Birchwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birchwood clear sky chart from 2005-11-06 to 2024-01-12.

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	13% 6% 3 6% 5% 3 5% 5% 6% 8% 40%	74850
	Night	13% 6% 3 5% 5% 3 5% 5% 6% 7% 43%	74873
Transparency	Day	40% 8% 12% 21% 16% 3	74573
	Night	39% 7% 12% 21% 18% 4	74669
Seeing	Day	16% 8% 21% 24% 20% 11%	13608
	Night	16% 7% 23% 31% 20% 3	23703

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	18% 8% 3 6% 5% 3 4 5% 4 6% 38%	1706
	11th to 20th	Day	20% 10% 4 6% 5% 3 4 3 3 5% 36%	1629
	21st to end	Day	17% 6% 3 5% 4 2 4 4 5% 6% 44%	1854
February	1st to 10th	Day	20% 9% 3 6% 5% 3 4 4 5% 6% 35%	1800
	11th to 20th	Day	21% 7% 3 6% 5% 3 5% 3 5% 6% 34%	1748
	21st to end	Day	21% 7% 3 6% 5% 3 3 4 4 6% 37%	1480
March	1st to 10th	Day	15% 6% 2 5% 5% 3 5% 4 4 7% 43%	1804
	11th to 20th	Day	20% 7% 3 6% 6% 3 4 4 5% 6% 36%	1980
	21st to end	Day	15% 8% 3 7% 5% 3 5% 5% 6% 7% 36%	2178
April	1st to 10th	Day	12% 5% 2 5% 5% 2 6% 5% 7% 8% 44%	2118
	11th to 20th	Day	14% 6% 2 5% 4 3 3 5% 6% 7% 45%	2134
	21st to end	Day	12% 4 2 6% 5% 3 4 5% 6% 9% 43%	2287
May	1st to 10th	Day	10% 5% 3 5% 5% 4 5% 5% 5% 7% 45%	2340
	11th to 20th	Day	11% 6% 3 6% 6% 4 6% 6% 6% 8% 39%	2301
	21st to end	Day	9% 5% 2 5% 6% 3 6% 6% 8% 10% 40%	2531
June	1st to 10th	Day	8% 5% 2 6% 6% 3 6% 7% 7% 11% 38%	2340
	11th to 20th	Day	5% 3 2 4 6% 4 7% 7% 9% 11% 44%	2340
	21st to end	Day	7% 4 2 5% 6% 4 7% 6% 7% 11% 40%	2340
July	1st to 10th	Day	7% 6% 2 8% 8% 5% 7% 7% 8% 11% 30%	2340
	11th to 20th	Day	10% 5% 3 5% 7% 4 7% 7% 9% 11% 32%	2340
	21st to end	Day	7% 5% 3 7% 8% 5% 8% 8% 9% 10% 31%	2547
August	1st to 10th	Day	8% 5% 2 6% 7% 5% 7% 8% 9% 12% 31%	2327
	11th to 20th	Day	9% 6% 3 7% 7% 4 7% 7% 8% 8% 34%	2147
	21st to end	Day	11% 6% 3 7% 6% 4 6% 7% 8% 9% 34%	2376
September	1st to 10th	Day	10% 5% 2 6% 7% 3 6% 6% 7% 8% 40%	2039
	11th to 20th	Day	15% 5% 1 5% 6% 4 6% 5% 6% 8% 40%	1980
	21st to end	Day	13% 4 2 4 4 3 4 4 5% 7% 49%	1980
October	1st to 10th	Day	10% 5% 2 5% 3 2 4 4 4 5% 54%	1980
	11th to 20th	Day	12% 5% 2 3 3 3 3 3 4 7% 54%	1904
	21st to end	Day	15% 4 1 4 4 2 4 4 5% 7% 50%	1901
November	1st to 10th	Day	10% 6% 2 4 4 2 3 5% 3 6% 54%	1672
	11th to 20th	Day	13% 5% 3 5% 6% 2 3 4 5% 6% 48%	1710
	21st to end	Day	17% 6% 2 4 4 3 4 4 5% 6% 45%	1710
December	1st to 10th	Day	21% 8% 3 6% 5% 3 4 4 5% 5% 36%	1710
	11th to 20th	Day	14% 9% 3 6% 6% 4 5% 4 5% 5% 38%	2153
	21st to end	Day	20% 9% 4 6% 6% 3 4 4 5% 8% 32%	3124
January	1st to 10th	Night	19% 8% 2 5% 5% 3 4 4 4 5% 40%	2459
	11th to 20th	Night	19% 8% 3 7% 5% 3 4 5% 5% 6% 34%	2360
	21st to end	Night	15% 8% 4 6% 5% 3 5% 4 5% 6% 40%	2498
February	1st to 10th	Night	16% 7% 3 5% 6% 3 4 4 5% 6% 41%	2160
	11th to 20th	Night	21% 7% 4 6% 5% 3 4 3 4 4 38%	2098
	21st to end	Night	24% 9% 3 5% 5% 3 4 4 4 6% 34%	1776
March	1st to 10th	Night	15% 7% 2 5% 4 3 4 4 5% 7% 45%	2155
	11th to 20th	Night	17% 7% 3 5% 6% 3 4 4 5% 7% 39%	1980
	21st to end	Night	14% 8% 3 5% 6% 4 5% 4 5% 6% 41%	2178
April	1st to 10th	Night	12% 4 2 4 4 3 4 4 5% 8% 50%	1855
	11th to 20th	Night	13% 6% 2 4 4 2 3 3 5% 8% 50%	1775
	21st to end	Night	11% 5% 2 5% 4 3 5% 5% 5% 8% 48%	1670
May	1st to 10th	Night	9% 5% 2 6% 5% 3 5% 5% 6% 8% 46%	1620
	11th to 20th	Night	10% 5% 2 6% 5% 3 6% 5% 6% 8% 43%	1592
	21st to end	Night	7% 6% 2 6% 5% 4 6% 5% 7% 9% 43%	1744
June	1st to 10th	Night	6% 5% 3 6% 6% 4 6% 7% 5% 10% 41%	1620
	11th to 20th	Night	4 3 1 4 4 3 6% 5% 8% 12% 51%	1620
	21st to end	Night	5% 3 2 5% 6% 4 6% 6% 8% 11% 45%	1620
July	1st to 10th	Night	9% 5% 2 6% 6% 3 7% 6% 9% 11% 35%	1620
	11th to 20th	Night	8% 5% 3 5% 5% 3 4 6% 8% 11% 40%	1620
	21st to end	Night	6% 5% 2 7% 6% 4 6% 6% 9% 12% 36%	1771
August	1st to 10th	Night	6% 5% 2 5% 6% 4 6% 6% 9% 13% 39%	1611
	11th to 20th	Night	7% 6% 3 6% 6% 4 5% 5% 7% 7% 43%	1750
	21st to end	Night	9% 5% 3 5% 6% 4 7% 6% 8% 10% 38%	1980
September	1st to 10th	Night	9% 6% 3 6% 6% 4 5% 5% 6% 8% 41%	1903
	11th to 20th	Night	13% 5% 2 6% 6% 3 5% 5% 6% 8% 41%	1980
	21st to end	Night	11% 5% 3 4 4 3 4 4 5% 7% 50%	1980
October	1st to 10th	Night	12% 4 2 4 3 3 2 3 4 7% 57%	1980
	11th to 20th	Night	13% 5% 2 5% 5% 2 3 5% 4 5% 52%	2056
	21st to end	Night	14% 4 2 4 4 3 3 4 5% 5% 52%	2452
November	1st to 10th	Night	12% 4 2 5% 4 3 3 4 4 6% 53%	2406
	11th to 20th	Night	12% 6% 2 4 4 2 4 3 5% 6% 51%	2470
	21st to end	Night	16% 7% 2 4 4 3 4 4 4 7% 44%	2470
December	1st to 10th	Night	20% 8% 3 4 5% 3 6% 4 5% 5% 36%	2470
	11th to 20th	Night	16% 6% 3 5% 4 2 4 5% 4 6% 43%	3042
	21st to end	Night	19% 8% 3 7% 6% 4 6% 6% 6% 7% 28%	4532

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	46% 5% 9% 21% 17% 3	1707
	11th to 20th	Day	52% 4 8% 20% 14% 2	1629
	21st to end	Day	41% 4 9% 26% 17% 2	1854
February	1st to 10th	Day	50% 4 9% 20% 16% 2	1800
	11th to 20th	Day	51% 5% 8% 18% 16% 3	1758
	21st to end	Day	49% 5% 8% 15% 17% 6%	1480
March	1st to 10th	Day	41% 5% 10% 18% 20% 6%	1804
	11th to 20th	Day	49% 6% 10% 18% 15% 3	1980
	21st to end	Day	46% 7% 10% 23% 13% 1	2178
April	1st to 10th	Day	37% 7% 11% 23% 18% 3	2118
	11th to 20th	Day	38% 6% 8% 19% 23% 5%	2134
	21st to end	Day	38% 7% 10% 27% 16% 2	2287
May	1st to 10th	Day	36% 9% 12% 26% 15% 3	2340
	11th to 20th	Day	41% 10% 14% 20% 13% 1	2305
	21st to end	Day	35% 11% 15% 23% 14% 2	2531
June	1st to 10th	Day	37% 10% 16% 22% 14% 1	2340
	11th to 20th	Day	29% 10% 20% 23% 15% 2	2340
	21st to end	Day	34% 12% 19% 17% 13% 4	2340
July	1st to 10th	Day	42% 12% 16% 17% 11% 2	2275
	11th to 20th	Day	41% 11% 20% 15% 10% 4	2271
	21st to end	Day	40% 13% 17% 21% 9% 1	2405
August	1st to 10th	Day	41% 13% 19% 19% 7% 1	2314
	11th to 20th	Day	42% 12% 14% 18% 13% 1	2147
	21st to end	Day	43% 10% 15% 17% 11% 3	2376
September	1st to 10th	Day	38% 10% 13% 18% 16% 5%	2039
	11th to 20th	Day	40% 7% 12% 20% 19% 2	1980
	21st to end	Day	34% 6% 11% 21% 21% 7%	1980
October	1st to 10th	Day	33% 5% 10% 21% 24% 8%	1980
	11th to 20th	Day	30% 5% 9% 24% 25% 7%	1904
	21st to end	Day	34% 4 8% 23% 27% 5%	1901
November	1st to 10th	Day	32% 5% 8% 21% 28% 6%	1672
	11th to 20th	Day	36% 4 7% 23% 25% 6%	1710
	21st to end	Day	40% 4 8% 23% 20% 4	1710
December	1st to 10th	Day	51% 4 7% 21% 14% 3	1710
	11th to 20th	Day	46% 5% 9% 20% 17% 3	2150
	21st to end	Day	48% 6% 11% 22% 10% 2	3124
January	1st to 10th	Night	46% 4 10% 20% 17% 3	2459
	11th to 20th	Night	50% 6% 9% 17% 16% 3	2360
	21st to end	Night	44% 4 7% 23% 19% 3	2498
February	1st to 10th	Night	45% 4 11% 20% 17% 3	2160
	11th to 20th	Night	51% 4 7% 18% 15% 5%	2099
	21st to end	Night	52% 5% 7% 17% 15% 5%	1776
March	1st to 10th	Night	40% 5% 9% 18% 22% 7%	2155
	11th to 20th	Night	45% 6% 10% 20% 17% 3	1980
	21st to end	Night	44% 6% 9% 23% 15% 2	2178
April	1st to 10th	Night	32% 5% 12% 24% 23% 4	1855
	11th to 20th	Night	34% 4 9% 24% 22% 8%	1775
	21st to end	Night	34% 6% 14% 28% 17% 2	1670
May	1st to 10th	Night	35% 6% 12% 24% 21% 3	1620
	11th to 20th	Night	36% 10% 17% 23% 13% 1	1593
	21st to end	Night	34% 11% 15% 23% 15% 2	1745
June	1st to 10th	Night	36% 11% 15% 22% 13% 2	1620
	11th to 20th	Night	23% 9% 23% 30% 14% 2	1620
	21st to end	Night	30% 11% 19% 21% 13% 5%	1620
July	1st to 10th	Night	38% 12% 18% 17% 12% 2	1575
	11th to 20th	Night	33% 11% 21% 18% 10% 6%	1566
	21st to end	Night	34% 13% 20% 22% 10% 2	1672
August	1st to 10th	Night	34% 11% 20% 26% 9% 1	1600
	11th to 20th	Night	36% 12% 18% 17% 15% 2	1750
	21st to end	Night	37% 12% 18% 20% 12% 2	1980
September	1st to 10th	Night	37% 8% 17% 20% 14% 4	1903
	11th to 20th	Night	38% 9% 13% 17% 22% 1	1980
	21st to end	Night	32% 6% 10% 22% 23% 7%	1980
October	1st to 10th	Night	29% 4 10% 22% 28% 8%	1980
	11th to 20th	Night	34% 5% 8% 21% 23% 9%	2056
	21st to end	Night	34% 4 5% 22% 26% 9%	2452
November	1st to 10th	Night	32% 4 8% 23% 25% 9%	2406
	11th to 20th	Night	35% 4 6% 22% 26% 8%	2470
	21st to end	Night	41% 4 7% 22% 22% 4	2470
December	1st to 10th	Night	49% 6% 8% 19% 15% 3	2470
	11th to 20th	Night	39% 6% 10% 20% 23% 2	3044
	21st to end	Night	49% 7% 12% 19% 12% 1	4532

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	23% 20% 29% 23% 4 1	270
	11th to 20th	Day	26% 21% 30% 13% 9% 2	243
	21st to end	Day	20% 16% 44% 14% 5%	264
February	1st to 10th	Day	27% 21% 28% 20% 4	240
	11th to 20th	Day	38% 14% 31% 15% 2	240
	21st to end	Day	32% 19% 34% 12% 3	198
March	1st to 10th	Day	23% 16% 38% 16% 5% 1	244
	11th to 20th	Day	22% 17% 31% 22% 7% 1	420
	21st to end	Day	21% 12% 34% 24% 8% 1	462
April	1st to 10th	Day	16% 15% 26% 29% 13% 2	420
	11th to 20th	Day	17% 14% 30% 24% 15% 2	412
	21st to end	Day	16% 10% 30% 28% 12% 5%	420
May	1st to 10th	Day	13% 11% 27% 28% 14% 7%	423
	11th to 20th	Day	12% 5% 24% 27% 22% 11%	447
	21st to end	Day	13% 5% 18% 23% 23% 20%	485
June	1st to 10th	Day	10% 4 16% 27% 27% 16%	450
	11th to 20th	Day	6% 2 13% 26% 29% 24%	450
	21st to end	Day	10% 3 12% 28% 27% 19%	450
July	1st to 10th	Day	9% 3 10% 21% 30% 26%	445
	11th to 20th	Day	10% 2 8% 23% 31% 25%	444
	21st to end	Day	9% 2 7% 22% 35% 26%	488
August	1st to 10th	Day	13% 3 7% 22% 35% 21%	446
	11th to 20th	Day	12% 2 11% 26% 34% 16%	439
	21st to end	Day	12% 2 10% 20% 35% 22%	495
September	1st to 10th	Day	10% 2 8% 24% 34% 21%	450
	11th to 20th	Day	16% 1 12% 21% 32% 19%	450
	21st to end	Day	15% 3 12% 24% 34% 12%	450
October	1st to 10th	Day	12% 4 18% 25% 27% 13%	450
	11th to 20th	Day	13% 9% 23% 30% 21% 5%	450
	21st to end	Day	18% 8% 21% 30% 19% 4	416
November	1st to 10th	Day	19% 7% 24% 29% 15% 6%	270
	11th to 20th	Day	12% 14% 32% 33% 9% 1	270
	21st to end	Day	13% 11% 36% 25% 14% 1	270
December	1st to 10th	Day	30% 10% 29% 24% 6% 1	270
	11th to 20th	Day	19% 15% 31% 23% 11% 1	270
	21st to end	Day	28% 15% 28% 20% 8% 1	297
January	1st to 10th	Night	23% 13% 37% 21% 5%	760
	11th to 20th	Night	21% 15% 31% 26% 6%	727
	21st to end	Night	19% 12% 36% 31% 2	792
February	1st to 10th	Night	20% 12% 35% 28% 5%	720
	11th to 20th	Night	25% 14% 36% 23% 2	703
	21st to end	Night	28% 13% 35% 21% 2	592
March	1st to 10th	Night	18% 12% 33% 33% 4	720
	11th to 20th	Night	21% 11% 33% 26% 8% 1	720
	21st to end	Night	18% 11% 31% 32% 8%	792
April	1st to 10th	Night	14% 10% 30% 36% 10%	595
	11th to 20th	Night	16% 7% 33% 30% 14%	533
	21st to end	Night	13% 6% 26% 40% 14% 1	540
May	1st to 10th	Night	10% 6% 27% 36% 20% 1	540
	11th to 20th	Night	13% 4 21% 31% 25% 5%	531
	21st to end	Night	10% 3 12% 33% 37% 6%	584
June	1st to 10th	Night	8% 2 14% 36% 35% 5%	540
	11th to 20th	Night	5% 1 11% 35% 37% 11%	540
	21st to end	Night	6% 2 8% 34% 40% 10%	540
July	1st to 10th	Night	12% 2 9% 25% 42% 11%	525
	11th to 20th	Night	9% 2 8% 25% 41% 14%	522
	21st to end	Night	7% 1 9% 25% 45% 12%	591
August	1st to 10th	Night	7% 1 6% 27% 47% 12%	537
	11th to 20th	Night	9% 2 10% 32% 40% 6%	534
	21st to end	Night	10% 2 8% 24% 45% 10%	594
September	1st to 10th	Night	9% 2 12% 28% 40% 9%	643
	11th to 20th	Night	15% 3 10% 28% 39% 5%	720
	21st to end	Night	13% 2 16% 31% 34% 3	720
October	1st to 10th	Night	13% 5% 16% 30% 28% 8%	720
	11th to 20th	Night	15% 6% 23% 34% 22% 2	720
	21st to end	Night	15% 7% 20% 39% 16% 2	792
November	1st to 10th	Night	14% 7% 20% 37% 20% 2	740
	11th to 20th	Night	14% 9% 31% 33% 12% 1	760
	21st to end	Night	20% 8% 26% 35% 10% 1	760
December	1st to 10th	Night	24% 12% 25% 30% 8%	760
	11th to 20th	Night	20% 8% 31% 33% 8%	760
	21st to end	Night	25% 11% 26% 31% 7%	836

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