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 2025-06-10.

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 10% 5% 4 4 3 5% 6% 30%	119638
	Night	17% 16% 5% 10% 5% 4 4 3 5% 5% 26%	118542
Transparency	Day	54% 6% 11% 18% 9% 1	123278
	Night	58% 6% 10% 16% 9% 1	123062
Seeing	Day	23% 17% 29% 20% 9% 2	15255
	Night	25% 16% 27% 22% 9% 1	24906

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	15% 26% 10% 27% 5% 3 2 2 2 2 7%	3826
	11th to 20th	Day	12% 24% 10% 21% 7% 5% 4 2 3 4 9%	4026
	21st to end	Day	19% 18% 9% 17% 7% 5% 4 3 4 3 10%	4466
February	1st to 10th	Day	18% 19% 6% 16% 10% 4 4 2 3 4 14%	1800
	11th to 20th	Day	16% 18% 10% 15% 7% 5% 4 3 3 3 16%	1753
	21st to end	Day	20% 16% 5% 12% 5% 4 4 4 5% 5% 21%	1603
March	1st to 10th	Day	15% 15% 6% 12% 6% 3 3 2 4 5% 28%	4932
	11th to 20th	Day	13% 16% 5% 9% 5% 4 4 3 4 6% 31%	4980
	21st to end	Day	22% 14% 6% 10% 4 4 3 3 3 5% 26%	5465
April	1st to 10th	Day	18% 13% 3 7% 4 4 4 3 5% 7% 31%	2315
	11th to 20th	Day	19% 12% 3 6% 4 4 4 4 4 6% 34%	2455
	21st to end	Day	20% 9% 3 7% 5% 4 4 3 5% 6% 35%	2600
May	1st to 10th	Day	20% 9% 3 8% 5% 4 4 4 5% 6% 31%	5888
	11th to 20th	Day	18% 8% 3 7% 4 4 5% 4 6% 8% 34%	5952
	21st to end	Day	20% 7% 2 7% 4 4 5% 4 5% 6% 37%	6949
June	1st to 10th	Day	16% 6% 2 6% 4 3 4 3 6% 9% 41%	2786
	11th to 20th	Day	12% 5% 3 6% 4 4 5% 4 7% 9% 41%	2660
	21st to end	Day	15% 5% 2 5% 4 4 4 4 7% 9% 42%	2660
July	1st to 10th	Day	9% 4 2 6% 4 4 4 4 7% 10% 45%	4440
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 6% 9% 45%	4190
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 45%	4466
August	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 4 6% 8% 38%	4104
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 10% 34%	4044
	21st to end	Day	12% 5% 2 5% 4 5% 4 4 6% 9% 43%	4357
September	1st to 10th	Day	16% 8% 2 6% 3 4 4 3 6% 8% 41%	2280
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 3 6% 8% 45%	2104
	21st to end	Day	19% 7% 3 8% 5% 4 5% 3 4 7% 34%	2085
October	1st to 10th	Day	19% 8% 2 7% 4 5% 5% 4 5% 8% 34%	1900
	11th to 20th	Day	23% 10% 3 7% 5% 4 4 3 5% 6% 29%	1900
	21st to end	Day	28% 13% 4 8% 5% 4 3 3 4 6% 21%	2090
November	1st to 10th	Day	17% 16% 4 8% 4 4 3 3 5% 7% 29%	1816
	11th to 20th	Day	17% 23% 3 8% 5% 4 5% 3 5% 6% 21%	1600
	21st to end	Day	22% 26% 5% 10% 5% 4 4 3 4 5% 13%	1600
December	1st to 10th	Day	19% 32% 11% 12% 5% 4 3 2 3 2 8%	2600
	11th to 20th	Day	22% 25% 9% 13% 4 3 4 3 3 3 11%	2986
	21st to end	Day	23% 24% 7% 15% 5% 4 4 3 2 3 10%	3960
January	1st to 10th	Night	15% 25% 12% 26% 4 3 2 2 2 3 7%	6105
	11th to 20th	Night	11% 23% 12% 20% 8% 6% 4 3 4 3 7%	5859
	21st to end	Night	21% 20% 8% 16% 5% 3 3 2 3 4 13%	6490
February	1st to 10th	Night	19% 21% 6% 14% 8% 4 3 2 3 4 16%	2600
	11th to 20th	Night	16% 19% 6% 16% 7% 4 4 2 4 4 18%	2533
	21st to end	Night	19% 16% 6% 14% 5% 4 3 3 4 4 21%	2022
March	1st to 10th	Night	19% 17% 4 9% 6% 3 3 2 4 5% 27%	4908
	11th to 20th	Night	13% 17% 5% 7% 3 3 4 3 5% 5% 35%	4979
	21st to end	Night	16% 16% 5% 10% 5% 4 3 3 4 4 30%	5467
April	1st to 10th	Night	19% 16% 5% 8% 4 4 4 3 4 5% 29%	2084
	11th to 20th	Night	20% 11% 3 7% 4 4 4 3 6% 6% 33%	1911
	21st to end	Night	16% 11% 3 6% 5% 5% 5% 4 5% 7% 34%	1800
May	1st to 10th	Night	23% 10% 3 7% 5% 4 4 4 5% 6% 29%	4053
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	3941
	21st to end	Night	20% 7% 2 5% 3 3 4 3 6% 9% 38%	3926
June	1st to 10th	Night	12% 7% 2 6% 5% 5% 4 4 7% 8% 39%	1598
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 4 5% 7% 42%	1520
	21st to end	Night	14% 6% 2 6% 5% 4 5% 4 7% 8% 39%	1520
July	1st to 10th	Night	11% 7% 2 6% 5% 4 4 3 6% 9% 43%	2520
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 4 6% 9% 42%	2766
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 5% 6% 8% 45%	3087
August	1st to 10th	Night	11% 7% 3 8% 5% 5% 5% 4 7% 8% 39%	2825
	11th to 20th	Night	17% 8% 3 7% 5% 4 5% 3 5% 6% 36%	2785
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 6% 9% 42%	3252
September	1st to 10th	Night	15% 7% 3 5% 3 3 4 4 5% 7% 45%	1900
	11th to 20th	Night	14% 5% 2 6% 4 3 4 3 5% 7% 48%	2071
	21st to end	Night	18% 8% 3 8% 5% 5% 5% 4 6% 6% 34%	2094
October	1st to 10th	Night	20% 9% 2 6% 4 3 4 3 5% 6% 38%	2280
	11th to 20th	Night	25% 9% 3 7% 5% 4 4 3 5% 5% 28%	2280
	21st to end	Night	30% 11% 3 7% 3 3 4 3 4 5% 26%	2508
November	1st to 10th	Night	20% 15% 4 7% 4 4 3 3 5% 6% 31%	2474
	11th to 20th	Night	20% 22% 3 8% 5% 4 4 3 5% 5% 21%	2800
	21st to end	Night	22% 26% 6% 7% 4 3 3 3 4 4 18%	2800
December	1st to 10th	Night	17% 32% 10% 10% 5% 3 4 3 3 3 10%	4580
	11th to 20th	Night	19% 29% 9% 12% 4 3 3 2 4 4 12%	5208
	21st to end	Night	22% 23% 6% 12% 6% 4 4 3 4 5% 12%	6996

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	85% 4 6% 4 1	3827
	11th to 20th	Day	81% 3 8% 6% 1	4060
	21st to end	Day	79% 4 7% 7% 3	4466
February	1st to 10th	Day	76% 5% 8% 8% 4	1800
	11th to 20th	Day	74% 5% 6% 10% 5%	1762
	21st to end	Day	65% 7% 10% 13% 4 2	1603
March	1st to 10th	Day	59% 5% 14% 18% 3	4935
	11th to 20th	Day	54% 6% 15% 18% 5% 1	4980
	21st to end	Day	62% 5% 7% 17% 9% 1	5465
April	1st to 10th	Day	55% 5% 13% 16% 10%	2315
	11th to 20th	Day	51% 7% 10% 20% 11% 2	2455
	21st to end	Day	51% 5% 10% 19% 12% 2	2600
May	1st to 10th	Day	52% 8% 9% 19% 9% 4	5897
	11th to 20th	Day	48% 9% 11% 22% 9% 1	5956
	21st to end	Day	46% 8% 12% 19% 12% 3	6949
June	1st to 10th	Day	42% 7% 13% 24% 13% 1	2786
	11th to 20th	Day	40% 8% 15% 23% 13% 1	2660
	21st to end	Day	39% 8% 13% 25% 14% 1	2660
July	1st to 10th	Day	34% 7% 15% 25% 17% 2	4370
	11th to 20th	Day	36% 6% 14% 26% 17% 1	4118
	21st to end	Day	36% 9% 15% 24% 15% 1	4324
August	1st to 10th	Day	43% 8% 15% 22% 12%	4091
	11th to 20th	Day	44% 9% 17% 24% 6% 1	4083
	21st to end	Day	37% 8% 13% 29% 12% 1	4357
September	1st to 10th	Day	42% 7% 15% 22% 13% 1	2280
	11th to 20th	Day	38% 7% 13% 25% 16%	2104
	21st to end	Day	50% 7% 12% 21% 10% 1	2085
October	1st to 10th	Day	41% 7% 15% 22% 14% 1	3160
	11th to 20th	Day	50% 5% 13% 19% 11% 2	3140
	21st to end	Day	50% 6% 14% 23% 7%	3428
November	1st to 10th	Day	55% 7% 10% 17% 10% 2	1816
	11th to 20th	Day	64% 6% 10% 12% 6% 2	1600
	21st to end	Day	74% 5% 8% 8% 4	1600
December	1st to 10th	Day	84% 5% 6% 5% 1	2600
	11th to 20th	Day	78% 5% 7% 9% 1	2986
	21st to end	Day	81% 5% 7% 6% 1	3960
January	1st to 10th	Night	86% 4 5% 5% 1	6105
	11th to 20th	Night	82% 4 7% 5% 1	5900
	21st to end	Night	77% 3 8% 8% 4	6490
February	1st to 10th	Night	73% 6% 9% 10% 3	2600
	11th to 20th	Night	71% 5% 8% 11% 4	2535
	21st to end	Night	68% 5% 9% 11% 6% 1	2022
March	1st to 10th	Night	60% 5% 12% 16% 7%	4918
	11th to 20th	Night	51% 6% 11% 21% 10% 1	4980
	21st to end	Night	55% 8% 9% 15% 12% 1	5467
April	1st to 10th	Night	58% 5% 9% 16% 10% 1	2084
	11th to 20th	Night	52% 6% 11% 17% 12% 2	1911
	21st to end	Night	49% 7% 11% 20% 11% 3	1800
May	1st to 10th	Night	55% 6% 8% 14% 11% 6%	4060
	11th to 20th	Night	47% 9% 11% 22% 10% 1	3942
	21st to end	Night	42% 8% 11% 24% 13% 2	3927
June	1st to 10th	Night	41% 8% 10% 22% 15% 3	1598
	11th to 20th	Night	40% 7% 12% 22% 17% 2	1520
	21st to end	Night	41% 8% 13% 23% 15% 1	1520
July	1st to 10th	Night	39% 6% 12% 21% 17% 4	2480
	11th to 20th	Night	36% 7% 12% 26% 16% 3	2715
	21st to end	Night	36% 8% 15% 21% 19% 1	2988
August	1st to 10th	Night	42% 7% 12% 27% 12% 1	2814
	11th to 20th	Night	47% 7% 13% 23% 9% 1	2812
	21st to end	Night	36% 7% 15% 28% 12% 2	3252
September	1st to 10th	Night	39% 7% 12% 25% 15% 2	1900
	11th to 20th	Night	36% 6% 13% 24% 19% 1	2071
	21st to end	Night	49% 7% 11% 19% 13% 1	2094
October	1st to 10th	Night	41% 7% 10% 22% 19% 2	3800
	11th to 20th	Night	51% 6% 11% 17% 14% 2	3779
	21st to end	Night	46% 4 14% 22% 13% 1	4120
November	1st to 10th	Night	54% 6% 11% 19% 9% 2	2474
	11th to 20th	Night	65% 6% 9% 12% 6% 3	2800
	21st to end	Night	71% 5% 9% 11% 3 1	2800
December	1st to 10th	Night	80% 6% 9% 5% 1	4580
	11th to 20th	Night	77% 5% 8% 9% 1	5208
	21st to end	Night	75% 6% 9% 9% 1	6996

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	29% 32% 28% 8% 3	250
	11th to 20th	Day	33% 34% 25% 6% 2	300
	21st to end	Day	28% 32% 32% 8% 1	330
February	1st to 10th	Day	33% 26% 33% 8%	300
	11th to 20th	Day	34% 28% 33% 5%	300
	21st to end	Day	35% 26% 32% 7%	256
March	1st to 10th	Day	26% 27% 35% 9% 4	500
	11th to 20th	Day	27% 24% 34% 12% 2 1	500
	21st to end	Day	34% 21% 27% 15% 2 1	550
April	1st to 10th	Day	25% 25% 32% 14% 4	500
	11th to 20th	Day	31% 19% 27% 14% 8% 1	492
	21st to end	Day	26% 18% 35% 16% 4 1	500
May	1st to 10th	Day	27% 17% 30% 19% 5% 2	503
	11th to 20th	Day	24% 14% 28% 24% 8% 2	527
	21st to end	Day	19% 9% 29% 27% 12% 3	573
June	1st to 10th	Day	19% 14% 31% 27% 8% 2	526
	11th to 20th	Day	16% 11% 27% 31% 13% 2	490
	21st to end	Day	17% 10% 32% 29% 10% 2	490
July	1st to 10th	Day	10% 7% 27% 33% 19% 4	485
	11th to 20th	Day	16% 8% 30% 26% 15% 4	484
	21st to end	Day	11% 7% 33% 29% 16% 4	532
August	1st to 10th	Day	12% 8% 26% 31% 16% 6%	486
	11th to 20th	Day	15% 6% 24% 31% 17% 6%	479
	21st to end	Day	14% 7% 27% 29% 19% 4	539
September	1st to 10th	Day	17% 11% 24% 27% 16% 5%	490
	11th to 20th	Day	12% 5% 25% 29% 22% 7%	490
	21st to end	Day	20% 14% 29% 21% 12% 4	490
October	1st to 10th	Day	21% 14% 26% 22% 13% 3	490
	11th to 20th	Day	28% 19% 29% 17% 6% 1	490
	21st to end	Day	36% 19% 28% 15% 2	539
November	1st to 10th	Day	30% 21% 27% 17% 4 1	354
	11th to 20th	Day	36% 23% 29% 11% 3	200
	21st to end	Day	34% 25% 33% 9%	200
December	1st to 10th	Day	37% 29% 23% 9% 1 1	200
	11th to 20th	Day	35% 28% 30% 8%	200
	21st to end	Day	33% 25% 31% 10% 1	220
January	1st to 10th	Night	30% 29% 28% 9% 3	800
	11th to 20th	Night	32% 24% 30% 13% 1	800
	21st to end	Night	25% 26% 32% 15% 3	880
February	1st to 10th	Night	31% 24% 29% 16% 2	800
	11th to 20th	Night	27% 25% 32% 13% 2	783
	21st to end	Night	28% 26% 30% 15% 1	660
March	1st to 10th	Night	25% 20% 34% 17% 4	800
	11th to 20th	Night	26% 21% 31% 18% 4	800
	21st to end	Night	27% 16% 29% 21% 5% 1	880
April	1st to 10th	Night	26% 17% 31% 21% 5%	800
	11th to 20th	Night	26% 13% 32% 21% 7% 1	727
	21st to end	Night	21% 12% 33% 28% 6% 1	600
May	1st to 10th	Night	26% 8% 25% 31% 10% 1	600
	11th to 20th	Night	18% 9% 30% 34% 8% 1	552
	21st to end	Night	16% 6% 27% 31% 18% 2	433
June	1st to 10th	Night	16% 5% 33% 32% 14% 1	400
	11th to 20th	Night	14% 6% 29% 36% 16%	380
	21st to end	Night	16% 6% 28% 35% 15% 1	380
July	1st to 10th	Night	10% 6% 19% 39% 22% 4	370
	11th to 20th	Night	13% 3 22% 34% 24% 3	507
	21st to end	Night	13% 4 20% 39% 23% 1	624
August	1st to 10th	Night	12% 3 21% 37% 23% 3	567
	11th to 20th	Night	13% 4 19% 34% 26% 4	564
	21st to end	Night	15% 6% 22% 30% 23% 4	703
September	1st to 10th	Night	18% 7% 19% 32% 19% 4	760
	11th to 20th	Night	14% 6% 20% 31% 26% 4	760
	21st to end	Night	21% 11% 23% 27% 15% 4	760
October	1st to 10th	Night	23% 9% 25% 25% 16% 2	760
	11th to 20th	Night	30% 17% 26% 20% 7% 1	760
	21st to end	Night	37% 15% 29% 17% 2	836
November	1st to 10th	Night	30% 16% 27% 20% 6%	780
	11th to 20th	Night	35% 20% 28% 12% 4 1	800
	21st to end	Night	37% 22% 29% 10% 4	800
December	1st to 10th	Night	38% 26% 25% 8% 3	800
	11th to 20th	Night	34% 28% 26% 11% 2	800
	21st to end	Night	33% 24% 31% 11% 1	880

Page generated at 2025-06-07 10:28:19. Version ac07

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