Maynard F. Jordan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maynard F. Jordan Observatory clear sky chart from 2005-11-06 to 2024-02-15.

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	19% 8% 3 6% 5% 3 4 4 5% 7% 37%	77953
	Night	18% 8% 3 6% 5% 3 4 5% 5% 6% 36%	81121
Transparency	Day	48% 3 11% 23% 13% 2	77658
	Night	49% 3 11% 22% 13% 2	80951
Seeing	Day	25% 15% 31% 20% 8% 2	14729
	Night	23% 12% 31% 26% 7%	23209

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	16% 10% 3 6% 5% 3 5% 4 5% 5% 38%	2495
	11th to 20th	Day	17% 7% 3 8% 6% 3 4 2 3 5% 41%	2520
	21st to end	Day	15% 10% 4 6% 5% 4 4 5% 5% 6% 35%	2808
February	1st to 10th	Day	17% 10% 4 6% 5% 3 5% 5% 6% 7% 33%	1710
	11th to 20th	Day	14% 11% 4 7% 6% 3 4 4 5% 7% 34%	1694
	21st to end	Day	14% 11% 4 7% 7% 4 5% 4 5% 6% 33%	1480
March	1st to 10th	Day	19% 9% 3 7% 5% 3 5% 3 4 5% 35%	1809
	11th to 20th	Day	17% 10% 4 6% 5% 4 4 4 4 6% 38%	1980
	21st to end	Day	19% 9% 3 5% 5% 3 4 4 5% 6% 36%	2178
April	1st to 10th	Day	22% 9% 5% 7% 6% 3 4 4 4 5% 31%	2136
	11th to 20th	Day	19% 8% 3 5% 5% 2 5% 4 4 6% 38%	2298
	21st to end	Day	25% 10% 3 6% 5% 2 3 3 4 5% 33%	2340
May	1st to 10th	Day	23% 7% 3 6% 5% 3 5% 4 4 7% 33%	2340
	11th to 20th	Day	22% 7% 3 5% 5% 3 4 4 4 6% 36%	2301
	21st to end	Day	19% 7% 2 6% 5% 3 5% 5% 5% 7% 36%	2820
June	1st to 10th	Day	24% 7% 3 7% 6% 3 4 4 5% 8% 29%	2700
	11th to 20th	Day	20% 6% 3 6% 5% 3 4 4 6% 7% 37%	2700
	21st to end	Day	25% 7% 3 6% 5% 3 4 5% 6% 8% 29%	2700
July	1st to 10th	Day	16% 5% 3 5% 5% 4 6% 5% 6% 8% 38%	2636
	11th to 20th	Day	15% 6% 3 5% 5% 4 4 5% 6% 8% 38%	2520
	21st to end	Day	15% 6% 3 5% 6% 4 5% 6% 6% 8% 35%	2712
August	1st to 10th	Day	16% 6% 2 5% 6% 4 5% 4 6% 8% 38%	2327
	11th to 20th	Day	15% 5% 3 5% 7% 4 5% 4 7% 9% 36%	2304
	21st to end	Day	14% 5% 3 5% 6% 3 4 4 4 8% 45%	2574
September	1st to 10th	Day	12% 5% 2 4 5% 3 4 5% 5% 8% 48%	2076
	11th to 20th	Day	15% 5% 2 4 4 2 4 4 4 4 51%	1980
	21st to end	Day	17% 7% 2 5% 4 2 4 3 5% 8% 43%	1980
October	1st to 10th	Day	18% 6% 2 5% 5% 3 4 4 4 6% 41%	1923
	11th to 20th	Day	19% 7% 3 5% 5% 2 4 4 4 7% 40%	1787
	21st to end	Day	24% 9% 3 5% 4 4 4 3 4 6% 33%	1782
November	1st to 10th	Day	20% 5% 3 5% 5% 2 5% 5% 5% 6% 39%	1671
	11th to 20th	Day	20% 6% 2 5% 5% 3 4 5% 5% 6% 39%	1649
	21st to end	Day	22% 7% 3 4 4 3 3 4 5% 5% 39%	1520
December	1st to 10th	Day	23% 10% 5% 6% 6% 3 3 3 3 6% 31%	1404
	11th to 20th	Day	23% 10% 2 6% 4 3 4 4 4 4 36%	1668
	21st to end	Day	25% 10% 3 5% 5% 3 4 3 4 5% 32%	2431
January	1st to 10th	Night	14% 10% 4 8% 6% 3 4 4 4 6% 37%	4448
	11th to 20th	Night	14% 9% 4 7% 7% 3 5% 5% 5% 6% 34%	4440
	21st to end	Night	15% 11% 4 8% 5% 4 4 6% 5% 6% 31%	4846
February	1st to 10th	Night	15% 10% 4 7% 6% 3 5% 5% 6% 6% 31%	2470
	11th to 20th	Night	13% 11% 5% 7% 6% 3 5% 4 4 5% 37%	2243
	21st to end	Night	15% 11% 4 7% 6% 3 5% 4 5% 4 35%	1776
March	1st to 10th	Night	16% 12% 4 7% 5% 3 4 4 4 6% 36%	2137
	11th to 20th	Night	17% 10% 4 6% 6% 3 5% 5% 5% 6% 34%	1979
	21st to end	Night	18% 9% 3 7% 5% 3 4 4 4 7% 36%	2178
April	1st to 10th	Night	24% 10% 3 6% 4 3 3 4 4 6% 33%	1822
	11th to 20th	Night	20% 6% 3 5% 6% 4 4 5% 5% 7% 34%	1611
	21st to end	Night	26% 9% 3 5% 4 3 4 4 6% 6% 30%	1620
May	1st to 10th	Night	23% 8% 3 5% 4 2 5% 5% 6% 7% 32%	1620
	11th to 20th	Night	22% 6% 3 7% 5% 3 6% 5% 6% 6% 31%	1593
	21st to end	Night	17% 8% 3 5% 6% 3 5% 4 6% 6% 35%	1466
June	1st to 10th	Night	23% 7% 3 8% 5% 3 4 5% 6% 6% 28%	1260
	11th to 20th	Night	17% 6% 3 5% 5% 2 5% 5% 7% 9% 36%	1260
	21st to end	Night	24% 7% 3 6% 5% 3 3 4 7% 7% 31%	1260
July	1st to 10th	Night	13% 6% 3 5% 5% 4 6% 5% 7% 10% 36%	1320
	11th to 20th	Night	14% 7% 4 5% 5% 4 4 5% 7% 9% 37%	1440
	21st to end	Night	17% 6% 3 6% 6% 3 6% 5% 7% 8% 35%	1587
August	1st to 10th	Night	14% 5% 2 5% 6% 3 5% 5% 6% 8% 41%	1612
	11th to 20th	Night	15% 6% 3 6% 6% 4 4 6% 7% 8% 35%	1596
	21st to end	Night	14% 5% 2 5% 4 3 4 4 4 7% 48%	1782
September	1st to 10th	Night	14% 5% 2 4 3 2 4 5% 6% 8% 49%	1880
	11th to 20th	Night	17% 5% 2 4 3 2 3 3 4 7% 50%	1980
	21st to end	Night	18% 5% 2 4 4 3 4 4 5% 7% 44%	1980
October	1st to 10th	Night	19% 6% 1 5% 4 3 5% 5% 5% 6% 41%	2036
	11th to 20th	Night	18% 7% 3 6% 5% 3 4 5% 5% 7% 39%	2168
	21st to end	Night	25% 9% 3 5% 5% 2 3 4 4 6% 34%	2574
November	1st to 10th	Night	18% 6% 3 5% 5% 3 4 4 4 6% 41%	2407
	11th to 20th	Night	19% 6% 3 5% 5% 3 5% 5% 5% 7% 38%	2529
	21st to end	Night	19% 8% 3 6% 4 2 4 5% 5% 7% 37%	2660
December	1st to 10th	Night	24% 9% 4 6% 4 3 4 4 4 6% 31%	2774
	11th to 20th	Night	24% 8% 3 5% 4 3 4 4 4 5% 37%	3531
	21st to end	Night	25% 10% 3 5% 4 2 4 4 4 5% 34%	5236

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	49% 2 15% 25% 9% 1	2496
	11th to 20th	Day	48% 2 9% 26% 12% 2	2520
	21st to end	Day	50% 2 14% 19% 13% 1	2808
February	1st to 10th	Day	51% 3 11% 20% 13% 2	1710
	11th to 20th	Day	51% 3 10% 21% 13% 1	1702
	21st to end	Day	52% 3 10% 20% 11% 5%	1480
March	1st to 10th	Day	53% 3 10% 18% 14% 3	1813
	11th to 20th	Day	49% 2 8% 20% 17% 3	1980
	21st to end	Day	50% 2 8% 19% 16% 4	2178
April	1st to 10th	Day	57% 3 7% 16% 14% 4	2136
	11th to 20th	Day	49% 3 9% 23% 15% 2	2298
	21st to end	Day	54% 3 7% 18% 14% 4	2340
May	1st to 10th	Day	52% 3 7% 20% 15% 4	2340
	11th to 20th	Day	51% 3 8% 21% 16% 3	2305
	21st to end	Day	47% 4 14% 21% 12% 2	2820
June	1st to 10th	Day	55% 3 11% 20% 10% 1	2700
	11th to 20th	Day	46% 4 10% 21% 17% 2	2700
	21st to end	Day	54% 4 11% 19% 10% 2	2700
July	1st to 10th	Day	43% 5% 14% 25% 13% 1	2564
	11th to 20th	Day	43% 4 13% 27% 12% 1	2445
	21st to end	Day	46% 4 13% 25% 11% 1	2560
August	1st to 10th	Day	44% 4 16% 28% 8%	2314
	11th to 20th	Day	44% 5% 12% 26% 13%	2304
	21st to end	Day	39% 4 11% 27% 17% 2	2574
September	1st to 10th	Day	35% 4 13% 29% 17% 1	2076
	11th to 20th	Day	37% 3 8% 29% 21% 3	1980
	21st to end	Day	41% 3 9% 28% 18% 1	1980
October	1st to 10th	Day	45% 3 9% 26% 15% 1	1923
	11th to 20th	Day	47% 2 13% 28% 9% 1	1787
	21st to end	Day	54% 3 9% 21% 12% 1	1782
November	1st to 10th	Day	46% 3 13% 22% 14% 1	1671
	11th to 20th	Day	46% 4 10% 22% 14% 3	1649
	21st to end	Day	48% 4 15% 24% 8% 1	1520
December	1st to 10th	Day	57% 3 9% 20% 10% 1	1404
	11th to 20th	Day	54% 3 9% 21% 12% 2	1668
	21st to end	Day	56% 2 11% 20% 8% 3	2431
January	1st to 10th	Night	51% 2 12% 24% 9% 1	4448
	11th to 20th	Night	52% 3 12% 20% 11% 2	4440
	21st to end	Night	54% 3 10% 21% 11% 1	4846
February	1st to 10th	Night	52% 3 11% 20% 12% 2	2470
	11th to 20th	Night	51% 3 9% 21% 14% 2	2246
	21st to end	Night	52% 3 7% 20% 15% 3	1776
March	1st to 10th	Night	52% 2 7% 22% 15% 3	2146
	11th to 20th	Night	51% 4 10% 17% 16% 3	1980
	21st to end	Night	50% 3 7% 21% 17% 3	2178
April	1st to 10th	Night	55% 3 7% 16% 16% 3	1822
	11th to 20th	Night	49% 4 7% 23% 15% 2	1611
	21st to end	Night	55% 3 10% 16% 13% 3	1620
May	1st to 10th	Night	51% 3 8% 20% 16% 2	1620
	11th to 20th	Night	53% 4 9% 21% 11% 2	1594
	21st to end	Night	48% 5% 13% 21% 12% 1	1467
June	1st to 10th	Night	56% 4 11% 20% 8% 1	1260
	11th to 20th	Night	44% 3 16% 22% 12% 2	1260
	21st to end	Night	52% 4 13% 20% 9% 2	1260
July	1st to 10th	Night	42% 6% 17% 24% 11%	1282
	11th to 20th	Night	43% 4 15% 26% 12%	1392
	21st to end	Night	46% 5% 17% 22% 10% 1	1499
August	1st to 10th	Night	42% 5% 17% 27% 9%	1600
	11th to 20th	Night	45% 6% 15% 24% 11%	1596
	21st to end	Night	37% 4 12% 27% 18% 1	1782
September	1st to 10th	Night	35% 5% 14% 32% 14% 1	1880
	11th to 20th	Night	37% 3 11% 28% 19% 2	1980
	21st to end	Night	40% 3 9% 32% 15% 1	1980
October	1st to 10th	Night	44% 4 10% 27% 14% 1	2036
	11th to 20th	Night	46% 3 12% 27% 10% 1	2168
	21st to end	Night	53% 3 10% 21% 12% 1	2574
November	1st to 10th	Night	46% 3 11% 25% 14% 1	2407
	11th to 20th	Night	46% 3 9% 23% 16% 2	2529
	21st to end	Night	47% 5% 14% 20% 13% 1	2660
December	1st to 10th	Night	56% 3 9% 21% 11%	2774
	11th to 20th	Night	51% 3 8% 21% 16% 1	3532
	21st to end	Night	55% 2 11% 19% 11% 2	5236

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	31% 14% 35% 17% 2 1	180
	11th to 20th	Day	28% 22% 35% 11% 3 1	180
	21st to end	Day	22% 16% 38% 19% 5%	221
February	1st to 10th	Day	31% 15% 27% 24% 3	370
	11th to 20th	Day	27% 24% 37% 10% 2	386
	21st to end	Day	23% 21% 32% 21% 3	346
March	1st to 10th	Day	30% 19% 32% 16% 2	420
	11th to 20th	Day	26% 21% 33% 14% 6% 1	420
	21st to end	Day	30% 21% 26% 14% 6% 3	462
April	1st to 10th	Day	36% 22% 24% 10% 5% 2	420
	11th to 20th	Day	32% 18% 31% 12% 7% 1	412
	21st to end	Day	34% 15% 29% 15% 5% 2	420
May	1st to 10th	Day	32% 15% 33% 15% 4 1	423
	11th to 20th	Day	28% 16% 34% 17% 5% 1	447
	21st to end	Day	26% 16% 32% 18% 7%	597
June	1st to 10th	Day	30% 12% 30% 19% 8% 1	630
	11th to 20th	Day	23% 14% 27% 24% 11% 1	630
	21st to end	Day	27% 10% 27% 25% 10% 1	630
July	1st to 10th	Day	19% 11% 34% 24% 11% 2	559
	11th to 20th	Day	20% 9% 37% 21% 10% 2	444
	21st to end	Day	16% 8% 36% 27% 10% 2	488
August	1st to 10th	Day	16% 9% 33% 29% 10% 2	446
	11th to 20th	Day	19% 8% 30% 28% 11% 4	439
	21st to end	Day	18% 11% 29% 25% 14% 3	495
September	1st to 10th	Day	15% 12% 33% 25% 12% 3	450
	11th to 20th	Day	22% 8% 31% 20% 14% 6%	450
	21st to end	Day	19% 15% 30% 20% 11% 6%	450
October	1st to 10th	Day	21% 16% 24% 21% 14% 4	421
	11th to 20th	Day	23% 17% 28% 20% 10% 2	360
	21st to end	Day	31% 15% 28% 19% 6% 1	396
November	1st to 10th	Day	25% 14% 26% 24% 8% 2	365
	11th to 20th	Day	30% 14% 32% 17% 6%	370
	21st to end	Day	30% 17% 26% 21% 5% 1	370
December	1st to 10th	Day	30% 18% 34% 15% 4	254
	11th to 20th	Day	35% 22% 29% 12% 1 1	180
	21st to end	Day	34% 19% 26% 15% 5% 2	198
January	1st to 10th	Night	23% 18% 30% 25% 4	763
	11th to 20th	Night	27% 16% 31% 22% 4	760
	21st to end	Night	19% 18% 34% 24% 4	836
February	1st to 10th	Night	22% 17% 34% 24% 3	760
	11th to 20th	Night	21% 20% 36% 20% 3 1	722
	21st to end	Night	23% 16% 35% 23% 3	592
March	1st to 10th	Night	26% 15% 34% 23% 2	720
	11th to 20th	Night	24% 17% 33% 21% 5%	720
	21st to end	Night	24% 15% 32% 22% 7% 1	792
April	1st to 10th	Night	31% 16% 30% 19% 3	720
	11th to 20th	Night	24% 15% 36% 20% 5%	547
	21st to end	Night	32% 12% 30% 21% 5%	540
May	1st to 10th	Night	26% 11% 30% 28% 4	540
	11th to 20th	Night	25% 11% 37% 23% 5%	532
	21st to end	Night	20% 7% 37% 27% 7%	420
June	1st to 10th	Night	25% 7% 33% 28% 7%	360
	11th to 20th	Night	19% 7% 31% 33% 9%	360
	21st to end	Night	27% 4 26% 38% 6%	360
July	1st to 10th	Night	17% 4 34% 36% 9%	350
	11th to 20th	Night	18% 2 26% 43% 11%	348
	21st to end	Night	18% 4 25% 41% 12%	407
August	1st to 10th	Night	16% 5% 30% 38% 11%	538
	11th to 20th	Night	16% 6% 28% 36% 12% 1	532
	21st to end	Night	15% 6% 26% 39% 12% 1	594
September	1st to 10th	Night	17% 6% 30% 34% 13% 1	710
	11th to 20th	Night	19% 8% 28% 28% 14% 3	720
	21st to end	Night	19% 8% 29% 30% 12% 1	720
October	1st to 10th	Night	22% 7% 25% 31% 14% 1	720
	11th to 20th	Night	22% 10% 30% 28% 9% 1	720
	21st to end	Night	31% 14% 27% 22% 5%	792
November	1st to 10th	Night	24% 11% 30% 26% 8% 1	740
	11th to 20th	Night	25% 14% 30% 23% 8% 1	789
	21st to end	Night	26% 17% 28% 24% 5%	850
December	1st to 10th	Night	29% 11% 33% 21% 5%	850
	11th to 20th	Night	26% 18% 31% 22% 4	850
	21st to end	Night	28% 16% 31% 19% 5%	935

Page generated at 2024-02-12 06:35:32. Version ac07

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