The Jewett Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Jewett 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	12% 9% 3 6% 5% 3 5% 5% 5% 7% 41%	77972
	Night	15% 10% 3 6% 5% 3 4 5% 5% 6% 37%	81042
Transparency	Day	43% 8% 13% 19% 13% 4	77699
	Night	46% 8% 13% 18% 11% 4	80895
Seeing	Day	15% 9% 22% 27% 20% 8%	13562
	Night	19% 6% 19% 33% 19% 3	23035

By Month

Percentage Cloud Cover NW (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% 19% 8% 10% 6% 4 5% 5% 5% 6% 16%	2539
	11th to 20th	Day	19% 15% 6% 9% 7% 4 5% 5% 5% 6% 20%	2516
	21st to end	Day	21% 18% 5% 9% 8% 4 4 4 4 5% 18%	2794
February	1st to 10th	Day	19% 15% 4 9% 5% 3 5% 5% 5% 7% 22%	1536
	11th to 20th	Day	17% 15% 4 8% 8% 4 6% 4 4 5% 25%	1752
	21st to end	Day	15% 17% 6% 7% 6% 4 5% 5% 6% 6% 25%	1480
March	1st to 10th	Day	18% 12% 5% 6% 5% 3 6% 6% 5% 7% 27%	1800
	11th to 20th	Day	15% 11% 5% 9% 7% 3 5% 6% 5% 9% 25%	1858
	21st to end	Day	18% 13% 5% 9% 8% 4 6% 5% 6% 6% 22%	2340
April	1st to 10th	Day	14% 11% 4 8% 6% 4 6% 6% 7% 8% 27%	2160
	11th to 20th	Day	12% 10% 5% 7% 6% 3 5% 5% 7% 7% 34%	2184
	21st to end	Day	13% 7% 4 7% 7% 4 6% 5% 6% 8% 34%	2340
May	1st to 10th	Day	9% 6% 3 5% 5% 3 5% 5% 6% 8% 44%	2421
	11th to 20th	Day	12% 7% 3 7% 7% 4 6% 6% 6% 9% 35%	2477
	21st to end	Day	12% 8% 3 5% 6% 4 5% 6% 6% 9% 36%	2723
June	1st to 10th	Day	13% 7% 3 7% 6% 3 5% 5% 5% 7% 39%	2596
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 6% 5% 7% 42%	2700
	21st to end	Day	6% 3 2 4 3 3 4 4 5% 6% 60%	2594
July	1st to 10th	Day	3 3 1 2 3 2 3 3 5% 5% 70%	2520
	11th to 20th	Day	3 2 1 3 2 2 3 3 4 5% 71%	2520
	21st to end	Day	1 1 1 2 2 1 2 3 2 5% 80%	2741
August	1st to 10th	Day	4 3 1 3 3 2 2 3 4 6% 71%	2507
	11th to 20th	Day	4 2 1 2 3 2 2 3 4 7% 69%	2339
	21st to end	Day	5% 4 1 3 3 1 4 4 5% 7% 63%	2413
September	1st to 10th	Day	4 2 1 2 3 2 3 3 5% 7% 69%	2160
	11th to 20th	Day	7% 5% 2 4 5% 3 4 5% 6% 7% 53%	2048
	21st to end	Day	8% 6% 2 5% 4 3 4 4 5% 8% 51%	1980
October	1st to 10th	Day	14% 7% 3 6% 5% 3 4 3 4 6% 46%	1931
	11th to 20th	Day	13% 6% 3 5% 4 3 5% 5% 5% 8% 44%	1697
	21st to end	Day	17% 9% 3 7% 5% 3 5% 5% 4 6% 36%	1782
November	1st to 10th	Day	24% 9% 3 6% 7% 3 5% 4 5% 7% 27%	1674
	11th to 20th	Day	20% 12% 5% 8% 7% 3 6% 5% 4 7% 23%	1668
	21st to end	Day	17% 14% 4 7% 7% 3 4 5% 6% 8% 25%	1463
December	1st to 10th	Day	20% 14% 5% 8% 8% 4 4 6% 5% 7% 19%	1330
	11th to 20th	Day	20% 16% 6% 9% 7% 4 6% 5% 3 4 21%	1682
	21st to end	Day	17% 13% 4 8% 7% 3 5% 5% 6% 8% 24%	2707
January	1st to 10th	Night	22% 16% 5% 9% 7% 4 5% 5% 5% 6% 17%	4406
	11th to 20th	Night	19% 17% 5% 7% 6% 3 4 4 5% 6% 24%	4382
	21st to end	Night	20% 14% 5% 7% 7% 4 5% 5% 6% 7% 21%	4862
February	1st to 10th	Night	19% 17% 5% 7% 6% 2 4 4 5% 5% 24%	2641
	11th to 20th	Night	16% 14% 4 6% 6% 3 4 5% 5% 5% 32%	2189
	21st to end	Night	16% 15% 5% 9% 6% 4 5% 4 5% 6% 26%	1776
March	1st to 10th	Night	17% 10% 4 7% 6% 4 5% 6% 5% 7% 29%	2160
	11th to 20th	Night	15% 10% 4 7% 6% 4 4 6% 5% 7% 33%	2101
	21st to end	Night	17% 10% 4 7% 7% 4 6% 5% 6% 7% 26%	2013
April	1st to 10th	Night	13% 10% 4 6% 6% 4 5% 7% 6% 8% 31%	1800
	11th to 20th	Night	14% 9% 4 7% 5% 3 4 4 5% 6% 38%	1726
	21st to end	Night	12% 7% 3 7% 6% 3 6% 6% 6% 8% 37%	1620
May	1st to 10th	Night	9% 6% 2 5% 7% 3 5% 5% 5% 6% 48%	1535
	11th to 20th	Night	10% 7% 3 6% 6% 3 5% 6% 6% 9% 40%	1416
	21st to end	Night	10% 6% 3 6% 6% 3 4 8% 6% 9% 39%	1552
June	1st to 10th	Night	11% 5% 3 6% 6% 3 5% 5% 7% 9% 41%	1360
	11th to 20th	Night	10% 5% 3 6% 7% 3 4 6% 6% 6% 44%	1260
	21st to end	Night	7% 3 3 4 4 3 5% 4 5% 6% 56%	1357
July	1st to 10th	Night	3 2 1 3 3 2 3 3 4 7% 69%	1440
	11th to 20th	Night	2 2 3 2 1 3 4 5% 6% 72%	1440
	21st to end	Night	1 2 1 2 3 2 3 2 3 5% 77%	1576
August	1st to 10th	Night	3 4 1 3 3 2 2 4 5% 6% 68%	1432
	11th to 20th	Night	3 2 1 3 3 3 3 3 4 5% 69%	1561
	21st to end	Night	5% 3 2 3 3 1 3 4 5% 5% 66%	1939
September	1st to 10th	Night	4 2 1 3 3 2 3 4 4 7% 67%	1800
	11th to 20th	Night	7% 3 2 4 5% 3 3 6% 5% 6% 56%	1912
	21st to end	Night	10% 4 2 4 4 3 4 4 6% 9% 51%	1980
October	1st to 10th	Night	11% 7% 3 4 5% 3 3 4 5% 6% 49%	2027
	11th to 20th	Night	14% 7% 2 6% 4 3 4 4 4 6% 45%	2262
	21st to end	Night	16% 8% 3 6% 6% 3 4 4 5% 5% 41%	2574
November	1st to 10th	Night	21% 10% 3 7% 6% 3 5% 5% 5% 6% 29%	2405
	11th to 20th	Night	23% 11% 4 7% 5% 3 4 4 4 6% 29%	2509
	21st to end	Night	19% 10% 4 6% 5% 3 4 4 6% 8% 31%	2693
December	1st to 10th	Night	17% 13% 4 6% 6% 4 6% 4 5% 7% 27%	2850
	11th to 20th	Night	20% 16% 5% 10% 7% 4 6% 5% 5% 6% 17%	3510
	21st to end	Night	19% 13% 4 7% 6% 3 5% 5% 5% 6% 26%	4976

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	66% 11% 14% 7% 1	2539
	11th to 20th	Day	62% 10% 12% 13% 2 1	2540
	21st to end	Day	69% 9% 12% 9% 1	2794
February	1st to 10th	Day	61% 11% 14% 10% 4	1536
	11th to 20th	Day	62% 7% 12% 14% 4	1762
	21st to end	Day	60% 9% 11% 15% 5% 1	1480
March	1st to 10th	Day	54% 11% 14% 15% 5%	1800
	11th to 20th	Day	56% 9% 15% 16% 4	1858
	21st to end	Day	63% 9% 14% 11% 3	2340
April	1st to 10th	Day	53% 10% 15% 17% 5%	2160
	11th to 20th	Day	48% 8% 14% 18% 11% 1	2184
	21st to end	Day	47% 8% 13% 22% 9% 1	2340
May	1st to 10th	Day	35% 7% 16% 29% 12% 1	2421
	11th to 20th	Day	43% 9% 16% 22% 11% 1	2480
	21st to end	Day	43% 9% 16% 23% 7% 1	2724
June	1st to 10th	Day	45% 7% 12% 23% 12%	2596
	11th to 20th	Day	38% 9% 14% 23% 12% 2	2700
	21st to end	Day	25% 5% 12% 30% 24% 4	2594
July	1st to 10th	Day	16% 4 10% 23% 30% 16%	2451
	11th to 20th	Day	15% 4 9% 17% 37% 18%	2445
	21st to end	Day	8% 2 6% 19% 44% 21%	2589
August	1st to 10th	Day	16% 4 8% 21% 36% 15%	2490
	11th to 20th	Day	16% 4 9% 25% 33% 12%	2339
	21st to end	Day	21% 5% 10% 28% 27% 8%	2413
September	1st to 10th	Day	16% 5% 11% 29% 27% 12%	2160
	11th to 20th	Day	29% 6% 14% 23% 22% 6%	2048
	21st to end	Day	31% 8% 17% 26% 14% 4	1980
October	1st to 10th	Day	39% 6% 13% 26% 14% 1	1931
	11th to 20th	Day	38% 7% 15% 28% 11% 1	1697
	21st to end	Day	48% 8% 15% 19% 8%	1782
November	1st to 10th	Day	57% 8% 16% 15% 4 1	1674
	11th to 20th	Day	61% 10% 15% 12% 2 1	1668
	21st to end	Day	58% 11% 14% 12% 4 1	1463
December	1st to 10th	Day	63% 10% 13% 11% 2 1	1330
	11th to 20th	Day	68% 11% 11% 8% 1	1684
	21st to end	Day	57% 11% 17% 12% 3	2707
January	1st to 10th	Night	66% 11% 11% 11% 1 1	4406
	11th to 20th	Night	60% 8% 15% 13% 4 1	4420
	21st to end	Night	61% 12% 14% 11% 2	4862
February	1st to 10th	Night	60% 8% 13% 13% 4 1	2641
	11th to 20th	Night	54% 7% 16% 17% 6%	2190
	21st to end	Night	58% 6% 13% 16% 6% 1	1776
March	1st to 10th	Night	53% 8% 13% 19% 7% 1	2160
	11th to 20th	Night	50% 8% 13% 22% 8%	2102
	21st to end	Night	56% 8% 15% 17% 4	2013
April	1st to 10th	Night	48% 8% 14% 22% 7%	1800
	11th to 20th	Night	45% 7% 12% 21% 13% 2	1726
	21st to end	Night	43% 7% 15% 19% 13% 3	1620
May	1st to 10th	Night	36% 7% 11% 27% 19% 1	1535
	11th to 20th	Night	37% 9% 13% 25% 14% 2	1416
	21st to end	Night	38% 10% 17% 26% 9% 1	1552
June	1st to 10th	Night	38% 9% 13% 24% 16% 1	1360
	11th to 20th	Night	38% 7% 10% 25% 16% 4	1260
	21st to end	Night	28% 5% 10% 24% 26% 7%	1357
July	1st to 10th	Night	15% 4 6% 21% 34% 20%	1400
	11th to 20th	Night	13% 4 9% 17% 30% 27%	1392
	21st to end	Night	12% 3 5% 13% 37% 31%	1488
August	1st to 10th	Night	17% 3 7% 16% 37% 21%	1424
	11th to 20th	Night	18% 4 7% 19% 33% 19%	1561
	21st to end	Night	19% 5% 9% 23% 31% 14%	1939
September	1st to 10th	Night	17% 4 10% 25% 27% 17%	1800
	11th to 20th	Night	27% 7% 9% 21% 26% 9%	1912
	21st to end	Night	30% 6% 14% 27% 16% 8%	1980
October	1st to 10th	Night	35% 6% 12% 28% 18% 1	2027
	11th to 20th	Night	40% 7% 11% 23% 17% 2	2262
	21st to end	Night	44% 7% 12% 22% 13% 1	2574
November	1st to 10th	Night	54% 6% 15% 19% 5% 1	2405
	11th to 20th	Night	57% 7% 14% 16% 5% 1	2509
	21st to end	Night	53% 8% 16% 18% 5%	2693
December	1st to 10th	Night	56% 10% 15% 15% 4 1	2850
	11th to 20th	Night	67% 10% 12% 11% 1	3507
	21st to end	Night	56% 11% 16% 13% 5%	4976

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	25% 16% 24% 17% 13% 6%	180
	11th to 20th	Day	28% 8% 27% 21% 9% 7%	180
	21st to end	Day	29% 12% 30% 22% 7% 1	198
February	1st to 10th	Day	26% 16% 34% 18% 6% 1	196
	11th to 20th	Day	30% 14% 20% 23% 10% 2	414
	21st to end	Day	20% 14% 26% 28% 9% 2	346
March	1st to 10th	Day	21% 13% 26% 24% 13% 2	420
	11th to 20th	Day	18% 11% 20% 28% 18% 4	420
	21st to end	Day	23% 13% 27% 24% 12% 2	462
April	1st to 10th	Day	22% 11% 23% 27% 13% 5%	420
	11th to 20th	Day	17% 13% 31% 23% 12% 5%	412
	21st to end	Day	13% 12% 27% 30% 14% 5%	420
May	1st to 10th	Day	12% 9% 23% 31% 22% 4	423
	11th to 20th	Day	18% 6% 20% 24% 19% 13%	448
	21st to end	Day	12% 8% 18% 24% 25% 13%	485
June	1st to 10th	Day	14% 8% 18% 28% 21% 11%	526
	11th to 20th	Day	14% 11% 27% 25% 18% 6%	630
	21st to end	Day	7% 4 19% 29% 31% 10%	524
July	1st to 10th	Day	5% 4 14% 37% 31% 10%	446
	11th to 20th	Day	4 3 19% 35% 29% 10%	444
	21st to end	Day	1 2 19% 33% 33% 12%	487
August	1st to 10th	Day	5% 4 18% 28% 33% 13%	447
	11th to 20th	Day	4 2 13% 28% 35% 18%	437
	21st to end	Day	8% 5% 19% 27% 26% 15%	495
September	1st to 10th	Day	5% 4 22% 28% 29% 12%	450
	11th to 20th	Day	10% 7% 19% 24% 28% 12%	450
	21st to end	Day	10% 9% 22% 31% 21% 7%	450
October	1st to 10th	Day	16% 8% 20% 30% 18% 7%	450
	11th to 20th	Day	15% 10% 22% 29% 16% 8%	347
	21st to end	Day	25% 8% 27% 23% 12% 4	297
November	1st to 10th	Day	30% 7% 27% 29% 7%	270
	11th to 20th	Day	23% 15% 25% 18% 15% 4	250
	21st to end	Day	22% 9% 26% 29% 12% 2	180
December	1st to 10th	Day	28% 13% 23% 20% 11% 6%	180
	11th to 20th	Day	31% 18% 23% 19% 6% 3	180
	21st to end	Day	30% 13% 25% 22% 8% 2	198
January	1st to 10th	Night	32% 10% 21% 24% 11% 2	760
	11th to 20th	Night	28% 8% 19% 28% 14% 4	760
	21st to end	Night	25% 6% 21% 33% 14% 2	836
February	1st to 10th	Night	27% 8% 23% 32% 9% 1	760
	11th to 20th	Night	25% 7% 21% 30% 14% 2	722
	21st to end	Night	25% 8% 20% 32% 12% 3	592
March	1st to 10th	Night	21% 8% 23% 32% 14% 3	720
	11th to 20th	Night	17% 10% 22% 30% 21% 1	720
	21st to end	Night	21% 8% 25% 33% 11% 2	792
April	1st to 10th	Night	18% 6% 22% 36% 16% 2	720
	11th to 20th	Night	16% 7% 26% 33% 15% 3	658
	21st to end	Night	15% 5% 26% 33% 19% 3	540
May	1st to 10th	Night	11% 6% 24% 33% 22% 4	455
	11th to 20th	Night	14% 3 13% 33% 29% 8%	354
	21st to end	Night	12% 5% 16% 33% 28% 5%	389
June	1st to 10th	Night	12% 4 22% 34% 23% 6%	360
	11th to 20th	Night	12% 6% 26% 34% 19% 3	360
	21st to end	Night	9% 4 13% 41% 28% 4	360
July	1st to 10th	Night	3 3 11% 42% 36% 5%	350
	11th to 20th	Night	2 2 17% 47% 29% 3	348
	21st to end	Night	1 2 12% 41% 38% 6%	394
August	1st to 10th	Night	6% 3 9% 38% 37% 7%	358
	11th to 20th	Night	3 1 10% 34% 43% 9%	493
	21st to end	Night	5% 3 17% 37% 34% 4	751
September	1st to 10th	Night	5% 2 17% 41% 29% 7%	720
	11th to 20th	Night	7% 4 13% 33% 35% 8%	720
	21st to end	Night	13% 2 16% 37% 27% 6%	720
October	1st to 10th	Night	15% 4 15% 41% 24% 2	720
	11th to 20th	Night	18% 7% 14% 31% 26% 4	720
	21st to end	Night	19% 7% 21% 34% 15% 5%	792
November	1st to 10th	Night	24% 8% 19% 37% 11% 1	740
	11th to 20th	Night	26% 9% 21% 29% 12% 2	760
	21st to end	Night	24% 7% 19% 31% 15% 3	793
December	1st to 10th	Night	25% 8% 19% 33% 15% 2	950
	11th to 20th	Night	31% 7% 21% 31% 9% 1	950
	21st to end	Night	34% 11% 20% 25% 8% 1	898

Page generated at 2024-02-12 03:35:30. Version ac07

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