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 2024-12-24.

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	16% 11% 4 8% 5% 4 4 4 5% 7% 31%	101574
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 32%	98484
Transparency	Day	52% 3 10% 22% 11% 1	105220
	Night	53% 3 10% 21% 11% 2	102977
Seeing	Day	24% 14% 28% 20% 10% 4	14784
	Night	23% 12% 26% 26% 11% 1	24509

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	10% 6% 3 7% 6% 5% 5% 5% 7% 9% 39%	4377
	11th to 20th	Day	11% 6% 2 6% 5% 5% 4 4 7% 10% 41%	4120
	21st to end	Day	11% 6% 2 8% 5% 5% 6% 5% 7% 8% 37%	4466
August	1st to 10th	Day	15% 9% 3 9% 6% 5% 5% 4 6% 7% 30%	4104
	11th to 20th	Day	13% 6% 3 8% 5% 5% 6% 5% 7% 9% 34%	4044
	21st to end	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 9% 40%	4318
September	1st to 10th	Day	16% 7% 3 5% 4 4 3 3 5% 8% 43%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	2104
	21st to end	Day	20% 8% 3 7% 4 5% 4 3 5% 6% 35%	2085
October	1st to 10th	Day	16% 6% 3 7% 4 4 4 4 6% 7% 39%	1900
	11th to 20th	Day	20% 9% 3 7% 5% 4 5% 4 4 6% 33%	1900
	21st to end	Day	24% 10% 3 7% 4 4 4 3 5% 6% 31%	2090
November	1st to 10th	Day	12% 9% 3 7% 5% 5% 4 3 4 7% 41%	1867
	11th to 20th	Day	18% 15% 4 8% 4 5% 4 2 5% 5% 30%	1642
	21st to end	Day	18% 18% 5% 11% 5% 4 4 3 5% 5% 21%	1600
December	1st to 10th	Day	19% 21% 8% 12% 5% 5% 4 2 3 4 16%	1600
	11th to 20th	Day	20% 17% 7% 13% 4 4 3 3 4 6% 18%	1990
	21st to end	Day	26% 25% 7% 10% 4 3 3 2 4 4 12%	2796
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	11% 8% 3 7% 5% 4 4 4 5% 7% 41%	2578
	11th to 20th	Night	10% 5% 2 6% 5% 5% 4 4 6% 9% 44%	2840
	21st to end	Night	9% 8% 3 7% 5% 5% 5% 4 6% 7% 42%	3087
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 4 7% 37%	2825
	11th to 20th	Night	13% 7% 3 7% 4 4 3 3 5% 7% 42%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 6% 4 6% 9% 45%	3304
September	1st to 10th	Night	14% 5% 2 5% 4 3 4 3 6% 7% 48%	1900
	11th to 20th	Night	10% 4 2 4 3 4 4 4 5% 8% 52%	2073
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 7% 39%	2095
October	1st to 10th	Night	15% 6% 2 7% 3 3 4 3 5% 7% 44%	2280
	11th to 20th	Night	17% 8% 2 6% 6% 4 4 3 4 6% 39%	2280
	21st to end	Night	23% 9% 3 6% 3 4 3 4 5% 6% 34%	2508
November	1st to 10th	Night	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	2426
	11th to 20th	Night	18% 13% 4 8% 5% 4 5% 4 5% 6% 29%	2755
	21st to end	Night	18% 16% 4 8% 5% 4 4 3 4 6% 27%	2800
December	1st to 10th	Night	21% 17% 8% 11% 6% 3 4 3 4 5% 18%	2800
	11th to 20th	Night	19% 17% 7% 11% 6% 4 4 3 4 5% 21%	3425
	21st to end	Night	25% 20% 7% 10% 5% 5% 4 3 5% 4 13%	4933

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	41% 6% 11% 24% 15% 3	4308
	11th to 20th	Day	38% 5% 11% 29% 17%	4051
	21st to end	Day	43% 5% 12% 21% 18% 1	4324
August	1st to 10th	Day	52% 4 12% 23% 9%	4091
	11th to 20th	Day	44% 4 12% 29% 10% 1	4083
	21st to end	Day	38% 6% 12% 28% 14% 2	4318
September	1st to 10th	Day	41% 5% 9% 24% 19% 3	2280
	11th to 20th	Day	39% 3 13% 27% 16% 1	2104
	21st to end	Day	51% 3 10% 23% 13%	2085
October	1st to 10th	Day	34% 4 10% 32% 19% 1	3160
	11th to 20th	Day	46% 4 9% 22% 15% 4	3140
	21st to end	Day	40% 3 13% 28% 15%	3428
November	1st to 10th	Day	45% 3 10% 23% 18% 2	1867
	11th to 20th	Day	57% 2 10% 19% 10% 2	1642
	21st to end	Day	64% 3 11% 16% 5% 1	1600
December	1st to 10th	Day	73% 2 9% 12% 4	1600
	11th to 20th	Day	68% 2 10% 16% 3	1986
	21st to end	Day	78% 2 9% 10% 2	2796
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	43% 5% 9% 23% 18% 2	2537
	11th to 20th	Night	36% 5% 14% 27% 17% 1	2786
	21st to end	Night	41% 5% 11% 18% 23% 2	2988
August	1st to 10th	Night	47% 5% 12% 23% 12% 1	2814
	11th to 20th	Night	41% 4 11% 30% 14% 1	2812
	21st to end	Night	35% 6% 13% 30% 13% 2	3304
September	1st to 10th	Night	35% 5% 10% 26% 20% 4	1900
	11th to 20th	Night	30% 4 13% 28% 23% 1	2073
	21st to end	Night	46% 3 12% 21% 17% 1	2095
October	1st to 10th	Night	36% 3 8% 27% 24% 1	3800
	11th to 20th	Night	40% 4 11% 24% 14% 8%	3779
	21st to end	Night	37% 3 10% 29% 19% 2	4120
November	1st to 10th	Night	42% 3 11% 24% 16% 3	2426
	11th to 20th	Night	56% 3 9% 20% 10% 3	2755
	21st to end	Night	60% 3 11% 20% 5% 1	2800
December	1st to 10th	Night	70% 2 10% 13% 4	2800
	11th to 20th	Night	68% 2 10% 16% 4	3428
	21st to end	Night	76% 2 9% 11% 2	4933

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% 27% 19% 8%	484
	21st to end	Day	13% 7% 25% 31% 17% 7%	532
August	1st to 10th	Day	14% 7% 22% 28% 20% 8%	486
	11th to 20th	Day	15% 6% 25% 30% 14% 10%	479
	21st to end	Day	12% 6% 24% 29% 19% 10%	539
September	1st to 10th	Day	18% 8% 22% 27% 18% 8%	490
	11th to 20th	Day	13% 3 22% 33% 19% 10%	490
	21st to end	Day	24% 12% 25% 21% 13% 4	490
October	1st to 10th	Day	18% 9% 28% 23% 17% 4	490
	11th to 20th	Day	24% 14% 30% 21% 8% 3	490
	21st to end	Day	33% 12% 31% 18% 6%	539
November	1st to 10th	Day	21% 19% 33% 15% 10% 2	405
	11th to 20th	Day	29% 20% 35% 12% 3 2	220
	21st to end	Day	29% 23% 36% 10% 2 1	200
December	1st to 10th	Day	37% 25% 27% 8% 3 2	200
	11th to 20th	Day	38% 18% 32% 12% 1 1	200
	21st to end	Day	37% 20% 30% 12% 1	204
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% 3 17% 36% 28% 5%	552
	21st to end	Night	13% 3 13% 42% 27% 2	624
August	1st to 10th	Night	12% 4 14% 42% 26% 3	567
	11th to 20th	Night	13% 4 11% 43% 24% 5%	564
	21st to end	Night	12% 3 21% 33% 27% 5%	729
September	1st to 10th	Night	16% 4 21% 33% 23% 4	760
	11th to 20th	Night	12% 3 17% 35% 30% 3	760
	21st to end	Night	22% 7% 23% 26% 18% 4	760
October	1st to 10th	Night	20% 6% 24% 30% 19% 2	760
	11th to 20th	Night	22% 11% 31% 26% 10% 1	760
	21st to end	Night	28% 12% 28% 25% 7%	836
November	1st to 10th	Night	19% 13% 28% 29% 10% 1	780
	11th to 20th	Night	29% 16% 27% 22% 6% 1	800
	21st to end	Night	29% 15% 31% 19% 6%	800
December	1st to 10th	Night	33% 19% 26% 18% 4	800
	11th to 20th	Night	29% 21% 28% 19% 4	800
	21st to end	Night	32% 17% 29% 20% 2	851

Page generated at 2024-12-21 04:13:51. Version ac07

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