% 43% 22% 3	540
July	1st to 10th	Night	5% 1 17% 41% 32% 4	525
	11th to 20th	Night	5% 1 18% 43% 28% 5%	522
	21st to end	Night	4 1 15% 41% 35% 4	591
August	1st to 10th	Night	4 1 19% 43% 30% 3	537
	11th to 20th	Night	8% 1 20% 43% 25% 4	534
	21st to end	Night	6% 2 19% 39% 30% 4	594
September	1st to 10th	Night	10% 2 17% 45% 24% 3	540
	11th to 20th	Night	13% 3 18% 39% 22% 5%	540
	21st to end	Night	9% 6% 26% 35% 22% 3	540
October	1st to 10th	Night	8% 7% 34% 28% 21% 2	713
	11th to 20th	Night	15% 15% 37% 25% 8%	720
	21st to end	Night	22% 14% 33% 22% 9% 1	792
November	1st to 10th	Night	13% 17% 37% 24% 8%	740
	11th to 20th	Night	16% 16% 38% 23% 6%	807
	21st to end	Night	22% 20% 32% 21% 6%	950
December	1st to 10th	Night	21% 19% 36% 20% 4	950
	11th to 20th	Night	21% 21% 32% 21% 4	950
	21st to end	Night	20% 25% 33% 18% 4	1045

Page generated at 2024-01-11 12:31:58. Version ac07

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