Ayer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ayer 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% 8% 2 6% 4 4 4 4 5% 7% 38%	119135
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 39%	119094
Transparency	Day	44% 4 11% 23% 15% 3	122713
	Night	45% 4 11% 22% 16% 3	123677
Seeing	Day	22% 16% 33% 20% 7% 2	15804
	Night	21% 14% 34% 24% 7% 1	24754

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% 7% 2 8% 5% 4 4 3 5% 7% 45%	3597
	11th to 20th	Day	14% 8% 3 6% 3 3 4 3 4 5% 48%	3570
	21st to end	Day	16% 9% 2 5% 3 4 4 3 5% 5% 42%	3960
February	1st to 10th	Day	19% 7% 3 6% 4 5% 5% 4 5% 6% 37%	1878
	11th to 20th	Day	14% 7% 3 6% 4 4 4 4 5% 7% 41%	1948
	21st to end	Day	17% 10% 2 6% 4 4 4 3 5% 6% 39%	1650
March	1st to 10th	Day	19% 12% 3 5% 4 4 4 3 5% 6% 35%	4476
	11th to 20th	Day	14% 9% 3 7% 4 4 4 4 6% 7% 39%	4795
	21st to end	Day	25% 9% 2 6% 5% 4 5% 4 6% 5% 30%	5730
April	1st to 10th	Day	19% 10% 3 6% 4 4 4 3 4 6% 37%	2400
	11th to 20th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	2399
	21st to end	Day	19% 7% 2 7% 4 4 4 3 6% 7% 38%	2600
May	1st to 10th	Day	27% 11% 3 7% 4 4 4 3 5% 6% 25%	5886
	11th to 20th	Day	19% 10% 3 8% 5% 5% 4 3 5% 6% 31%	5898
	21st to end	Day	19% 7% 3 6% 4 4 4 3 6% 7% 36%	6949
June	1st to 10th	Day	19% 6% 2 6% 4 4 4 5% 7% 8% 34%	2786
	11th to 20th	Day	16% 5% 2 6% 5% 5% 5% 5% 6% 8% 37%	2660
	21st to end	Day	15% 7% 2 7% 5% 5% 5% 4 6% 9% 35%	2660
July	1st to 10th	Day	11% 8% 3 6% 6% 5% 6% 5% 8% 9% 34%	4440
	11th to 20th	Day	10% 7% 3 6% 5% 5% 5% 5% 6% 10% 39%	4440
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 34%	4814
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 5% 7% 8% 31%	4113
	11th to 20th	Day	12% 8% 3 7% 4 5% 5% 4 6% 8% 39%	4018
	21st to end	Day	8% 4 2 5% 4 4 5% 5% 7% 10% 46%	4180
September	1st to 10th	Day	13% 7% 2 5% 4 4 4 3 5% 7% 47%	2263
	11th to 20th	Day	15% 6% 1 5% 2 3 3 2 5% 5% 52%	2090
	21st to end	Day	18% 6% 2 5% 3 3 4 3 5% 6% 43%	2090
October	1st to 10th	Day	23% 5% 3 5% 3 3 4 3 4 6% 41%	2090
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 6% 48%	1864
	21st to end	Day	19% 7% 2 5% 4 3 4 3 5% 6% 41%	1881
November	1st to 10th	Day	16% 6% 3 4 3 3 3 3 4 5% 50%	1761
	11th to 20th	Day	17% 7% 2 4 4 3 3 3 4 6% 47%	1800
	21st to end	Day	18% 7% 2 5% 5% 3 4 4 5% 6% 43%	1800
December	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 4 5% 49%	2703
	11th to 20th	Day	17% 8% 2 4 3 3 3 3 4 5% 48%	2986
	21st to end	Day	20% 11% 3 7% 4 4 4 2 4 5% 38%	3960
January	1st to 10th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 51%	6349
	11th to 20th	Night	13% 9% 4 9% 5% 4 4 3 5% 5% 40%	6315
	21st to end	Night	15% 11% 3 7% 5% 5% 4 3 4 5% 38%	6996
February	1st to 10th	Night	17% 9% 2 6% 4 4 4 3 5% 7% 38%	2515
	11th to 20th	Night	16% 8% 3 6% 4 5% 4 4 5% 6% 39%	2338
	21st to end	Night	17% 9% 3 5% 4 4 4 4 4 5% 42%	1980
March	1st to 10th	Night	24% 9% 2 4 3 4 3 3 4 5% 37%	5364
	11th to 20th	Night	15% 11% 3 7% 4 4 4 3 5% 6% 38%	5163
	21st to end	Night	25% 10% 2 6% 4 4 4 4 5% 4 32%	5197
April	1st to 10th	Night	19% 8% 3 7% 4 5% 4 3 5% 6% 36%	2000
	11th to 20th	Night	18% 8% 2 7% 5% 4 3 3 5% 6% 38%	1949
	21st to end	Night	20% 7% 2 7% 4 3 4 3 5% 5% 40%	1800
May	1st to 10th	Night	28% 11% 3 6% 4 4 3 3 5% 6% 26%	4054
	11th to 20th	Night	18% 9% 3 8% 6% 5% 6% 5% 7% 7% 28%	4033
	21st to end	Night	19% 8% 3 8% 5% 4 5% 3 5% 8% 31%	3930
June	1st to 10th	Night	17% 8% 2 7% 5% 4 4 4 6% 7% 36%	1598
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 6% 8% 37%	1520
	21st to end	Night	17% 5% 2 5% 4 4 5% 4 6% 9% 39%	1520
July	1st to 10th	Night	10% 7% 2 9% 5% 4 6% 5% 7% 10% 35%	2520
	11th to 20th	Night	10% 7% 3 7% 6% 4 5% 4 7% 10% 38%	2520
	21st to end	Night	15% 9% 3 8% 5% 6% 4 4 6% 7% 33%	2739
August	1st to 10th	Night	17% 7% 4 7% 5% 5% 5% 4 7% 7% 31%	2784
	11th to 20th	Night	8% 7% 3 9% 6% 5% 6% 5% 7% 8% 36%	2805
	21st to end	Night	8% 4 2 5% 4 3 4 3 5% 8% 54%	3476
September	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 6% 45%	1916
	11th to 20th	Night	14% 6% 1 5% 2 2 3 2 4 7% 53%	2090
	21st to end	Night	17% 6% 2 6% 3 4 4 4 6% 5% 43%	2090
October	1st to 10th	Night	20% 6% 2 6% 4 4 4 2 4 6% 42%	2090
	11th to 20th	Night	15% 4 2 5% 4 4 4 3 5% 5% 48%	2310
	21st to end	Night	20% 8% 2 6% 3 4 3 3 4 6% 40%	2717
November	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 5% 6% 49%	2537
	11th to 20th	Night	16% 8% 2 5% 3 4 4 3 5% 5% 46%	2600
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 45%	2600
December	1st to 10th	Night	17% 9% 2 5% 4 3 3 3 4 5% 44%	4475
	11th to 20th	Night	20% 9% 3 6% 3 3 3 3 4 4 41%	5208
	21st to end	Night	17% 10% 4 6% 4 4 3 3 4 6% 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	40% 3 6% 28% 20% 2	3598
	11th to 20th	Day	40% 3 10% 23% 20% 4	3600
	21st to end	Day	45% 3 10% 22% 19% 2	3960
February	1st to 10th	Day	49% 4 11% 19% 15% 2	1878
	11th to 20th	Day	43% 4 11% 24% 16% 3	1957
	21st to end	Day	47% 4 10% 21% 15% 3	1650
March	1st to 10th	Day	51% 3 10% 15% 13% 9%	4479
	11th to 20th	Day	45% 4 11% 27% 11% 2	4795
	21st to end	Day	55% 3 6% 16% 13% 7%	5730
April	1st to 10th	Day	48% 4 8% 19% 17% 4	2400
	11th to 20th	Day	44% 4 10% 23% 17% 3	2399
	21st to end	Day	45% 3 8% 22% 17% 4	2600
May	1st to 10th	Day	60% 4 13% 15% 7% 1	5897
	11th to 20th	Day	52% 4 13% 18% 11% 2	5902
	21st to end	Day	46% 5% 11% 23% 13% 1	6949
June	1st to 10th	Day	45% 6% 13% 24% 12% 1	2786
	11th to 20th	Day	42% 5% 14% 21% 15% 2	2660
	21st to end	Day	44% 6% 14% 22% 13% 1	2660
July	1st to 10th	Day	43% 8% 11% 21% 15% 2	4370
	11th to 20th	Day	39% 6% 14% 25% 14% 2	4365
	21st to end	Day	46% 6% 12% 21% 13% 2	4661
August	1st to 10th	Day	47% 7% 16% 22% 8% 1	4099
	11th to 20th	Day	42% 6% 13% 28% 10% 1	4057
	21st to end	Day	31% 5% 13% 30% 18% 1	4180
September	1st to 10th	Day	37% 5% 10% 25% 21% 2	2263
	11th to 20th	Day	35% 4 8% 29% 23% 3	2090
	21st to end	Day	42% 4 8% 21% 23% 2	2090
October	1st to 10th	Day	40% 5% 9% 26% 17% 3	3480
	11th to 20th	Day	32% 2 6% 30% 25% 4	3067
	21st to end	Day	33% 4 11% 30% 20% 3	3081
November	1st to 10th	Day	39% 3 12% 26% 18% 2	1761
	11th to 20th	Day	39% 3 8% 24% 23% 3	1800
	21st to end	Day	43% 5% 13% 25% 13% 2	1800
December	1st to 10th	Day	40% 3 14% 28% 14% 1	2703
	11th to 20th	Day	40% 3 14% 17% 20% 5%	2986
	21st to end	Day	51% 2 7% 22% 13% 4	3960
January	1st to 10th	Night	35% 2 9% 35% 17% 2	6349
	11th to 20th	Night	47% 3 11% 22% 13% 4	6360
	21st to end	Night	51% 2 8% 17% 17% 4	6996
February	1st to 10th	Night	47% 4 12% 21% 14% 2	2515
	11th to 20th	Night	46% 4 8% 21% 16% 4	2340
	21st to end	Night	45% 3 7% 24% 16% 4	1980
March	1st to 10th	Night	51% 2 8% 16% 16% 7%	5374
	11th to 20th	Night	49% 3 15% 19% 13% 2	5164
	21st to end	Night	56% 2 4 13% 19% 6%	5197
April	1st to 10th	Night	49% 4 8% 20% 16% 4	2000
	11th to 20th	Night	47% 4 8% 20% 18% 3	1949
	21st to end	Night	48% 3 9% 20% 17% 4	1800
May	1st to 10th	Night	60% 3 13% 14% 8% 2	4060
	11th to 20th	Night	54% 5% 15% 16% 9% 2	4034
	21st to end	Night	53% 4 12% 19% 10% 1	3931
June	1st to 10th	Night	47% 5% 13% 22% 12% 1	1598
	11th to 20th	Night	45% 5% 13% 22% 14% 1	1520
	21st to end	Night	41% 6% 14% 25% 13% 1	1520
July	1st to 10th	Night	41% 7% 13% 21% 16%	2480
	11th to 20th	Night	40% 6% 17% 25% 13% 1	2472
	21st to end	Night	48% 6% 11% 20% 14% 1	2651
August	1st to 10th	Night	51% 6% 14% 19% 10% 1	2774
	11th to 20th	Night	41% 9% 15% 24% 10%	2832
	21st to end	Night	27% 5% 15% 29% 24% 1	3476
September	1st to 10th	Night	37% 5% 11% 25% 19% 2	1916
	11th to 20th	Night	34% 3 11% 24% 26% 2	2090
	21st to end	Night	41% 5% 8% 24% 20% 3	2090
October	1st to 10th	Night	40% 4 12% 25% 15% 3	3480
	11th to 20th	Night	27% 3 10% 28% 29% 3	3846
	21st to end	Night	34% 3 11% 30% 20% 2	4467
November	1st to 10th	Night	38% 4 11% 24% 20% 3	2537
	11th to 20th	Night	41% 3 9% 22% 21% 4	2600
	21st to end	Night	41% 3 13% 27% 13% 2	2600
December	1st to 10th	Night	43% 3 11% 28% 14% 1	4475
	11th to 20th	Night	47% 3 14% 15% 20% 1	5208
	21st to end	Night	46% 4 7% 24% 17% 2	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% 25% 28% 14% 4	200
	11th to 20th	Day	22% 30% 38% 10% 2 1	200
	21st to end	Day	22% 26% 39% 10% 3	220
February	1st to 10th	Day	28% 23% 35% 12% 2	423
	11th to 20th	Day	27% 28% 34% 9% 1	496
	21st to end	Day	25% 23% 37% 12% 3	414
March	1st to 10th	Day	27% 25% 34% 11% 3	500
	11th to 20th	Day	26% 25% 33% 12% 3 1	500
	21st to end	Day	29% 23% 33% 10% 5% 1	550
April	1st to 10th	Day	30% 22% 30% 12% 6%	500
	11th to 20th	Day	27% 24% 32% 14% 3	492
	21st to end	Day	26% 17% 37% 17% 3 1	500
May	1st to 10th	Day	30% 23% 31% 11% 3 1	503
	11th to 20th	Day	21% 16% 40% 17% 5% 1	527
	21st to end	Day	19% 13% 35% 28% 5% 1	573
June	1st to 10th	Day	21% 12% 35% 24% 7% 1	526
	11th to 20th	Day	14% 14% 40% 23% 8% 1	490
	21st to end	Day	15% 10% 42% 23% 8% 1	490
July	1st to 10th	Day	16% 8% 32% 28% 13% 2	485
	11th to 20th	Day	14% 6% 32% 35% 12% 2	484
	21st to end	Day	12% 7% 36% 33% 11% 1	532
August	1st to 10th	Day	11% 9% 32% 36% 10% 2	486
	11th to 20th	Day	16% 7% 30% 29% 15% 3	479
	21st to end	Day	14% 7% 33% 27% 15% 4	539
September	1st to 10th	Day	18% 10% 28% 26% 15% 3	490
	11th to 20th	Day	21% 10% 26% 21% 15% 8%	490
	21st to end	Day	22% 10% 32% 22% 9% 4	490
October	1st to 10th	Day	25% 12% 28% 22% 9% 4	490
	11th to 20th	Day	19% 15% 33% 21% 9% 3	427
	21st to end	Day	24% 16% 34% 17% 8% 1	429
November	1st to 10th	Day	20% 16% 34% 18% 11% 1	395
	11th to 20th	Day	22% 23% 35% 15% 5%	400
	21st to end	Day	25% 22% 30% 16% 6% 1	400
December	1st to 10th	Day	25% 22% 33% 16% 4	264
	11th to 20th	Day	31% 28% 32% 8% 2	200
	21st to end	Day	24% 20% 35% 17% 3 3	220
January	1st to 10th	Night	22% 21% 39% 16% 3	800
	11th to 20th	Night	26% 20% 36% 17% 2	800
	21st to end	Night	19% 20% 37% 19% 4	880
February	1st to 10th	Night	23% 20% 36% 19% 1	800
	11th to 20th	Night	25% 20% 40% 14% 1	783
	21st to end	Night	24% 21% 35% 17% 3	660
March	1st to 10th	Night	25% 20% 35% 17% 4	800
	11th to 20th	Night	23% 19% 37% 17% 4 1	800
	21st to end	Night	26% 17% 34% 18% 5%	880
April	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	24% 16% 37% 17% 5%	765
	21st to end	Night	24% 17% 35% 22% 2	600
May	1st to 10th	Night	28% 13% 35% 20% 5%	600
	11th to 20th	Night	21% 13% 40% 21% 5%	592
	21st to end	Night	19% 9% 38% 27% 7%	437
June	1st to 10th	Night	20% 7% 34% 33% 6% 1	400
	11th to 20th	Night	17% 8% 36% 33% 6% 1	380
	21st to end	Night	18% 5% 36% 35% 6%	380
July	1st to 10th	Night	13% 3 33% 41% 9% 1	370
	11th to 20th	Night	15% 3 30% 42% 9% 1	368
	21st to end	Night	14% 2 32% 42% 10%	416
August	1st to 10th	Night	11% 5% 32% 40% 11%	539
	11th to 20th	Night	13% 5% 28% 42% 12% 1	572
	21st to end	Night	14% 5% 23% 38% 19% 2	836
September	1st to 10th	Night	14% 4 30% 36% 14% 1	760
	11th to 20th	Night	16% 6% 27% 29% 19% 2	760
	21st to end	Night	22% 6% 26% 31% 13% 3	760
October	1st to 10th	Night	24% 9% 26% 26% 13% 2	760
	11th to 20th	Night	17% 13% 33% 25% 10% 2	760
	21st to end	Night	25% 12% 34% 22% 7%	836
November	1st to 10th	Night	17% 14% 35% 24% 8% 1	780
	11th to 20th	Night	21% 22% 31% 19% 6% 1	800
	21st to end	Night	21% 22% 34% 19% 5% 1	800
December	1st to 10th	Night	25% 18% 35% 18% 5%	800
	11th to 20th	Night	24% 20% 33% 19% 4	800
	21st to end	Night	24% 18% 38% 18% 2	880

Page generated at 2025-06-07 10:28:17. Version ac07

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