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 2025-03-27.

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	10% 8% 3 6% 4 4 4 3 5% 7% 45%	112328
	Night	12% 10% 3 7% 4 4 4 4 5% 6% 41%	115655
Transparency	Day	21% 45% 7% 13% 9% 5%	112095
	Night	25% 43% 7% 13% 8% 4	115534
Seeing	Day	15% 9% 22% 27% 20% 7%	15000
	Night	18% 6% 19% 34% 19% 3	24457

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	19% 22% 8% 10% 4 4 5% 3 5% 6% 15%	3639
	11th to 20th	Day	14% 12% 4 9% 6% 4 5% 4 5% 6% 31%	3616
	21st to end	Day	15% 14% 3 9% 5% 4 3 3 4 5% 33%	4004
February	1st to 10th	Day	18% 16% 4 9% 5% 4 5% 4 5% 7% 21%	1617
	11th to 20th	Day	17% 17% 4 9% 7% 5% 5% 3 5% 5% 23%	1909
	21st to end	Day	14% 17% 5% 8% 6% 5% 5% 4 6% 5% 25%	1650
March	1st to 10th	Day	11% 12% 4 8% 5% 5% 6% 5% 7% 9% 29%	3238
	11th to 20th	Day	9% 8% 3 7% 5% 4 4 3 5% 7% 44%	3393
	21st to end	Day	14% 15% 4 11% 7% 6% 6% 4 7% 7% 20%	4175
April	1st to 10th	Day	14% 12% 4 9% 5% 5% 6% 5% 7% 7% 27%	2280
	11th to 20th	Day	11% 11% 4 8% 5% 5% 5% 4 7% 7% 35%	2307
	21st to end	Day	12% 8% 3 8% 6% 6% 6% 4 6% 8% 33%	2470
May	1st to 10th	Day	10% 7% 2 5% 4 4 5% 4 6% 8% 44%	4256
	11th to 20th	Day	7% 5% 2 5% 4 4 4 3 5% 8% 52%	4377
	21st to end	Day	9% 8% 3 8% 6% 6% 6% 5% 6% 9% 35%	4813
June	1st to 10th	Day	13% 8% 3 7% 6% 4 4 4 5% 7% 39%	2741
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 7% 42%	2850
	21st to end	Day	6% 3 2 4 3 4 4 4 5% 6% 59%	2738
July	1st to 10th	Day	1 2 2 2 2 2 2 4 3 80%	4420
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	4420
	21st to end	Day	1 1 1 2 2 2 3 3 3 6% 76%	4790
August	1st to 10th	Day	3 3 1 3 2 2 3 2 4 6% 70%	4401
	11th to 20th	Day	3 2 2 3 3 2 3 3 4 7% 67%	4092
	21st to end	Day	3 3 1 3 2 2 3 3 4 6% 68%	4243
September	1st to 10th	Day	4 2 1 3 3 2 2 3 5% 7% 68%	2280
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 6% 7% 51%	2161
	21st to end	Day	8% 6% 2 6% 3 4 4 3 5% 8% 52%	2090
October	1st to 10th	Day	9% 5% 2 5% 3 3 3 3 4 7% 58%	3402
	11th to 20th	Day	9% 6% 3 6% 5% 5% 5% 4 6% 8% 45%	2975
	21st to end	Day	15% 10% 3 7% 4 4 5% 4 5% 6% 37%	3101
November	1st to 10th	Day	23% 10% 3 7% 7% 4 5% 3 5% 7% 27%	1764
	11th to 20th	Day	19% 14% 5% 9% 6% 5% 5% 3 4 7% 22%	1755
	21st to end	Day	17% 16% 4 9% 5% 4 4 4 6% 8% 24%	1540
December	1st to 10th	Day	14% 11% 4 8% 5% 5% 3 4 6% 8% 31%	2290
	11th to 20th	Day	20% 18% 5% 10% 6% 5% 6% 4 4 4 19%	2642
	21st to end	Day	20% 15% 4 9% 6% 5% 4 4 5% 7% 20%	3889
January	1st to 10th	Night	22% 19% 6% 11% 5% 5% 4 4 5% 6% 13%	6306
	11th to 20th	Night	14% 13% 4 8% 5% 4 4 4 5% 6% 33%	6282
	21st to end	Night	15% 13% 3 7% 4 4 4 3 5% 5% 37%	6952
February	1st to 10th	Night	19% 19% 5% 8% 5% 4 4 3 5% 5% 23%	2780
	11th to 20th	Night	16% 15% 4 7% 5% 4 4 4 5% 5% 31%	2378
	21st to end	Night	15% 16% 4 10% 5% 5% 4 3 5% 6% 26%	1980
March	1st to 10th	Night	10% 11% 3 8% 6% 4 5% 4 6% 7% 36%	3856
	11th to 20th	Night	10% 9% 3 6% 4 4 3 4 4 5% 48%	3806
	21st to end	Night	13% 11% 4 10% 7% 7% 6% 5% 7% 8% 22%	3600
April	1st to 10th	Night	13% 12% 3 7% 5% 5% 5% 5% 6% 8% 31%	1900
	11th to 20th	Night	13% 10% 3 7% 5% 5% 4 3 5% 7% 39%	1822
	21st to end	Night	12% 8% 3 8% 6% 4 6% 4 6% 7% 37%	1710
May	1st to 10th	Night	11% 5% 2 6% 6% 4 5% 4 6% 7% 44%	2705
	11th to 20th	Night	7% 6% 2 6% 4 3 4 4 6% 9% 49%	2516
	21st to end	Night	13% 6% 2 7% 5% 5% 5% 5% 6% 8% 37%	2762
June	1st to 10th	Night	11% 5% 2 7% 4 4 5% 4 7% 9% 41%	1435
	11th to 20th	Night	10% 6% 3 7% 5% 5% 3 5% 6% 6% 45%	1330
	21st to end	Night	6% 4 2 5% 4 4 4 4 5% 6% 56%	1433
July	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 1 2 1 2 2 2 3 4 81%	2540
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 76%	2762
August	1st to 10th	Night	4 3 1 4 2 2 3 3 5% 5% 67%	2529
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 5% 66%	2744
	21st to end	Night	4 3 1 4 3 3 4 3 5% 5% 65%	3429
September	1st to 10th	Night	4 2 1 3 2 2 4 3 5% 7% 67%	1900
	11th to 20th	Night	7% 4 1 5% 4 4 4 5% 5% 6% 54%	2019
	21st to end	Night	9% 4 2 5% 3 3 4 3 6% 8% 52%	2090
October	1st to 10th	Night	6% 4 2 3 3 3 4 3 5% 6% 60%	3576
	11th to 20th	Night	10% 5% 2 6% 4 5% 5% 4 5% 8% 47%	3956
	21st to end	Night	13% 6% 3 7% 5% 4 4 3 5% 6% 43%	4457
November	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 4 5% 6% 30%	2535
	11th to 20th	Night	22% 12% 4 8% 4 4 4 3 5% 6% 29%	2642
	21st to end	Night	18% 12% 4 7% 5% 4 4 4 7% 7% 29%	2835
December	1st to 10th	Night	11% 10% 3 5% 4 4 4 3 6% 7% 43%	4890
	11th to 20th	Night	21% 18% 4 11% 6% 6% 5% 4 5% 7% 14%	5550
	21st to end	Night	20% 16% 5% 9% 5% 4 5% 4 4 5% 22%	7108

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	53% 30% 10% 6%	3639
	11th to 20th	Day	35% 31% 12% 18% 4	3640
	21st to end	Day	38% 30% 6% 17% 9%	4004
February	1st to 10th	Day	28% 63% 4 3 2	1617
	11th to 20th	Day	28% 61% 4 6% 1	1919
	21st to end	Day	23% 64% 4 6% 2 1	1650
March	1st to 10th	Day	27% 43% 7% 16% 6%	3238
	11th to 20th	Day	16% 42% 10% 27% 5%	3393
	21st to end	Day	36% 47% 8% 5% 4	4175
April	1st to 10th	Day	18% 66% 8% 6% 2	2280
	11th to 20th	Day	17% 67% 5% 6% 4 1	2307
	21st to end	Day	17% 66% 3 9% 5%	2470
May	1st to 10th	Day	20% 43% 9% 19% 8% 1	4256
	11th to 20th	Day	12% 41% 9% 25% 10% 2	4380
	21st to end	Day	21% 48% 14% 15% 2	4814
June	1st to 10th	Day	11% 65% 6% 11% 7% 1	2741
	11th to 20th	Day	12% 66% 4 10% 7% 2	2850
	21st to end	Day	9% 64% 4 10% 12% 1	2738
July	1st to 10th	Day	4 35% 3 8% 18% 32%	4351
	11th to 20th	Day	5% 35% 2 4 16% 38%	4345
	21st to end	Day	8% 35% 4 12% 21% 21%	4639
August	1st to 10th	Day	11% 37% 2 6% 28% 17%	4384
	11th to 20th	Day	13% 37% 4 29% 14% 3	4132
	21st to end	Day	12% 37% 7% 21% 19% 5%	4242
September	1st to 10th	Day	8% 61% 5% 14% 10% 3	2280
	11th to 20th	Day	16% 61% 7% 8% 7% 2	2161
	21st to end	Day	14% 61% 7% 12% 5% 1	2090
October	1st to 10th	Day	15% 37% 7% 23% 16% 2	3402
	11th to 20th	Day	25% 38% 12% 17% 9%	2975
	21st to end	Day	30% 38% 5% 19% 6% 1	3101
November	1st to 10th	Day	26% 58% 7% 8% 1	1764
	11th to 20th	Day	26% 60% 6% 5% 1 1	1755
	21st to end	Day	24% 60% 6% 7% 3 1	1540
December	1st to 10th	Day	33% 39% 13% 14% 2	2290
	11th to 20th	Day	48% 34% 9% 8% 1	2644
	21st to end	Day	45% 31% 14% 8% 2	3889
January	1st to 10th	Night	53% 31% 8% 6%	6306
	11th to 20th	Night	34% 30% 14% 15% 6% 1	6320
	21st to end	Night	33% 32% 8% 18% 9%	6952
February	1st to 10th	Night	28% 63% 4 3 2	2780
	11th to 20th	Night	24% 61% 5% 6% 3	2379
	21st to end	Night	23% 62% 5% 6% 3 1	1980
March	1st to 10th	Night	24% 41% 5% 19% 9% 1	3856
	11th to 20th	Night	17% 40% 8% 26% 8%	3807
	21st to end	Night	31% 47% 9% 10% 2	3600
April	1st to 10th	Night	17% 66% 7% 8% 3	1900
	11th to 20th	Night	14% 66% 5% 8% 5% 2	1822
	21st to end	Night	15% 66% 3 8% 6% 1	1710
May	1st to 10th	Night	21% 43% 9% 16% 9% 2	2705
	11th to 20th	Night	14% 44% 8% 19% 13% 2	2516
	21st to end	Night	24% 47% 13% 13% 4	2762
June	1st to 10th	Night	9% 65% 4 11% 9% 1	1435
	11th to 20th	Night	11% 65% 4 9% 9% 2	1330
	21st to end	Night	10% 64% 4 8% 12% 2	1433
July	1st to 10th	Night	4 34% 1 7% 17% 35%	2500
	11th to 20th	Night	4 35% 2 4 10% 44%	2492
	21st to end	Night	9% 34% 1 10% 18% 28%	2674
August	1st to 10th	Night	13% 36% 2 5% 25% 20%	2521
	11th to 20th	Night	13% 37% 3 24% 19% 4	2770
	21st to end	Night	15% 37% 5% 17% 17% 9%	3429
September	1st to 10th	Night	8% 60% 5% 12% 9% 6%	1900
	11th to 20th	Night	14% 61% 5% 8% 10% 2	2019
	21st to end	Night	13% 61% 7% 12% 7% 1	2090
October	1st to 10th	Night	14% 37% 5% 21% 21% 2	3576
	11th to 20th	Night	22% 38% 11% 13% 15%	3956
	21st to end	Night	27% 38% 4 24% 8%	4457
November	1st to 10th	Night	24% 57% 7% 8% 3	2535
	11th to 20th	Night	23% 58% 7% 8% 2 1	2642
	21st to end	Night	22% 58% 6% 9% 4	2835
December	1st to 10th	Night	23% 37% 17% 19% 4	4890
	11th to 20th	Night	49% 34% 8% 8% 1	5547
	21st to end	Night	45% 31% 12% 9% 3	7108

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	26% 15% 24% 17% 13% 6%	200
	11th to 20th	Day	26% 9% 27% 23% 10% 6%	200
	21st to end	Day	27% 11% 28% 25% 8% 1	220
February	1st to 10th	Day	25% 17% 32% 18% 7%	217
	11th to 20th	Day	29% 15% 21% 24% 9% 2	475
	21st to end	Day	20% 16% 27% 26% 9% 2	414
March	1st to 10th	Day	19% 13% 27% 26% 13% 2	500
	11th to 20th	Day	17% 14% 18% 27% 18% 5%	500
	21st to end	Day	21% 14% 27% 24% 12% 2	531
April	1st to 10th	Day	22% 12% 23% 26% 13% 4	460
	11th to 20th	Day	15% 13% 30% 25% 12% 5%	452
	21st to end	Day	12% 12% 28% 29% 14% 5%	460
May	1st to 10th	Day	11% 9% 24% 30% 21% 4	463
	11th to 20th	Day	17% 7% 20% 24% 19% 13%	488
	21st to end	Day	12% 9% 19% 23% 25% 12%	529
June	1st to 10th	Day	15% 7% 17% 28% 21% 11%	571
	11th to 20th	Day	14% 10% 27% 25% 18% 6%	680
	21st to end	Day	7% 5% 19% 30% 30% 9%	568
July	1st to 10th	Day	4 3 15% 36% 31% 10%	486
	11th to 20th	Day	4 3 19% 35% 29% 10%	484
	21st to end	Day	1 3 18% 33% 33% 12%	531
August	1st to 10th	Day	5% 4 18% 28% 33% 12%	487
	11th to 20th	Day	4 2 14% 29% 34% 18%	477
	21st to end	Day	8% 6% 20% 26% 26% 15%	539
September	1st to 10th	Day	4 3 21% 28% 28% 14%	490
	11th to 20th	Day	11% 7% 19% 25% 27% 11%	490
	21st to end	Day	9% 9% 21% 31% 22% 7%	490
October	1st to 10th	Day	15% 8% 20% 30% 19% 8%	490
	11th to 20th	Day	15% 9% 21% 28% 17% 9%	381
	21st to end	Day	25% 8% 26% 25% 11% 4	330
November	1st to 10th	Day	29% 8% 26% 29% 8%	300
	11th to 20th	Day	23% 16% 25% 18% 14% 3	277
	21st to end	Day	22% 10% 27% 30% 11% 2	200
December	1st to 10th	Day	26% 12% 22% 19% 14% 8%	200
	11th to 20th	Day	32% 19% 24% 19% 5% 3	200
	21st to end	Day	31% 13% 24% 22% 7% 2	220
January	1st to 10th	Night	32% 10% 21% 25% 11% 2	800
	11th to 20th	Night	27% 8% 19% 29% 13% 3	800
	21st to end	Night	24% 5% 20% 34% 14% 2	880
February	1st to 10th	Night	27% 8% 23% 31% 9% 1	800
	11th to 20th	Night	25% 7% 21% 31% 13% 2	784
	21st to end	Night	25% 9% 20% 32% 12% 2	660
March	1st to 10th	Night	19% 7% 23% 34% 14% 3	800
	11th to 20th	Night	16% 10% 22% 30% 20% 1	800
	21st to end	Night	21% 8% 25% 34% 11% 2	862
April	1st to 10th	Night	18% 7% 22% 36% 16% 2	760
	11th to 20th	Night	16% 7% 27% 34% 14% 3	694
	21st to end	Night	14% 5% 26% 33% 18% 3	570
May	1st to 10th	Night	10% 6% 24% 34% 22% 4	480
	11th to 20th	Night	13% 4 13% 33% 29% 7%	374
	21st to end	Night	12% 6% 16% 33% 27% 5%	411
June	1st to 10th	Night	12% 4 21% 34% 23% 6%	380
	11th to 20th	Night	12% 6% 26% 34% 20% 2	380
	21st to end	Night	9% 5% 14% 40% 28% 4	380
July	1st to 10th	Night	3 2 12% 42% 35% 5%	370
	11th to 20th	Night	2 2 16% 47% 30% 2	368
	21st to end	Night	1 2 13% 42% 38% 5%	416
August	1st to 10th	Night	5% 3 10% 39% 36% 7%	378
	11th to 20th	Night	3 1 12% 35% 41% 8%	521
	21st to end	Night	5% 3 17% 37% 34% 4	793
September	1st to 10th	Night	5% 2 16% 40% 30% 7%	760
	11th to 20th	Night	8% 4 13% 33% 34% 8%	760
	21st to end	Night	13% 2 16% 37% 27% 6%	760
October	1st to 10th	Night	14% 4 15% 41% 24% 2	760
	11th to 20th	Night	18% 6% 14% 32% 26% 4	760
	21st to end	Night	19% 7% 21% 34% 14% 4	836
November	1st to 10th	Night	24% 8% 18% 37% 11% 1	780
	11th to 20th	Night	26% 10% 22% 28% 11% 2	800
	21st to end	Night	24% 7% 19% 31% 15% 3	835
December	1st to 10th	Night	24% 7% 19% 33% 15% 2	1000
	11th to 20th	Night	31% 7% 22% 30% 9% 1	1000
	21st to end	Night	34% 12% 21% 25% 8% 1	945

Page generated at 2025-03-24 06:04:27. Version ac07

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