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 2026-04-17.

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 10% 5% 5% 4 4 5% 7% 30%	131540
	Night	16% 12% 5% 10% 5% 4 4 3 5% 6% 30%	130608
Transparency	Day	54% 3 10% 21% 11% 1	135142
	Night	55% 3 10% 20% 11% 1	135076
Seeing	Day	25% 15% 28% 20% 10% 4	16461
	Night	23% 12% 26% 26% 11% 1	26247

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	17% 19% 8% 23% 6% 4 3 2 3 4 12%	5158
	11th to 20th	Day	11% 19% 9% 22% 9% 6% 3 2 3 3 13%	5280
	21st to end	Day	18% 17% 6% 12% 10% 8% 3 3 3 4 16%	5780
February	1st to 10th	Day	18% 18% 5% 11% 8% 5% 3 3 3 4 22%	1890
	11th to 20th	Day	17% 14% 7% 15% 7% 4 4 3 5% 5% 20%	1843
	21st to end	Day	19% 16% 4 11% 4 4 4 3 5% 6% 24%	1719
March	1st to 10th	Day	17% 11% 6% 9% 6% 4 4 3 4 6% 31%	5045
	11th to 20th	Day	13% 10% 5% 9% 5% 5% 5% 3 5% 6% 34%	5090
	21st to end	Day	18% 11% 4 9% 5% 5% 4 3 4 6% 30%	5586
April	1st to 10th	Day	17% 13% 3 8% 5% 4 4 3 6% 7% 29%	2427
	11th to 20th	Day	18% 11% 3 8% 5% 4 4 4 5% 6% 32%	2486
	21st to end	Day	21% 10% 2 7% 4 4 4 3 5% 6% 33%	2600
May	1st to 10th	Day	21% 10% 3 9% 5% 5% 5% 4 6% 6% 27%	5862
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 6% 9% 28%	5861
	21st to end	Day	19% 8% 3 6% 5% 5% 5% 4 6% 8% 32%	6885
June	1st to 10th	Day	15% 6% 2 8% 5% 5% 5% 5% 6% 9% 34%	2800
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 7% 9% 33%	2800
	21st to end	Day	14% 6% 2 6% 5% 6% 5% 4 6% 9% 37%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	6269
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 10% 34%	5900
	21st to end	Day	11% 6% 2 7% 4 5% 5% 4 7% 8% 41%	6424
August	1st to 10th	Day	15% 9% 3 9% 6% 5% 5% 4 6% 8% 31%	4234
	11th to 20th	Day	12% 6% 3 8% 5% 5% 6% 5% 7% 9% 34%	4174
	21st to end	Day	8% 5% 2 7% 6% 5% 6% 5% 7% 9% 40%	4454
September	1st to 10th	Day	16% 7% 3 5% 4 4 3 3 5% 8% 43%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	2215
	21st to end	Day	21% 8% 3 7% 4 5% 4 3 5% 6% 34%	2194
October	1st to 10th	Day	16% 6% 3 6% 4 4 4 4 6% 7% 41%	2000
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 4 4 5% 34%	2000
	21st to end	Day	23% 10% 3 7% 4 4 4 3 5% 6% 31%	2200
November	1st to 10th	Day	12% 9% 3 7% 5% 5% 4 4 4 7% 41%	1962
	11th to 20th	Day	18% 15% 4 8% 5% 5% 4 2 5% 5% 29%	1724
	21st to end	Day	19% 18% 5% 11% 5% 4 4 3 5% 5% 21%	1680
December	1st to 10th	Day	19% 17% 9% 21% 5% 4 3 2 3 3 15%	2680
	11th to 20th	Day	24% 17% 6% 11% 4 3 3 2 3 5% 21%	3070
	21st to end	Day	23% 24% 6% 11% 5% 4 4 3 4 4 13%	4048
January	1st to 10th	Night	14% 21% 8% 23% 5% 4 4 3 3 4 12%	7717
	11th to 20th	Night	12% 18% 9% 19% 9% 7% 5% 3 3 3 12%	7680
	21st to end	Night	19% 16% 6% 12% 8% 7% 5% 2 3 4 20%	8401
February	1st to 10th	Night	18% 14% 5% 11% 7% 5% 4 4 4 4 23%	2730
	11th to 20th	Night	18% 14% 5% 14% 6% 5% 4 3 4 5% 22%	2663
	21st to end	Night	19% 14% 4 13% 5% 5% 3 4 4 4 25%	2082
March	1st to 10th	Night	21% 13% 5% 8% 5% 3 3 2 3 5% 30%	5028
	11th to 20th	Night	14% 10% 5% 7% 4 4 4 3 4 6% 38%	5090
	21st to end	Night	18% 11% 4 8% 5% 4 4 3 4 5% 34%	5588
April	1st to 10th	Night	19% 13% 4 8% 4 4 4 3 4 6% 31%	2191
	11th to 20th	Night	16% 10% 3 6% 5% 4 5% 4 5% 6% 35%	2012
	21st to end	Night	16% 9% 3 7% 4 4 4 3 7% 7% 35%	1800
May	1st to 10th	Night	27% 11% 4 7% 4 4 4 3 5% 6% 26%	4033
	11th to 20th	Night	18% 10% 3 7% 4 4 3 3 5% 7% 37%	4032
	21st to end	Night	20% 8% 2 5% 3 3 4 4 6% 9% 36%	3968
June	1st to 10th	Night	11% 6% 2 6% 4 5% 4 4 5% 8% 44%	1600
	11th to 20th	Night	13% 5% 2 7% 5% 5% 4 4 7% 8% 41%	1600
	21st to end	Night	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	1600
July	1st to 10th	Night	11% 8% 3 7% 5% 4 4 4 5% 7% 42%	3684
	11th to 20th	Night	11% 5% 2 7% 6% 5% 5% 4 7% 10% 38%	4057
	21st to end	Night	8% 7% 3 7% 4 5% 4 4 5% 8% 45%	4427
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 4 7% 38%	2915
	11th to 20th	Night	13% 7% 3 7% 4 4 3 3 5% 7% 43%	2875
	21st to end	Night	8% 5% 2 6% 4 5% 6% 4 6% 9% 44%	3409
September	1st to 10th	Night	13% 5% 2 5% 4 3 4 3 6% 7% 49%	2000
	11th to 20th	Night	10% 4 2 4 3 4 4 4 5% 8% 53%	2182
	21st to end	Night	18% 8% 3 7% 4 4 4 3 4 6% 38%	2205
October	1st to 10th	Night	15% 6% 2 6% 3 3 4 3 4 7% 46%	2400
	11th to 20th	Night	17% 8% 2 6% 6% 4 4 3 4 6% 40%	2400
	21st to end	Night	23% 9% 3 5% 3 4 3 4 5% 6% 35%	2640
November	1st to 10th	Night	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	2551
	11th to 20th	Night	18% 13% 4 8% 5% 4 5% 4 5% 6% 28%	2893
	21st to end	Night	19% 16% 4 8% 5% 4 4 3 4 6% 27%	2940
December	1st to 10th	Night	17% 17% 10% 15% 6% 4 4 3 4 5% 16%	4720
	11th to 20th	Night	22% 17% 7% 11% 5% 4 3 2 4 5% 20%	5345
	21st to end	Night	23% 19% 6% 11% 5% 4 4 3 4 5% 15%	7150

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	78% 2 7% 7% 4	5159
	11th to 20th	Day	80% 1 5% 8% 4 1	5280
	21st to end	Day	74% 2 8% 12% 4	5780
February	1st to 10th	Day	68% 2 8% 16% 7%	1890
	11th to 20th	Day	67% 3 8% 14% 7%	1852
	21st to end	Day	63% 2 10% 17% 7% 1	1719
March	1st to 10th	Day	56% 2 10% 23% 8% 1	5045
	11th to 20th	Day	52% 2 13% 23% 8% 2	5090
	21st to end	Day	56% 2 8% 21% 12% 1	5586
April	1st to 10th	Day	55% 3 11% 17% 12% 2	2427
	11th to 20th	Day	53% 3 9% 19% 13% 2	2486
	21st to end	Day	52% 3 8% 23% 12% 2	2600
May	1st to 10th	Day	57% 4 9% 18% 9% 3	5897
	11th to 20th	Day	53% 4 9% 23% 9% 1	5865
	21st to end	Day	50% 3 12% 21% 11% 3	6900
June	1st to 10th	Day	47% 4 12% 24% 12% 1	2800
	11th to 20th	Day	47% 4 15% 22% 12% 1	2800
	21st to end	Day	43% 5% 11% 27% 13% 1	2800
July	1st to 10th	Day	40% 5% 11% 28% 15% 2	6200
	11th to 20th	Day	43% 6% 12% 26% 13%	5831
	21st to end	Day	40% 5% 10% 25% 20% 1	6240
August	1st to 10th	Day	51% 4 12% 23% 9%	4221
	11th to 20th	Day	44% 4 12% 29% 10% 1	4213
	21st to end	Day	38% 6% 12% 29% 14% 2	4454
September	1st to 10th	Day	40% 5% 9% 25% 19% 3	2400
	11th to 20th	Day	38% 3 14% 27% 17% 1	2215
	21st to end	Day	51% 3 10% 22% 13%	2194
October	1st to 10th	Day	33% 4 10% 32% 20% 1	3260
	11th to 20th	Day	46% 4 9% 22% 15% 4	3240
	21st to end	Day	40% 3 13% 27% 16% 1	3538
November	1st to 10th	Day	45% 3 11% 23% 17% 2	1962
	11th to 20th	Day	58% 2 9% 19% 10% 2	1724
	21st to end	Day	64% 3 11% 16% 5% 1	1680
December	1st to 10th	Day	76% 2 8% 10% 3	2680
	11th to 20th	Day	68% 2 11% 15% 5%	3066
	21st to end	Day	77% 2 8% 10% 4	4048
January	1st to 10th	Night	78% 2 6% 10% 3	7718
	11th to 20th	Night	78% 1 8% 9% 3	7680
	21st to end	Night	71% 2 7% 14% 5% 1	8401
February	1st to 10th	Night	66% 3 8% 17% 6%	2730
	11th to 20th	Night	66% 2 9% 16% 7%	2665
	21st to end	Night	63% 3 8% 15% 10% 1	2082
March	1st to 10th	Night	59% 2 10% 21% 8%	5028
	11th to 20th	Night	48% 2 12% 26% 9% 3	5090
	21st to end	Night	54% 2 8% 18% 16% 1	5588
April	1st to 10th	Night	55% 3 9% 19% 13% 2	2191
	11th to 20th	Night	49% 4 11% 19% 15% 3	2012
	21st to end	Night	47% 4 9% 25% 13% 3	1800
May	1st to 10th	Night	60% 3 8% 16% 10% 4	4060
	11th to 20th	Night	49% 3 9% 26% 12% 1	4033
	21st to end	Night	46% 3 11% 25% 12% 3	3975
June	1st to 10th	Night	39% 4 13% 26% 17% 2	1600
	11th to 20th	Night	40% 5% 14% 24% 15% 2	1600
	21st to end	Night	39% 5% 14% 24% 17% 1	1600
July	1st to 10th	Night	42% 4 9% 24% 19% 1	3645
	11th to 20th	Night	40% 6% 17% 24% 12% 1	4006
	21st to end	Night	37% 5% 11% 21% 25% 2	4297
August	1st to 10th	Night	46% 5% 12% 24% 12% 1	2904
	11th to 20th	Night	41% 4 11% 30% 14% 1	2902
	21st to end	Night	35% 6% 13% 31% 13% 2	3409
September	1st to 10th	Night	34% 5% 10% 26% 20% 4	2000
	11th to 20th	Night	30% 4 14% 30% 22% 1	2182
	21st to end	Night	47% 3 11% 21% 17% 1	2205
October	1st to 10th	Night	36% 3 8% 27% 25% 1	3920
	11th to 20th	Night	40% 4 11% 23% 15% 7%	3899
	21st to end	Night	37% 3 10% 29% 19% 2	4252
November	1st to 10th	Night	42% 3 11% 24% 16% 3	2551
	11th to 20th	Night	56% 3 9% 20% 9% 3	2893
	21st to end	Night	60% 3 11% 20% 5% 1	2940
December	1st to 10th	Night	71% 3 11% 12% 3	4720
	11th to 20th	Night	69% 2 10% 15% 3	5348
	21st to end	Night	73% 2 9% 14% 2	7150

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	34% 28% 25% 8% 4 1	311
	11th to 20th	Day	37% 31% 23% 6% 2 1	330
	21st to end	Day	36% 28% 27% 7% 2	363
February	1st to 10th	Day	34% 24% 33% 8% 2	330
	11th to 20th	Day	42% 24% 27% 6% 1	330
	21st to end	Day	39% 24% 29% 8%	304
March	1st to 10th	Day	32% 23% 30% 12% 3	540
	11th to 20th	Day	36% 19% 29% 9% 5% 1	540
	21st to end	Day	33% 20% 29% 13% 4 1	594
April	1st to 10th	Day	33% 19% 27% 15% 5% 1	540
	11th to 20th	Day	27% 20% 28% 16% 6% 3	516
	21st to end	Day	29% 17% 32% 16% 5% 2	500
May	1st to 10th	Day	30% 16% 29% 18% 4 3	503
	11th to 20th	Day	25% 13% 32% 18% 9% 3	527
	21st to end	Day	19% 9% 28% 26% 14% 5%	573
June	1st to 10th	Day	16% 13% 31% 26% 12% 3	530
	11th to 20th	Day	21% 10% 26% 26% 13% 4	530
	21st to end	Day	15% 9% 34% 26% 13% 3	530
July	1st to 10th	Day	13% 8% 19% 30% 20% 10%	525
	11th to 20th	Day	15% 6% 25% 28% 19% 8%	524
	21st to end	Day	13% 7% 24% 32% 18% 7%	576
August	1st to 10th	Day	13% 7% 23% 29% 20% 8%	526
	11th to 20th	Day	14% 6% 24% 31% 15% 10%	519
	21st to end	Day	11% 6% 25% 29% 19% 10%	583
September	1st to 10th	Day	18% 8% 22% 26% 18% 8%	530
	11th to 20th	Day	12% 2 21% 32% 22% 11%	530
	21st to end	Day	25% 12% 25% 22% 13% 4	530
October	1st to 10th	Day	18% 9% 27% 24% 18% 5%	530
	11th to 20th	Day	23% 14% 31% 21% 8% 2	530
	21st to end	Day	33% 13% 32% 17% 6%	583
November	1st to 10th	Day	21% 20% 33% 15% 9% 2	440
	11th to 20th	Day	32% 19% 33% 12% 3 2	242
	21st to end	Day	31% 22% 35% 10% 1	220
December	1st to 10th	Day	36% 25% 27% 7% 3 1	220
	11th to 20th	Day	38% 19% 32% 10%	220
	21st to end	Day	39% 21% 29% 11% 1	242
January	1st to 10th	Night	31% 20% 33% 13% 2	840
	11th to 20th	Night	32% 21% 31% 14% 2	840
	21st to end	Night	31% 20% 30% 17% 2	924
February	1st to 10th	Night	31% 17% 28% 22% 2	840
	11th to 20th	Night	31% 20% 33% 15% 1	823
	21st to end	Night	32% 18% 31% 16% 2	692
March	1st to 10th	Night	28% 19% 30% 20% 2	840
	11th to 20th	Night	30% 18% 26% 21% 5%	840
	21st to end	Night	25% 16% 34% 19% 4 1	924
April	1st to 10th	Night	28% 16% 30% 23% 4	840
	11th to 20th	Night	23% 13% 34% 22% 7% 1	792
	21st to end	Night	23% 8% 35% 29% 5% 1	600
May	1st to 10th	Night	24% 11% 24% 32% 9% 1	600
	11th to 20th	Night	21% 8% 26% 32% 12% 1	591
	21st to end	Night	13% 5% 25% 34% 21% 3	456
June	1st to 10th	Night	11% 5% 28% 38% 18% 1	400
	11th to 20th	Night	15% 7% 22% 40% 17% 1	400
	21st to end	Night	11% 4 25% 38% 21% 2	400
July	1st to 10th	Night	9% 4 18% 41% 24% 5%	423
	11th to 20th	Night	10% 3 17% 35% 30% 5%	582
	21st to end	Night	12% 3 14% 42% 27% 2	657
August	1st to 10th	Night	11% 4 14% 42% 26% 3	597
	11th to 20th	Night	13% 4 11% 42% 25% 5%	594
	21st to end	Night	12% 4 21% 33% 26% 5%	768
September	1st to 10th	Night	15% 4 21% 33% 23% 4	800
	11th to 20th	Night	11% 3 16% 36% 30% 3	800
	21st to end	Night	22% 8% 23% 27% 17% 4	800
October	1st to 10th	Night	19% 5% 23% 30% 20% 2	800
	11th to 20th	Night	21% 10% 32% 26% 9% 1	800
	21st to end	Night	28% 12% 28% 25% 6%	880
November	1st to 10th	Night	20% 13% 28% 29% 10%	820
	11th to 20th	Night	29% 16% 26% 22% 6% 1	840
	21st to end	Night	30% 15% 31% 18% 5%	840
December	1st to 10th	Night	32% 19% 26% 18% 4	840
	11th to 20th	Night	28% 21% 28% 19% 4	840
	21st to end	Night	32% 17% 28% 20% 2	924

Page generated at 2026-04-14 02:44:28. Version ac07

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