Blythewood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blythewood clear sky chart from 2005-11-06 to 2025-10-19.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	126612
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 44%	123115
Transparency	Day	38% 7% 12% 19% 18% 6%	130271
	Night	38% 6% 11% 18% 20% 7%	127525
Seeing	Day	16% 10% 25% 25% 18% 7%	16628
	Night	15% 8% 24% 31% 18% 2	26672

By Month

Percentage Cloud Cover SE (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% 5% 1 5% 3 3 2 2 4 6% 54%	4056
	11th to 20th	Day	11% 6% 2 4 3 3 2 2 3 5% 58%	4060
	21st to end	Day	16% 8% 3 6% 4 3 3 3 4 5% 45%	4466
February	1st to 10th	Day	14% 7% 3 6% 5% 4 5% 3 6% 6% 42%	1803
	11th to 20th	Day	22% 6% 2 5% 3 3 3 2 4 5% 44%	1948
	21st to end	Day	18% 7% 3 5% 4 4 3 3 5% 5% 43%	1715
March	1st to 10th	Day	21% 8% 3 7% 4 4 4 3 4 4 40%	4935
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 6% 54%	4980
	21st to end	Day	15% 6% 2 7% 4 4 3 3 4 5% 47%	5465
April	1st to 10th	Day	10% 4 2 5% 4 3 5% 4 6% 7% 50%	2200
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 7% 49%	2354
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 8% 50%	2517
May	1st to 10th	Day	10% 7% 2 7% 5% 5% 5% 4 7% 10% 39%	5862
	11th to 20th	Day	18% 7% 3 7% 4 5% 5% 4 7% 8% 31%	5861
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 6% 8% 44%	6433
June	1st to 10th	Day	9% 6% 2 7% 6% 5% 6% 5% 8% 10% 36%	2600
	11th to 20th	Day	7% 5% 2 5% 4 5% 6% 5% 8% 11% 41%	2600
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 5% 8% 12% 39%	2600
July	1st to 10th	Day	12% 6% 3 8% 6% 6% 6% 6% 9% 10% 28%	5900
	11th to 20th	Day	5% 4 2 7% 6% 6% 7% 7% 11% 15% 30%	5900
	21st to end	Day	8% 7% 3 8% 6% 6% 7% 6% 9% 13% 27%	6424
August	1st to 10th	Day	22% 7% 2 8% 5% 5% 6% 5% 7% 10% 22%	4234
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 8% 11% 33%	4174
	21st to end	Day	7% 4 2 5% 4 5% 5% 5% 8% 11% 46%	4350
September	1st to 10th	Day	7% 6% 2 7% 5% 6% 5% 4 7% 10% 41%	2204
	11th to 20th	Day	12% 7% 3 7% 4 4 5% 5% 6% 8% 40%	2200
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 7% 41%	2200
October	1st to 10th	Day	13% 6% 2 5% 4 4 3 4 5% 6% 49%	2085
	11th to 20th	Day	11% 5% 1 5% 3 3 3 2 4 5% 59%	1980
	21st to end	Day	11% 5% 2 5% 3 3 4 2 3 4 58%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 3 2 4 6% 53%	1957
	11th to 20th	Day	15% 6% 2 5% 2 2 3 3 4 6% 53%	1890
	21st to end	Day	14% 6% 2 5% 4 3 3 3 4 6% 50%	1800
December	1st to 10th	Day	12% 5% 2 6% 3 4 3 3 4 5% 51%	2930
	11th to 20th	Day	15% 6% 3 6% 3 4 4 3 5% 5% 46%	3373
	21st to end	Day	21% 6% 2 4 3 3 3 2 4 6% 45%	4466
January	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 3 5% 57%	5889
	11th to 20th	Night	12% 6% 2 6% 3 3 4 3 5% 6% 51%	5900
	21st to end	Night	15% 9% 2 6% 4 3 3 2 4 5% 46%	6490
February	1st to 10th	Night	16% 6% 2 5% 4 4 3 3 5% 6% 46%	2595
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 4 6% 43%	2338
	21st to end	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	1914
March	1st to 10th	Night	19% 8% 2 6% 4 3 3 3 5% 6% 40%	4918
	11th to 20th	Night	11% 4 1 3 3 3 4 3 5% 7% 55%	4980
	21st to end	Night	15% 5% 2 4 4 4 3 3 6% 7% 48%	5467
April	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 6% 7% 51%	2200
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 52%	1994
	21st to end	Night	9% 5% 2 5% 5% 3 4 4 6% 7% 52%	1898
May	1st to 10th	Night	10% 6% 3 6% 4 5% 5% 4 7% 10% 41%	4033
	11th to 20th	Night	18% 7% 2 6% 4 5% 5% 4 6% 9% 33%	4032
	21st to end	Night	12% 5% 2 5% 4 5% 4 4 7% 9% 42%	4421
June	1st to 10th	Night	12% 6% 3 6% 5% 5% 5% 4 9% 10% 34%	1800
	11th to 20th	Night	8% 6% 2 6% 6% 6% 5% 5% 9% 10% 38%	1800
	21st to end	Night	6% 6% 2 6% 5% 5% 7% 6% 10% 11% 35%	1800
July	1st to 10th	Night	13% 9% 3 9% 6% 6% 7% 5% 7% 10% 26%	4058
	11th to 20th	Night	7% 5% 2 7% 6% 6% 8% 7% 11% 15% 26%	4057
	21st to end	Night	10% 9% 4 11% 7% 7% 6% 6% 8% 10% 22%	4427
August	1st to 10th	Night	22% 8% 3 9% 6% 5% 5% 5% 7% 9% 22%	2915
	11th to 20th	Night	11% 7% 2 8% 5% 5% 5% 5% 8% 10% 34%	2875
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 11% 50%	3515
September	1st to 10th	Night	8% 6% 2 6% 5% 6% 5% 4 7% 11% 40%	2191
	11th to 20th	Night	12% 7% 2 8% 5% 5% 5% 3 6% 7% 40%	2200
	21st to end	Night	16% 7% 2 6% 4 3 5% 3 5% 6% 43%	2200
October	1st to 10th	Night	11% 7% 2 5% 3 3 3 3 5% 6% 52%	2315
	11th to 20th	Night	13% 5% 2 4 3 2 2 3 5% 6% 55%	2377
	21st to end	Night	10% 6% 2 4 3 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 2 4 5% 54%	2341
	11th to 20th	Night	14% 6% 3 4 3 3 3 3 4 5% 53%	2505
	21st to end	Night	13% 5% 2 5% 3 3 4 3 6% 7% 48%	2600
December	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 54%	4250
	11th to 20th	Night	17% 7% 3 6% 3 3 3 3 4 6% 45%	4822
	21st to end	Night	20% 7% 2 6% 3 3 4 3 4 5% 43%	6490

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	32% 3 5% 21% 31% 7%	4057
	11th to 20th	Day	31% 3 4 15% 37% 10%	4060
	21st to end	Day	42% 3 4 18% 22% 11%	4466
February	1st to 10th	Day	42% 4 9% 20% 19% 6%	1803
	11th to 20th	Day	44% 4 5% 17% 25% 5%	1958
	21st to end	Day	43% 4 7% 16% 21% 9%	1715
March	1st to 10th	Day	48% 3 6% 13% 20% 10%	4935
	11th to 20th	Day	30% 4 7% 14% 26% 18%	4980
	21st to end	Day	41% 4 6% 19% 23% 8%	5465
April	1st to 10th	Day	32% 6% 11% 20% 26% 6%	2200
	11th to 20th	Day	33% 5% 10% 21% 20% 10%	2354
	21st to end	Day	31% 6% 11% 25% 22% 4	2517
May	1st to 10th	Day	38% 7% 17% 24% 12% 2	5897
	11th to 20th	Day	48% 7% 16% 17% 12% 1	5865
	21st to end	Day	37% 7% 13% 19% 21% 4	6447
June	1st to 10th	Day	39% 9% 16% 24% 10% 2	2600
	11th to 20th	Day	33% 10% 17% 26% 12% 2	2600
	21st to end	Day	34% 11% 18% 21% 15% 2	2600
July	1st to 10th	Day	46% 11% 17% 19% 6% 1	5835
	11th to 20th	Day	37% 15% 31% 14% 2 1	5831
	21st to end	Day	43% 13% 16% 21% 6% 1	6240
August	1st to 10th	Day	54% 9% 18% 14% 4	4221
	11th to 20th	Day	41% 11% 15% 20% 13%	4213
	21st to end	Day	29% 9% 15% 30% 15% 2	4350
September	1st to 10th	Day	36% 9% 13% 19% 19% 4	2204
	11th to 20th	Day	41% 8% 11% 17% 19% 4	2200
	21st to end	Day	42% 5% 10% 22% 15% 7%	2200
October	1st to 10th	Day	33% 3 8% 18% 28% 9%	3397
	11th to 20th	Day	19% 2 5% 17% 28% 28%	3220
	21st to end	Day	26% 3 4 22% 30% 15%	3428
November	1st to 10th	Day	34% 4 7% 18% 26% 12%	1957
	11th to 20th	Day	33% 4 7% 21% 26% 10%	1890
	21st to end	Day	36% 4 8% 19% 24% 9%	1800
December	1st to 10th	Day	35% 4 6% 17% 28% 10%	2930
	11th to 20th	Day	39% 3 11% 20% 19% 6%	3370
	21st to end	Day	41% 3 8% 23% 24% 2	4466
January	1st to 10th	Night	31% 3 5% 19% 35% 7%	5889
	11th to 20th	Night	35% 3 6% 15% 33% 7%	5900
	21st to end	Night	41% 3 4 14% 29% 8%	6490
February	1st to 10th	Night	40% 4 8% 20% 20% 8%	2595
	11th to 20th	Night	44% 4 7% 17% 21% 7%	2339
	21st to end	Night	39% 4 9% 17% 21% 11%	1914
March	1st to 10th	Night	44% 4 7% 11% 25% 8%	4918
	11th to 20th	Night	29% 3 8% 12% 29% 19%	4980
	21st to end	Night	36% 4 7% 20% 21% 11%	5467
April	1st to 10th	Night	33% 5% 11% 20% 22% 9%	2200
	11th to 20th	Night	31% 5% 10% 21% 22% 11%	1994
	21st to end	Night	30% 6% 12% 27% 21% 5%	1898
May	1st to 10th	Night	38% 7% 15% 25% 14% 2	4060
	11th to 20th	Night	46% 8% 18% 17% 11%	4033
	21st to end	Night	35% 8% 13% 23% 19% 3	4429
June	1st to 10th	Night	40% 11% 17% 19% 12% 2	1800
	11th to 20th	Night	37% 11% 18% 22% 11% 2	1800
	21st to end	Night	36% 13% 19% 18% 12% 2	1800
July	1st to 10th	Night	49% 11% 19% 14% 5% 1	4015
	11th to 20th	Night	41% 17% 28% 11% 3 1	4006
	21st to end	Night	54% 12% 15% 14% 5%	4297
August	1st to 10th	Night	56% 9% 18% 13% 3 1	2904
	11th to 20th	Night	42% 9% 21% 19% 7% 1	2902
	21st to end	Night	28% 7% 15% 31% 17% 3	3515
September	1st to 10th	Night	36% 10% 15% 20% 14% 5%	2191
	11th to 20th	Night	42% 7% 12% 17% 18% 3	2200
	21st to end	Night	41% 7% 12% 18% 16% 5%	2200
October	1st to 10th	Night	33% 4 9% 18% 30% 6%	3783
	11th to 20th	Night	21% 3 5% 15% 26% 29%	3876
	21st to end	Night	25% 4 5% 19% 28% 19%	4120
November	1st to 10th	Night	33% 4 8% 15% 26% 13%	2341
	11th to 20th	Night	34% 4 6% 17% 26% 13%	2505
	21st to end	Night	34% 4 8% 20% 25% 9%	2600
December	1st to 10th	Night	34% 4 6% 17% 30% 9%	4250
	11th to 20th	Night	41% 4 10% 17% 19% 9%	4824
	21st to end	Night	43% 4 7% 23% 20% 3	6490

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% 26% 39% 11% 3	300
	11th to 20th	Day	22% 20% 38% 17% 3	300
	21st to end	Day	21% 27% 38% 12% 2	330
February	1st to 10th	Day	17% 23% 39% 16% 4	300
	11th to 20th	Day	37% 17% 37% 8% 1	300
	21st to end	Day	25% 22% 33% 16% 4	314
March	1st to 10th	Day	22% 17% 32% 22% 6%	500
	11th to 20th	Day	20% 18% 40% 17% 4 1	500
	21st to end	Day	20% 18% 36% 18% 7% 1	550
April	1st to 10th	Day	19% 18% 35% 18% 9%	500
	11th to 20th	Day	13% 17% 30% 25% 12% 3	492
	21st to end	Day	12% 10% 30% 28% 15% 4	500
May	1st to 10th	Day	10% 14% 35% 24% 14% 4	503
	11th to 20th	Day	18% 6% 22% 29% 18% 7%	527
	21st to end	Day	15% 6% 19% 26% 23% 11%	573
June	1st to 10th	Day	12% 5% 20% 29% 26% 9%	530
	11th to 20th	Day	8% 6% 19% 27% 26% 14%	530
	21st to end	Day	8% 2 14% 32% 30% 14%	530
July	1st to 10th	Day	9% 2 11% 25% 31% 22%	525
	11th to 20th	Day	8% 2 11% 27% 34% 17%	524
	21st to end	Day	7% 3 9% 34% 33% 14%	576
August	1st to 10th	Day	16% 5% 15% 22% 28% 14%	526
	11th to 20th	Day	14% 3 10% 28% 28% 16%	519
	21st to end	Day	13% 2 13% 28% 29% 15%	583
September	1st to 10th	Day	9% 4 13% 32% 30% 11%	530
	11th to 20th	Day	15% 4 14% 28% 29% 10%	530
	21st to end	Day	18% 5% 18% 27% 25% 7%	530
October	1st to 10th	Day	15% 6% 22% 29% 20% 8%	530
	11th to 20th	Day	11% 9% 25% 30% 20% 6%	522
	21st to end	Day	13% 6% 28% 30% 18% 5%	539
November	1st to 10th	Day	18% 8% 28% 26% 15% 4	495
	11th to 20th	Day	19% 14% 32% 25% 8% 1	390
	21st to end	Day	13% 15% 37% 24% 8% 2	300
December	1st to 10th	Day	26% 15% 40% 15% 4	300
	11th to 20th	Day	21% 13% 38% 19% 7% 2	300
	21st to end	Day	27% 16% 33% 16% 5% 2	330
January	1st to 10th	Night	21% 14% 32% 27% 6%	800
	11th to 20th	Night	20% 13% 32% 29% 6%	800
	21st to end	Night	20% 15% 34% 28% 3	880
February	1st to 10th	Night	19% 12% 34% 31% 4	800
	11th to 20th	Night	23% 15% 38% 23% 2	783
	21st to end	Night	18% 16% 33% 28% 5%	660
March	1st to 10th	Night	18% 13% 35% 30% 6%	800
	11th to 20th	Night	18% 13% 38% 24% 8%	800
	21st to end	Night	16% 14% 37% 25% 8%	880
April	1st to 10th	Night	16% 15% 33% 29% 7%	800
	11th to 20th	Night	13% 12% 32% 29% 13%	791
	21st to end	Night	14% 6% 30% 38% 12%	698
May	1st to 10th	Night	11% 8% 30% 38% 13%	600
	11th to 20th	Night	13% 5% 30% 34% 17% 2	591
	21st to end	Night	10% 3 17% 35% 32% 3	650
June	1st to 10th	Night	13% 4 16% 39% 25% 5%	600
	11th to 20th	Night	10% 3 15% 33% 31% 8%	600
	21st to end	Night	9% 2 14% 38% 31% 6%	600
July	1st to 10th	Night	9% 1 10% 38% 34% 7%	585
	11th to 20th	Night	10% 1 7% 35% 40% 8%	582
	21st to end	Night	8% 2 10% 35% 40% 5%	657
August	1st to 10th	Night	14% 3 10% 30% 37% 6%	597
	11th to 20th	Night	15% 3 9% 30% 37% 6%	594
	21st to end	Night	12% 2 12% 31% 37% 7%	835
September	1st to 10th	Night	10% 2 13% 32% 38% 6%	800
	11th to 20th	Night	15% 2 14% 30% 35% 4	800
	21st to end	Night	20% 4 13% 34% 25% 3	800
October	1st to 10th	Night	15% 5% 21% 31% 26% 4	800
	11th to 20th	Night	16% 7% 22% 32% 22% 2	793
	21st to end	Night	12% 8% 23% 32% 23% 2	836
November	1st to 10th	Night	16% 7% 25% 31% 18% 3	780
	11th to 20th	Night	16% 12% 28% 31% 12% 1	800
	21st to end	Night	14% 9% 28% 33% 15% 1	800
December	1st to 10th	Night	18% 13% 28% 32% 9% 1	800
	11th to 20th	Night	20% 9% 26% 34% 10% 1	800
	21st to end	Night	25% 14% 24% 29% 9%	880

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