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 2024-11-03.

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 8% 5% 4 4 3 5% 7% 31%	101275
	Night	18% 15% 5% 9% 5% 4 4 3 5% 6% 28%	97700
Transparency	Day	53% 7% 12% 18% 9% 1	101046
	Night	58% 6% 10% 16% 9% 1	97548
Seeing	Day	23% 16% 29% 21% 9% 2	14552
	Night	25% 15% 27% 22% 9% 1	24105

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	19% 29% 10% 14% 6% 3 3 2 3 3 9%	2676
	11th to 20th	Day	12% 25% 10% 19% 6% 3 3 2 3 5% 11%	2840
	21st to end	Day	24% 19% 11% 18% 7% 4 3 3 3 2 7%	3124
February	1st to 10th	Day	18% 19% 6% 16% 10% 4 4 2 3 4 14%	1710
	11th to 20th	Day	15% 18% 10% 15% 7% 5% 4 3 3 3 16%	1663
	21st to end	Day	20% 16% 5% 12% 5% 4 4 4 5% 5% 22%	1524
March	1st to 10th	Day	19% 15% 5% 8% 4 4 3 3 4 6% 28%	3432
	11th to 20th	Day	12% 20% 5% 9% 4 4 4 3 4 6% 29%	3480
	21st to end	Day	20% 14% 5% 10% 4 4 4 3 4 5% 27%	3815
April	1st to 10th	Day	18% 13% 3 7% 4 4 4 3 5% 7% 32%	2199
	11th to 20th	Day	19% 12% 3 6% 4 4 4 4 4 6% 34%	2331
	21st to end	Day	21% 9% 3 7% 5% 4 4 3 5% 5% 34%	2470
May	1st to 10th	Day	19% 9% 3 7% 5% 4 4 4 5% 7% 32%	4108
	11th to 20th	Day	19% 9% 3 7% 4 4 4 3 5% 7% 35%	4144
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 6% 7% 44%	4837
June	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 6% 9% 40%	2660
	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%	1744
	11th to 20th	Day	17% 24% 3 8% 5% 4 4 3 5% 5% 21%	1520
	21st to end	Day	21% 25% 4 10% 5% 4 4 3 4 5% 14%	1520
December	1st to 10th	Day	20% 32% 8% 9% 4 3 3 2 4 3 12%	1520
	11th to 20th	Day	19% 25% 11% 11% 4 3 4 3 3 4 13%	1906
	21st to end	Day	25% 24% 8% 14% 5% 4 3 3 2 3 11%	2772
January	1st to 10th	Night	17% 29% 11% 16% 5% 3 2 2 2 3 9%	4269
	11th to 20th	Night	14% 23% 10% 15% 7% 6% 5% 3 4 4 9%	4120
	21st to end	Night	26% 22% 8% 16% 6% 3 3 2 2 2 11%	4532
February	1st to 10th	Night	19% 21% 6% 14% 8% 4 3 2 3 4 17%	2470
	11th to 20th	Night	16% 19% 6% 16% 7% 4 4 2 4 4 18%	2403
	21st to end	Night	19% 16% 6% 14% 5% 4 3 3 4 4 22%	1925
March	1st to 10th	Night	22% 16% 4 8% 4 3 3 2 4 5% 30%	3408
	11th to 20th	Night	17% 20% 6% 8% 3 3 3 2 4 5% 29%	3479
	21st to end	Night	16% 14% 5% 10% 4 4 3 3 4 4 34%	3817
April	1st to 10th	Night	18% 16% 5% 7% 4 3 4 3 4 5% 30%	1980
	11th to 20th	Night	20% 11% 3 7% 4 4 3 3 6% 6% 33%	1815
	21st to end	Night	16% 11% 3 6% 5% 5% 5% 4 5% 7% 33%	1710
May	1st to 10th	Night	25% 9% 3 6% 4 4 3 3 5% 7% 32%	2833
	11th to 20th	Night	16% 8% 2 7% 5% 5% 4 3 6% 8% 35%	2749
	21st to end	Night	13% 5% 2 5% 3 3 4 4 6% 10% 45%	2738
June	1st to 10th	Night	12% 7% 3 6% 5% 5% 5% 4 7% 8% 39%	1520
	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%	2372
	11th to 20th	Night	20% 22% 3 8% 5% 4 4 3 5% 5% 21%	2660
	21st to end	Night	22% 25% 6% 7% 3 3 3 3 4 4 19%	2660
December	1st to 10th	Night	22% 32% 8% 8% 5% 2 3 2 3 3 12%	2660
	11th to 20th	Night	18% 27% 10% 11% 4 3 3 3 3 4 13%	3288
	21st to end	Night	25% 25% 6% 11% 6% 4 4 3 4 4 8%	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	79% 6% 8% 5% 1	2677
	11th to 20th	Day	78% 3 9% 8% 1	2840
	21st to end	Day	85% 5% 5% 3 2	3124
February	1st to 10th	Day	76% 5% 8% 7% 4	1710
	11th to 20th	Day	74% 5% 6% 10% 5%	1672
	21st to end	Day	64% 7% 10% 13% 4 2	1524
March	1st to 10th	Day	57% 6% 11% 22% 4	3435
	11th to 20th	Day	56% 7% 17% 14% 5% 1	3480
	21st to end	Day	59% 6% 8% 17% 9% 1	3815
April	1st to 10th	Day	54% 6% 13% 16% 11%	2199
	11th to 20th	Day	51% 7% 10% 19% 11% 2	2331
	21st to end	Day	52% 5% 10% 18% 12% 2	2470
May	1st to 10th	Day	50% 7% 9% 23% 9% 2	4120
	11th to 20th	Day	49% 8% 12% 22% 7% 1	4148
	21st to end	Day	37% 9% 13% 21% 15% 4	4837
June	1st to 10th	Day	42% 7% 13% 25% 12% 1	2660
	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	49% 7% 14% 17% 11% 1	1900
	11th to 20th	Day	57% 7% 13% 15% 7% 1	1900
	21st to end	Day	65% 6% 11% 12% 5% 1	2090
November	1st to 10th	Day	55% 7% 10% 17% 10% 2	1744
	11th to 20th	Day	64% 6% 10% 12% 6% 2	1520
	21st to end	Day	73% 5% 8% 9% 4	1520
December	1st to 10th	Day	78% 5% 8% 8% 1	1520
	11th to 20th	Day	76% 5% 8% 8% 1	1906
	21st to end	Day	81% 5% 7% 6%	2772
January	1st to 10th	Night	82% 4 7% 6% 2	4269
	11th to 20th	Night	79% 5% 8% 6% 1	4120
	21st to end	Night	83% 3 6% 6% 1	4532
February	1st to 10th	Night	73% 6% 9% 10% 3	2470
	11th to 20th	Night	71% 5% 8% 11% 4	2405
	21st to end	Night	67% 5% 9% 11% 7% 1	1925
March	1st to 10th	Night	58% 5% 11% 19% 6%	3418
	11th to 20th	Night	59% 7% 11% 16% 7% 1	3480
	21st to end	Night	51% 8% 10% 17% 12% 2	3817
April	1st to 10th	Night	57% 5% 10% 17% 10% 1	1980
	11th to 20th	Night	51% 6% 11% 17% 12% 2	1815
	21st to end	Night	49% 7% 11% 19% 10% 3	1710
May	1st to 10th	Night	51% 7% 9% 18% 10% 4	2840
	11th to 20th	Night	47% 7% 13% 22% 9% 2	2750
	21st to end	Night	32% 9% 13% 26% 16% 3	2739
June	1st to 10th	Night	41% 8% 11% 23% 15% 2	1520
	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	44% 8% 12% 20% 15% 1	2280
	11th to 20th	Night	55% 7% 12% 15% 10% 1	2280
	21st to end	Night	62% 4 10% 15% 8% 1	2508
November	1st to 10th	Night	54% 6% 11% 18% 8% 2	2372
	11th to 20th	Night	65% 6% 9% 12% 5% 2	2660
	21st to end	Night	70% 5% 10% 12% 3 1	2660
December	1st to 10th	Night	79% 5% 8% 6% 2	2660
	11th to 20th	Night	76% 6% 9% 7% 1	3288
	21st to end	Night	80% 7% 7% 5%	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	28% 30% 30% 9% 3	225
	11th to 20th	Day	31% 34% 26% 7% 2	270
	21st to end	Day	27% 32% 31% 9% 1	297
February	1st to 10th	Day	32% 24% 35% 9%	270
	11th to 20th	Day	31% 29% 34% 6%	270
	21st to end	Day	35% 26% 32% 7%	231
March	1st to 10th	Day	27% 26% 35% 9% 4	460
	11th to 20th	Day	26% 24% 36% 12% 2 1	460
	21st to end	Day	35% 20% 26% 16% 2 1	506
April	1st to 10th	Day	22% 25% 33% 15% 4	460
	11th to 20th	Day	30% 19% 27% 14% 8% 1	452
	21st to end	Day	25% 18% 36% 16% 4 1	460
May	1st to 10th	Day	27% 16% 30% 20% 5% 2	463
	11th to 20th	Day	24% 13% 28% 24% 8% 2	487
	21st to end	Day	16% 8% 31% 29% 13% 4	529
June	1st to 10th	Day	18% 13% 31% 28% 8% 2	490
	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% 26% 18% 4 2	330
	11th to 20th	Day	36% 23% 27% 11% 3	180
	21st to end	Day	31% 24% 34% 10%	180
December	1st to 10th	Day	37% 29% 22% 9% 1 1	180
	11th to 20th	Day	33% 29% 30% 8%	180
	21st to end	Day	33% 25% 31% 10% 1	198
January	1st to 10th	Night	30% 28% 28% 10% 3	760
	11th to 20th	Night	32% 24% 29% 14% 1	760
	21st to end	Night	25% 25% 33% 15% 3	836
February	1st to 10th	Night	31% 23% 29% 16% 2	760
	11th to 20th	Night	26% 26% 32% 14% 2	743
	21st to end	Night	29% 25% 31% 14% 1	628
March	1st to 10th	Night	25% 20% 34% 17% 4	760
	11th to 20th	Night	26% 20% 32% 17% 5%	760
	21st to end	Night	26% 16% 30% 22% 6% 1	836
April	1st to 10th	Night	25% 16% 32% 21% 5%	760
	11th to 20th	Night	26% 13% 33% 21% 7% 1	691
	21st to end	Night	21% 11% 33% 28% 6% 1	570
May	1st to 10th	Night	26% 8% 25% 30% 10% 1	570
	11th to 20th	Night	18% 9% 30% 34% 8% 1	524
	21st to end	Night	14% 7% 26% 32% 19% 2	411
June	1st to 10th	Night	16% 4 32% 32% 14% 1	380
	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% 21% 7%	749
	11th to 20th	Night	34% 21% 28% 12% 4 1	760
	21st to end	Night	35% 22% 29% 10% 4	760
December	1st to 10th	Night	39% 25% 25% 8% 4	760
	11th to 20th	Night	33% 28% 27% 11% 2	760
	21st to end	Night	33% 25% 31% 10% 1	836

Page generated at 2024-10-31 03:39:24. Version ac07

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