Birch Rd. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birch Rd. Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	74857
	Night	15% 9% 4 7% 6% 3 4 4 5% 7% 36%	74906
Transparency	Day	47% 7% 12% 20% 12% 2	74561
	Night	48% 6% 12% 20% 13% 2	74723
Seeing	Day	17% 10% 27% 24% 15% 6%	14529
	Night	19% 9% 29% 29% 13% 1	24488

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% 14% 5% 11% 8% 3 3 5% 6% 6% 20%	1517
	11th to 20th	Day	16% 18% 7% 10% 7% 5% 5% 4 5% 5% 16%	1461
	21st to end	Day	16% 18% 8% 10% 6% 5% 5% 4 4 7% 19%	1598
February	1st to 10th	Day	16% 15% 6% 10% 8% 4 5% 4 4 4 22%	1754
	11th to 20th	Day	16% 12% 5% 12% 8% 4 5% 4 5% 6% 24%	1748
	21st to end	Day	17% 15% 8% 9% 5% 3 4 4 5% 5% 25%	1480
March	1st to 10th	Day	17% 12% 5% 8% 7% 4 5% 4 4 7% 28%	1800
	11th to 20th	Day	16% 10% 5% 7% 6% 4 5% 5% 6% 7% 30%	1909
	21st to end	Day	24% 12% 4 7% 6% 3 4 3 3 7% 26%	2272
April	1st to 10th	Day	17% 11% 5% 6% 6% 3 4 5% 5% 6% 32%	2160
	11th to 20th	Day	17% 10% 2 6% 6% 3 4 5% 4 7% 34%	2137
	21st to end	Day	19% 9% 3 6% 5% 3 5% 5% 5% 7% 33%	2340
May	1st to 10th	Day	17% 9% 4 7% 5% 4 5% 5% 6% 8% 31%	2340
	11th to 20th	Day	19% 8% 3 5% 7% 3 4 4 5% 6% 35%	2301
	21st to end	Day	12% 5% 3 6% 6% 3 5% 5% 6% 9% 39%	2659
June	1st to 10th	Day	11% 5% 2 5% 6% 3 5% 5% 5% 8% 43%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 5% 7% 8% 44%	2520
	21st to end	Day	11% 5% 2 5% 6% 4 6% 5% 6% 9% 40%	2520
July	1st to 10th	Day	8% 5% 2 5% 5% 3 5% 6% 6% 9% 46%	2520
	11th to 20th	Day	8% 4 2 4 5% 4 5% 6% 7% 10% 45%	2520
	21st to end	Day	7% 4 2 5% 6% 4 6% 6% 6% 10% 44%	2724
August	1st to 10th	Day	7% 4 3 4 6% 4 6% 5% 6% 10% 45%	2327
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 6% 7% 9% 42%	2256
	21st to end	Day	8% 5% 2 5% 5% 3 5% 5% 6% 10% 46%	2376
September	1st to 10th	Day	11% 6% 2 5% 5% 3 6% 4 5% 8% 44%	2138
	11th to 20th	Day	11% 5% 2 4 5% 3 4 5% 6% 8% 46%	1980
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 5% 8% 44%	1980
October	1st to 10th	Day	14% 6% 3 5% 5% 3 4 4 6% 7% 43%	1980
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 5% 6% 6% 37%	1799
	21st to end	Day	21% 10% 3 8% 7% 3 4 5% 5% 6% 28%	1782
November	1st to 10th	Day	11% 7% 3 6% 5% 3 3 5% 4 7% 46%	1672
	11th to 20th	Day	17% 13% 4 7% 6% 3 4 3 4 7% 31%	1710
	21st to end	Day	20% 11% 6% 8% 6% 3 5% 4 5% 6% 28%	1710
December	1st to 10th	Day	16% 15% 6% 10% 7% 4 4 4 4 6% 25%	1646
	11th to 20th	Day	13% 16% 9% 11% 7% 2 4 3 4 6% 24%	1918
	21st to end	Day	23% 21% 9% 12% 7% 3 3 3 3 4 12%	2783
January	1st to 10th	Night	17% 12% 5% 11% 8% 3 5% 5% 6% 6% 23%	2648
	11th to 20th	Night	18% 16% 6% 12% 7% 3 4 2 4 5% 24%	2561
	21st to end	Night	16% 13% 8% 11% 8% 4 5% 5% 4 5% 22%	2755
February	1st to 10th	Night	19% 13% 6% 9% 8% 3 4 4 5% 6% 24%	2204
	11th to 20th	Night	14% 13% 5% 7% 7% 4 5% 4 4 6% 31%	2098
	21st to end	Night	21% 12% 4 9% 5% 3 4 4 4 5% 30%	1776
March	1st to 10th	Night	15% 10% 5% 7% 7% 4 4 4 5% 5% 33%	2160
	11th to 20th	Night	14% 9% 3 6% 6% 3 5% 5% 6% 7% 38%	2049
	21st to end	Night	21% 10% 4 6% 5% 3 4 5% 5% 6% 32%	2098
April	1st to 10th	Night	15% 7% 3 5% 6% 3 5% 5% 5% 8% 37%	1800
	11th to 20th	Night	20% 6% 3 5% 5% 3 4 4 5% 7% 38%	1771
	21st to end	Night	19% 8% 3 6% 4 2 5% 5% 6% 6% 36%	1620
May	1st to 10th	Night	19% 7% 3 7% 6% 3 5% 5% 6% 6% 32%	1620
	11th to 20th	Night	19% 6% 3 5% 6% 3 5% 4 6% 7% 36%	1592
	21st to end	Night	10% 5% 3 5% 5% 4 5% 6% 6% 9% 41%	1613
June	1st to 10th	Night	9% 5% 1 5% 5% 3 4 3 5% 9% 50%	1440
	11th to 20th	Night	9% 6% 2 6% 5% 3 4 5% 5% 9% 47%	1440
	21st to end	Night	10% 4 2 5% 6% 2 5% 6% 6% 10% 43%	1440
July	1st to 10th	Night	5% 5% 3 5% 6% 3 5% 4 6% 9% 50%	1440
	11th to 20th	Night	9% 4 2 4 3 3 5% 4 6% 9% 52%	1440
	21st to end	Night	5% 4 2 4 4 3 6% 5% 6% 9% 50%	1593
August	1st to 10th	Night	7% 3 2 4 5% 3 4 6% 7% 10% 50%	1611
	11th to 20th	Night	7% 4 2 4 5% 2 5% 5% 5% 9% 50%	1642
	21st to end	Night	8% 4 2 4 4 3 4 5% 6% 8% 52%	1980
September	1st to 10th	Night	11% 6% 2 4 5% 3 4 4 4 8% 50%	1804
	11th to 20th	Night	9% 4 2 3 3 2 3 4 4 8% 55%	1980
	21st to end	Night	12% 6% 2 4 4 3 4 4 5% 6% 49%	1980
October	1st to 10th	Night	12% 5% 2 5% 4 3 5% 3 5% 6% 49%	1980
	11th to 20th	Night	17% 8% 3 5% 4 3 4 4 4 6% 42%	2157
	21st to end	Night	21% 9% 3 5% 5% 3 4 4 5% 6% 36%	2574
November	1st to 10th	Night	13% 8% 3 6% 5% 3 4 4 6% 6% 44%	2406
	11th to 20th	Night	14% 12% 4 7% 5% 3 4 4 5% 8% 33%	2470
	21st to end	Night	18% 12% 5% 7% 5% 3 5% 4 5% 6% 29%	2470
December	1st to 10th	Night	17% 13% 6% 7% 7% 4 5% 4 5% 7% 26%	2531
	11th to 20th	Night	16% 15% 8% 10% 6% 3 4 4 4 4 24%	3279
	21st to end	Night	26% 18% 7% 9% 6% 3 4 3 4 5% 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	63% 6% 12% 12% 7%	1518
	11th to 20th	Day	68% 8% 11% 9% 4	1461
	21st to end	Day	66% 5% 12% 11% 5%	1598
February	1st to 10th	Day	66% 5% 9% 12% 7% 1	1754
	11th to 20th	Day	60% 6% 10% 16% 7%	1758
	21st to end	Day	60% 6% 11% 16% 6% 1	1480
March	1st to 10th	Day	58% 5% 10% 20% 7%	1800
	11th to 20th	Day	52% 7% 13% 17% 10% 1	1909
	21st to end	Day	60% 5% 11% 18% 5% 1	2272
April	1st to 10th	Day	52% 6% 11% 19% 11%	2160
	11th to 20th	Day	49% 6% 11% 21% 11% 3	2137
	21st to end	Day	49% 8% 13% 16% 11% 3	2340
May	1st to 10th	Day	51% 8% 12% 21% 8% 1	2340
	11th to 20th	Day	49% 7% 10% 20% 13% 1	2305
	21st to end	Day	41% 8% 12% 23% 14% 2	2659
June	1st to 10th	Day	37% 8% 12% 24% 16% 2	2520
	11th to 20th	Day	35% 8% 13% 23% 18% 3	2520
	21st to end	Day	39% 10% 16% 18% 15% 2	2520
July	1st to 10th	Day	33% 8% 14% 24% 18% 4	2451
	11th to 20th	Day	31% 9% 16% 25% 16% 2	2445
	21st to end	Day	34% 9% 15% 25% 16% 2	2573
August	1st to 10th	Day	33% 8% 16% 28% 13% 2	2314
	11th to 20th	Day	34% 10% 16% 24% 15% 2	2256
	21st to end	Day	31% 9% 14% 23% 19% 5%	2376
September	1st to 10th	Day	37% 7% 16% 20% 16% 3	2138
	11th to 20th	Day	34% 7% 12% 26% 19% 2	1980
	21st to end	Day	37% 6% 11% 24% 19% 2	1980
October	1st to 10th	Day	39% 6% 13% 22% 17% 2	1980
	11th to 20th	Day	44% 7% 11% 21% 16%	1799
	21st to end	Day	55% 6% 11% 17% 9% 1	1782
November	1st to 10th	Day	39% 7% 13% 24% 17% 1	1672
	11th to 20th	Day	54% 5% 11% 19% 10% 1	1710
	21st to end	Day	56% 6% 13% 18% 6% 1	1710
December	1st to 10th	Day	61% 7% 11% 15% 6%	1646
	11th to 20th	Day	62% 7% 10% 18% 3	1915
	21st to end	Day	79% 4 7% 7% 3	2783
January	1st to 10th	Night	61% 6% 10% 15% 8%	2648
	11th to 20th	Night	65% 4 12% 13% 6%	2561
	21st to end	Night	64% 6% 11% 13% 6%	2755
February	1st to 10th	Night	61% 6% 11% 13% 7% 1	2204
	11th to 20th	Night	54% 6% 11% 18% 9% 1	2099
	21st to end	Night	57% 5% 11% 15% 10% 2	1776
March	1st to 10th	Night	52% 6% 12% 19% 10% 1	2160
	11th to 20th	Night	44% 6% 13% 22% 12% 3	2049
	21st to end	Night	53% 6% 10% 20% 10% 1	2098
April	1st to 10th	Night	43% 8% 13% 21% 13% 2	1800
	11th to 20th	Night	45% 7% 11% 19% 17% 2	1771
	21st to end	Night	46% 6% 12% 20% 13% 3	1620
May	1st to 10th	Night	50% 8% 10% 20% 10% 1	1620
	11th to 20th	Night	45% 8% 11% 21% 14% 2	1593
	21st to end	Night	37% 9% 14% 21% 15% 3	1614
June	1st to 10th	Night	31% 6% 13% 27% 19% 3	1440
	11th to 20th	Night	34% 7% 12% 25% 18% 3	1440
	21st to end	Night	35% 10% 16% 22% 15% 3	1440
July	1st to 10th	Night	32% 7% 14% 24% 18% 5%	1400
	11th to 20th	Night	29% 7% 15% 26% 21% 3	1392
	21st to end	Night	28% 8% 17% 23% 20% 6%	1504
August	1st to 10th	Night	27% 9% 14% 31% 19% 1	1600
	11th to 20th	Night	30% 8% 15% 25% 20% 2	1642
	21st to end	Night	28% 9% 12% 24% 22% 5%	1980
September	1st to 10th	Night	33% 7% 14% 24% 17% 6%	1804
	11th to 20th	Night	26% 7% 9% 31% 25% 2	1980
	21st to end	Night	35% 7% 11% 24% 21% 3	1980
October	1st to 10th	Night	35% 6% 10% 27% 20% 1	1980
	11th to 20th	Night	44% 5% 9% 23% 18% 1	2157
	21st to end	Night	48% 5% 12% 20% 13% 1	2574
November	1st to 10th	Night	40% 6% 12% 23% 18% 2	2406
	11th to 20th	Night	49% 5% 12% 20% 12% 1	2470
	21st to end	Night	55% 5% 13% 19% 7%	2470
December	1st to 10th	Night	57% 7% 12% 15% 8%	2531
	11th to 20th	Night	62% 7% 11% 13% 6% 1	3281
	21st to end	Night	74% 6% 10% 8% 3	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% 19% 37% 17% 6% 1	270
	11th to 20th	Day	24% 13% 42% 17% 3 2	248
	21st to end	Day	29% 14% 34% 18% 4	264
February	1st to 10th	Day	27% 17% 38% 17% 1 1	240
	11th to 20th	Day	28% 17% 39% 13% 3	240
	21st to end	Day	26% 19% 42% 10% 3	198
March	1st to 10th	Day	23% 20% 37% 18% 2	240
	11th to 20th	Day	24% 13% 32% 23% 7% 1	349
	21st to end	Day	33% 16% 26% 17% 6% 2	462
April	1st to 10th	Day	21% 14% 33% 21% 9% 1	420
	11th to 20th	Day	19% 18% 31% 19% 9% 3	412
	21st to end	Day	27% 13% 31% 19% 8% 3	420
May	1st to 10th	Day	25% 13% 31% 19% 10% 3	423
	11th to 20th	Day	23% 8% 27% 23% 16% 4	447
	21st to end	Day	13% 5% 28% 26% 18% 9%	613
June	1st to 10th	Day	12% 8% 27% 29% 19% 6%	630
	11th to 20th	Day	11% 6% 28% 27% 20% 7%	630
	21st to end	Day	14% 6% 26% 31% 17% 7%	630
July	1st to 10th	Day	10% 3 18% 28% 25% 17%	621
	11th to 20th	Day	8% 3 24% 31% 22% 12%	618
	21st to end	Day	8% 4 20% 32% 26% 10%	665
August	1st to 10th	Day	8% 2 13% 28% 33% 17%	446
	11th to 20th	Day	11% 3 17% 31% 26% 13%	439
	21st to end	Day	10% 2 16% 28% 31% 14%	495
September	1st to 10th	Day	14% 5% 18% 26% 26% 12%	450
	11th to 20th	Day	9% 6% 15% 25% 30% 16%	450
	21st to end	Day	13% 10% 20% 26% 22% 8%	450
October	1st to 10th	Day	14% 5% 29% 28% 16% 8%	450
	11th to 20th	Day	19% 20% 28% 25% 8%	365
	21st to end	Day	31% 14% 33% 20% 2 1	297
November	1st to 10th	Day	11% 14% 36% 25% 11% 3	270
	11th to 20th	Day	23% 19% 31% 19% 9%	270
	21st to end	Day	30% 14% 34% 19% 4	270
December	1st to 10th	Day	20% 18% 39% 14% 9%	270
	11th to 20th	Day	20% 19% 40% 16% 6%	270
	21st to end	Day	27% 19% 38% 14% 1	297
January	1st to 10th	Night	23% 15% 36% 20% 5%	950
	11th to 20th	Night	25% 12% 38% 20% 4 1	915
	21st to end	Night	21% 14% 36% 25% 4	973
February	1st to 10th	Night	28% 11% 35% 23% 3	720
	11th to 20th	Night	22% 14% 38% 24% 3	703
	21st to end	Night	28% 14% 36% 18% 3	592
March	1st to 10th	Night	20% 12% 36% 26% 6% 1	720
	11th to 20th	Night	19% 12% 37% 26% 6%	720
	21st to end	Night	26% 10% 31% 27% 5%	712
April	1st to 10th	Night	18% 12% 37% 26% 7% 1	540
	11th to 20th	Night	20% 11% 35% 25% 7% 1	533
	21st to end	Night	21% 9% 34% 25% 10% 1	540
May	1st to 10th	Night	22% 8% 32% 29% 9%	540
	11th to 20th	Night	20% 5% 28% 31% 14% 1	531
	21st to end	Night	12% 3 25% 40% 18% 1	584
June	1st to 10th	Night	11% 3 25% 43% 18%	540
	11th to 20th	Night	10% 4 25% 41% 19% 1	540
	21st to end	Night	11% 3 24% 37% 24% 1	540
July	1st to 10th	Night	7% 1 16% 42% 29% 4	525
	11th to 20th	Night	11% 2 16% 39% 29% 3	522
	21st to end	Night	6% 2 15% 43% 32% 2	591
August	1st to 10th	Night	8% 1 11% 44% 32% 5%	537
	11th to 20th	Night	9% 3 14% 41% 30% 4	534
	21st to end	Night	9% 3 16% 35% 35% 4	594
September	1st to 10th	Night	12% 4 16% 37% 28% 3	544
	11th to 20th	Night	11% 4 15% 35% 31% 5%	720
	21st to end	Night	14% 6% 24% 32% 21% 3	720
October	1st to 10th	Night	14% 6% 25% 35% 18% 3	720
	11th to 20th	Night	21% 13% 31% 27% 9%	720
	21st to end	Night	25% 13% 31% 25% 6%	792
November	1st to 10th	Night	16% 12% 35% 26% 11% 1	740
	11th to 20th	Night	21% 15% 32% 24% 6% 1	760
	21st to end	Night	24% 14% 32% 24% 6%	760
December	1st to 10th	Night	24% 14% 36% 21% 5%	821
	11th to 20th	Night	23% 12% 37% 23% 5%	950
	21st to end	Night	30% 14% 33% 20% 3	1045

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