Weather and Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Weather and Sky Observatory clear sky chart from 2005-11-06 to 2024-08-15.

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	14% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	95529
	Night	15% 10% 4 8% 5% 5% 4 4 5% 7% 34%	93186
Transparency	Day	48% 8% 13% 20% 10% 1	95258
	Night	49% 7% 13% 19% 11% 1	93010
Seeing	Day	18% 12% 28% 24% 13% 5%	14160
	Night	19% 10% 29% 28% 12% 1	23591

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	15% 17% 5% 10% 6% 4 4 3 4 6% 25%	2517
	11th to 20th	Day	12% 21% 7% 14% 7% 6% 5% 3 4 6% 15%	2813
	21st to end	Day	22% 19% 6% 12% 6% 6% 5% 3 4 3 14%	3124
February	1st to 10th	Day	17% 17% 5% 11% 7% 6% 5% 4 4 5% 19%	1710
	11th to 20th	Day	11% 16% 6% 14% 7% 5% 4 5% 4 5% 23%	1663
	21st to end	Day	18% 14% 4 11% 6% 5% 4 4 5% 5% 23%	1486
March	1st to 10th	Day	18% 12% 5% 9% 6% 4 4 4 4 5% 29%	3432
	11th to 20th	Day	11% 16% 5% 8% 6% 5% 5% 3 5% 6% 31%	3480
	21st to end	Day	17% 10% 3 8% 5% 4 4 3 5% 6% 35%	3815
April	1st to 10th	Day	18% 12% 4 8% 5% 5% 5% 3 5% 6% 31%	2235
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 8% 35%	2318
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 3 4 5% 34%	2470
May	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	4108
	11th to 20th	Day	14% 10% 4 9% 5% 5% 5% 3 5% 7% 32%	4322
	21st to end	Day	10% 6% 2 6% 4 4 4 4 5% 8% 46%	4837
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 4 6% 8% 40%	2660
	11th to 20th	Day	11% 5% 3 7% 5% 4 5% 4 8% 9% 40%	2660
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 8% 42%	2660
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 5% 7% 10% 44%	4440
	11th to 20th	Day	11% 5% 2 5% 4 5% 5% 4 7% 9% 44%	4411
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	4466
August	1st to 10th	Day	10% 4 2 6% 4 5% 6% 4 7% 9% 41%	2457
	11th to 20th	Day	8% 6% 2 7% 5% 5% 7% 4 8% 11% 38%	2357
	21st to end	Day	11% 6% 2 6% 4 5% 4 4 5% 9% 44%	2475
September	1st to 10th	Day	13% 7% 3 5% 4 4 5% 4 6% 8% 40%	2160
	11th to 20th	Day	10% 5% 2 5% 5% 5% 4 3 5% 7% 48%	2044
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 3 7% 7% 37%	1949
October	1st to 10th	Day	19% 9% 3 7% 5% 4 5% 3 6% 6% 34%	1800
	11th to 20th	Day	17% 10% 4 10% 5% 5% 5% 3 5% 6% 30%	1800
	21st to end	Day	24% 12% 3 9% 5% 4 5% 4 4 6% 24%	1980
November	1st to 10th	Day	12% 10% 3 9% 5% 4 4 4 6% 8% 34%	1642
	11th to 20th	Day	12% 13% 3 8% 7% 5% 5% 3 6% 7% 29%	1520
	21st to end	Day	19% 15% 3 8% 5% 4 4 5% 6% 6% 25%	1520
December	1st to 10th	Day	16% 15% 6% 11% 6% 5% 5% 4 5% 6% 21%	1520
	11th to 20th	Day	17% 15% 6% 9% 7% 5% 6% 4 5% 6% 21%	1906
	21st to end	Day	28% 19% 7% 12% 5% 4 4 3 4 4 12%	2772
January	1st to 10th	Night	17% 15% 5% 10% 6% 5% 5% 3 4 5% 24%	4429
	11th to 20th	Night	13% 17% 6% 11% 7% 6% 6% 4 5% 4 21%	4143
	21st to end	Night	23% 17% 5% 10% 5% 5% 4 3 4 5% 18%	4532
February	1st to 10th	Night	15% 16% 5% 10% 7% 5% 4 4 6% 6% 23%	2470
	11th to 20th	Night	14% 14% 5% 10% 6% 6% 4 4 5% 7% 27%	2403
	21st to end	Night	16% 13% 5% 11% 5% 5% 4 4 6% 7% 24%	1964
March	1st to 10th	Night	20% 9% 3 8% 6% 4 5% 4 4 5% 32%	3408
	11th to 20th	Night	15% 12% 3 7% 4 4 4 3 5% 7% 36%	3479
	21st to end	Night	15% 8% 3 7% 4 4 4 4 5% 5% 42%	3817
April	1st to 10th	Night	18% 11% 4 7% 5% 5% 3 4 5% 6% 32%	1944
	11th to 20th	Night	19% 7% 3 6% 4 5% 4 4 5% 6% 36%	1827
	21st to end	Night	16% 9% 3 7% 5% 4 5% 4 5% 7% 36%	1710
May	1st to 10th	Night	19% 10% 3 8% 4 3 3 3 5% 6% 36%	2833
	11th to 20th	Night	14% 8% 4 9% 5% 6% 5% 4 6% 7% 32%	2569
	21st to end	Night	10% 6% 2 5% 4 3 4 3 5% 8% 49%	2738
June	1st to 10th	Night	11% 6% 2 7% 6% 5% 5% 5% 7% 8% 39%	1520
	11th to 20th	Night	9% 6% 2 6% 6% 6% 4 4 7% 8% 43%	1520
	21st to end	Night	12% 5% 2 5% 4 4 5% 4 6% 8% 44%	1520
July	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 6% 8% 49%	2520
	11th to 20th	Night	10% 4 2 6% 4 3 3 4 6% 9% 49%	2545
	21st to end	Night	8% 4 2 6% 4 4 4 4 7% 8% 49%	3087
August	1st to 10th	Night	8% 4 2 6% 4 5% 4 5% 7% 9% 47%	1701
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 6% 9% 49%	1643
	21st to end	Night	8% 5% 2 6% 5% 4 5% 3 6% 9% 49%	1862
September	1st to 10th	Night	13% 7% 2 6% 5% 4 6% 3 5% 7% 44%	1800
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 4 6% 54%	1912
	21st to end	Night	14% 7% 2 9% 5% 4 4 4 5% 7% 40%	2011
October	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 5% 6% 40%	2160
	11th to 20th	Night	20% 10% 3 7% 5% 4 4 4 5% 7% 32%	2160
	21st to end	Night	24% 9% 3 7% 5% 4 5% 4 5% 5% 28%	2376
November	1st to 10th	Night	15% 8% 3 6% 4 5% 4 4 5% 7% 39%	2431
	11th to 20th	Night	14% 11% 3 8% 6% 5% 5% 4 6% 8% 31%	2660
	21st to end	Night	18% 11% 4 8% 5% 5% 4 4 6% 7% 28%	2660
December	1st to 10th	Night	17% 15% 5% 10% 5% 6% 4 4 5% 7% 22%	2660
	11th to 20th	Night	14% 18% 6% 10% 5% 5% 4 4 5% 6% 23%	3288
	21st to end	Night	29% 16% 6% 10% 5% 5% 4 4 4 4 13%	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	59% 9% 14% 11% 7%	2518
	11th to 20th	Day	71% 8% 9% 8% 3	2813
	21st to end	Day	73% 8% 9% 7% 3	3124
February	1st to 10th	Day	68% 6% 9% 12% 4 1	1710
	11th to 20th	Day	62% 6% 11% 15% 6% 1	1672
	21st to end	Day	62% 7% 11% 13% 7% 1	1486
March	1st to 10th	Day	55% 7% 12% 20% 5% 1	3435
	11th to 20th	Day	52% 8% 15% 17% 7% 1	3480
	21st to end	Day	48% 8% 10% 22% 11% 1	3815
April	1st to 10th	Day	54% 5% 11% 17% 12% 1	2235
	11th to 20th	Day	48% 6% 11% 19% 14% 2	2318
	21st to end	Day	51% 6% 11% 17% 12% 2	2470
May	1st to 10th	Day	47% 7% 12% 19% 12% 2	4120
	11th to 20th	Day	49% 9% 14% 19% 7% 1	4326
	21st to end	Day	34% 8% 13% 25% 17% 3	4837
June	1st to 10th	Day	40% 8% 12% 22% 15% 2	2660
	11th to 20th	Day	38% 9% 14% 24% 14% 1	2660
	21st to end	Day	37% 8% 14% 25% 15% 1	2660
July	1st to 10th	Day	33% 8% 13% 27% 17% 2	4370
	11th to 20th	Day	34% 8% 14% 30% 13% 1	4336
	21st to end	Day	35% 9% 15% 22% 18% 1	4324
August	1st to 10th	Day	35% 8% 17% 25% 13% 2	2444
	11th to 20th	Day	37% 10% 17% 25% 11% 1	2357
	21st to end	Day	36% 8% 13% 28% 13% 1	2475
September	1st to 10th	Day	39% 9% 15% 24% 12% 1	2160
	11th to 20th	Day	35% 7% 14% 29% 16% 1	2044
	21st to end	Day	42% 8% 15% 24% 10% 1	1949
October	1st to 10th	Day	49% 6% 13% 19% 11% 1	1800
	11th to 20th	Day	53% 7% 12% 20% 8% 1	1800
	21st to end	Day	60% 7% 12% 15% 6% 1	1980
November	1st to 10th	Day	46% 7% 14% 22% 9% 1	1642
	11th to 20th	Day	52% 8% 15% 15% 10% 1	1520
	21st to end	Day	57% 7% 16% 15% 5%	1520
December	1st to 10th	Day	63% 8% 14% 12% 3	1520
	11th to 20th	Day	62% 8% 15% 10% 4	1906
	21st to end	Day	75% 8% 9% 7% 1	2772
January	1st to 10th	Night	60% 8% 15% 13% 4	4429
	11th to 20th	Night	62% 10% 13% 11% 3	4143
	21st to end	Night	68% 7% 10% 12% 3	4532
February	1st to 10th	Night	61% 7% 11% 15% 5% 1	2470
	11th to 20th	Night	56% 7% 12% 17% 7%	2405
	21st to end	Night	59% 6% 13% 15% 7%	1964
March	1st to 10th	Night	53% 6% 13% 21% 6% 1	3418
	11th to 20th	Night	47% 7% 15% 18% 11% 2	3480
	21st to end	Night	42% 6% 9% 24% 17% 2	3817
April	1st to 10th	Night	52% 6% 9% 19% 12% 1	1944
	11th to 20th	Night	49% 6% 8% 19% 15% 2	1827
	21st to end	Night	46% 7% 10% 19% 15% 3	1710
May	1st to 10th	Night	48% 6% 10% 19% 13% 4	2840
	11th to 20th	Night	45% 10% 15% 19% 8% 2	2570
	21st to end	Night	32% 7% 11% 25% 22% 2	2739
June	1st to 10th	Night	39% 8% 12% 22% 17% 1	1520
	11th to 20th	Night	37% 6% 12% 25% 18% 2	1520
	21st to end	Night	37% 6% 13% 25% 17% 2	1520
July	1st to 10th	Night	33% 6% 12% 23% 23% 4	2480
	11th to 20th	Night	31% 7% 14% 31% 16% 1	2497
	21st to end	Night	31% 8% 16% 24% 19% 2	2988
August	1st to 10th	Night	32% 7% 16% 28% 15% 2	1690
	11th to 20th	Night	31% 8% 16% 31% 13% 1	1643
	21st to end	Night	31% 8% 14% 29% 16% 2	1862
September	1st to 10th	Night	39% 7% 13% 25% 15% 2	1800
	11th to 20th	Night	31% 5% 13% 33% 18% 1	1912
	21st to end	Night	43% 6% 12% 24% 14% 1	2011
October	1st to 10th	Night	44% 6% 12% 22% 15% 1	2160
	11th to 20th	Night	50% 5% 15% 21% 9% 1	2160
	21st to end	Night	55% 6% 11% 16% 10% 1	2376
November	1st to 10th	Night	43% 6% 15% 23% 13% 1	2431
	11th to 20th	Night	49% 7% 15% 18% 10% 1	2660
	21st to end	Night	54% 8% 17% 16% 5%	2660
December	1st to 10th	Night	61% 8% 14% 13% 4	2660
	11th to 20th	Night	60% 9% 13% 13% 5%	3288
	21st to end	Night	72% 8% 11% 6% 2	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	21% 21% 31% 17% 9% 2	180
	11th to 20th	Day	25% 22% 35% 13% 4 1	264
	21st to end	Day	22% 21% 31% 21% 4 1	297
February	1st to 10th	Day	24% 20% 41% 11% 4	270
	11th to 20th	Day	24% 23% 37% 14% 1	270
	21st to end	Day	31% 25% 28% 15% 1	246
March	1st to 10th	Day	23% 15% 34% 21% 7%	460
	11th to 20th	Day	22% 16% 38% 18% 6% 1	460
	21st to end	Day	28% 13% 31% 20% 7% 1	506
April	1st to 10th	Day	21% 15% 34% 21% 8% 1	460
	11th to 20th	Day	23% 20% 24% 20% 9% 4	452
	21st to end	Day	25% 10% 31% 23% 10% 2	460
May	1st to 10th	Day	22% 9% 29% 25% 10% 4	463
	11th to 20th	Day	17% 8% 23% 29% 19% 4	487
	21st to end	Day	12% 9% 24% 29% 18% 7%	529
June	1st to 10th	Day	12% 10% 29% 32% 14% 3	490
	11th to 20th	Day	12% 7% 28% 29% 20% 4	490
	21st to end	Day	16% 7% 29% 30% 14% 5%	490
July	1st to 10th	Day	9% 5% 17% 31% 26% 11%	485
	11th to 20th	Day	15% 6% 21% 28% 20% 10%	484
	21st to end	Day	9% 4 24% 36% 19% 8%	532
August	1st to 10th	Day	11% 3 21% 30% 21% 13%	486
	11th to 20th	Day	12% 4 21% 30% 20% 14%	456
	21st to end	Day	13% 5% 22% 30% 19% 11%	495
September	1st to 10th	Day	14% 7% 21% 26% 20% 12%	450
	11th to 20th	Day	10% 6% 20% 23% 26% 15%	450
	21st to end	Day	15% 10% 28% 24% 16% 8%	450
October	1st to 10th	Day	20% 12% 25% 19% 19% 6%	450
	11th to 20th	Day	18% 18% 33% 21% 8% 2	450
	21st to end	Day	32% 14% 26% 20% 8%	495
November	1st to 10th	Day	16% 23% 32% 18% 8% 3	285
	11th to 20th	Day	22% 25% 26% 19% 7% 2	180
	21st to end	Day	24% 19% 39% 14% 3	180
December	1st to 10th	Day	26% 17% 34% 16% 5% 1	180
	11th to 20th	Day	26% 26% 32% 14% 3	180
	21st to end	Day	23% 22% 38% 14% 3	198
January	1st to 10th	Night	24% 16% 36% 17% 6% 1	760
	11th to 20th	Night	28% 17% 31% 20% 4	760
	21st to end	Night	19% 14% 38% 22% 6%	836
February	1st to 10th	Night	24% 19% 35% 20% 3	760
	11th to 20th	Night	21% 15% 35% 25% 4	743
	21st to end	Night	23% 15% 34% 25% 3	628
March	1st to 10th	Night	20% 11% 35% 27% 7%	760
	11th to 20th	Night	23% 13% 33% 23% 7% 1	760
	21st to end	Night	19% 11% 33% 27% 9% 1	836
April	1st to 10th	Night	22% 12% 32% 29% 5%	760
	11th to 20th	Night	21% 10% 34% 25% 9% 1	703
	21st to end	Night	19% 7% 31% 34% 9%	570
May	1st to 10th	Night	19% 5% 27% 30% 17% 1	570
	11th to 20th	Night	14% 4 25% 41% 14%	422
	21st to end	Night	14% 4 24% 35% 21% 2	411
June	1st to 10th	Night	12% 3 31% 37% 16% 1	380
	11th to 20th	Night	13% 3 28% 38% 18%	380
	21st to end	Night	12% 5% 24% 42% 17% 1	380
July	1st to 10th	Night	6% 3 20% 45% 25% 1	370
	11th to 20th	Night	12% 3 23% 40% 20% 3	380
	21st to end	Night	10% 3 18% 44% 23% 3	624
August	1st to 10th	Night	8% 4 13% 41% 29% 3	567
	11th to 20th	Night	8% 2 17% 35% 35% 3	549
	21st to end	Night	9% 3 20% 36% 29% 3	674
September	1st to 10th	Night	14% 6% 17% 33% 24% 6%	720
	11th to 20th	Night	13% 3 18% 31% 28% 6%	720
	21st to end	Night	15% 6% 23% 29% 21% 6%	720
October	1st to 10th	Night	18% 7% 26% 27% 20% 2	720
	11th to 20th	Night	22% 13% 28% 26% 10% 1	720
	21st to end	Night	28% 11% 29% 26% 6%	792
November	1st to 10th	Night	19% 11% 30% 27% 12% 2	740
	11th to 20th	Night	20% 15% 33% 21% 9% 2	760
	21st to end	Night	24% 12% 36% 21% 7% 1	760
December	1st to 10th	Night	27% 19% 31% 18% 6%	760
	11th to 20th	Night	23% 19% 33% 20% 5%	760
	21st to end	Night	30% 14% 32% 22% 2	836

Page generated at 2024-08-12 09:52:53. Version ac07

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