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Carr Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Carr Astronomical Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	13% 11% 4 10% 6% 5% 5% 4 5% 6% 29%	131582
	Night	12% 12% 5% 12% 7% 6% 5% 4 5% 6% 26%	129921
Transparency	Day	54% 7% 11% 18% 10% 1	135145
	Night	56% 7% 11% 15% 9% 1	134448
Seeing	Day	19% 14% 27% 24% 12% 4	16692
	Night	19% 14% 28% 26% 11% 1	25913

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	12% 14% 7% 22% 13% 7% 5% 3 3 4 9%	4094
	11th to 20th	Day	6% 15% 10% 25% 12% 10% 6% 4 4 3 6%	5104
	21st to end	Day	10% 15% 7% 17% 11% 12% 8% 4 4 4 8%	5780
February	1st to 10th	Day	10% 17% 7% 16% 10% 6% 6% 5% 5% 5% 13%	1890
	11th to 20th	Day	11% 13% 7% 20% 12% 6% 5% 4 5% 4 14%	1843
	21st to end	Day	12% 17% 7% 13% 9% 7% 5% 3 4 5% 17%	1641
March	1st to 10th	Day	13% 13% 5% 12% 8% 6% 5% 3 5% 5% 25%	4983
	11th to 20th	Day	8% 15% 8% 13% 6% 6% 4 4 5% 6% 24%	5057
	21st to end	Day	15% 15% 6% 11% 6% 4 4 3 4 5% 27%	5465
April	1st to 10th	Day	14% 16% 4 10% 5% 5% 4 3 5% 6% 27%	2328
	11th to 20th	Day	16% 13% 3 5% 5% 4 4 3 5% 6% 35%	2527
	21st to end	Day	17% 12% 4 7% 4 4 4 3 5% 6% 33%	2600
May	1st to 10th	Day	14% 10% 3 8% 5% 4 4 3 5% 7% 37%	5888
	11th to 20th	Day	12% 11% 4 8% 5% 5% 4 3 5% 6% 36%	6219
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 7% 37%	6949
June	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 5% 6% 7% 37%	2842
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 5% 6% 9% 40%	3000
	21st to end	Day	15% 7% 2 6% 4 5% 5% 5% 6% 8% 38%	3000
July	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 9% 38%	6819
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 47%	6576
	21st to end	Day	9% 5% 2 6% 4 4 4 3 5% 9% 49%	6424
August	1st to 10th	Day	12% 7% 2 7% 4 5% 5% 5% 6% 8% 38%	4234
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	4174
	21st to end	Day	11% 4 2 6% 4 4 5% 4 7% 9% 42%	4583
September	1st to 10th	Day	15% 7% 3 7% 5% 4 5% 3 6% 8% 38%	2400
	11th to 20th	Day	13% 5% 2 6% 3 4 5% 4 5% 8% 43%	2225
	21st to end	Day	16% 8% 2 7% 6% 5% 5% 4 6% 7% 34%	2195
October	1st to 10th	Day	17% 10% 3 7% 4 4 3 5% 5% 8% 33%	2000
	11th to 20th	Day	17% 14% 4 9% 7% 5% 6% 4 6% 6% 24%	2000
	21st to end	Day	22% 19% 4 8% 5% 5% 4 4 6% 5% 17%	2171
November	1st to 10th	Day	13% 18% 4 10% 6% 6% 4 3 5% 5% 25%	1763
	11th to 20th	Day	12% 18% 5% 13% 6% 6% 7% 4 5% 6% 18%	1680
	21st to end	Day	15% 18% 7% 14% 7% 7% 6% 4 5% 6% 11%	1680
December	1st to 10th	Day	13% 24% 13% 16% 6% 6% 5% 3 4 4 5%	2680
	11th to 20th	Day	13% 20% 7% 18% 7% 6% 5% 3 4 4 13%	3066
	21st to end	Day	20% 17% 6% 16% 8% 6% 4 3 4 5% 11%	3702
January	1st to 10th	Night	11% 14% 7% 22% 13% 6% 5% 4 4 5% 9%	8831
	11th to 20th	Night	7% 14% 8% 25% 12% 7% 7% 5% 5% 4 7%	7745
	21st to end	Night	8% 12% 7% 14% 13% 9% 10% 5% 5% 5% 12%	8401
February	1st to 10th	Night	10% 15% 6% 15% 9% 9% 5% 5% 5% 5% 15%	2730
	11th to 20th	Night	9% 13% 7% 15% 11% 8% 6% 5% 5% 5% 16%	2663
	21st to end	Night	11% 13% 5% 14% 8% 7% 6% 5% 5% 7% 20%	2160
March	1st to 10th	Night	13% 11% 6% 11% 6% 6% 6% 4 6% 6% 26%	5075
	11th to 20th	Night	7% 12% 6% 9% 6% 6% 6% 5% 6% 7% 30%	5057
	21st to end	Night	14% 11% 5% 12% 6% 5% 6% 4 5% 5% 27%	5467
April	1st to 10th	Night	17% 12% 4 10% 6% 5% 4 3 5% 6% 29%	2071
	11th to 20th	Night	16% 10% 3 7% 5% 5% 3 3 5% 6% 36%	1837
	21st to end	Night	16% 12% 4 8% 5% 4 4 4 5% 6% 32%	1800
May	1st to 10th	Night	15% 9% 2 7% 5% 5% 5% 4 5% 7% 36%	4053
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 44%	3672
	21st to end	Night	18% 8% 2 6% 3 3 4 3 4 6% 43%	3926
June	1st to 10th	Night	13% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	1553
	11th to 20th	Night	11% 4 3 5% 4 5% 6% 5% 7% 10% 41%	1400
	21st to end	Night	12% 7% 3 7% 4 4 4 4 7% 8% 42%	1400
July	1st to 10th	Night	11% 7% 2 7% 4 4 4 3 5% 8% 45%	3160
	11th to 20th	Night	12% 4 1 5% 4 4 3 3 5% 6% 50%	3379
	21st to end	Night	9% 5% 2 6% 3 4 4 3 6% 9% 49%	4424
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 46%	2915
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	2875
	21st to end	Night	13% 5% 2 6% 4 5% 4 4 6% 8% 44%	3280
September	1st to 10th	Night	12% 5% 2 5% 3 4 3 4 7% 10% 45%	2000
	11th to 20th	Night	12% 5% 1 4 3 3 4 5% 5% 8% 49%	2172
	21st to end	Night	17% 8% 2 6% 4 4 4 4 6% 6% 38%	2204
October	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 5% 6% 39%	2400
	11th to 20th	Night	19% 12% 3 7% 6% 5% 6% 3 6% 7% 28%	2400
	21st to end	Night	21% 18% 3 7% 5% 5% 4 4 5% 6% 22%	2665
November	1st to 10th	Night	14% 17% 4 9% 5% 5% 5% 4 5% 6% 27%	2755
	11th to 20th	Night	13% 17% 5% 11% 7% 5% 5% 4 6% 6% 19%	2940
	21st to end	Night	16% 18% 6% 12% 6% 6% 4 5% 5% 6% 15%	2940
December	1st to 10th	Night	12% 20% 9% 18% 7% 6% 6% 4 5% 4 8%	4720
	11th to 20th	Night	9% 17% 8% 17% 8% 7% 7% 5% 5% 6% 12%	5348
	21st to end	Night	17% 19% 7% 19% 7% 7% 5% 3 4 3 9%	7503

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% 5% 9% 6% 2	4095
	11th to 20th	Day	82% 7% 7% 3 1	5138
	21st to end	Day	77% 6% 9% 5% 3	5780
February	1st to 10th	Day	73% 7% 8% 10% 2	1890
	11th to 20th	Day	73% 7% 8% 8% 4	1852
	21st to end	Day	69% 5% 9% 11% 5%	1641
March	1st to 10th	Day	60% 5% 15% 11% 9%	4986
	11th to 20th	Day	58% 6% 12% 17% 6% 2	5057
	21st to end	Day	58% 5% 12% 16% 9% 1	5465
April	1st to 10th	Day	59% 5% 10% 17% 8% 1	2328
	11th to 20th	Day	50% 5% 9% 21% 14% 1	2527
	21st to end	Day	52% 6% 11% 18% 12% 2	2600
May	1st to 10th	Day	45% 7% 13% 18% 12% 5%	5897
	11th to 20th	Day	47% 8% 11% 16% 17% 1	6223
	21st to end	Day	45% 6% 11% 19% 15% 4	6949
June	1st to 10th	Day	43% 8% 11% 22% 15% 1	2842
	11th to 20th	Day	39% 8% 13% 23% 16% 1	3000
	21st to end	Day	43% 7% 14% 23% 13% 1	3000
July	1st to 10th	Day	41% 7% 14% 23% 14% 1	6746
	11th to 20th	Day	33% 6% 12% 33% 15% 1	6496
	21st to end	Day	33% 6% 12% 31% 18% 1	6240
August	1st to 10th	Day	41% 8% 15% 23% 12% 1	4221
	11th to 20th	Day	44% 9% 16% 22% 9%	4213
	21st to end	Day	36% 7% 14% 31% 11% 1	4583
September	1st to 10th	Day	44% 7% 13% 24% 10% 2	2400
	11th to 20th	Day	38% 7% 13% 29% 13%	2225
	21st to end	Day	49% 8% 12% 23% 8% 1	2195
October	1st to 10th	Day	45% 7% 13% 24% 11% 1	3260
	11th to 20th	Day	54% 6% 10% 17% 12%	3240
	21st to end	Day	62% 7% 12% 15% 5%	3485
November	1st to 10th	Day	60% 7% 11% 15% 6%	1763
	11th to 20th	Day	66% 8% 10% 10% 5%	1680
	21st to end	Day	75% 8% 9% 7% 1	1680
December	1st to 10th	Day	82% 8% 6% 3 1	2680
	11th to 20th	Day	76% 6% 8% 6% 4	3066
	21st to end	Day	77% 7% 6% 7% 3	3702
January	1st to 10th	Night	77% 8% 9% 5% 1	8831
	11th to 20th	Night	75% 11% 9% 4 1	7786
	21st to end	Night	69% 6% 12% 6% 6%	8401
February	1st to 10th	Night	70% 7% 10% 10% 2	2730
	11th to 20th	Night	69% 7% 10% 11% 4	2665
	21st to end	Night	62% 9% 11% 12% 6%	2160
March	1st to 10th	Night	56% 7% 13% 15% 8% 1	5085
	11th to 20th	Night	50% 9% 16% 16% 8% 1	5058
	21st to end	Night	55% 7% 13% 14% 9% 1	5467
April	1st to 10th	Night	56% 5% 10% 17% 11% 1	2071
	11th to 20th	Night	49% 6% 8% 18% 17% 2	1837
	21st to end	Night	53% 6% 10% 14% 16% 2	1800
May	1st to 10th	Night	46% 7% 12% 15% 15% 6%	4060
	11th to 20th	Night	38% 9% 13% 23% 16% 2	3673
	21st to end	Night	42% 7% 10% 25% 14% 2	3927
June	1st to 10th	Night	42% 7% 11% 20% 18% 2	1553
	11th to 20th	Night	36% 6% 13% 25% 18% 1	1400
	21st to end	Night	38% 9% 13% 25% 14% 1	1400
July	1st to 10th	Night	37% 6% 13% 26% 16% 2	3125
	11th to 20th	Night	34% 6% 12% 29% 19% 1	3337
	21st to end	Night	32% 7% 11% 32% 17% 1	4294
August	1st to 10th	Night	37% 6% 14% 27% 16% 1	2904
	11th to 20th	Night	39% 7% 17% 23% 13% 1	2902
	21st to end	Night	38% 7% 13% 30% 12% 1	3280
September	1st to 10th	Night	33% 9% 15% 29% 13% 1	2000
	11th to 20th	Night	32% 8% 15% 28% 17% 1	2172
	21st to end	Night	45% 8% 12% 23% 10% 1	2204
October	1st to 10th	Night	41% 6% 10% 24% 18% 1	3920
	11th to 20th	Night	50% 6% 12% 18% 14%	3899
	21st to end	Night	53% 7% 15% 19% 6%	4301
November	1st to 10th	Night	58% 8% 11% 16% 6% 1	2755
	11th to 20th	Night	64% 9% 12% 10% 4 1	2940
	21st to end	Night	69% 10% 12% 7% 2	2940
December	1st to 10th	Night	78% 9% 8% 4 1	4720
	11th to 20th	Night	71% 9% 11% 7% 3	5348
	21st to end	Night	80% 6% 6% 7% 1	7503

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	20% 25% 32% 16% 5% 1	220
	11th to 20th	Day	19% 23% 40% 13% 4 1	314
	21st to end	Day	23% 20% 32% 22% 3	363
February	1st to 10th	Day	22% 24% 36% 14% 4	330
	11th to 20th	Day	22% 23% 37% 15% 2	330
	21st to end	Day	18% 31% 33% 17%	273
March	1st to 10th	Day	21% 17% 36% 21% 5% 1	505
	11th to 20th	Day	21% 19% 31% 23% 5% 1	528
	21st to end	Day	27% 14% 32% 20% 7%	550
April	1st to 10th	Day	21% 18% 32% 21% 7%	500
	11th to 20th	Day	28% 16% 23% 18% 10% 4	492
	21st to end	Day	25% 15% 28% 23% 9% 1	500
May	1st to 10th	Day	18% 15% 28% 25% 12% 2	503
	11th to 20th	Day	18% 9% 25% 26% 17% 4	527
	21st to end	Day	18% 8% 25% 27% 18% 5%	573
June	1st to 10th	Day	18% 11% 26% 27% 16% 2	572
	11th to 20th	Day	13% 10% 25% 34% 15% 3	730
	21st to end	Day	18% 6% 28% 31% 14% 3	730
July	1st to 10th	Day	11% 8% 23% 34% 20% 5%	721
	11th to 20th	Day	12% 8% 24% 31% 17% 7%	618
	21st to end	Day	9% 6% 25% 37% 18% 5%	576
August	1st to 10th	Day	11% 5% 21% 33% 21% 9%	526
	11th to 20th	Day	13% 5% 18% 33% 20% 12%	519
	21st to end	Day	13% 9% 22% 31% 19% 6%	583
September	1st to 10th	Day	17% 10% 21% 25% 20% 8%	530
	11th to 20th	Day	10% 6% 20% 26% 25% 12%	530
	21st to end	Day	15% 12% 24% 27% 18% 5%	530
October	1st to 10th	Day	23% 13% 23% 22% 14% 5%	530
	11th to 20th	Day	21% 21% 31% 20% 7%	530
	21st to end	Day	31% 23% 24% 18% 4	554
November	1st to 10th	Day	24% 25% 27% 16% 7% 1	283
	11th to 20th	Day	20% 30% 28% 17% 3 1	220
	21st to end	Day	26% 24% 37% 11% 2	220
December	1st to 10th	Day	21% 29% 30% 17% 3	220
	11th to 20th	Day	23% 32% 28% 13% 3	220
	21st to end	Day	28% 25% 34% 10% 2	242
January	1st to 10th	Night	19% 25% 34% 18% 4	1050
	11th to 20th	Night	22% 20% 34% 19% 4	855
	21st to end	Night	15% 17% 36% 24% 7% 1	924
February	1st to 10th	Night	19% 22% 35% 20% 3	840
	11th to 20th	Night	17% 18% 35% 24% 5%	823
	21st to end	Night	21% 18% 36% 21% 3	692
March	1st to 10th	Night	18% 18% 36% 23% 5%	840
	11th to 20th	Night	19% 17% 35% 21% 6% 1	829
	21st to end	Night	19% 14% 28% 29% 9% 1	880
April	1st to 10th	Night	23% 14% 32% 27% 5%	800
	11th to 20th	Night	21% 10% 30% 25% 12% 1	653
	21st to end	Night	24% 12% 24% 27% 13%	600
May	1st to 10th	Night	18% 6% 23% 36% 14% 3	600
	11th to 20th	Night	18% 5% 23% 39% 13% 2	439
	21st to end	Night	13% 4 22% 39% 20% 1	433
June	1st to 10th	Night	15% 8% 24% 38% 13% 2	400
	11th to 20th	Night	12% 7% 19% 40% 22% 1	400
	21st to end	Night	16% 5% 22% 36% 20% 2	400
July	1st to 10th	Night	9% 5% 16% 45% 24% 1	390
	11th to 20th	Night	9% 5% 22% 39% 22% 3	388
	21st to end	Night	9% 3 21% 43% 23% 2	653
August	1st to 10th	Night	12% 3 17% 37% 28% 2	597
	11th to 20th	Night	9% 5% 16% 38% 28% 5%	594
	21st to end	Night	15% 5% 21% 32% 25% 2	691
September	1st to 10th	Night	14% 6% 23% 32% 19% 7%	800
	11th to 20th	Night	14% 5% 17% 34% 24% 6%	800
	21st to end	Night	19% 9% 21% 29% 17% 5%	800
October	1st to 10th	Night	22% 10% 25% 27% 15% 2	800
	11th to 20th	Night	26% 16% 27% 23% 8% 1	800
	21st to end	Night	31% 17% 28% 20% 4 1	880
November	1st to 10th	Night	25% 16% 31% 20% 8% 1	820
	11th to 20th	Night	22% 26% 29% 17% 5% 1	840
	21st to end	Night	26% 20% 32% 18% 3	840
December	1st to 10th	Night	24% 24% 34% 15% 3	840
	11th to 20th	Night	20% 24% 38% 15% 2	840
	21st to end	Night	26% 22% 33% 16% 2	1082

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