Winthrop
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winthrop clear sky chart from 2005-11-06 to 2024-11-04.

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	5% 7% 3 6% 5% 5% 5% 4 6% 7% 47%	105790
	Night	5% 9% 3 7% 4 4 4 4 6% 7% 45%	101823
Transparency	Day	36% 8% 13% 21% 16% 6%	105558
	Night	38% 9% 14% 22% 13% 3	101701
Seeing	Day	13% 20% 33% 23% 9% 2	14636
	Night	12% 12% 33% 30% 12% 1	23199

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	7% 16% 6% 13% 7% 6% 5% 5% 6% 6% 23%	2539
	11th to 20th	Day	7% 15% 5% 11% 6% 5% 4 2 4 6% 35%	2516
	21st to end	Day	14% 23% 5% 9% 5% 4 4 3 5% 6% 22%	2794
February	1st to 10th	Day	3 12% 4 10% 6% 5% 6% 5% 8% 9% 33%	1629
	11th to 20th	Day	3 12% 3 7% 5% 5% 5% 4 8% 9% 39%	1665
	21st to end	Day	3 9% 4 10% 7% 6% 5% 5% 8% 10% 32%	1551
March	1st to 10th	Day	3 11% 3 9% 5% 6% 7% 6% 7% 7% 36%	3138
	11th to 20th	Day	2 8% 2 6% 4 4 5% 4 7% 8% 50%	3383
	21st to end	Day	8% 11% 3 8% 6% 5% 6% 5% 7% 8% 33%	4089
April	1st to 10th	Day	6% 10% 3 7% 7% 7% 7% 5% 7% 9% 32%	2280
	11th to 20th	Day	4 8% 3 8% 6% 6% 6% 6% 7% 9% 37%	2252
	21st to end	Day	4 8% 3 7% 5% 6% 6% 6% 7% 9% 39%	2560
May	1st to 10th	Day	5% 5% 2 6% 4 5% 5% 5% 6% 8% 49%	4420
	11th to 20th	Day	4 5% 2 6% 4 5% 4 4 7% 9% 48%	4377
	21st to end	Day	10% 10% 3 8% 6% 6% 6% 4 7% 7% 33%	4813
June	1st to 10th	Day	9% 6% 3 6% 6% 6% 6% 6% 7% 8% 38%	2660
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 7% 9% 41%	2736
	21st to end	Day	5% 4 2 5% 4 5% 5% 3 6% 8% 53%	2850
July	1st to 10th	Day	2 2 1 3 2 2 3 2 4 5% 74%	4585
	11th to 20th	Day	2 2 1 3 2 3 3 2 4 4 75%	4420
	21st to end	Day	2 2 1 3 2 3 3 2 4 6% 73%	4790
August	1st to 10th	Day	1 1 1 3 3 3 4 4 6% 7% 67%	4401
	11th to 20th	Day	2 1 1 4 4 4 5% 4 7% 8% 60%	4187
	21st to end	Day	5% 4 2 4 4 3 5% 3 5% 7% 58%	4220
September	1st to 10th	Day	4 3 1 4 3 2 3 3 4 7% 66%	2280
	11th to 20th	Day	4 4 2 6% 4 4 5% 4 6% 8% 52%	2279
	21st to end	Day	6% 6% 2 6% 4 3 4 4 5% 6% 54%	2047
October	1st to 10th	Day	4 2 1 5% 4 4 5% 4 5% 7% 58%	3180
	11th to 20th	Day	5% 5% 2 6% 5% 5% 6% 5% 6% 8% 46%	3139
	21st to end	Day	6% 7% 2 6% 5% 5% 6% 4 6% 8% 45%	3101
November	1st to 10th	Day	8% 11% 3 6% 5% 6% 6% 4 8% 9% 35%	1630
	11th to 20th	Day	5% 14% 4 8% 6% 6% 4 4 5% 9% 36%	1520
	21st to end	Day	3 12% 6% 11% 6% 5% 6% 4 6% 8% 33%	1520
December	1st to 10th	Day	4 13% 4 9% 6% 5% 4 4 7% 7% 38%	1520
	11th to 20th	Day	6% 13% 6% 9% 5% 5% 6% 4 6% 7% 32%	1925
	21st to end	Day	8% 19% 6% 11% 5% 5% 5% 4 5% 6% 25%	2794
January	1st to 10th	Night	5% 19% 7% 13% 6% 6% 5% 5% 6% 6% 21%	4406
	11th to 20th	Night	7% 16% 5% 10% 5% 4 5% 5% 6% 7% 31%	4382
	21st to end	Night	13% 24% 6% 12% 6% 4 4 3 4 5% 20%	4862
February	1st to 10th	Night	6% 13% 3 8% 6% 7% 5% 5% 7% 8% 32%	2549
	11th to 20th	Night	4 10% 3 7% 4 5% 6% 4 7% 9% 42%	2402
	21st to end	Night	4 11% 5% 10% 6% 5% 5% 5% 9% 8% 33%	1903
March	1st to 10th	Night	2 10% 4 9% 7% 5% 5% 5% 8% 10% 36%	3736
	11th to 20th	Night	3 7% 2 5% 5% 4 4 4 4 7% 55%	3592
	21st to end	Night	5% 12% 3 9% 6% 6% 6% 6% 7% 8% 32%	3564
April	1st to 10th	Night	3 8% 2 8% 5% 5% 6% 5% 8% 10% 39%	1900
	11th to 20th	Night	3 6% 3 6% 5% 5% 6% 4 7% 8% 46%	1879
	21st to end	Night	4 7% 2 6% 5% 4 3 5% 8% 8% 49%	1617
May	1st to 10th	Night	5% 5% 2 6% 4 4 4 4 6% 8% 54%	2540
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 7% 59%	2516
	21st to end	Night	5% 5% 2 5% 5% 4 5% 5% 8% 10% 46%	2762
June	1st to 10th	Night	6% 5% 3 5% 5% 5% 5% 4 8% 9% 45%	1520
	11th to 20th	Night	3 4 1 4 4 5% 5% 5% 7% 8% 52%	1437
	21st to end	Night	3 3 1 3 2 4 3 4 8% 7% 62%	1330
July	1st to 10th	Night	2 1 1 2 1 2 2 2 4 5% 79%	2370
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 5% 81%	2540
	21st to end	Night	1 1 1 2 2 2 2 3 5% 5% 77%	2762
August	1st to 10th	Night	1 2 1 3 2 3 4 3 4 6% 71%	2529
	11th to 20th	Night	1 2 1 3 2 3 2 3 6% 9% 70%	2643
	21st to end	Night	5% 2 1 3 3 4 4 4 5% 7% 64%	3456
September	1st to 10th	Night	3 2 1 3 2 3 4 3 4 7% 68%	1900
	11th to 20th	Night	5% 3 1 5% 3 4 3 3 6% 7% 60%	1900
	21st to end	Night	5% 4 2 5% 4 4 3 4 7% 8% 55%	2130
October	1st to 10th	Night	4 4 1 4 3 3 4 4 6% 7% 59%	3800
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 6% 7% 49%	3797
	21st to end	Night	7% 7% 2 6% 5% 5% 4 4 5% 8% 46%	4457
November	1st to 10th	Night	10% 14% 3 7% 4 5% 5% 4 7% 8% 34%	2520
	11th to 20th	Night	6% 14% 3 7% 5% 6% 6% 5% 6% 8% 36%	2660
	21st to end	Night	4 16% 6% 11% 6% 5% 5% 4 6% 6% 32%	2660
December	1st to 10th	Night	6% 13% 4 9% 6% 5% 5% 4 6% 7% 35%	2660
	11th to 20th	Night	6% 17% 7% 12% 7% 6% 7% 5% 6% 6% 20%	3280
	21st to end	Night	9% 22% 6% 11% 5% 4 5% 3 5% 5% 24%	4862

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	62% 10% 11% 10% 7%	2539
	11th to 20th	Day	53% 7% 12% 20% 5% 3	2540
	21st to end	Day	65% 9% 10% 11% 5%	2794
February	1st to 10th	Day	50% 10% 17% 16% 5% 1	1629
	11th to 20th	Day	42% 11% 18% 18% 9% 1	1674
	21st to end	Day	48% 12% 16% 15% 7% 1	1551
March	1st to 10th	Day	46% 10% 13% 13% 15% 3	3138
	11th to 20th	Day	32% 9% 17% 36% 7%	3383
	21st to end	Day	48% 12% 16% 14% 9% 1	4089
April	1st to 10th	Day	50% 10% 16% 18% 6% 1	2280
	11th to 20th	Day	45% 10% 16% 16% 10% 2	2252
	21st to end	Day	42% 11% 14% 20% 11% 2	2560
May	1st to 10th	Day	33% 9% 14% 19% 24% 1	4420
	11th to 20th	Day	31% 9% 18% 25% 15% 1	4380
	21st to end	Day	48% 11% 13% 22% 6%	4814
June	1st to 10th	Day	45% 9% 10% 18% 13% 4	2660
	11th to 20th	Day	40% 11% 13% 21% 13% 2	2736
	21st to end	Day	30% 7% 14% 26% 19% 4	2850
July	1st to 10th	Day	15% 4 7% 17% 27% 30%	4515
	11th to 20th	Day	15% 3 5% 14% 27% 36%	4345
	21st to end	Day	14% 4 7% 25% 32% 19%	4639
August	1st to 10th	Day	16% 6% 6% 26% 34% 12%	4384
	11th to 20th	Day	19% 6% 16% 34% 21% 4	4229
	21st to end	Day	27% 6% 14% 25% 23% 5%	4219
September	1st to 10th	Day	21% 6% 11% 29% 27% 5%	2280
	11th to 20th	Day	31% 8% 11% 24% 22% 5%	2279
	21st to end	Day	31% 8% 15% 25% 17% 4	2047
October	1st to 10th	Day	27% 5% 13% 36% 17% 1	3180
	11th to 20th	Day	35% 9% 13% 24% 18%	3139
	21st to end	Day	37% 8% 12% 26% 16% 2	3101
November	1st to 10th	Day	47% 12% 14% 16% 10% 1	1630
	11th to 20th	Day	49% 9% 16% 17% 8% 1	1520
	21st to end	Day	50% 12% 13% 14% 8% 2	1520
December	1st to 10th	Day	47% 11% 14% 18% 10%	1520
	11th to 20th	Day	52% 12% 18% 12% 6%	1928
	21st to end	Day	62% 10% 9% 11% 7% 1	2794
January	1st to 10th	Night	60% 10% 10% 13% 6% 1	4406
	11th to 20th	Night	52% 6% 14% 19% 8% 2	4420
	21st to end	Night	68% 10% 8% 10% 4 1	4862
February	1st to 10th	Night	49% 11% 17% 16% 7% 2	2549
	11th to 20th	Night	41% 10% 17% 21% 10% 2	2404
	21st to end	Night	46% 12% 15% 16% 10% 1	1903
March	1st to 10th	Night	42% 10% 14% 16% 15% 2	3736
	11th to 20th	Night	30% 8% 18% 34% 10%	3593
	21st to end	Night	48% 13% 15% 18% 4 1	3564
April	1st to 10th	Night	39% 14% 17% 21% 8% 1	1900
	11th to 20th	Night	37% 10% 16% 21% 14% 2	1879
	21st to end	Night	33% 12% 15% 27% 13%	1617
May	1st to 10th	Night	28% 8% 14% 24% 25% 1	2540
	11th to 20th	Night	25% 7% 18% 32% 15% 2	2516
	21st to end	Night	28% 14% 21% 29% 6% 1	2762
June	1st to 10th	Night	35% 10% 14% 25% 14% 3	1520
	11th to 20th	Night	30% 10% 17% 27% 15% 1	1437
	21st to end	Night	21% 8% 17% 30% 21% 3	1330
July	1st to 10th	Night	11% 4 7% 23% 30% 24%	2331
	11th to 20th	Night	10% 3 7% 19% 32% 30%	2492
	21st to end	Night	11% 3 9% 28% 30% 18%	2674
August	1st to 10th	Night	16% 4 7% 30% 31% 11%	2521
	11th to 20th	Night	13% 6% 24% 36% 18% 3	2667
	21st to end	Night	21% 6% 16% 32% 23% 3	3456
September	1st to 10th	Night	19% 7% 11% 31% 27% 5%	1900
	11th to 20th	Night	25% 7% 10% 30% 23% 4	1900
	21st to end	Night	28% 9% 14% 31% 14% 5%	2130
October	1st to 10th	Night	26% 6% 18% 33% 16% 1	3800
	11th to 20th	Night	35% 8% 15% 28% 13% 1	3797
	21st to end	Night	38% 7% 14% 26% 14% 1	4457
November	1st to 10th	Night	48% 10% 16% 14% 10% 2	2520
	11th to 20th	Night	46% 11% 15% 18% 6% 3	2660
	21st to end	Night	52% 11% 12% 15% 8% 2	2660
December	1st to 10th	Night	50% 10% 14% 18% 8%	2660
	11th to 20th	Night	59% 14% 11% 11% 6%	3276
	21st to end	Night	61% 9% 9% 13% 8% 1	4862

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	16% 23% 23% 22% 12% 4	180
	11th to 20th	Day	17% 17% 27% 24% 12% 3	180
	21st to end	Day	20% 15% 31% 23% 11%	198
February	1st to 10th	Day	15% 27% 38% 15% 5%	289
	11th to 20th	Day	17% 23% 36% 18% 5% 1	365
	21st to end	Day	13% 25% 37% 20% 6%	373
March	1st to 10th	Day	18% 23% 39% 15% 5% 1	460
	11th to 20th	Day	15% 26% 28% 21% 9% 1	460
	21st to end	Day	17% 26% 35% 17% 5%	506
April	1st to 10th	Day	24% 22% 34% 13% 5% 1	460
	11th to 20th	Day	18% 30% 26% 19% 5% 2	452
	21st to end	Day	13% 29% 32% 18% 7% 1	460
May	1st to 10th	Day	13% 22% 35% 25% 5% 1	463
	11th to 20th	Day	19% 18% 28% 25% 8% 2	488
	21st to end	Day	16% 21% 30% 22% 9% 2	529
June	1st to 10th	Day	16% 25% 33% 19% 7%	490
	11th to 20th	Day	15% 27% 35% 15% 6% 1	566
	21st to end	Day	9% 20% 36% 25% 9% 1	680
July	1st to 10th	Day	6% 19% 34% 33% 7% 1	585
	11th to 20th	Day	7% 17% 34% 28% 11% 2	484
	21st to end	Day	3 16% 35% 31% 13% 2	531
August	1st to 10th	Day	6% 12% 35% 31% 13% 3	487
	11th to 20th	Day	4 11% 32% 32% 17% 5%	477
	21st to end	Day	9% 18% 35% 27% 9% 3	539
September	1st to 10th	Day	9% 17% 35% 23% 12% 4	490
	11th to 20th	Day	10% 18% 31% 26% 13% 3	490
	21st to end	Day	13% 21% 31% 24% 9% 2	490
October	1st to 10th	Day	14% 17% 33% 26% 10%	490
	11th to 20th	Day	15% 20% 30% 22% 10% 3	480
	21st to end	Day	18% 19% 33% 20% 8% 2	330
November	1st to 10th	Day	13% 23% 35% 21% 7%	246
	11th to 20th	Day	16% 22% 31% 18% 11% 3	180
	21st to end	Day	16% 14% 34% 19% 12% 4	180
December	1st to 10th	Day	17% 14% 23% 22% 18% 6%	180
	11th to 20th	Day	9% 17% 36% 29% 7% 2	180
	21st to end	Day	10% 12% 38% 26% 14% 1	198
January	1st to 10th	Night	17% 15% 32% 23% 11% 1	760
	11th to 20th	Night	17% 11% 33% 27% 10% 1	760
	21st to end	Night	15% 11% 32% 30% 11% 1	836
February	1st to 10th	Night	15% 14% 38% 23% 8% 2	760
	11th to 20th	Night	11% 11% 36% 29% 11%	744
	21st to end	Night	14% 14% 32% 25% 13% 2	628
March	1st to 10th	Night	11% 12% 35% 31% 11% 1	760
	11th to 20th	Night	11% 12% 37% 28% 12% 1	760
	21st to end	Night	11% 15% 37% 29% 7% 1	836
April	1st to 10th	Night	10% 14% 35% 30% 10% 1	760
	11th to 20th	Night	9% 17% 34% 29% 9% 2	751
	21st to end	Night	10% 17% 40% 25% 8% 1	477
May	1st to 10th	Night	10% 13% 34% 32% 11%	380
	11th to 20th	Night	11% 10% 30% 32% 16% 1	374
	21st to end	Night	11% 12% 32% 29% 15%	411
June	1st to 10th	Night	11% 14% 35% 30% 10% 1	380
	11th to 20th	Night	7% 16% 37% 29% 10%	380
	21st to end	Night	6% 11% 36% 32% 17%	380
July	1st to 10th	Night	4 9% 32% 41% 14% 1	370
	11th to 20th	Night	4 12% 32% 38% 15%	368
	21st to end	Night	2 7% 32% 39% 19%	416
August	1st to 10th	Night	4 7% 21% 43% 24%	378
	11th to 20th	Night	2 6% 27% 37% 26% 2	457
	21st to end	Night	6% 9% 29% 36% 18% 2	807
September	1st to 10th	Night	5% 9% 29% 33% 20% 3	760
	11th to 20th	Night	7% 8% 24% 31% 25% 6%	760
	21st to end	Night	9% 10% 29% 32% 17% 3	760
October	1st to 10th	Night	12% 9% 28% 31% 18% 2	760
	11th to 20th	Night	13% 12% 28% 31% 14% 2	760
	21st to end	Night	15% 14% 33% 26% 9% 3	836
November	1st to 10th	Night	20% 12% 36% 25% 6%	754
	11th to 20th	Night	19% 11% 37% 21% 10% 2	760
	21st to end	Night	14% 13% 34% 25% 10% 4	760
December	1st to 10th	Night	15% 9% 37% 28% 10% 3	760
	11th to 20th	Night	18% 10% 37% 29% 5% 1	760
	21st to end	Night	17% 14% 36% 28% 6%	836

Page generated at 2024-11-01 19:47:30. Version ac07

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