Mt. Chestnut Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. Chestnut Observatory clear sky chart from 2005-11-06 to 2024-07-16.

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	17% 11% 4 8% 5% 4 4 4 5% 7% 30%	89720
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 31%	89724
Transparency	Day	55% 3 10% 20% 10% 1	89489
	Night	55% 3 10% 20% 10% 1	89559
Seeing	Day	24% 14% 28% 20% 10% 4	14230
	Night	23% 12% 26% 26% 11% 1	23906

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	18% 22% 11% 12% 4 3 3 2 3 4 17%	2776
	11th to 20th	Day	13% 22% 9% 16% 8% 4 2 2 3 4 17%	2840
	21st to end	Day	25% 21% 8% 13% 6% 4 3 2 3 4 10%	3124
February	1st to 10th	Day	18% 19% 4 11% 9% 5% 3 2 3 4 21%	1710
	11th to 20th	Day	16% 13% 6% 14% 7% 4 5% 3 5% 6% 21%	1663
	21st to end	Day	21% 16% 4 11% 4 4 4 3 4 6% 24%	1560
March	1st to 10th	Day	21% 11% 5% 8% 6% 3 4 3 4 6% 28%	3435
	11th to 20th	Day	13% 12% 5% 10% 5% 5% 5% 3 4 6% 31%	3480
	21st to end	Day	20% 10% 3 8% 4 4 4 3 5% 6% 31%	3815
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 4 5% 7% 30%	2195
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 4 5% 6% 32%	2292
	21st to end	Day	22% 10% 2 7% 4 4 4 3 4 6% 32%	2470
May	1st to 10th	Day	23% 9% 3 8% 5% 4 4 4 5% 6% 28%	4082
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 5% 6% 8% 28%	4081
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 7% 9% 38%	4788
June	1st to 10th	Day	16% 6% 2 8% 5% 5% 5% 5% 6% 9% 33%	2660
	11th to 20th	Day	14% 7% 3 6% 6% 5% 5% 4 7% 9% 34%	2660
	21st to end	Day	15% 6% 2 6% 4 6% 5% 5% 6% 9% 37%	2660
July	1st to 10th	Day	11% 6% 3 6% 6% 6% 4 4 6% 8% 39%	2623
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 10% 37%	2407
	21st to end	Day	11% 6% 2 8% 6% 5% 6% 5% 8% 8% 34%	2547
August	1st to 10th	Day	10% 6% 3 7% 6% 6% 6% 4 7% 9% 35%	2327
	11th to 20th	Day	14% 6% 2 9% 5% 5% 5% 5% 7% 9% 32%	2303
	21st to end	Day	12% 6% 2 7% 6% 5% 6% 5% 6% 9% 36%	2458
September	1st to 10th	Day	17% 7% 3 5% 4 4 3 3 5% 8% 41%	2160
	11th to 20th	Day	13% 6% 2 6% 4 4 6% 4 6% 8% 41%	1994
	21st to end	Day	19% 7% 3 7% 4 5% 4 3 5% 6% 37%	1976
October	1st to 10th	Day	17% 6% 3 7% 4 4 4 4 6% 7% 38%	1800
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 4 4 6% 32%	1800
	21st to end	Day	25% 11% 3 7% 4 4 4 3 5% 5% 29%	1980
November	1st to 10th	Day	12% 9% 3 7% 5% 5% 4 4 4 7% 40%	1772
	11th to 20th	Day	18% 15% 4 7% 4 4 4 2 5% 5% 31%	1560
	21st to end	Day	18% 18% 5% 10% 5% 4 4 3 5% 5% 22%	1520
December	1st to 10th	Day	19% 22% 8% 11% 5% 5% 4 2 3 4 16%	1520
	11th to 20th	Day	19% 18% 7% 13% 4 4 4 3 4 6% 18%	1910
	21st to end	Day	26% 26% 7% 10% 4 3 3 2 3 4 12%	2772
January	1st to 10th	Night	16% 23% 9% 14% 4 4 4 3 3 5% 16%	4164
	11th to 20th	Night	17% 18% 8% 15% 6% 7% 5% 3 3 3 14%	4120
	21st to end	Night	28% 17% 9% 12% 6% 3 3 2 2 3 15%	4532
February	1st to 10th	Night	19% 15% 5% 11% 7% 5% 3 4 4 4 23%	2470
	11th to 20th	Night	18% 14% 5% 13% 6% 4 4 3 4 5% 23%	2403
	21st to end	Night	20% 14% 4 12% 6% 5% 3 4 4 4 25%	1890
March	1st to 10th	Night	22% 13% 3 6% 4 3 3 2 4 6% 32%	3418
	11th to 20th	Night	18% 12% 7% 7% 4 4 3 3 4 6% 32%	3480
	21st to end	Night	18% 11% 4 7% 4 4 4 3 4 5% 38%	3817
April	1st to 10th	Night	18% 13% 4 7% 4 4 4 3 4 6% 32%	1983
	11th to 20th	Night	17% 10% 3 6% 5% 4 4 4 5% 6% 36%	1852
	21st to end	Night	16% 9% 4 7% 4 4 4 3 7% 7% 35%	1710
May	1st to 10th	Night	27% 9% 3 6% 4 3 4 3 5% 6% 29%	2813
	11th to 20th	Night	18% 9% 3 7% 5% 4 3 3 5% 7% 36%	2812
	21st to end	Night	12% 7% 2 5% 3 4 4 4 7% 11% 41%	2766
June	1st to 10th	Night	11% 5% 2 6% 4 5% 4 4 6% 8% 44%	1520
	11th to 20th	Night	12% 5% 2 7% 4 5% 4 4 7% 8% 42%	1520
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 4 7% 8% 41%	1520
July	1st to 10th	Night	8% 7% 2 7% 5% 5% 5% 5% 6% 6% 45%	1552
	11th to 20th	Night	9% 5% 2 5% 5% 4 4 4 6% 10% 46%	1674
	21st to end	Night	9% 7% 3 6% 4 5% 5% 5% 7% 10% 40%	1771
August	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 4 5% 8% 47%	1611
	11th to 20th	Night	10% 6% 3 6% 4 4 4 3 6% 8% 45%	1598
	21st to end	Night	10% 6% 2 5% 3 4 5% 4 6% 9% 46%	1878
September	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 6% 7% 47%	1800
	11th to 20th	Night	11% 4 2 4 3 4 4 4 6% 8% 51%	1963
	21st to end	Night	16% 7% 3 6% 4 4 4 3 4 7% 41%	1984
October	1st to 10th	Night	15% 6% 2 7% 3 4 4 3 5% 8% 43%	2160
	11th to 20th	Night	18% 8% 2 6% 6% 4 4 3 4 6% 38%	2160
	21st to end	Night	24% 9% 3 6% 4 4 4 4 5% 6% 32%	2376
November	1st to 10th	Night	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	2301
	11th to 20th	Night	18% 13% 4 8% 5% 4 5% 4 5% 6% 29%	2617
	21st to end	Night	18% 16% 4 7% 5% 4 4 3 4 6% 28%	2660
December	1st to 10th	Night	22% 17% 7% 10% 6% 3 4 3 4 5% 18%	2660
	11th to 20th	Night	19% 18% 6% 11% 6% 4 4 3 4 5% 21%	3285
	21st to end	Night	25% 20% 7% 10% 5% 5% 4 3 5% 4 12%	4884

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	73% 2 8% 11% 5%	2777
	11th to 20th	Day	75% 2 7% 12% 4 1	2840
	21st to end	Day	81% 2 8% 7% 2	3124
February	1st to 10th	Day	69% 2 8% 15% 7%	1710
	11th to 20th	Day	66% 3 8% 15% 8%	1672
	21st to end	Day	63% 2 10% 17% 7% 2	1560
March	1st to 10th	Day	58% 2 9% 25% 6%	3435
	11th to 20th	Day	56% 2 12% 21% 8% 1	3480
	21st to end	Day	55% 1 9% 22% 12% 1	3815
April	1st to 10th	Day	55% 3 10% 18% 12% 2	2195
	11th to 20th	Day	53% 3 9% 18% 14% 2	2292
	21st to end	Day	53% 3 8% 23% 11% 2	2470
May	1st to 10th	Day	57% 2 9% 21% 9% 1	4120
	11th to 20th	Day	55% 3 11% 22% 8% 1	4085
	21st to end	Day	43% 3 13% 24% 12% 3	4802
June	1st to 10th	Day	48% 4 12% 24% 12% 1	2660
	11th to 20th	Day	45% 4 15% 22% 12% 1	2660
	21st to end	Day	43% 5% 11% 28% 12% 1	2660
July	1st to 10th	Day	43% 5% 11% 23% 15% 3	2554
	11th to 20th	Day	42% 5% 11% 26% 15% 1	2338
	21st to end	Day	44% 4 13% 22% 16% 1	2405
August	1st to 10th	Day	44% 4 13% 29% 9% 1	2314
	11th to 20th	Day	46% 4 14% 25% 10% 1	2303
	21st to end	Day	42% 6% 11% 23% 15% 3	2458
September	1st to 10th	Day	42% 5% 9% 23% 17% 3	2160
	11th to 20th	Day	40% 3 13% 27% 15% 1	1994
	21st to end	Day	49% 3 10% 24% 14% 1	1976
October	1st to 10th	Day	44% 4 12% 23% 16% 1	1800
	11th to 20th	Day	54% 4 10% 22% 10%	1800
	21st to end	Day	57% 3 13% 16% 10% 1	1980
November	1st to 10th	Day	45% 3 11% 22% 17% 2	1772
	11th to 20th	Day	57% 2 10% 20% 10% 2	1560
	21st to end	Day	63% 3 11% 17% 6% 1	1520
December	1st to 10th	Day	73% 2 9% 12% 4	1520
	11th to 20th	Day	69% 2 9% 16% 3	1906
	21st to end	Day	78% 2 9% 10% 2	2772
January	1st to 10th	Night	73% 1 8% 15% 3	4164
	11th to 20th	Night	76% 2 8% 11% 3	4120
	21st to end	Night	77% 2 6% 12% 3	4532
February	1st to 10th	Night	65% 3 9% 17% 6%	2470
	11th to 20th	Night	64% 2 9% 17% 7%	2405
	21st to end	Night	63% 3 8% 16% 10% 1	1890
March	1st to 10th	Night	57% 2 9% 24% 8%	3418
	11th to 20th	Night	55% 2 12% 23% 7% 1	3480
	21st to end	Night	51% 2 7% 24% 15% 1	3817
April	1st to 10th	Night	53% 3 9% 20% 14% 2	1983
	11th to 20th	Night	49% 3 11% 19% 15% 3	1852
	21st to end	Night	48% 4 9% 24% 13% 3	1710
May	1st to 10th	Night	57% 3 7% 21% 11% 2	2840
	11th to 20th	Night	49% 4 10% 26% 10% 2	2813
	21st to end	Night	37% 3 13% 27% 15% 4	2773
June	1st to 10th	Night	38% 4 14% 26% 16% 2	1520
	11th to 20th	Night	39% 5% 15% 25% 16% 2	1520
	21st to end	Night	39% 5% 14% 24% 16% 1	1520
July	1st to 10th	Night	39% 6% 10% 23% 19% 3	1511
	11th to 20th	Night	34% 5% 14% 26% 19% 2	1620
	21st to end	Night	39% 5% 15% 24% 14% 3	1672
August	1st to 10th	Night	34% 5% 14% 29% 17% 1	1600
	11th to 20th	Night	37% 4 16% 27% 15% 2	1598
	21st to end	Night	35% 5% 12% 28% 16% 4	1878
September	1st to 10th	Night	35% 5% 11% 26% 19% 4	1800
	11th to 20th	Night	30% 4 14% 28% 22% 1	1963
	21st to end	Night	44% 3 12% 22% 18% 1	1984
October	1st to 10th	Night	41% 4 12% 25% 16% 2	2160
	11th to 20th	Night	48% 3 11% 24% 13% 1	2160
	21st to end	Night	54% 4 10% 20% 11% 2	2376
November	1st to 10th	Night	42% 3 11% 24% 16% 3	2301
	11th to 20th	Night	55% 3 9% 21% 10% 3	2617
	21st to end	Night	59% 3 11% 21% 6% 1	2660
December	1st to 10th	Night	69% 2 11% 13% 5%	2660
	11th to 20th	Night	68% 2 10% 16% 4	3288
	21st to end	Night	76% 2 9% 11% 2	4884

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	33% 28% 24% 9% 5% 1	254
	11th to 20th	Day	34% 31% 25% 7% 2 1	270
	21st to end	Day	35% 26% 29% 7% 2	297
February	1st to 10th	Day	35% 24% 33% 7% 1	270
	11th to 20th	Day	41% 24% 27% 7% 1	270
	21st to end	Day	43% 23% 25% 9%	253
March	1st to 10th	Day	32% 22% 30% 13% 3	460
	11th to 20th	Day	35% 18% 32% 10% 5% 1	460
	21st to end	Day	33% 20% 28% 15% 5% 1	506
April	1st to 10th	Day	32% 19% 27% 16% 6% 1	460
	11th to 20th	Day	26% 20% 27% 17% 7% 3	452
	21st to end	Day	30% 16% 33% 16% 5% 2	460
May	1st to 10th	Day	31% 15% 28% 19% 4 3	463
	11th to 20th	Day	25% 12% 31% 18% 10% 4	487
	21st to end	Day	16% 8% 29% 27% 15% 5%	529
June	1st to 10th	Day	17% 12% 30% 26% 12% 3	490
	11th to 20th	Day	18% 10% 26% 28% 13% 4	490
	21st to end	Day	16% 9% 35% 26% 10% 3	490
July	1st to 10th	Day	13% 8% 19% 31% 19% 10%	485
	11th to 20th	Day	15% 5% 25% 28% 19% 8%	465
	21st to end	Day	13% 7% 25% 31% 17% 8%	488
August	1st to 10th	Day	13% 6% 21% 29% 21% 9%	446
	11th to 20th	Day	15% 5% 23% 31% 15% 11%	439
	21st to end	Day	13% 6% 25% 28% 18% 10%	495
September	1st to 10th	Day	18% 7% 22% 27% 17% 8%	450
	11th to 20th	Day	14% 2 22% 31% 20% 10%	450
	21st to end	Day	21% 12% 26% 22% 15% 4	450
October	1st to 10th	Day	18% 9% 28% 24% 17% 4	450
	11th to 20th	Day	26% 15% 30% 21% 7% 1	450
	21st to end	Day	35% 12% 30% 17% 5%	495
November	1st to 10th	Day	21% 19% 31% 16% 11% 3	370
	11th to 20th	Day	30% 20% 31% 13% 3 3	198
	21st to end	Day	26% 23% 38% 11% 2 1	180
December	1st to 10th	Day	37% 24% 26% 8% 3 2	180
	11th to 20th	Day	35% 19% 33% 12% 1 1	180
	21st to end	Day	38% 20% 30% 11% 1	198
January	1st to 10th	Night	31% 19% 34% 13% 2	760
	11th to 20th	Night	32% 20% 31% 15% 3	760
	21st to end	Night	30% 18% 30% 18% 3	836
February	1st to 10th	Night	31% 17% 28% 22% 2	760
	11th to 20th	Night	31% 20% 34% 14% 1	743
	21st to end	Night	32% 19% 29% 17% 3	628
March	1st to 10th	Night	27% 20% 30% 21% 3	760
	11th to 20th	Night	30% 18% 26% 21% 5%	760
	21st to end	Night	25% 16% 34% 20% 5% 1	836
April	1st to 10th	Night	26% 16% 30% 23% 5%	760
	11th to 20th	Night	24% 12% 34% 22% 7% 1	728
	21st to end	Night	24% 7% 36% 28% 4 1	570
May	1st to 10th	Night	23% 11% 24% 32% 9% 1	570
	11th to 20th	Night	21% 8% 25% 33% 12% 1	561
	21st to end	Night	11% 5% 25% 34% 22% 3	433
June	1st to 10th	Night	10% 4 28% 38% 19% 1	380
	11th to 20th	Night	13% 7% 22% 40% 18% 1	380
	21st to end	Night	11% 4 26% 38% 19% 2	380
July	1st to 10th	Night	8% 4 19% 40% 24% 5%	401
	11th to 20th	Night	10% 4 16% 36% 29% 5%	540
	21st to end	Night	13% 3 13% 42% 27% 2	591
August	1st to 10th	Night	12% 4 13% 42% 27% 3	537
	11th to 20th	Night	13% 4 10% 43% 25% 5%	534
	21st to end	Night	12% 3 21% 31% 27% 5%	690
September	1st to 10th	Night	16% 4 20% 33% 24% 4	720
	11th to 20th	Night	12% 3 18% 35% 29% 4	720
	21st to end	Night	20% 7% 24% 26% 18% 4	720
October	1st to 10th	Night	20% 6% 23% 31% 19% 2	720
	11th to 20th	Night	23% 11% 31% 26% 9% 1	720
	21st to end	Night	30% 13% 27% 24% 6%	792
November	1st to 10th	Night	20% 13% 26% 29% 11% 1	740
	11th to 20th	Night	29% 16% 26% 22% 6% 1	760
	21st to end	Night	28% 15% 31% 19% 6%	760
December	1st to 10th	Night	34% 18% 26% 18% 4	760
	11th to 20th	Night	28% 21% 28% 20% 4	760
	21st to end	Night	33% 17% 28% 20% 2	836

Page generated at 2024-07-13 10:19:49. Version ac07

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