Mitchell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mitchell Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	149404
	Night	17% 15% 5% 10% 5% 4 4 3 5% 6% 27%	142282
Transparency	Day	53% 6% 11% 19% 9% 1	153003
	Night	58% 6% 10% 16% 9% 1	146770
Seeing	Day	24% 17% 29% 20% 8% 2	17046
	Night	25% 16% 27% 22% 9% 1	26569

By Month

Percentage Cloud Cover NE (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% 24% 9% 28% 5% 3 2 2 3 2 6%	4960
	11th to 20th	Day	11% 23% 10% 21% 8% 5% 5% 3 3 4 8%	5246
	21st to end	Day	16% 17% 8% 18% 9% 7% 5% 3 4 3 10%	5780
February	1st to 10th	Day	17% 18% 5% 16% 10% 5% 4 2 3 4 14%	1890
	11th to 20th	Day	15% 18% 10% 15% 7% 5% 4 3 3 3 16%	1843
	21st to end	Day	19% 16% 5% 12% 5% 4 4 4 5% 5% 21%	1681
March	1st to 10th	Day	15% 15% 6% 12% 6% 3 3 2 4 5% 28%	5042
	11th to 20th	Day	13% 16% 5% 9% 5% 4 4 3 4 6% 30%	5090
	21st to end	Day	21% 14% 6% 10% 4 4 3 3 3 5% 26%	5586
April	1st to 10th	Day	18% 13% 3 7% 4 4 4 3 5% 7% 31%	2431
	11th to 20th	Day	19% 12% 3 6% 4 4 4 4 4 6% 34%	2579
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 35%	2730
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 4 4 6% 6% 31%	7642
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 6% 8% 35%	7745
	21st to end	Day	18% 7% 2 6% 4 4 5% 3 5% 6% 39%	9060
June	1st to 10th	Day	16% 6% 2 6% 4 3 4 3 6% 9% 42%	2940
	11th to 20th	Day	13% 6% 3 6% 4 4 5% 4 6% 9% 39%	2940
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 7% 9% 42%	2940
July	1st to 10th	Day	11% 6% 3 6% 4 4 4 4 6% 9% 44%	8280
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 6% 9% 46%	7806
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 8% 44%	8340
August	1st to 10th	Day	19% 7% 3 7% 5% 5% 4 4 5% 7% 35%	6014
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 7% 10% 33%	5951
	21st to end	Day	14% 6% 3 6% 4 5% 4 4 6% 9% 39%	6344
September	1st to 10th	Day	16% 7% 2 6% 3 4 4 4 6% 8% 41%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 3 6% 7% 46%	2326
	21st to end	Day	18% 8% 3 9% 5% 4 5% 3 4 7% 34%	2305
October	1st to 10th	Day	18% 7% 2 7% 4 4 4 4 5% 8% 37%	2100
	11th to 20th	Day	23% 10% 3 7% 5% 5% 4 3 5% 6% 29%	2030
	21st to end	Day	28% 13% 3 8% 5% 4 4 3 4 6% 22%	2200
November	1st to 10th	Day	18% 16% 4 7% 4 4 3 3 5% 7% 29%	1909
	11th to 20th	Day	17% 24% 3 8% 5% 4 5% 3 5% 6% 21%	1680
	21st to end	Day	22% 25% 5% 10% 5% 4 4 3 4 5% 13%	1680
December	1st to 10th	Day	19% 32% 11% 12% 5% 4 3 2 3 2 7%	2680
	11th to 20th	Day	21% 25% 9% 13% 4 3 4 3 3 3 11%	3066
	21st to end	Day	23% 23% 7% 16% 5% 4 4 3 3 3 10%	4048
January	1st to 10th	Night	16% 24% 11% 28% 4 3 2 2 2 2 6%	7920
	11th to 20th	Night	11% 22% 11% 21% 8% 6% 4 2 3 3 7%	7639
	21st to end	Night	17% 18% 7% 16% 8% 5% 4 3 4 5% 14%	8401
February	1st to 10th	Night	18% 21% 6% 14% 7% 4 3 2 3 4 17%	2730
	11th to 20th	Night	16% 19% 6% 15% 7% 5% 4 2 4 4 17%	2663
	21st to end	Night	19% 17% 6% 14% 5% 4 4 3 4 4 21%	2120
March	1st to 10th	Night	19% 16% 4 9% 6% 3 3 2 4 5% 27%	5018
	11th to 20th	Night	14% 17% 5% 8% 3 3 4 3 5% 6% 34%	5089
	21st to end	Night	16% 16% 5% 10% 5% 4 4 3 4 4 30%	5588
April	1st to 10th	Night	19% 16% 5% 8% 4 4 4 3 4 5% 28%	2188
	11th to 20th	Night	20% 11% 3 7% 4 4 4 3 6% 6% 32%	2008
	21st to end	Night	15% 11% 3 6% 5% 5% 5% 4 5% 7% 34%	1890
May	1st to 10th	Night	23% 9% 3 7% 5% 5% 4 4 5% 6% 29%	5257
	11th to 20th	Night	18% 7% 2 8% 5% 4 4 4 6% 7% 35%	5115
	21st to end	Night	19% 8% 2 6% 3 3 4 4 5% 8% 37%	5114
June	1st to 10th	Night	11% 7% 3 6% 6% 5% 4 4 7% 8% 40%	1680
	11th to 20th	Night	11% 7% 3 7% 5% 4 5% 4 6% 7% 40%	1680
	21st to end	Night	13% 6% 2 6% 5% 4 5% 4 7% 8% 39%	1680
July	1st to 10th	Night	10% 7% 3 7% 5% 4 4 4 6% 9% 40%	4680
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 4 7% 9% 43%	5136
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 7% 47%	5737
August	1st to 10th	Night	15% 8% 3 8% 5% 5% 4 3 6% 7% 36%	4135
	11th to 20th	Night	16% 8% 3 8% 5% 4 5% 4 6% 7% 35%	4095
	21st to end	Night	14% 6% 3 6% 4 5% 5% 4 6% 9% 38%	4720
September	1st to 10th	Night	14% 7% 2 5% 4 4 4 4 5% 7% 45%	2100
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 7% 47%	2289
	21st to end	Night	17% 8% 3 7% 4 5% 5% 4 6% 7% 34%	2314
October	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 5% 6% 40%	2520
	11th to 20th	Night	25% 9% 3 7% 5% 4 4 3 5% 5% 29%	2437
	21st to end	Night	30% 11% 3 7% 3 3 4 3 4 5% 27%	2640
November	1st to 10th	Night	20% 15% 4 7% 4 4 3 3 5% 6% 31%	2601
	11th to 20th	Night	20% 22% 3 8% 5% 4 4 3 5% 5% 20%	2940
	21st to end	Night	23% 26% 6% 7% 4 3 3 3 4 3 18%	2940
December	1st to 10th	Night	17% 32% 10% 10% 5% 3 4 3 3 3 10%	4720
	11th to 20th	Night	18% 28% 9% 12% 4 3 3 2 4 4 12%	5348
	21st to end	Night	22% 23% 6% 12% 6% 4 4 3 4 5% 12%	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	86% 4 6% 4 1	4961
	11th to 20th	Day	82% 3 8% 5% 2	5280
	21st to end	Day	79% 4 6% 7% 3	5780
February	1st to 10th	Day	75% 5% 8% 8% 4	1890
	11th to 20th	Day	74% 5% 6% 11% 5%	1852
	21st to end	Day	65% 6% 11% 13% 4 2	1681
March	1st to 10th	Day	59% 5% 14% 18% 4	5045
	11th to 20th	Day	55% 6% 15% 17% 5% 1	5090
	21st to end	Day	62% 5% 7% 17% 9% 1	5586
April	1st to 10th	Day	55% 5% 13% 16% 10%	2431
	11th to 20th	Day	52% 6% 10% 19% 11% 2	2579
	21st to end	Day	51% 5% 11% 20% 12% 2	2730
May	1st to 10th	Day	51% 7% 9% 20% 9% 3	7651
	11th to 20th	Day	47% 8% 11% 23% 10% 1	7749
	21st to end	Day	44% 7% 11% 21% 15% 2	9061
June	1st to 10th	Day	41% 7% 12% 25% 14% 1	2940
	11th to 20th	Day	42% 8% 15% 22% 12% 1	2940
	21st to end	Day	38% 9% 14% 25% 14% 1	2940
July	1st to 10th	Day	37% 6% 13% 26% 17% 1	8210
	11th to 20th	Day	34% 6% 14% 31% 15% 1	7734
	21st to end	Day	38% 7% 13% 24% 17% 1	8156
August	1st to 10th	Day	48% 7% 14% 18% 12% 1	6001
	11th to 20th	Day	45% 8% 16% 25% 5% 1	5990
	21st to end	Day	41% 7% 14% 28% 9%	6344
September	1st to 10th	Day	42% 7% 14% 23% 13% 1	2520
	11th to 20th	Day	38% 7% 13% 27% 15%	2326
	21st to end	Day	50% 7% 11% 20% 10% 1	2305
October	1st to 10th	Day	40% 7% 14% 24% 15% 1	3360
	11th to 20th	Day	49% 5% 13% 19% 11% 2	3270
	21st to end	Day	50% 5% 14% 23% 7% 1	3538
November	1st to 10th	Day	55% 6% 9% 18% 9% 2	1909
	11th to 20th	Day	64% 6% 10% 13% 6% 2	1680
	21st to end	Day	74% 5% 8% 8% 4	1680
December	1st to 10th	Day	84% 5% 6% 5% 1	2680
	11th to 20th	Day	78% 5% 7% 9% 1	3066
	21st to end	Day	81% 5% 8% 6% 1	4048
January	1st to 10th	Night	87% 3 5% 4 1	7920
	11th to 20th	Night	83% 3 7% 5% 2	7680
	21st to end	Night	75% 4 9% 10% 3	8401
February	1st to 10th	Night	72% 6% 8% 11% 3	2730
	11th to 20th	Night	72% 5% 9% 11% 3	2665
	21st to end	Night	68% 5% 9% 11% 6% 1	2120
March	1st to 10th	Night	59% 5% 12% 17% 7%	5028
	11th to 20th	Night	51% 6% 11% 21% 9% 1	5090
	21st to end	Night	55% 8% 9% 15% 12% 1	5588
April	1st to 10th	Night	59% 5% 9% 16% 10% 1	2188
	11th to 20th	Night	52% 6% 11% 17% 12% 2	2008
	21st to end	Night	49% 7% 11% 20% 10% 3	1890
May	1st to 10th	Night	54% 6% 8% 16% 11% 5%	5264
	11th to 20th	Night	47% 8% 11% 22% 10% 2	5116
	21st to end	Night	44% 7% 10% 22% 14% 2	5115
June	1st to 10th	Night	40% 8% 10% 22% 16% 3	1680
	11th to 20th	Night	41% 7% 13% 22% 16% 2	1680
	21st to end	Night	41% 8% 13% 23% 15% 1	1680
July	1st to 10th	Night	39% 7% 12% 23% 16% 2	4640
	11th to 20th	Night	35% 6% 11% 28% 18% 2	5085
	21st to end	Night	36% 6% 12% 20% 25% 2	5606
August	1st to 10th	Night	47% 5% 11% 20% 13% 3	4124
	11th to 20th	Night	46% 8% 15% 23% 7% 1	4122
	21st to end	Night	40% 7% 15% 26% 11% 1	4720
September	1st to 10th	Night	38% 7% 13% 26% 14% 2	2100
	11th to 20th	Night	36% 6% 13% 25% 18% 1	2289
	21st to end	Night	48% 7% 11% 20% 13% 1	2314
October	1st to 10th	Night	40% 7% 9% 23% 19% 2	4040
	11th to 20th	Night	50% 6% 11% 17% 14% 2	3936
	21st to end	Night	46% 3 14% 22% 13% 1	4252
November	1st to 10th	Night	54% 6% 11% 19% 9% 2	2601
	11th to 20th	Night	65% 6% 9% 12% 5% 2	2940
	21st to end	Night	71% 5% 9% 11% 3 1	2940
December	1st to 10th	Night	79% 5% 9% 5% 2	4720
	11th to 20th	Night	76% 5% 9% 9% 1	5348
	21st to end	Night	75% 6% 9% 9% 1	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	33% 29% 28% 7% 3	275
	11th to 20th	Day	34% 35% 24% 5% 2	330
	21st to end	Day	29% 31% 30% 9% 1	363
February	1st to 10th	Day	31% 25% 32% 11% 1	330
	11th to 20th	Day	32% 28% 33% 6%	330
	21st to end	Day	34% 25% 34% 7%	280
March	1st to 10th	Day	26% 26% 34% 9% 4	540
	11th to 20th	Day	29% 24% 32% 12% 2 1	540
	21st to end	Day	35% 22% 27% 14% 2 1	594
April	1st to 10th	Day	25% 25% 32% 14% 4	540
	11th to 20th	Day	31% 20% 27% 14% 7% 1	532
	21st to end	Day	26% 17% 35% 16% 4 1	540
May	1st to 10th	Day	27% 18% 31% 18% 5% 1	543
	11th to 20th	Day	25% 14% 28% 23% 7% 2	567
	21st to end	Day	19% 10% 30% 26% 11% 3	617
June	1st to 10th	Day	18% 15% 30% 28% 7% 2	570
	11th to 20th	Day	18% 13% 28% 28% 11% 1	570
	21st to end	Day	16% 10% 33% 29% 10% 2	570
July	1st to 10th	Day	10% 7% 29% 32% 18% 3	565
	11th to 20th	Day	16% 8% 31% 27% 15% 4	564
	21st to end	Day	11% 7% 33% 29% 16% 3	620
August	1st to 10th	Day	13% 8% 27% 31% 16% 6%	566
	11th to 20th	Day	16% 7% 25% 30% 16% 6%	559
	21st to end	Day	15% 8% 27% 29% 17% 4	627
September	1st to 10th	Day	17% 12% 24% 26% 16% 5%	570
	11th to 20th	Day	13% 5% 26% 29% 21% 7%	570
	21st to end	Day	19% 14% 30% 21% 12% 4	570
October	1st to 10th	Day	20% 14% 27% 23% 13% 4	570
	11th to 20th	Day	27% 19% 30% 16% 5% 1	542
	21st to end	Day	37% 19% 28% 14% 2	583
November	1st to 10th	Day	32% 22% 26% 16% 3 1	387
	11th to 20th	Day	39% 21% 27% 10% 3	220
	21st to end	Day	36% 24% 32% 8%	220
December	1st to 10th	Day	37% 27% 25% 9% 1 1	220
	11th to 20th	Day	35% 28% 28% 8%	220
	21st to end	Day	34% 26% 30% 9% 1	242
January	1st to 10th	Night	31% 29% 28% 9% 3	840
	11th to 20th	Night	32% 25% 29% 13% 1	840
	21st to end	Night	26% 26% 31% 15% 2	924
February	1st to 10th	Night	30% 23% 29% 17% 2	840
	11th to 20th	Night	27% 26% 32% 13% 2	823
	21st to end	Night	28% 26% 30% 15% 1	692
March	1st to 10th	Night	26% 20% 34% 17% 4	840
	11th to 20th	Night	27% 20% 31% 17% 4	840
	21st to end	Night	27% 16% 30% 21% 5% 1	924
April	1st to 10th	Night	27% 17% 31% 20% 5%	840
	11th to 20th	Night	26% 13% 32% 20% 7% 1	764
	21st to end	Night	22% 11% 32% 28% 6% 1	630
May	1st to 10th	Night	26% 8% 25% 31% 9% 1	630
	11th to 20th	Night	18% 9% 30% 33% 8% 1	580
	21st to end	Night	17% 6% 26% 31% 17% 2	455
June	1st to 10th	Night	15% 5% 32% 33% 14%	420
	11th to 20th	Night	15% 7% 31% 33% 15%	420
	21st to end	Night	15% 6% 27% 36% 15% 1	420
July	1st to 10th	Night	10% 6% 19% 40% 21% 4	410
	11th to 20th	Night	13% 4 22% 36% 22% 3	562
	21st to end	Night	13% 4 20% 40% 22% 1	690
August	1st to 10th	Night	12% 3 21% 38% 22% 3	627
	11th to 20th	Night	13% 4 19% 35% 25% 4	624
	21st to end	Night	15% 7% 21% 30% 23% 4	777
September	1st to 10th	Night	17% 8% 20% 34% 18% 4	840
	11th to 20th	Night	14% 6% 19% 31% 26% 4	840
	21st to end	Night	20% 10% 24% 28% 15% 3	840
October	1st to 10th	Night	22% 9% 25% 26% 16% 1	840
	11th to 20th	Night	29% 17% 27% 20% 6% 1	813
	21st to end	Night	37% 15% 29% 17% 2	880
November	1st to 10th	Night	31% 16% 27% 20% 6%	820
	11th to 20th	Night	35% 20% 28% 12% 4 1	840
	21st to end	Night	37% 22% 28% 9% 3	840
December	1st to 10th	Night	38% 25% 25% 8% 3	840
	11th to 20th	Night	33% 28% 26% 10% 2	840
	21st to end	Night	33% 24% 31% 11% 1	924

Page generated at 2026-10-11 05:06:32. Version ac07

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