John J. McCarthy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the John J. McCarthy 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 5% 5% 5% 7% 39%	78596
	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 40%	81288
Transparency	Day	44% 6% 11% 22% 15% 2	78301
	Night	44% 5% 11% 23% 15% 2	81117
Seeing	Day	25% 17% 30% 19% 8% 2	14084
	Night	22% 16% 30% 24% 7% 1	23316

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	15% 7% 2 5% 5% 2 4 4 5% 6% 45%	2517
	11th to 20th	Day	16% 10% 4 6% 7% 4 5% 5% 4 6% 35%	2520
	21st to end	Day	21% 12% 5% 8% 6% 3 3 3 4 5% 31%	2903
February	1st to 10th	Day	17% 7% 3 6% 5% 3 5% 5% 5% 7% 35%	1738
	11th to 20th	Day	15% 6% 3 6% 5% 3 4 5% 5% 7% 41%	1848
	21st to end	Day	21% 8% 3 7% 5% 2 4 4 5% 6% 36%	1570
March	1st to 10th	Day	15% 7% 3 5% 6% 4 4 4 5% 6% 41%	1897
	11th to 20th	Day	16% 6% 3 6% 6% 4 5% 5% 6% 8% 37%	1965
	21st to end	Day	19% 8% 3 5% 5% 3 3 3 4 6% 39%	2400
April	1st to 10th	Day	18% 9% 3 6% 5% 2 4 4 5% 7% 37%	2160
	11th to 20th	Day	18% 5% 2 5% 5% 3 5% 4 5% 8% 40%	2134
	21st to end	Day	21% 6% 2 5% 4 3 3 5% 4 6% 39%	2232
May	1st to 10th	Day	23% 8% 3 5% 4 3 5% 4 5% 6% 33%	2340
	11th to 20th	Day	19% 7% 2 5% 5% 2 5% 4 6% 7% 40%	2443
	21st to end	Day	16% 6% 2 5% 5% 3 5% 5% 5% 8% 41%	2725
June	1st to 10th	Day	19% 5% 2 4 4 4 5% 6% 7% 9% 34%	2520
	11th to 20th	Day	17% 7% 3 5% 6% 2 6% 5% 6% 9% 35%	2520
	21st to end	Day	13% 6% 3 5% 6% 4 5% 5% 5% 10% 38%	2520
July	1st to 10th	Day	13% 6% 3 5% 6% 3 5% 5% 5% 8% 40%	2520
	11th to 20th	Day	12% 4 3 5% 5% 3 6% 6% 6% 10% 40%	2520
	21st to end	Day	12% 5% 4 6% 6% 3 6% 6% 6% 8% 39%	2744
August	1st to 10th	Day	12% 6% 3 5% 6% 3 6% 6% 8% 9% 37%	2428
	11th to 20th	Day	12% 6% 2 4 5% 4 5% 6% 7% 9% 38%	2126
	21st to end	Day	13% 5% 3 5% 6% 3 5% 5% 5% 9% 42%	2376
September	1st to 10th	Day	14% 6% 2 5% 6% 4 5% 5% 6% 6% 41%	2160
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 5% 6% 50%	2058
	21st to end	Day	17% 6% 2 5% 5% 3 4 5% 5% 6% 42%	1980
October	1st to 10th	Day	24% 6% 2 5% 4 2 4 4 4 5% 40%	1917
	11th to 20th	Day	15% 6% 2 5% 5% 3 4 4 6% 8% 44%	1800
	21st to end	Day	23% 5% 2 5% 5% 3 5% 5% 5% 5% 38%	1792
November	1st to 10th	Day	13% 6% 3 4 4 3 3 2 5% 6% 50%	1671
	11th to 20th	Day	19% 6% 2 4 3 2 4 4 4 7% 45%	1710
	21st to end	Day	17% 6% 2 4 5% 2 4 3 6% 7% 43%	1644
December	1st to 10th	Day	20% 7% 4 5% 6% 3 3 5% 4 6% 36%	1520
	11th to 20th	Day	22% 6% 2 4 2 2 4 4 4 6% 42%	1906
	21st to end	Day	20% 10% 4 7% 7% 3 4 4 5% 7% 30%	2772
January	1st to 10th	Night	17% 6% 2 5% 4 3 4 4 5% 6% 45%	4429
	11th to 20th	Night	16% 8% 3 6% 5% 3 4 4 5% 6% 39%	4440
	21st to end	Night	24% 11% 4 6% 5% 2 3 3 4 5% 32%	4752
February	1st to 10th	Night	16% 9% 3 6% 5% 3 5% 4 5% 6% 40%	2441
	11th to 20th	Night	15% 8% 3 5% 5% 3 4 4 6% 6% 40%	2218
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 6% 40%	1884
March	1st to 10th	Night	17% 6% 2 5% 5% 3 4 5% 5% 6% 42%	2270
	11th to 20th	Night	15% 7% 2 6% 5% 3 5% 5% 5% 7% 41%	2212
	21st to end	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 40%	2001
April	1st to 10th	Night	18% 8% 3 5% 5% 3 5% 5% 5% 7% 37%	1800
	11th to 20th	Night	18% 6% 3 6% 5% 4 4 5% 5% 7% 39%	1775
	21st to end	Night	20% 7% 2 6% 5% 2 4 4 5% 6% 39%	1728
May	1st to 10th	Night	22% 7% 3 6% 6% 4 4 4 5% 6% 32%	1620
	11th to 20th	Night	19% 8% 2 6% 4 3 4 5% 4 7% 38%	1465
	21st to end	Night	16% 7% 3 5% 5% 3 4 4 5% 8% 41%	1550
June	1st to 10th	Night	18% 6% 3 6% 5% 3 4 5% 4 7% 41%	1440
	11th to 20th	Night	16% 5% 2 5% 5% 3 4 5% 6% 9% 39%	1440
	21st to end	Night	13% 5% 2 4 4 3 4 5% 6% 9% 43%	1440
July	1st to 10th	Night	13% 6% 2 5% 5% 3 5% 5% 6% 9% 41%	1440
	11th to 20th	Night	13% 6% 3 5% 5% 3 4 5% 7% 10% 39%	1440
	21st to end	Night	10% 6% 2 5% 5% 4 5% 5% 7% 9% 42%	1574
August	1st to 10th	Night	9% 5% 2 4 6% 4 5% 6% 7% 9% 44%	1488
	11th to 20th	Night	12% 5% 3 7% 7% 4 5% 6% 6% 9% 37%	1774
	21st to end	Night	17% 5% 2 4 4 2 4 4 5% 8% 45%	1980
September	1st to 10th	Night	15% 7% 3 4 5% 3 4 4 4 7% 45%	1800
	11th to 20th	Night	13% 5% 2 4 3 2 3 4 3 6% 54%	1900
	21st to end	Night	18% 7% 2 4 4 2 5% 4 4 5% 46%	1980
October	1st to 10th	Night	22% 6% 3 4 4 3 3 3 4 5% 42%	2041
	11th to 20th	Night	16% 5% 2 5% 5% 3 4 4 4 7% 46%	2160
	21st to end	Night	23% 6% 2 4 4 3 3 4 5% 6% 38%	2562
November	1st to 10th	Night	13% 6% 2 5% 4 3 4 5% 5% 6% 47%	2407
	11th to 20th	Night	19% 6% 2 4 4 2 4 4 4 7% 44%	2470
	21st to end	Night	15% 6% 2 6% 4 3 4 4 5% 7% 44%	2535
December	1st to 10th	Night	20% 7% 2 5% 5% 2 4 4 5% 6% 39%	2660
	11th to 20th	Night	20% 7% 2 4 4 3 5% 5% 6% 6% 38%	3288
	21st to end	Night	22% 10% 3 6% 5% 3 4 4 5% 6% 33%	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	40% 4 9% 34% 11% 2	2518
	11th to 20th	Day	50% 5% 10% 24% 10% 1	2520
	21st to end	Day	56% 4 8% 20% 11% 2	2903
February	1st to 10th	Day	47% 5% 12% 21% 13% 2	1738
	11th to 20th	Day	42% 4 11% 23% 17% 3	1857
	21st to end	Day	49% 4 10% 22% 13% 3	1570
March	1st to 10th	Day	45% 4 12% 21% 14% 4	1900
	11th to 20th	Day	43% 5% 11% 24% 15% 2	1965
	21st to end	Day	48% 3 8% 18% 18% 5%	2400
April	1st to 10th	Day	46% 5% 10% 20% 15% 4	2160
	11th to 20th	Day	42% 6% 10% 22% 18% 3	2134
	21st to end	Day	44% 6% 10% 21% 16% 4	2232
May	1st to 10th	Day	51% 5% 8% 20% 14% 2	2340
	11th to 20th	Day	43% 7% 10% 21% 17% 3	2447
	21st to end	Day	41% 7% 15% 21% 14% 2	2725
June	1st to 10th	Day	44% 7% 14% 22% 12% 1	2520
	11th to 20th	Day	45% 7% 12% 23% 11% 1	2520
	21st to end	Day	41% 9% 13% 24% 13% 1	2520
July	1st to 10th	Day	42% 8% 11% 21% 16% 2	2450
	11th to 20th	Day	39% 8% 14% 25% 14% 2	2445
	21st to end	Day	41% 8% 13% 19% 17% 2	2591
August	1st to 10th	Day	40% 9% 13% 24% 13% 1	2414
	11th to 20th	Day	40% 8% 17% 23% 12% 1	2126
	21st to end	Day	40% 7% 10% 24% 18% 2	2376
September	1st to 10th	Day	41% 8% 10% 22% 17% 1	2160
	11th to 20th	Day	35% 6% 9% 23% 24% 2	2058
	21st to end	Day	42% 6% 9% 21% 18% 3	1980
October	1st to 10th	Day	46% 5% 8% 22% 16% 3	1917
	11th to 20th	Day	38% 5% 12% 28% 15% 2	1800
	21st to end	Day	47% 4 10% 21% 15% 3	1792
November	1st to 10th	Day	36% 5% 12% 26% 19% 3	1671
	11th to 20th	Day	40% 4 11% 23% 19% 3	1710
	21st to end	Day	41% 4 13% 28% 12% 2	1644
December	1st to 10th	Day	48% 4 14% 18% 14% 1	1520
	11th to 20th	Day	43% 4 15% 20% 16% 2	1906
	21st to end	Day	53% 5% 10% 22% 9% 1	2772
January	1st to 10th	Night	40% 4 11% 31% 11% 4	4429
	11th to 20th	Night	44% 5% 12% 25% 11% 3	4440
	21st to end	Night	55% 4 8% 20% 11% 2	4752
February	1st to 10th	Night	46% 4 12% 23% 12% 2	2441
	11th to 20th	Night	44% 5% 10% 19% 18% 4	2220
	21st to end	Night	45% 4 9% 23% 14% 5%	1884
March	1st to 10th	Night	41% 5% 12% 23% 15% 3	2280
	11th to 20th	Night	44% 4 10% 22% 17% 3	2213
	21st to end	Night	43% 4 9% 18% 20% 5%	2001
April	1st to 10th	Night	46% 5% 10% 20% 16% 3	1800
	11th to 20th	Night	43% 6% 11% 19% 19% 1	1775
	21st to end	Night	46% 5% 8% 22% 15% 4	1728
May	1st to 10th	Night	53% 4 10% 17% 14% 2	1620
	11th to 20th	Night	47% 6% 9% 19% 17% 2	1466
	21st to end	Night	43% 7% 15% 22% 12% 1	1551
June	1st to 10th	Night	46% 5% 9% 22% 16% 1	1440
	11th to 20th	Night	41% 7% 15% 25% 10% 1	1440
	21st to end	Night	35% 9% 14% 26% 15% 1	1440
July	1st to 10th	Night	39% 9% 14% 22% 14% 2	1400
	11th to 20th	Night	40% 8% 15% 26% 10% 1	1392
	21st to end	Night	36% 9% 15% 23% 15% 3	1486
August	1st to 10th	Night	36% 9% 15% 25% 15% 1	1478
	11th to 20th	Night	42% 9% 15% 22% 10% 1	1774
	21st to end	Night	37% 8% 11% 23% 19% 1	1980
September	1st to 10th	Night	41% 6% 9% 26% 17% 1	1800
	11th to 20th	Night	33% 6% 10% 23% 26% 2	1900
	21st to end	Night	41% 6% 7% 22% 22% 2	1980
October	1st to 10th	Night	45% 6% 10% 23% 14% 3	2041
	11th to 20th	Night	38% 6% 11% 29% 14% 1	2160
	21st to end	Night	46% 5% 11% 22% 15% 1	2562
November	1st to 10th	Night	37% 6% 11% 22% 21% 4	2407
	11th to 20th	Night	41% 4 9% 24% 19% 4	2470
	21st to end	Night	40% 4 13% 29% 12% 1	2535
December	1st to 10th	Night	46% 5% 12% 22% 14% 1	2660
	11th to 20th	Night	45% 5% 13% 19% 16% 3	3288
	21st to end	Night	52% 4 11% 21% 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	36% 22% 26% 14% 2	180
	11th to 20th	Day	34% 28% 25% 11% 1	180
	21st to end	Day	27% 27% 33% 9% 3 1	236
February	1st to 10th	Day	35% 29% 28% 7% 1	298
	11th to 20th	Day	31% 26% 33% 9% 1	456
	21st to end	Day	34% 22% 31% 10% 3	382
March	1st to 10th	Day	31% 19% 35% 12% 3	460
	11th to 20th	Day	28% 26% 31% 11% 2 1	460
	21st to end	Day	33% 21% 30% 10% 4 1	470
April	1st to 10th	Day	31% 25% 25% 10% 6% 1	420
	11th to 20th	Day	28% 23% 28% 14% 5% 1	412
	21st to end	Day	32% 20% 31% 12% 4 1	420
May	1st to 10th	Day	35% 20% 28% 10% 5% 2	423
	11th to 20th	Day	24% 20% 33% 17% 5% 1	447
	21st to end	Day	21% 12% 29% 26% 11% 2	485
June	1st to 10th	Day	22% 12% 31% 23% 11% 1	450
	11th to 20th	Day	20% 14% 33% 22% 8% 2	450
	21st to end	Day	17% 12% 31% 27% 11% 2	450
July	1st to 10th	Day	16% 9% 32% 26% 14% 3	445
	11th to 20th	Day	13% 6% 33% 30% 15% 3	444
	21st to end	Day	14% 10% 36% 27% 11% 2	488
August	1st to 10th	Day	13% 10% 29% 31% 12% 5%	446
	11th to 20th	Day	16% 7% 30% 28% 14% 5%	439
	21st to end	Day	14% 10% 30% 24% 15% 6%	495
September	1st to 10th	Day	19% 12% 26% 22% 15% 6%	450
	11th to 20th	Day	15% 12% 29% 24% 12% 8%	450
	21st to end	Day	21% 15% 29% 21% 9% 5%	450
October	1st to 10th	Day	33% 9% 28% 14% 14% 3	450
	11th to 20th	Day	19% 21% 28% 22% 9% 1	450
	21st to end	Day	32% 13% 27% 19% 8%	401
November	1st to 10th	Day	22% 17% 26% 24% 10% 2	365
	11th to 20th	Day	31% 22% 26% 16% 4 1	370
	21st to end	Day	24% 23% 30% 18% 5%	304
December	1st to 10th	Day	31% 22% 31% 13% 4	180
	11th to 20th	Day	41% 22% 26% 10% 1	180
	21st to end	Day	31% 26% 25% 13% 6%	198
January	1st to 10th	Night	24% 19% 38% 16% 3	760
	11th to 20th	Night	27% 21% 31% 18% 2	760
	21st to end	Night	23% 23% 34% 17% 3	836
February	1st to 10th	Night	26% 20% 34% 18% 1	760
	11th to 20th	Night	25% 23% 37% 13% 1	743
	21st to end	Night	29% 23% 29% 17% 2	628
March	1st to 10th	Night	26% 22% 33% 17% 3	760
	11th to 20th	Night	23% 25% 33% 17% 2	760
	21st to end	Night	25% 22% 31% 17% 4	801
April	1st to 10th	Night	27% 22% 31% 18% 3	720
	11th to 20th	Night	23% 19% 33% 20% 5%	711
	21st to end	Night	24% 22% 32% 18% 4 1	648
May	1st to 10th	Night	28% 16% 31% 20% 5%	540
	11th to 20th	Night	22% 16% 36% 22% 3	404
	21st to end	Night	20% 8% 29% 33% 10% 1	389
June	1st to 10th	Night	20% 7% 31% 30% 11%	360
	11th to 20th	Night	19% 7% 29% 32% 12%	360
	21st to end	Night	16% 3 28% 43% 10%	360
July	1st to 10th	Night	16% 4 21% 37% 21%	350
	11th to 20th	Night	15% 5% 23% 41% 15% 1	348
	21st to end	Night	11% 6% 28% 39% 16%	394
August	1st to 10th	Night	11% 4 26% 42% 15% 1	414
	11th to 20th	Night	14% 4 23% 37% 19% 2	710
	21st to end	Night	16% 7% 21% 36% 18% 2	792
September	1st to 10th	Night	18% 7% 29% 32% 12% 3	720
	11th to 20th	Night	16% 9% 25% 30% 17% 3	720
	21st to end	Night	22% 8% 25% 28% 15% 2	720
October	1st to 10th	Night	25% 11% 26% 25% 12% 1	720
	11th to 20th	Night	18% 15% 30% 27% 8% 1	720
	21st to end	Night	28% 14% 30% 21% 7%	792
November	1st to 10th	Night	19% 15% 31% 26% 8% 1	740
	11th to 20th	Night	24% 18% 31% 19% 7% 1	760
	21st to end	Night	23% 18% 31% 22% 5% 1	760
December	1st to 10th	Night	29% 18% 31% 18% 4 1	760
	11th to 20th	Night	26% 21% 30% 21% 3	760
	21st to end	Night	29% 18% 34% 17% 2	836

Page generated at 2024-03-19 23:31:08. Version ac07

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