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 2024-12-06.

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% 7% 30%	102008
	Night	13% 12% 5% 10% 6% 5% 5% 4 5% 6% 28%	97707
Transparency	Day	52% 7% 12% 18% 10% 1	105581
	Night	55% 8% 11% 16% 9% 1	102225
Seeing	Day	18% 14% 27% 25% 13% 4	15100
	Night	19% 14% 28% 26% 11% 1	24218

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	14% 15% 9% 19% 11% 6% 6% 4 3 4 9%	2208
	11th to 20th	Day	7% 15% 9% 24% 13% 8% 5% 4 4 3 8%	2752
	21st to end	Day	16% 19% 9% 16% 8% 7% 6% 3 4 4 9%	3124
February	1st to 10th	Day	10% 18% 7% 16% 10% 6% 6% 5% 5% 5% 13%	1710
	11th to 20th	Day	10% 13% 7% 20% 11% 7% 5% 4 5% 5% 14%	1663
	21st to end	Day	12% 17% 7% 13% 9% 7% 5% 3 4 5% 18%	1489
March	1st to 10th	Day	13% 15% 5% 11% 7% 5% 4 3 4 6% 27%	3389
	11th to 20th	Day	9% 17% 9% 13% 6% 6% 4 3 4 5% 23%	3480
	21st to end	Day	13% 13% 5% 9% 6% 5% 6% 4 5% 6% 29%	3815
April	1st to 10th	Day	15% 16% 4 10% 5% 5% 4 3 5% 6% 27%	2211
	11th to 20th	Day	17% 13% 3 5% 5% 4 4 3 5% 6% 35%	2399
	21st to end	Day	18% 13% 4 7% 4 4 4 3 5% 6% 33%	2470
May	1st to 10th	Day	14% 10% 3 8% 5% 4 4 3 6% 7% 37%	4108
	11th to 20th	Day	14% 9% 3 7% 4 5% 5% 4 6% 7% 35%	4327
	21st to end	Day	12% 7% 2 6% 4 4 4 4 6% 8% 44%	4837
June	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 5% 6% 8% 36%	2700
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	2850
	21st to end	Day	15% 7% 2 6% 4 5% 4 5% 6% 8% 38%	2850
July	1st to 10th	Day	12% 6% 2 7% 6% 6% 5% 5% 7% 8% 35%	4760
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 45%	4586
	21st to end	Day	10% 6% 2 7% 5% 4 4 3 6% 9% 44%	4466
August	1st to 10th	Day	12% 7% 2 7% 4 5% 5% 5% 6% 8% 38%	4104
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	4044
	21st to end	Day	11% 4 2 6% 5% 4 5% 4 6% 9% 42%	4443
September	1st to 10th	Day	16% 7% 3 7% 5% 4 5% 3 6% 8% 38%	2280
	11th to 20th	Day	14% 5% 2 7% 3 5% 5% 4 5% 8% 41%	2114
	21st to end	Day	15% 8% 2 7% 6% 5% 5% 4 6% 7% 33%	2085
October	1st to 10th	Day	18% 11% 3 7% 5% 4 3 5% 5% 8% 32%	1900
	11th to 20th	Day	17% 14% 4 9% 7% 6% 6% 4 6% 5% 23%	1900
	21st to end	Day	23% 19% 4 8% 5% 5% 4 4 6% 5% 16%	2063
November	1st to 10th	Day	12% 18% 4 10% 6% 6% 5% 4 5% 5% 25%	1677
	11th to 20th	Day	12% 17% 5% 13% 6% 6% 7% 5% 5% 6% 19%	1600
	21st to end	Day	15% 18% 8% 13% 7% 7% 6% 4 6% 6% 11%	1600
December	1st to 10th	Day	11% 21% 10% 17% 7% 6% 5% 4 6% 5% 8%	1560
	11th to 20th	Day	14% 18% 7% 18% 7% 6% 6% 4 5% 5% 11%	1906
	21st to end	Day	19% 20% 7% 17% 6% 6% 4 3 4 4 9%	2538
January	1st to 10th	Night	12% 18% 8% 17% 9% 6% 6% 4 6% 5% 9%	4748
	11th to 20th	Night	10% 12% 9% 22% 11% 7% 7% 4 5% 4 9%	4198
	21st to end	Night	15% 15% 9% 16% 11% 7% 6% 4 5% 5% 8%	4532
February	1st to 10th	Night	10% 16% 6% 15% 9% 9% 5% 5% 5% 5% 14%	2470
	11th to 20th	Night	9% 13% 6% 15% 10% 8% 6% 5% 5% 5% 17%	2403
	21st to end	Night	12% 12% 5% 13% 8% 6% 6% 5% 5% 7% 21%	1960
March	1st to 10th	Night	13% 13% 6% 9% 5% 4 5% 4 5% 6% 31%	3449
	11th to 20th	Night	9% 14% 5% 9% 6% 5% 4 4 5% 6% 32%	3479
	21st to end	Night	11% 10% 5% 11% 6% 5% 6% 4 6% 5% 30%	3817
April	1st to 10th	Night	17% 13% 4 9% 6% 5% 4 3 5% 6% 30%	1968
	11th to 20th	Night	17% 10% 3 7% 4 5% 3 3 5% 6% 37%	1744
	21st to end	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 32%	1710
May	1st to 10th	Night	14% 8% 2 7% 5% 4 4 4 5% 7% 40%	2833
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 4 5% 6% 42%	2564
	21st to end	Night	9% 6% 2 7% 4 3 4 3 5% 7% 49%	2738
June	1st to 10th	Night	14% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	1476
	11th to 20th	Night	11% 4 2 5% 4 4 6% 5% 7% 10% 42%	1330
	21st to end	Night	12% 7% 3 7% 4 4 3 4 7% 8% 42%	1330
July	1st to 10th	Night	8% 6% 2 7% 4 4 4 3 5% 8% 48%	2210
	11th to 20th	Night	12% 5% 1 5% 5% 3 3 3 5% 6% 52%	2364
	21st to end	Night	8% 5% 2 6% 4 5% 4 4 7% 8% 48%	3083
August	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 4 5% 7% 43%	2785
	21st to end	Night	13% 5% 2 6% 4 5% 3 4 6% 8% 45%	3179
September	1st to 10th	Night	12% 5% 2 5% 3 4 3 4 7% 9% 45%	1900
	11th to 20th	Night	12% 6% 1 4 3 3 4 4 5% 8% 48%	2063
	21st to end	Night	17% 8% 2 6% 4 4 4 3 6% 7% 38%	2094
October	1st to 10th	Night	19% 8% 3 6% 5% 4 5% 3 4 6% 38%	2280
	11th to 20th	Night	19% 13% 3 7% 5% 5% 6% 4 6% 7% 27%	2280
	21st to end	Night	21% 18% 3 7% 5% 6% 4 4 5% 6% 20%	2532
November	1st to 10th	Night	14% 16% 4 9% 4 5% 5% 4 5% 6% 27%	2621
	11th to 20th	Night	13% 17% 5% 11% 7% 6% 5% 5% 6% 6% 19%	2800
	21st to end	Night	16% 18% 6% 12% 6% 6% 4 5% 5% 6% 15%	2800
December	1st to 10th	Night	14% 19% 8% 14% 7% 6% 6% 4 6% 5% 11%	2737
	11th to 20th	Night	10% 19% 7% 15% 8% 6% 7% 4 5% 6% 12%	3288
	21st to end	Night	20% 19% 8% 19% 6% 5% 5% 3 4 4 8%	5117

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% 8% 10% 4	2209
	11th to 20th	Day	81% 10% 5% 2 2	2752
	21st to end	Day	79% 8% 8% 4 1	3124
February	1st to 10th	Day	73% 7% 8% 9% 1	1710
	11th to 20th	Day	71% 7% 8% 8% 5% 1	1672
	21st to end	Day	69% 6% 9% 11% 5%	1489
March	1st to 10th	Day	59% 7% 12% 14% 8%	3392
	11th to 20th	Day	61% 6% 10% 16% 6% 1	3480
	21st to end	Day	53% 6% 15% 14% 9% 2	3815
April	1st to 10th	Day	59% 5% 10% 17% 8% 1	2211
	11th to 20th	Day	50% 5% 9% 20% 14% 1	2399
	21st to end	Day	52% 6% 10% 18% 12% 3	2470
May	1st to 10th	Day	44% 7% 13% 20% 15% 1	4120
	11th to 20th	Day	45% 9% 14% 21% 10% 1	4331
	21st to end	Day	37% 8% 12% 21% 18% 4	4837
June	1st to 10th	Day	43% 9% 11% 22% 15% 1	2700
	11th to 20th	Day	38% 8% 12% 23% 17% 1	2850
	21st to end	Day	42% 7% 14% 23% 13% 1	2850
July	1st to 10th	Day	43% 8% 14% 19% 14% 1	4686
	11th to 20th	Day	35% 7% 13% 33% 11% 1	4506
	21st to end	Day	36% 7% 12% 25% 19% 1	4324
August	1st to 10th	Day	41% 8% 15% 23% 12% 1	4091
	11th to 20th	Day	44% 9% 16% 22% 9%	4083
	21st to end	Day	37% 7% 14% 31% 11% 1	4443
September	1st to 10th	Day	44% 7% 13% 23% 10% 2	2280
	11th to 20th	Day	39% 7% 13% 27% 13%	2114
	21st to end	Day	49% 8% 12% 23% 8% 1	2085
October	1st to 10th	Day	46% 7% 13% 23% 11% 1	3160
	11th to 20th	Day	55% 6% 10% 17% 12%	3140
	21st to end	Day	63% 7% 12% 15% 4	3377
November	1st to 10th	Day	60% 8% 11% 15% 6%	1677
	11th to 20th	Day	66% 9% 10% 10% 6%	1600
	21st to end	Day	74% 8% 9% 7% 1	1600
December	1st to 10th	Day	80% 8% 8% 4 1	1560
	11th to 20th	Day	77% 8% 8% 6% 1	1906
	21st to end	Day	81% 7% 6% 5%	2538
January	1st to 10th	Night	78% 11% 7% 4	4748
	11th to 20th	Night	77% 15% 5% 2 1	4198
	21st to end	Night	78% 8% 9% 3 2	4532
February	1st to 10th	Night	72% 8% 9% 10% 2	2470
	11th to 20th	Night	69% 7% 10% 11% 4	2405
	21st to end	Night	61% 9% 11% 12% 6%	1960
March	1st to 10th	Night	52% 9% 13% 18% 8%	3459
	11th to 20th	Night	51% 10% 14% 16% 8% 1	3480
	21st to end	Night	50% 8% 15% 17% 9% 1	3817
April	1st to 10th	Night	56% 5% 9% 18% 11% 1	1968
	11th to 20th	Night	49% 6% 8% 19% 17% 2	1744
	21st to end	Night	53% 6% 10% 14% 16% 2	1710
May	1st to 10th	Night	41% 8% 13% 18% 16% 3	2840
	11th to 20th	Night	41% 7% 14% 23% 13% 2	2565
	21st to end	Night	33% 8% 12% 24% 20% 2	2739
June	1st to 10th	Night	43% 7% 11% 19% 18% 2	1476
	11th to 20th	Night	36% 7% 13% 26% 18% 1	1330
	21st to end	Night	38% 9% 12% 26% 14% 1	1330
July	1st to 10th	Night	34% 6% 14% 24% 19% 3	2175
	11th to 20th	Night	32% 6% 13% 34% 15% 1	2322
	21st to end	Night	33% 8% 14% 28% 17% 1	2985
August	1st to 10th	Night	37% 6% 13% 27% 16% 1	2814
	11th to 20th	Night	39% 7% 17% 22% 13% 1	2812
	21st to end	Night	38% 7% 13% 29% 12% 1	3179
September	1st to 10th	Night	33% 9% 15% 28% 14% 1	1900
	11th to 20th	Night	32% 8% 15% 27% 17% 1	2063
	21st to end	Night	45% 9% 13% 24% 10%	2094
October	1st to 10th	Night	42% 6% 10% 24% 18% 1	3800
	11th to 20th	Night	50% 6% 12% 18% 13%	3779
	21st to end	Night	54% 8% 15% 18% 5%	4168
November	1st to 10th	Night	57% 8% 11% 16% 7% 1	2621
	11th to 20th	Night	63% 10% 11% 10% 4 1	2800
	21st to end	Night	69% 10% 12% 7% 2	2800
December	1st to 10th	Night	76% 10% 9% 4 2	2737
	11th to 20th	Night	73% 10% 10% 6% 1	3288
	21st to end	Night	83% 6% 6% 5%	5117

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	17% 24% 31% 19% 7% 1	180
	11th to 20th	Day	16% 20% 44% 14% 5% 1	256
	21st to end	Day	22% 15% 35% 25% 4	297
February	1st to 10th	Day	19% 26% 37% 14% 4	270
	11th to 20th	Day	17% 24% 40% 16% 3	270
	21st to end	Day	17% 33% 32% 18%	225
March	1st to 10th	Day	18% 17% 37% 22% 6% 1	428
	11th to 20th	Day	16% 19% 33% 25% 6% 1	460
	21st to end	Day	25% 14% 32% 22% 7%	506
April	1st to 10th	Day	20% 18% 31% 23% 8%	460
	11th to 20th	Day	28% 16% 23% 18% 10% 5%	452
	21st to end	Day	24% 13% 28% 24% 9% 1	460
May	1st to 10th	Day	18% 13% 26% 27% 13% 2	463
	11th to 20th	Day	19% 7% 25% 27% 17% 5%	487
	21st to end	Day	15% 7% 25% 29% 19% 5%	529
June	1st to 10th	Day	18% 9% 25% 28% 17% 2	530
	11th to 20th	Day	12% 9% 24% 34% 16% 3	680
	21st to end	Day	18% 5% 27% 31% 15% 3	680
July	1st to 10th	Day	11% 7% 21% 35% 21% 5%	671
	11th to 20th	Day	12% 8% 23% 31% 18% 8%	573
	21st to end	Day	9% 4 24% 38% 19% 5%	532
August	1st to 10th	Day	12% 5% 21% 32% 21% 9%	486
	11th to 20th	Day	12% 5% 17% 33% 21% 12%	479
	21st to end	Day	13% 8% 22% 31% 19% 6%	539
September	1st to 10th	Day	17% 10% 21% 24% 21% 8%	490
	11th to 20th	Day	11% 6% 21% 25% 24% 13%	490
	21st to end	Day	14% 12% 24% 27% 18% 6%	490
October	1st to 10th	Day	23% 14% 22% 21% 14% 5%	490
	11th to 20th	Day	21% 21% 30% 21% 7%	490
	21st to end	Day	32% 23% 23% 17% 5%	512
November	1st to 10th	Day	21% 25% 28% 17% 8% 1	257
	11th to 20th	Day	19% 31% 29% 18% 3 2	200
	21st to end	Day	25% 24% 38% 13% 2	200
December	1st to 10th	Day	17% 29% 31% 19% 3	190
	11th to 20th	Day	19% 32% 29% 16% 4	180
	21st to end	Day	25% 26% 37% 10% 2	198
January	1st to 10th	Night	18% 25% 33% 19% 5%	950
	11th to 20th	Night	22% 19% 34% 20% 5% 1	775
	21st to end	Night	14% 15% 37% 25% 7% 1	836
February	1st to 10th	Night	20% 21% 36% 21% 3	760
	11th to 20th	Night	16% 18% 37% 24% 5%	743
	21st to end	Night	19% 19% 36% 22% 4	628
March	1st to 10th	Night	17% 16% 37% 24% 6%	760
	11th to 20th	Night	17% 17% 36% 22% 6% 1	760
	21st to end	Night	18% 13% 29% 30% 9% 1	836
April	1st to 10th	Night	22% 13% 32% 28% 5%	760
	11th to 20th	Night	21% 10% 30% 26% 13% 1	620
	21st to end	Night	24% 11% 24% 28% 13%	570
May	1st to 10th	Night	17% 5% 23% 36% 15% 3	570
	11th to 20th	Night	18% 5% 21% 40% 14% 2	417
	21st to end	Night	11% 4 22% 40% 21% 1	411
June	1st to 10th	Night	16% 7% 23% 39% 13% 2	380
	11th to 20th	Night	11% 7% 19% 40% 22% 1	380
	21st to end	Night	16% 4 22% 35% 21% 2	380
July	1st to 10th	Night	8% 5% 15% 46% 25% 1	370
	11th to 20th	Night	9% 5% 22% 39% 23% 4	368
	21st to end	Night	9% 2 20% 43% 24% 2	620
August	1st to 10th	Night	12% 3 18% 36% 28% 2	567
	11th to 20th	Night	9% 5% 15% 37% 29% 5%	564
	21st to end	Night	16% 5% 20% 31% 26% 2	656
September	1st to 10th	Night	14% 5% 23% 33% 19% 7%	760
	11th to 20th	Night	15% 5% 18% 34% 23% 6%	760
	21st to end	Night	19% 9% 21% 29% 17% 5%	760
October	1st to 10th	Night	23% 10% 26% 25% 15% 2	760
	11th to 20th	Night	26% 15% 26% 23% 8% 1	760
	21st to end	Night	32% 17% 27% 19% 4 1	836
November	1st to 10th	Night	24% 16% 31% 20% 8% 1	780
	11th to 20th	Night	21% 26% 30% 18% 6% 1	800
	21st to end	Night	26% 19% 33% 19% 3	800
December	1st to 10th	Night	24% 24% 34% 15% 3	783
	11th to 20th	Night	19% 24% 38% 16% 2 1	760
	21st to end	Night	25% 22% 35% 16% 2	978

Page generated at 2024-12-03 06:45:14. Version ac07

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