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 2024-03-23.

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% 7% 3 5% 5% 3 4 5% 5% 7% 40%	78578
	Night	17% 8% 3 5% 5% 3 4 4 5% 6% 40%	81293
Transparency	Day	44% 4 11% 23% 15% 2	78283
	Night	45% 4 11% 23% 15% 2	81122
Seeing	Day	22% 16% 33% 20% 7% 2	14202
	Night	21% 14% 34% 24% 7% 1	23245

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% 8% 3 6% 5% 3 4 4 4 5% 46%	2517
	11th to 20th	Day	17% 7% 3 6% 4 3 4 5% 5% 5% 41%	2520
	21st to end	Day	20% 12% 3 6% 5% 4 4 3 5% 5% 34%	2772
February	1st to 10th	Day	19% 7% 3 5% 5% 3 5% 5% 4 7% 37%	1783
	11th to 20th	Day	14% 7% 3 6% 5% 3 5% 4 5% 6% 42%	1848
	21st to end	Day	18% 9% 3 5% 5% 3 4 4 4 6% 38%	1570
March	1st to 10th	Day	17% 8% 3 4 5% 3 4 5% 5% 8% 38%	1897
	11th to 20th	Day	16% 6% 3 5% 5% 2 5% 5% 5% 7% 40%	2013
	21st to end	Day	18% 8% 2 6% 5% 2 5% 4 5% 5% 41%	2298
April	1st to 10th	Day	18% 9% 3 6% 4 3 4 4 4 7% 38%	2160
	11th to 20th	Day	17% 6% 3 5% 5% 3 4 5% 6% 7% 39%	2156
	21st to end	Day	21% 7% 2 5% 5% 3 4 4 6% 7% 36%	2340
May	1st to 10th	Day	24% 9% 3 6% 5% 3 4 4 4 7% 32%	2340
	11th to 20th	Day	18% 7% 3 5% 5% 3 5% 4 5% 6% 41%	2310
	21st to end	Day	15% 5% 3 4 5% 3 4 4 5% 8% 44%	2725
June	1st to 10th	Day	20% 6% 2 5% 5% 3 5% 6% 6% 8% 33%	2520
	11th to 20th	Day	17% 5% 3 5% 5% 3 6% 6% 6% 7% 37%	2520
	21st to end	Day	15% 6% 3 6% 6% 4 6% 5% 6% 9% 35%	2520
July	1st to 10th	Day	13% 6% 2 4 6% 3 6% 5% 7% 9% 40%	2520
	11th to 20th	Day	12% 6% 3 5% 5% 3 6% 6% 7% 9% 38%	2520
	21st to end	Day	11% 5% 2 6% 6% 3 6% 6% 6% 9% 39%	2744
August	1st to 10th	Day	10% 6% 2 5% 6% 4 5% 8% 7% 9% 38%	2335
	11th to 20th	Day	13% 6% 2 7% 4 4 5% 6% 6% 9% 39%	2291
	21st to end	Day	13% 5% 2 5% 5% 3 4 5% 5% 9% 44%	2376
September	1st to 10th	Day	13% 6% 2 4 5% 3 5% 4 5% 7% 46%	2144
	11th to 20th	Day	15% 6% 1 4 3 2 3 3 5% 5% 52%	1980
	21st to end	Day	18% 5% 2 4 4 2 4 4 5% 6% 45%	1980
October	1st to 10th	Day	24% 5% 3 4 4 2 4 3 3 5% 41%	1980
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 5% 6% 47%	1767
	21st to end	Day	20% 7% 3 5% 5% 3 3 5% 5% 6% 40%	1782
November	1st to 10th	Day	17% 6% 3 4 4 3 3 3 4 4 49%	1671
	11th to 20th	Day	18% 6% 2 4 4 2 3 4 4 6% 45%	1710
	21st to end	Day	18% 6% 2 5% 5% 3 4 4 5% 6% 43%	1710
December	1st to 10th	Day	18% 8% 3 6% 5% 3 4 4 4 6% 40%	1581
	11th to 20th	Day	20% 8% 3 4 4 2 3 3 3 6% 44%	1906
	21st to end	Day	21% 10% 3 6% 5% 3 4 3 4 6% 35%	2772
January	1st to 10th	Night	17% 8% 3 4 3 2 3 3 4 5% 48%	4429
	11th to 20th	Night	15% 9% 4 7% 6% 3 3 3 4 5% 40%	4440
	21st to end	Night	21% 14% 4 7% 6% 3 4 4 4 5% 30%	4884
February	1st to 10th	Night	17% 9% 2 6% 5% 4 4 4 5% 7% 38%	2390
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 5% 4 6% 40%	2218
	21st to end	Night	18% 8% 3 4 4 3 4 4 4 5% 43%	1884
March	1st to 10th	Night	17% 7% 3 4 5% 3 4 5% 4 6% 40%	2270
	11th to 20th	Night	15% 9% 4 6% 5% 3 4 5% 5% 6% 38%	2165
	21st to end	Night	17% 10% 3 6% 5% 3 4 4 3 5% 42%	2098
April	1st to 10th	Night	18% 7% 3 6% 5% 4 4 4 5% 7% 38%	1800
	11th to 20th	Night	19% 7% 2 5% 6% 3 4 4 4 7% 40%	1753
	21st to end	Night	21% 7% 2 6% 5% 3 4 4 5% 5% 38%	1620
May	1st to 10th	Night	23% 9% 3 6% 5% 3 4 3 5% 6% 32%	1620
	11th to 20th	Night	18% 6% 2 6% 6% 3 5% 5% 6% 7% 36%	1593
	21st to end	Night	17% 8% 3 6% 5% 2 3 3 6% 8% 38%	1554
June	1st to 10th	Night	18% 7% 3 6% 5% 2 6% 5% 6% 7% 36%	1440
	11th to 20th	Night	18% 7% 4 5% 4 3 4 5% 6% 8% 36%	1440
	21st to end	Night	16% 5% 2 5% 4 3 5% 5% 6% 9% 41%	1440
July	1st to 10th	Night	12% 6% 2 6% 6% 3 5% 6% 6% 8% 40%	1440
	11th to 20th	Night	11% 7% 4 6% 6% 3 5% 5% 7% 10% 36%	1440
	21st to end	Night	12% 5% 2 5% 6% 4 6% 5% 6% 8% 40%	1574
August	1st to 10th	Night	8% 5% 2 5% 6% 3 5% 5% 7% 9% 45%	1584
	11th to 20th	Night	10% 5% 4 6% 7% 4 6% 6% 7% 7% 38%	1605
	21st to end	Night	13% 5% 2 5% 5% 2 4 4 5% 8% 47%	1980
September	1st to 10th	Night	12% 5% 2 5% 5% 4 4 5% 5% 6% 44%	1815
	11th to 20th	Night	14% 6% 2 4 3 2 3 3 4 7% 53%	1980
	21st to end	Night	16% 6% 3 5% 4 3 4 5% 6% 5% 45%	1980
October	1st to 10th	Night	21% 6% 2 5% 5% 3 4 3 3 6% 42%	1980
	11th to 20th	Night	16% 4 2 5% 5% 3 4 4 5% 6% 46%	2187
	21st to end	Night	21% 8% 3 5% 4 3 4 3 5% 6% 38%	2574
November	1st to 10th	Night	15% 5% 2 5% 4 3 4 4 5% 6% 48%	2407
	11th to 20th	Night	17% 7% 3 5% 4 2 4 4 4 5% 45%	2470
	21st to end	Night	17% 6% 2 4 4 2 4 4 5% 6% 45%	2470
December	1st to 10th	Night	18% 8% 3 5% 5% 3 4 4 5% 6% 39%	2597
	11th to 20th	Night	19% 8% 2 5% 3 3 4 4 5% 6% 41%	3288
	21st to end	Night	20% 10% 4 6% 5% 3 4 3 4 6% 35%	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	42% 3 7% 33% 13% 2	2518
	11th to 20th	Day	47% 4 9% 26% 13% 2	2520
	21st to end	Day	54% 3 10% 22% 9% 1	2772
February	1st to 10th	Day	49% 4 11% 19% 14% 2	1783
	11th to 20th	Day	43% 4 12% 24% 16% 2	1857
	21st to end	Day	48% 4 10% 20% 15% 3	1570
March	1st to 10th	Day	46% 3 12% 21% 13% 4	1900
	11th to 20th	Day	44% 4 11% 22% 16% 4	2013
	21st to end	Day	45% 4 9% 19% 17% 6%	2298
April	1st to 10th	Day	47% 4 8% 20% 17% 4	2160
	11th to 20th	Day	44% 4 9% 24% 16% 3	2156
	21st to end	Day	47% 3 9% 21% 16% 4	2340
May	1st to 10th	Day	53% 3 8% 21% 13% 2	2340
	11th to 20th	Day	43% 5% 9% 20% 19% 4	2314
	21st to end	Day	38% 5% 15% 25% 14% 2	2725
June	1st to 10th	Day	46% 6% 13% 23% 11% 1	2520
	11th to 20th	Day	43% 5% 13% 21% 15% 2	2520
	21st to end	Day	44% 6% 14% 23% 12% 1	2520
July	1st to 10th	Day	39% 6% 11% 26% 15% 3	2450
	11th to 20th	Day	40% 6% 13% 26% 14% 2	2445
	21st to end	Day	39% 7% 13% 24% 14% 4	2591
August	1st to 10th	Day	39% 6% 14% 26% 13% 1	2321
	11th to 20th	Day	42% 6% 12% 25% 14% 1	2291
	21st to end	Day	37% 5% 11% 26% 19% 2	2376
September	1st to 10th	Day	38% 5% 10% 25% 20% 2	2144
	11th to 20th	Day	35% 4 8% 28% 23% 3	1980
	21st to end	Day	41% 4 7% 22% 24% 2	1980
October	1st to 10th	Day	46% 4 8% 24% 14% 4	1980
	11th to 20th	Day	39% 3 10% 31% 16% 2	1767
	21st to end	Day	45% 4 12% 20% 15% 4	1782
November	1st to 10th	Day	40% 3 12% 26% 17% 2	1671
	11th to 20th	Day	41% 3 8% 24% 21% 2	1710
	21st to end	Day	43% 4 13% 24% 13% 2	1710
December	1st to 10th	Day	47% 3 13% 17% 18% 1	1581
	11th to 20th	Day	45% 3 13% 18% 17% 5%	1906
	21st to end	Day	54% 2 10% 22% 11% 1	2772
January	1st to 10th	Night	41% 2 11% 31% 13% 3	4429
	11th to 20th	Night	48% 4 10% 24% 12% 3	4440
	21st to end	Night	60% 2 8% 17% 11% 2	4884
February	1st to 10th	Night	47% 4 12% 21% 14% 2	2390
	11th to 20th	Night	46% 4 8% 21% 16% 4	2220
	21st to end	Night	45% 3 7% 25% 16% 4	1884
March	1st to 10th	Night	47% 3 8% 23% 15% 4	2280
	11th to 20th	Night	47% 3 11% 17% 18% 4	2166
	21st to end	Night	47% 3 7% 19% 19% 6%	2098
April	1st to 10th	Night	47% 4 8% 20% 17% 3	1800
	11th to 20th	Night	46% 3 8% 21% 18% 3	1753
	21st to end	Night	49% 3 9% 20% 15% 4	1620
May	1st to 10th	Night	54% 3 9% 18% 13% 3	1620
	11th to 20th	Night	46% 5% 12% 19% 14% 4	1594
	21st to end	Night	45% 5% 15% 20% 13% 1	1555
June	1st to 10th	Night	47% 5% 14% 22% 11%	1440
	11th to 20th	Night	46% 5% 12% 21% 14% 1	1440
	21st to end	Night	40% 6% 15% 25% 13% 1	1440
July	1st to 10th	Night	40% 6% 14% 23% 17% 1	1400
	11th to 20th	Night	43% 5% 14% 28% 9% 1	1392
	21st to end	Night	40% 6% 12% 29% 12% 2	1486
August	1st to 10th	Night	36% 6% 16% 27% 14% 2	1574
	11th to 20th	Night	41% 8% 16% 21% 13%	1605
	21st to end	Night	36% 5% 11% 28% 20% 1	1980
September	1st to 10th	Night	38% 5% 12% 24% 18% 2	1815
	11th to 20th	Night	34% 3 11% 23% 26% 3	1980
	21st to end	Night	39% 5% 7% 25% 21% 3	1980
October	1st to 10th	Night	45% 4 10% 23% 14% 4	1980
	11th to 20th	Night	39% 4 13% 29% 15% 1	2187
	21st to end	Night	48% 3 11% 21% 15% 2	2574
November	1st to 10th	Night	39% 4 12% 24% 19% 3	2407
	11th to 20th	Night	42% 3 9% 23% 20% 3	2470
	21st to end	Night	41% 3 13% 27% 13% 2	2470
December	1st to 10th	Night	47% 4 12% 22% 14% 1	2597
	11th to 20th	Night	45% 4 14% 18% 18% 1	3288
	21st to end	Night	51% 4 7% 27% 11% 1	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% 24% 28% 16% 4	180
	11th to 20th	Day	22% 29% 37% 9% 2 1	180
	21st to end	Day	22% 25% 39% 11% 3 1	198
February	1st to 10th	Day	29% 22% 35% 12% 2	388
	11th to 20th	Day	26% 28% 35% 10% 1	456
	21st to end	Day	26% 23% 36% 12% 3	382
March	1st to 10th	Day	26% 24% 35% 12% 4	460
	11th to 20th	Day	26% 25% 34% 11% 3 1	460
	21st to end	Day	27% 22% 33% 11% 6% 1	470
April	1st to 10th	Day	29% 22% 29% 13% 7%	420
	11th to 20th	Day	25% 23% 33% 16% 3	412
	21st to end	Day	28% 16% 36% 16% 3 1	420
May	1st to 10th	Day	32% 23% 30% 11% 3 1	423
	11th to 20th	Day	21% 16% 40% 17% 5%	447
	21st to end	Day	18% 13% 34% 28% 6% 1	485
June	1st to 10th	Day	22% 11% 34% 24% 7% 2	450
	11th to 20th	Day	16% 14% 41% 21% 7% 1	450
	21st to end	Day	15% 9% 41% 25% 9% 1	450
July	1st to 10th	Day	17% 7% 30% 29% 14% 2	445
	11th to 20th	Day	15% 5% 30% 36% 12% 2	444
	21st to end	Day	11% 7% 35% 34% 11% 1	488
August	1st to 10th	Day	11% 8% 32% 37% 11% 2	446
	11th to 20th	Day	16% 7% 29% 29% 15% 3	439
	21st to end	Day	16% 7% 32% 26% 15% 4	495
September	1st to 10th	Day	18% 9% 27% 27% 15% 4	450
	11th to 20th	Day	21% 10% 28% 21% 13% 6%	450
	21st to end	Day	22% 10% 32% 22% 10% 4	450
October	1st to 10th	Day	26% 12% 27% 22% 9% 4	450
	11th to 20th	Day	19% 14% 33% 22% 10% 2	394
	21st to end	Day	26% 16% 33% 17% 8% 1	396
November	1st to 10th	Day	20% 16% 33% 19% 11% 1	365
	11th to 20th	Day	24% 22% 33% 16% 5%	370
	21st to end	Day	24% 21% 32% 16% 6% 1	370
December	1st to 10th	Day	24% 24% 34% 14% 4	241
	11th to 20th	Day	32% 26% 34% 7% 2	180
	21st to end	Day	25% 21% 34% 15% 3 3	198
January	1st to 10th	Night	21% 20% 39% 17% 3	760
	11th to 20th	Night	26% 19% 36% 17% 2	760
	21st to end	Night	20% 20% 37% 19% 4	836
February	1st to 10th	Night	23% 19% 37% 19% 1	760
	11th to 20th	Night	24% 20% 41% 14% 1	743
	21st to end	Night	25% 21% 34% 16% 4	628
March	1st to 10th	Night	25% 19% 35% 18% 4	760
	11th to 20th	Night	23% 19% 38% 17% 3 1	760
	21st to end	Night	24% 17% 34% 19% 5%	801
April	1st to 10th	Night	23% 19% 34% 20% 4	720
	11th to 20th	Night	23% 16% 38% 19% 5%	689
	21st to end	Night	25% 14% 36% 22% 2	540
May	1st to 10th	Night	28% 12% 34% 20% 5%	540
	11th to 20th	Night	22% 13% 39% 21% 5%	532
	21st to end	Night	19% 8% 38% 28% 7%	393
June	1st to 10th	Night	21% 6% 32% 34% 7% 1	360
	11th to 20th	Night	17% 8% 36% 33% 6% 1	360
	21st to end	Night	17% 4 37% 36% 6%	360
July	1st to 10th	Night	13% 3 32% 42% 9% 1	350
	11th to 20th	Night	16% 2 29% 42% 9% 1	348
	21st to end	Night	13% 2 33% 42% 10%	394
August	1st to 10th	Night	10% 5% 33% 41% 12%	510
	11th to 20th	Night	13% 4 27% 43% 12% 1	541
	21st to end	Night	15% 5% 22% 38% 18% 2	792
September	1st to 10th	Night	15% 4 29% 37% 14% 1	720
	11th to 20th	Night	16% 7% 28% 29% 18% 2	720
	21st to end	Night	21% 6% 27% 31% 13% 3	720
October	1st to 10th	Night	25% 9% 26% 25% 13% 2	720
	11th to 20th	Night	18% 13% 34% 25% 10% 2	720
	21st to end	Night	26% 12% 34% 22% 6% 1	792
November	1st to 10th	Night	18% 14% 33% 25% 8% 1	740
	11th to 20th	Night	21% 21% 31% 19% 6% 1	760
	21st to end	Night	20% 21% 34% 19% 5% 1	760
December	1st to 10th	Night	25% 18% 35% 18% 4	760
	11th to 20th	Night	23% 20% 33% 20% 3	760
	21st to end	Night	24% 18% 38% 18% 2	836

Page generated at 2024-03-19 23:31:06. Version ac07

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