Carpenter Museum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Carpenter Museum clear sky chart from 2005-11-06 to 2025-07-21.

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	18% 8% 2 6% 4 4 4 3 5% 7% 39%	119679
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	119415
Transparency	Day	44% 6% 12% 20% 15% 3	123270
	Night	44% 6% 12% 20% 16% 3	123985
Seeing	Day	24% 19% 33% 17% 6% 2	16003
	Night	22% 16% 33% 22% 6%	24894

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	11% 9% 3 7% 3 4 3 3 5% 5% 47%	3597
	11th to 20th	Day	15% 9% 2 5% 3 3 3 3 4 4 48%	3570
	21st to end	Day	17% 9% 2 6% 3 4 4 3 4 5% 42%	3960
February	1st to 10th	Day	20% 7% 2 5% 4 4 5% 4 5% 7% 36%	1907
	11th to 20th	Day	14% 9% 3 5% 4 4 5% 3 4 6% 42%	1948
	21st to end	Day	19% 9% 3 6% 4 3 4 3 5% 6% 38%	1650
March	1st to 10th	Day	22% 11% 2 6% 3 3 3 2 5% 5% 37%	4476
	11th to 20th	Day	17% 9% 2 5% 4 4 4 4 5% 6% 41%	4786
	21st to end	Day	25% 10% 2 6% 4 4 4 3 5% 5% 32%	5754
April	1st to 10th	Day	21% 9% 3 6% 4 4 3 4 5% 7% 37%	2400
	11th to 20th	Day	19% 6% 2 6% 3 4 4 4 5% 7% 39%	2381
	21st to end	Day	21% 7% 2 6% 4 4 4 4 4 5% 39%	2600
May	1st to 10th	Day	28% 11% 3 7% 3 3 3 3 4 7% 27%	5886
	11th to 20th	Day	22% 9% 3 7% 4 4 4 3 5% 6% 33%	5875
	21st to end	Day	21% 7% 2 5% 4 4 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	19% 7% 2 6% 4 4 5% 4 7% 8% 34%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 5% 5% 4 6% 9% 37%	2800
	21st to end	Day	15% 6% 3 5% 4 4 5% 4 6% 9% 38%	2800
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 10% 34%	4580
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 7% 10% 37%	4580
	21st to end	Day	20% 8% 3 6% 4 5% 5% 4 6% 8% 31%	4814
August	1st to 10th	Day	18% 7% 3 7% 5% 5% 5% 5% 6% 8% 31%	4106
	11th to 20th	Day	13% 6% 3 7% 6% 5% 5% 4 5% 8% 37%	3965
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 6% 9% 46%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 6% 5% 45%	2269
	11th to 20th	Day	14% 6% 2 5% 3 3 3 2 5% 6% 51%	2090
	21st to end	Day	21% 6% 2 5% 3 3 4 3 4 7% 42%	2090
October	1st to 10th	Day	23% 7% 2 4 3 3 4 3 3 5% 43%	2090
	11th to 20th	Day	16% 6% 2 4 3 4 3 3 4 7% 48%	1874
	21st to end	Day	19% 6% 2 5% 4 3 4 3 5% 6% 43%	1881
November	1st to 10th	Day	16% 8% 2 4 4 3 3 3 4 4 48%	1761
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 4 6% 47%	1800
	21st to end	Day	18% 8% 2 4 4 3 4 3 4 6% 44%	1800
December	1st to 10th	Day	15% 9% 2 4 3 2 3 3 4 6% 48%	2714
	11th to 20th	Day	20% 7% 2 5% 3 2 3 2 4 5% 49%	2986
	21st to end	Day	20% 11% 4 7% 3 3 3 2 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 4 3 3 3 3 4 6% 51%	6349
	11th to 20th	Night	15% 10% 3 8% 4 4 4 2 4 4 42%	6315
	21st to end	Night	16% 10% 3 7% 4 4 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 3 7% 3 4 3 3 5% 6% 39%	2486
	11th to 20th	Night	18% 8% 3 5% 4 4 4 4 5% 6% 39%	2338
	21st to end	Night	18% 9% 2 6% 3 4 4 3 4 4 41%	1980
March	1st to 10th	Night	24% 9% 2 4 4 3 4 3 5% 6% 37%	5364
	11th to 20th	Night	18% 11% 3 6% 4 3 4 3 5% 5% 39%	5171
	21st to end	Night	25% 9% 2 6% 4 4 4 3 5% 4 33%	5174
April	1st to 10th	Night	20% 8% 2 6% 4 4 5% 3 5% 6% 36%	2000
	11th to 20th	Night	20% 8% 2 6% 4 4 4 3 5% 6% 38%	1966
	21st to end	Night	21% 7% 2 7% 4 3 3 4 4 5% 40%	1800
May	1st to 10th	Night	29% 9% 3 7% 4 4 4 3 5% 6% 26%	4054
	11th to 20th	Night	20% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4033
	21st to end	Night	22% 8% 3 8% 4 5% 4 4 6% 6% 31%	3953
June	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 6% 7% 36%	1600
	11th to 20th	Night	19% 6% 3 7% 5% 4 4 3 5% 8% 37%	1600
	21st to end	Night	17% 8% 2 5% 3 3 5% 4 6% 9% 39%	1600
July	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 5% 7% 9% 32%	2600
	11th to 20th	Night	16% 8% 2 6% 5% 4 4 4 7% 9% 37%	2600
	21st to end	Night	17% 10% 4 7% 5% 5% 4 4 6% 7% 31%	2739
August	1st to 10th	Night	23% 8% 3 6% 4 5% 5% 4 6% 7% 29%	2789
	11th to 20th	Night	9% 7% 3 7% 6% 6% 6% 5% 7% 7% 37%	2846
	21st to end	Night	9% 4 2 5% 3 4 4 3 5% 8% 55%	3476
September	1st to 10th	Night	15% 6% 3 6% 4 4 4 4 5% 6% 44%	1910
	11th to 20th	Night	15% 6% 2 5% 4 2 3 2 4 6% 51%	2090
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 5% 41%	2090
October	1st to 10th	Night	21% 7% 2 6% 5% 3 3 3 3 6% 42%	2090
	11th to 20th	Night	16% 5% 2 5% 3 4 4 3 5% 8% 47%	2300
	21st to end	Night	20% 8% 3 6% 4 3 4 3 4 5% 40%	2717
November	1st to 10th	Night	15% 5% 2 6% 4 3 3 4 4 5% 48%	2537
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 4 5% 47%	2600
	21st to end	Night	16% 6% 2 5% 3 3 4 3 5% 7% 46%	2600
December	1st to 10th	Night	19% 8% 2 4 3 3 3 2 4 4 47%	4448
	11th to 20th	Night	23% 9% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	18% 11% 3 7% 3 3 3 2 4 5% 40%	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	38% 4 7% 26% 21% 3	3598
	11th to 20th	Day	39% 6% 10% 21% 17% 7%	3600
	21st to end	Day	46% 4 9% 20% 19% 2	3960
February	1st to 10th	Day	46% 6% 14% 16% 14% 3	1907
	11th to 20th	Day	42% 6% 12% 21% 16% 3	1957
	21st to end	Day	47% 5% 11% 19% 15% 3	1650
March	1st to 10th	Day	49% 4 11% 13% 12% 11%	4479
	11th to 20th	Day	40% 8% 14% 26% 10% 2	4786
	21st to end	Day	54% 4 7% 14% 14% 7%	5754
April	1st to 10th	Day	47% 6% 11% 18% 16% 3	2400
	11th to 20th	Day	43% 7% 9% 20% 18% 3	2381
	21st to end	Day	46% 6% 10% 18% 16% 4	2600
May	1st to 10th	Day	58% 7% 14% 13% 7% 1	5897
	11th to 20th	Day	51% 7% 15% 15% 11% 2	5879
	21st to end	Day	46% 8% 12% 20% 13% 1	6949
June	1st to 10th	Day	45% 9% 12% 22% 12% 1	2800
	11th to 20th	Day	42% 8% 14% 21% 13% 2	2800
	21st to end	Day	41% 9% 15% 21% 14% 1	2800
July	1st to 10th	Day	45% 8% 13% 19% 13% 2	4510
	11th to 20th	Day	41% 8% 16% 21% 12% 1	4505
	21st to end	Day	50% 7% 13% 17% 12% 2	4661
August	1st to 10th	Day	50% 8% 16% 18% 7%	4092
	11th to 20th	Day	42% 8% 14% 26% 10% 1	4004
	21st to end	Day	32% 7% 15% 26% 17% 2	4180
September	1st to 10th	Day	38% 7% 11% 21% 21% 1	2269
	11th to 20th	Day	35% 5% 10% 23% 24% 3	2090
	21st to end	Day	43% 5% 9% 20% 21% 2	2090
October	1st to 10th	Day	39% 5% 10% 22% 20% 4	3480
	11th to 20th	Day	31% 4 7% 27% 28% 3	3090
	21st to end	Day	31% 4 14% 27% 21% 3	3081
November	1st to 10th	Day	40% 5% 13% 21% 19% 2	1761
	11th to 20th	Day	38% 5% 10% 21% 22% 4	1800
	21st to end	Day	42% 5% 12% 26% 12% 3	1800
December	1st to 10th	Day	39% 4 14% 27% 15% 1	2714
	11th to 20th	Day	40% 4 14% 17% 18% 7%	2986
	21st to end	Day	49% 4 9% 20% 13% 4	3960
January	1st to 10th	Night	34% 3 10% 33% 17% 2	6349
	11th to 20th	Night	45% 4 11% 21% 13% 5%	6360
	21st to end	Night	49% 5% 9% 16% 16% 5%	6996
February	1st to 10th	Night	46% 5% 13% 21% 13% 2	2486
	11th to 20th	Night	46% 5% 9% 19% 17% 4	2340
	21st to end	Night	45% 4 9% 23% 15% 4	1980
March	1st to 10th	Night	48% 4 11% 16% 15% 7%	5374
	11th to 20th	Night	45% 7% 16% 17% 12% 2	5172
	21st to end	Night	54% 4 6% 11% 19% 6%	5174
April	1st to 10th	Night	48% 6% 10% 18% 15% 3	2000
	11th to 20th	Night	46% 7% 9% 18% 18% 3	1966
	21st to end	Night	46% 7% 9% 18% 16% 4	1800
May	1st to 10th	Night	57% 8% 15% 11% 7% 2	4060
	11th to 20th	Night	54% 8% 14% 14% 7% 2	4034
	21st to end	Night	51% 8% 14% 16% 9% 1	3954
June	1st to 10th	Night	45% 8% 13% 20% 13% 1	1600
	11th to 20th	Night	45% 7% 12% 22% 13% 1	1600
	21st to end	Night	41% 8% 16% 22% 13% 1	1600
July	1st to 10th	Night	47% 8% 12% 18% 14%	2560
	11th to 20th	Night	42% 7% 17% 22% 10% 1	2552
	21st to end	Night	51% 7% 12% 15% 12% 2	2651
August	1st to 10th	Night	52% 9% 12% 17% 8% 1	2779
	11th to 20th	Night	42% 9% 15% 25% 8%	2873
	21st to end	Night	28% 6% 16% 27% 22% 1	3476
September	1st to 10th	Night	40% 7% 11% 23% 18% 2	1910
	11th to 20th	Night	35% 6% 11% 22% 25% 2	2090
	21st to end	Night	45% 6% 8% 19% 19% 3	2090
October	1st to 10th	Night	40% 5% 13% 24% 15% 3	3480
	11th to 20th	Night	26% 4 9% 26% 32% 3	3823
	21st to end	Night	34% 5% 11% 28% 21% 2	4467
November	1st to 10th	Night	38% 7% 13% 21% 19% 3	2537
	11th to 20th	Night	40% 4 10% 22% 20% 4	2600
	21st to end	Night	38% 5% 14% 27% 13% 2	2600
December	1st to 10th	Night	43% 4 12% 27% 15% 1	4448
	11th to 20th	Night	46% 4 14% 14% 19% 2	5208
	21st to end	Night	46% 5% 9% 20% 17% 3	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	30% 28% 27% 14% 2	200
	11th to 20th	Day	29% 30% 33% 7% 2	200
	21st to end	Day	26% 27% 33% 10% 2 1	220
February	1st to 10th	Day	32% 24% 31% 12% 1	439
	11th to 20th	Day	31% 31% 30% 6% 1	496
	21st to end	Day	28% 26% 32% 11% 3	414
March	1st to 10th	Day	34% 22% 30% 13% 2	500
	11th to 20th	Day	30% 26% 29% 11% 3	500
	21st to end	Day	31% 24% 31% 9% 4 1	550
April	1st to 10th	Day	34% 23% 27% 10% 5% 1	500
	11th to 20th	Day	29% 26% 31% 11% 2 1	492
	21st to end	Day	27% 23% 32% 14% 2 2	500
May	1st to 10th	Day	34% 23% 29% 11% 2 1	503
	11th to 20th	Day	25% 19% 40% 12% 4 1	527
	21st to end	Day	21% 16% 37% 20% 6% 1	573
June	1st to 10th	Day	21% 16% 36% 20% 6% 1	530
	11th to 20th	Day	17% 20% 39% 19% 4 1	530
	21st to end	Day	18% 15% 37% 23% 6% 1	530
July	1st to 10th	Day	18% 10% 34% 27% 9% 2	525
	11th to 20th	Day	15% 9% 36% 27% 12% 2	524
	21st to end	Day	15% 12% 39% 26% 8% 1	532
August	1st to 10th	Day	14% 11% 36% 28% 9% 1	486
	11th to 20th	Day	15% 11% 33% 27% 11% 3	479
	21st to end	Day	12% 12% 35% 24% 12% 5%	539
September	1st to 10th	Day	20% 11% 30% 23% 12% 4	490
	11th to 20th	Day	17% 12% 29% 25% 11% 7%	490
	21st to end	Day	25% 13% 32% 20% 6% 4	490
October	1st to 10th	Day	28% 13% 27% 19% 9% 5%	490
	11th to 20th	Day	19% 19% 34% 17% 11% 2	435
	21st to end	Day	25% 20% 29% 18% 7% 2	429
November	1st to 10th	Day	23% 15% 34% 16% 9% 2	395
	11th to 20th	Day	26% 23% 32% 14% 5%	400
	21st to end	Day	26% 22% 31% 14% 7% 1	400
December	1st to 10th	Day	23% 30% 32% 12% 4	275
	11th to 20th	Day	34% 25% 32% 7% 2 1	200
	21st to end	Day	27% 24% 33% 11% 3 2	220
January	1st to 10th	Night	23% 23% 34% 16% 3	800
	11th to 20th	Night	29% 22% 33% 14% 2	800
	21st to end	Night	22% 24% 35% 17% 3	880
February	1st to 10th	Night	25% 21% 35% 18% 1	800
	11th to 20th	Night	28% 23% 35% 13% 1	783
	21st to end	Night	24% 24% 33% 16% 3	660
March	1st to 10th	Night	26% 20% 35% 16% 4	800
	11th to 20th	Night	22% 23% 35% 17% 3	800
	21st to end	Night	27% 21% 32% 16% 3	880
April	1st to 10th	Night	25% 22% 34% 17% 3	800
	11th to 20th	Night	24% 21% 35% 17% 3	782
	21st to end	Night	24% 17% 39% 17% 3	600
May	1st to 10th	Night	30% 13% 34% 19% 4	600
	11th to 20th	Night	21% 17% 39% 20% 3	592
	21st to end	Night	22% 9% 36% 28% 6%	447
June	1st to 10th	Night	18% 10% 32% 32% 8%	400
	11th to 20th	Night	20% 7% 35% 30% 7%	400
	21st to end	Night	20% 7% 32% 36% 5% 1	400
July	1st to 10th	Night	18% 5% 33% 36% 8%	390
	11th to 20th	Night	16% 5% 32% 36% 10% 1	388
	21st to end	Night	16% 5% 30% 40% 8%	416
August	1st to 10th	Night	15% 6% 36% 36% 7%	544
	11th to 20th	Night	13% 6% 30% 39% 12%	600
	21st to end	Night	15% 6% 26% 36% 16% 1	836
September	1st to 10th	Night	18% 5% 33% 32% 12% 1	760
	11th to 20th	Night	17% 8% 28% 27% 17% 3	760
	21st to end	Night	25% 8% 27% 26% 12% 2	760
October	1st to 10th	Night	24% 12% 26% 25% 12% 2	760
	11th to 20th	Night	19% 12% 34% 22% 11% 1	760
	21st to end	Night	25% 16% 33% 19% 7%	836
November	1st to 10th	Night	18% 15% 34% 22% 9% 1	780
	11th to 20th	Night	23% 21% 33% 17% 6%	800
	21st to end	Night	23% 20% 33% 20% 4 1	800
December	1st to 10th	Night	26% 19% 34% 16% 5% 1	800
	11th to 20th	Night	28% 19% 32% 18% 2 1	800
	21st to end	Night	26% 21% 33% 17% 3	880

Page generated at 2025-07-18 01:09:57. Version ac07

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