Windy Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Windy Sky Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	16% 7% 2 6% 4 4 4 3 5% 7% 40%	101457
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	98143
Transparency	Day	40% 7% 13% 23% 15% 2	105016
	Night	43% 6% 12% 23% 15% 2	102673
Seeing	Day	20% 12% 30% 24% 11% 3	15172
	Night	20% 9% 30% 28% 11% 1	24245

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% 6% 3 6% 4 4 3 3 4 5% 45%	2517
	11th to 20th	Day	16% 9% 2 6% 5% 4 5% 3 5% 5% 39%	2520
	21st to end	Day	21% 12% 5% 8% 3 4 3 2 4 4 33%	2900
February	1st to 10th	Day	17% 8% 2 7% 4 4 4 3 5% 6% 40%	1791
	11th to 20th	Day	15% 6% 3 6% 4 4 4 3 5% 5% 46%	1848
	21st to end	Day	19% 9% 3 6% 4 4 4 2 5% 6% 37%	1570
March	1st to 10th	Day	26% 9% 3 6% 4 3 2 3 4 5% 35%	3116
	11th to 20th	Day	9% 4 2 5% 4 3 5% 4 6% 9% 48%	3265
	21st to end	Day	19% 8% 2 6% 3 3 3 3 5% 6% 41%	4166
April	1st to 10th	Day	18% 8% 2 6% 4 4 4 3 5% 7% 38%	2280
	11th to 20th	Day	18% 7% 2 6% 4 4 4 3 5% 5% 43%	2254
	21st to end	Day	20% 7% 2 4 3 3 3 4 6% 6% 42%	2364
May	1st to 10th	Day	27% 10% 3 5% 3 3 3 3 5% 8% 30%	4106
	11th to 20th	Day	24% 9% 3 6% 4 4 4 3 5% 6% 33%	4225
	21st to end	Day	14% 5% 2 5% 4 3 5% 4 7% 9% 43%	4837
June	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 7% 9% 39%	2660
	11th to 20th	Day	13% 7% 3 4 5% 4 5% 5% 6% 8% 40%	2660
	21st to end	Day	11% 7% 2 5% 5% 3 4 4 7% 10% 42%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 7% 9% 41%	4440
	21st to end	Day	16% 8% 3 6% 5% 5% 5% 4 6% 8% 35%	4814
August	1st to 10th	Day	17% 7% 3 6% 4 5% 4 4 7% 9% 35%	4189
	11th to 20th	Day	11% 6% 3 7% 4 4 4 3 6% 8% 44%	3730
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 47%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	2280
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 5% 7% 51%	2147
	21st to end	Day	19% 7% 3 7% 4 3 4 3 5% 6% 40%	2090
October	1st to 10th	Day	20% 6% 2 5% 4 3 3 3 5% 7% 43%	2065
	11th to 20th	Day	14% 6% 1 5% 4 3 3 3 6% 7% 47%	1900
	21st to end	Day	18% 5% 2 6% 4 4 3 4 5% 7% 42%	1881
November	1st to 10th	Day	13% 6% 2 5% 5% 4 3 2 4 5% 50%	1761
	11th to 20th	Day	16% 7% 2 4 4 3 4 3 4 6% 49%	1800
	21st to end	Day	16% 7% 2 5% 4 5% 3 5% 5% 7% 44%	1798
December	1st to 10th	Day	19% 6% 2 5% 4 4 4 3 7% 8% 37%	1525
	11th to 20th	Day	22% 7% 3 3 3 3 3 2 4 6% 45%	1906
	21st to end	Day	18% 10% 4 8% 5% 5% 4 3 5% 6% 32%	2772
January	1st to 10th	Night	18% 6% 2 5% 3 4 3 3 4 6% 46%	4429
	11th to 20th	Night	17% 9% 3 7% 4 3 3 3 3 4 43%	4440
	21st to end	Night	23% 11% 4 7% 4 4 3 2 4 5% 33%	4752
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 5% 6% 41%	2387
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2218
	21st to end	Night	21% 6% 2 5% 4 4 3 3 4 6% 42%	1884
March	1st to 10th	Night	24% 12% 3 5% 3 3 3 3 3 5% 36%	3724
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 6% 46%	3690
	21st to end	Night	21% 9% 3 6% 3 3 4 3 5% 5% 39%	3466
April	1st to 10th	Night	19% 7% 2 6% 5% 5% 4 3 5% 7% 38%	1900
	11th to 20th	Night	18% 8% 2 6% 3 4 4 3 5% 7% 41%	1875
	21st to end	Night	19% 7% 2 6% 4 3 3 3 5% 7% 41%	1815
May	1st to 10th	Night	23% 12% 4 8% 4 4 4 3 5% 7% 27%	2834
	11th to 20th	Night	25% 9% 3 7% 4 4 4 4 5% 6% 29%	2696
	21st to end	Night	18% 8% 3 6% 4 4 4 4 6% 8% 38%	2738
June	1st to 10th	Night	18% 8% 2 5% 5% 3 4 3 4 8% 39%	1520
	11th to 20th	Night	15% 7% 2 4 4 3 4 4 7% 10% 40%	1520
	21st to end	Night	11% 7% 2 6% 5% 4 5% 5% 6% 9% 38%	1520
July	1st to 10th	Night	14% 11% 3 7% 5% 4 5% 4 6% 8% 33%	2520
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 5% 7% 9% 35%	2520
	21st to end	Night	20% 9% 3 7% 4 4 5% 3 6% 8% 32%	2739
August	1st to 10th	Night	23% 6% 3 6% 4 5% 5% 4 7% 8% 30%	2692
	11th to 20th	Night	12% 6% 2 6% 5% 5% 5% 4 7% 8% 40%	3097
	21st to end	Night	8% 5% 1 5% 4 3 4 4 5% 8% 53%	3476
September	1st to 10th	Night	14% 5% 2 6% 5% 4 4 4 5% 7% 43%	1900
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 5% 6% 52%	2031
	21st to end	Night	19% 6% 2 6% 4 4 4 3 5% 6% 41%	2090
October	1st to 10th	Night	18% 9% 2 7% 4 3 4 2 4 7% 40%	2113
	11th to 20th	Night	15% 5% 2 6% 4 3 3 3 5% 6% 49%	2280
	21st to end	Night	20% 7% 2 7% 4 3 3 3 4 6% 42%	2717
November	1st to 10th	Night	12% 6% 3 5% 5% 4 3 3 5% 6% 49%	2537
	11th to 20th	Night	18% 6% 2 5% 3 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 46%	2598
December	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 39%	2653
	11th to 20th	Night	20% 7% 2 5% 4 3 4 3 5% 6% 42%	3288
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 5% 38%	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 13% 29% 12% 2	2518
	11th to 20th	Day	44% 7% 12% 25% 10% 1	2520
	21st to end	Day	52% 7% 9% 21% 9% 1	2900
February	1st to 10th	Day	44% 6% 15% 22% 12% 1	1791
	11th to 20th	Day	41% 5% 10% 25% 16% 2	1857
	21st to end	Day	46% 5% 11% 21% 13% 3	1570
March	1st to 10th	Day	47% 6% 13% 23% 10% 2	3119
	11th to 20th	Day	29% 8% 14% 32% 15% 1	3265
	21st to end	Day	44% 5% 8% 19% 20% 5%	4166
April	1st to 10th	Day	41% 6% 12% 20% 17% 3	2280
	11th to 20th	Day	40% 6% 13% 22% 17% 3	2254
	21st to end	Day	41% 6% 10% 21% 18% 4	2364
May	1st to 10th	Day	52% 10% 13% 14% 10% 1	4120
	11th to 20th	Day	50% 7% 15% 18% 9% 1	4229
	21st to end	Day	36% 10% 13% 23% 16% 2	4837
June	1st to 10th	Day	40% 8% 13% 21% 15% 1	2660
	11th to 20th	Day	38% 8% 15% 24% 13% 2	2660
	21st to end	Day	35% 8% 17% 24% 15% 1	2660
July	1st to 10th	Day	46% 10% 10% 17% 16% 1	4370
	11th to 20th	Day	38% 7% 19% 23% 12% 1	4365
	21st to end	Day	44% 8% 13% 18% 14% 2	4661
August	1st to 10th	Day	43% 8% 17% 22% 8%	4175
	11th to 20th	Day	36% 7% 14% 32% 10% 1	3766
	21st to end	Day	28% 10% 14% 29% 18% 1	4180
September	1st to 10th	Day	37% 7% 14% 21% 19% 2	2280
	11th to 20th	Day	32% 6% 13% 25% 23% 2	2147
	21st to end	Day	46% 6% 9% 18% 18% 2	2090
October	1st to 10th	Day	33% 6% 13% 23% 21% 5%	3429
	11th to 20th	Day	26% 5% 11% 23% 32% 3	3140
	21st to end	Day	31% 5% 15% 27% 20% 2	3081
November	1st to 10th	Day	37% 5% 14% 23% 20% 2	1761
	11th to 20th	Day	38% 5% 12% 22% 20% 3	1800
	21st to end	Day	40% 5% 15% 28% 10% 2	1798
December	1st to 10th	Day	45% 6% 15% 19% 13% 1	1525
	11th to 20th	Day	41% 5% 15% 20% 17% 2	1906
	21st to end	Day	49% 8% 12% 23% 8%	2772
January	1st to 10th	Night	39% 5% 12% 30% 12% 3	4429
	11th to 20th	Night	46% 5% 11% 25% 11% 2	4440
	21st to end	Night	53% 6% 10% 20% 10% 1	4752
February	1st to 10th	Night	44% 7% 14% 23% 12% 2	2387
	11th to 20th	Night	45% 5% 10% 21% 15% 4	2220
	21st to end	Night	45% 4 10% 22% 16% 4	1884
March	1st to 10th	Night	52% 4 10% 21% 10% 2	3734
	11th to 20th	Night	36% 7% 12% 27% 14% 3	3691
	21st to end	Night	46% 5% 7% 15% 21% 6%	3466
April	1st to 10th	Night	45% 6% 10% 19% 16% 3	1900
	11th to 20th	Night	43% 6% 9% 20% 20% 2	1875
	21st to end	Night	44% 4 10% 21% 15% 5%	1815
May	1st to 10th	Night	56% 8% 10% 15% 9% 1	2840
	11th to 20th	Night	55% 8% 13% 14% 9% 2	2697
	21st to end	Night	43% 8% 14% 21% 13% 2	2739
June	1st to 10th	Night	44% 7% 10% 20% 18% 2	1520
	11th to 20th	Night	39% 7% 14% 26% 13% 1	1520
	21st to end	Night	40% 8% 13% 26% 12% 1	1520
July	1st to 10th	Night	48% 8% 13% 18% 13% 1	2480
	11th to 20th	Night	40% 9% 16% 25% 10% 1	2472
	21st to end	Night	50% 6% 13% 16% 13% 1	2651
August	1st to 10th	Night	49% 8% 17% 16% 9% 1	2682
	11th to 20th	Night	39% 8% 15% 29% 9%	3127
	21st to end	Night	28% 7% 12% 34% 18% 1	3476
September	1st to 10th	Night	41% 7% 10% 24% 17% 2	1900
	11th to 20th	Night	33% 6% 10% 25% 23% 3	2031
	21st to end	Night	45% 6% 9% 20% 18% 2	2090
October	1st to 10th	Night	40% 4 11% 24% 18% 2	3529
	11th to 20th	Night	26% 4 10% 26% 30% 4	3779
	21st to end	Night	31% 5% 13% 27% 21% 2	4467
November	1st to 10th	Night	37% 6% 12% 23% 18% 4	2537
	11th to 20th	Night	40% 5% 10% 22% 18% 4	2600
	21st to end	Night	38% 5% 14% 29% 13% 1	2598
December	1st to 10th	Night	46% 6% 15% 19% 12% 1	2653
	11th to 20th	Night	43% 6% 12% 20% 17% 1	3288
	21st to end	Night	46% 6% 12% 24% 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	22% 19% 37% 16% 6% 1	180
	11th to 20th	Day	24% 26% 32% 13% 4	180
	21st to end	Day	23% 16% 38% 16% 7%	234
February	1st to 10th	Day	25% 18% 37% 16% 4	351
	11th to 20th	Day	24% 18% 40% 16% 3	456
	21st to end	Day	28% 18% 32% 17% 4 1	382
March	1st to 10th	Day	22% 20% 33% 20% 5%	460
	11th to 20th	Day	23% 17% 37% 18% 5% 1	460
	21st to end	Day	26% 17% 33% 16% 5% 1	506
April	1st to 10th	Day	23% 19% 32% 18% 8% 1	460
	11th to 20th	Day	24% 15% 33% 17% 9% 1	452
	21st to end	Day	27% 13% 33% 20% 5% 2	460
May	1st to 10th	Day	30% 15% 30% 17% 7% 2	463
	11th to 20th	Day	21% 11% 35% 24% 8% 1	487
	21st to end	Day	19% 7% 26% 26% 17% 4	529
June	1st to 10th	Day	18% 7% 31% 27% 15% 3	490
	11th to 20th	Day	15% 8% 30% 29% 14% 3	490
	21st to end	Day	16% 7% 26% 32% 16% 4	490
July	1st to 10th	Day	15% 5% 23% 33% 18% 6%	485
	11th to 20th	Day	11% 3 24% 37% 18% 8%	484
	21st to end	Day	14% 8% 25% 33% 18% 3	532
August	1st to 10th	Day	13% 5% 22% 35% 20% 6%	486
	11th to 20th	Day	14% 4 22% 35% 17% 8%	479
	21st to end	Day	11% 6% 25% 32% 17% 9%	539
September	1st to 10th	Day	18% 6% 24% 27% 18% 7%	490
	11th to 20th	Day	15% 5% 26% 28% 16% 11%	490
	21st to end	Day	23% 7% 29% 23% 13% 4	490
October	1st to 10th	Day	25% 9% 24% 20% 15% 7%	490
	11th to 20th	Day	14% 14% 31% 24% 12% 4	490
	21st to end	Day	21% 11% 34% 20% 12% 2	429
November	1st to 10th	Day	16% 14% 29% 28% 11% 3	395
	11th to 20th	Day	22% 17% 31% 22% 8% 1	400
	21st to end	Day	20% 16% 32% 24% 8% 1	400
December	1st to 10th	Day	22% 18% 35% 18% 6% 1	185
	11th to 20th	Day	29% 20% 32% 14% 3 1	180
	21st to end	Day	27% 15% 35% 16% 6% 1	198
January	1st to 10th	Night	19% 14% 38% 23% 5%	760
	11th to 20th	Night	27% 14% 31% 23% 4	760
	21st to end	Night	21% 13% 39% 24% 3	836
February	1st to 10th	Night	21% 13% 38% 25% 3	760
	11th to 20th	Night	24% 16% 39% 19% 2	743
	21st to end	Night	26% 15% 33% 22% 4	628
March	1st to 10th	Night	23% 13% 37% 21% 6%	760
	11th to 20th	Night	21% 13% 38% 23% 5% 1	760
	21st to end	Night	22% 13% 37% 20% 7%	836
April	1st to 10th	Night	22% 11% 37% 24% 5%	760
	11th to 20th	Night	24% 9% 32% 28% 7%	751
	21st to end	Night	23% 7% 33% 30% 6% 1	675
May	1st to 10th	Night	24% 9% 30% 26% 10% 2	570
	11th to 20th	Night	24% 7% 36% 27% 7%	497
	21st to end	Night	20% 4 19% 39% 17% 1	411
June	1st to 10th	Night	21% 4 21% 37% 16% 1	380
	11th to 20th	Night	18% 3 18% 42% 17% 1	380
	21st to end	Night	12% 4 24% 44% 14% 1	380
July	1st to 10th	Night	12% 6% 20% 40% 19% 2	370
	11th to 20th	Night	14% 4 14% 45% 22% 1	368
	21st to end	Night	16% 4 18% 42% 19% 1	416
August	1st to 10th	Night	9% 3 19% 45% 23% 1	486
	11th to 20th	Night	15% 3 15% 40% 24% 3	750
	21st to end	Night	14% 3 15% 38% 25% 5%	836
September	1st to 10th	Night	17% 3 24% 32% 20% 4	760
	11th to 20th	Night	16% 4 21% 31% 22% 7%	760
	21st to end	Night	22% 5% 23% 31% 17% 3	760
October	1st to 10th	Night	23% 7% 25% 24% 18% 3	760
	11th to 20th	Night	19% 10% 26% 32% 12% 1	760
	21st to end	Night	25% 7% 31% 26% 10%	836
November	1st to 10th	Night	15% 10% 32% 28% 13% 2	780
	11th to 20th	Night	22% 13% 32% 24% 10% 1	800
	21st to end	Night	18% 16% 32% 26% 8% 1	800
December	1st to 10th	Night	25% 14% 31% 23% 7% 1	760
	11th to 20th	Night	22% 15% 34% 24% 5%	760
	21st to end	Night	25% 11% 36% 24% 3	836

Page generated at 2024-11-27 17:30:51. Version ac07

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