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 2024-06-03.

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% 4 8% 5% 4 4 4 5% 7% 34%	88886
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 34%	89652
Transparency	Day	48% 7% 12% 20% 11% 1	88628
	Night	49% 7% 12% 20% 12% 2	89496
Seeing	Day	17% 10% 28% 24% 15% 6%	15045
	Night	19% 10% 30% 29% 12% 1	25003

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% 15% 10% 12% 6% 4 4 4 6% 6% 18%	2527
	11th to 20th	Day	14% 16% 7% 14% 7% 6% 5% 7% 8% 5% 11%	2530
	21st to end	Day	26% 23% 6% 12% 3 4 3 2 3 4 14%	2810
February	1st to 10th	Day	16% 16% 5% 11% 7% 5% 5% 4 4 4 24%	1851
	11th to 20th	Day	15% 13% 5% 12% 6% 5% 4 3 5% 6% 26%	1848
	21st to end	Day	16% 15% 6% 10% 5% 4 4 3 5% 5% 26%	1570
March	1st to 10th	Day	19% 16% 5% 9% 5% 4 4 3 4 5% 26%	3120
	11th to 20th	Day	12% 10% 5% 8% 5% 4 4 3 6% 7% 36%	3367
	21st to end	Day	19% 11% 5% 10% 6% 5% 5% 3 5% 6% 25%	3991
April	1st to 10th	Day	16% 13% 4 8% 5% 4 5% 4 5% 6% 31%	2280
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 5% 4 7% 35%	2258
	21st to end	Day	19% 9% 3 7% 4 4 4 4 5% 8% 33%	2470
May	1st to 10th	Day	18% 9% 3 7% 4 5% 5% 4 7% 8% 30%	4082
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 4 6% 8% 38%	4081
	21st to end	Day	10% 5% 2 6% 5% 4 5% 4 6% 9% 44%	4691
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 8% 43%	2549
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	2520
	21st to end	Day	11% 6% 2 6% 6% 5% 5% 4 6% 9% 40%	2520
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 5% 6% 9% 46%	2520
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 10% 45%	2520
	21st to end	Day	7% 5% 1 6% 5% 6% 6% 5% 6% 9% 44%	2724
August	1st to 10th	Day	7% 4 2 5% 5% 6% 5% 4 6% 10% 45%	2327
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 5% 7% 9% 42%	2256
	21st to end	Day	7% 5% 2 6% 5% 4 4 4 6% 10% 46%	2376
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 3 5% 8% 44%	2138
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 7% 8% 46%	1980
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 44%	1980
October	1st to 10th	Day	14% 6% 2 6% 4 5% 4 3 6% 7% 43%	1980
	11th to 20th	Day	17% 8% 3 7% 5% 4 4 4 6% 6% 37%	1799
	21st to end	Day	21% 11% 3 9% 6% 4 4 4 5% 7% 28%	1782
November	1st to 10th	Day	11% 8% 2 7% 4 3 4 3 4 7% 46%	1672
	11th to 20th	Day	16% 15% 4 8% 5% 4 4 3 4 7% 31%	1710
	21st to end	Day	19% 12% 5% 9% 5% 4 4 3 5% 6% 28%	1710
December	1st to 10th	Day	15% 16% 5% 11% 6% 5% 4 3 5% 6% 24%	1646
	11th to 20th	Day	13% 17% 8% 12% 6% 4 4 3 4 6% 24%	1918
	21st to end	Day	23% 22% 8% 14% 6% 4 3 3 3 3 11%	2783
January	1st to 10th	Night	17% 15% 7% 12% 6% 4 4 3 5% 5% 20%	4428
	11th to 20th	Night	14% 20% 8% 16% 6% 4 5% 3 5% 4 15%	4440
	21st to end	Night	25% 17% 9% 10% 4 3 4 3 3 4 17%	4853
February	1st to 10th	Night	18% 14% 5% 10% 6% 4 3 3 5% 6% 26%	2327
	11th to 20th	Night	13% 13% 4 8% 6% 6% 4 3 5% 6% 33%	2218
	21st to end	Night	21% 13% 3 9% 5% 4 4 3 4 5% 30%	1884
March	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 5% 6% 34%	3733
	11th to 20th	Night	10% 8% 3 7% 5% 5% 4 4 6% 7% 43%	3591
	21st to end	Night	19% 10% 4 8% 4 4 4 4 5% 6% 32%	3668
April	1st to 10th	Night	15% 9% 2 6% 5% 4 5% 4 6% 8% 36%	1900
	11th to 20th	Night	20% 7% 3 6% 4 5% 3 3 6% 7% 38%	1870
	21st to end	Night	18% 9% 3 7% 4 4 4 4 6% 6% 36%	1710
May	1st to 10th	Night	17% 8% 2 7% 6% 4 6% 3 6% 8% 32%	2813
	11th to 20th	Night	19% 7% 3 5% 4 3 4 3 6% 8% 38%	2812
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 5% 9% 44%	2843
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 9% 49%	1461
	11th to 20th	Night	9% 6% 2 7% 4 4 4 4 5% 9% 46%	1440
	21st to end	Night	10% 5% 2 6% 5% 3 5% 4 7% 10% 43%	1440
July	1st to 10th	Night	5% 5% 2 6% 5% 5% 4 3 6% 8% 50%	1440
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 6% 9% 52%	1440
	21st to end	Night	5% 4 2 5% 4 5% 6% 5% 7% 9% 50%	1593
August	1st to 10th	Night	7% 3 2 5% 5% 4 4 4 7% 10% 50%	1611
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 8% 50%	1642
	21st to end	Night	8% 4 2 5% 4 4 3 4 6% 8% 52%	1980
September	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 4 8% 50%	1804
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 8% 55%	1980
	21st to end	Night	12% 6% 2 5% 4 3 4 3 5% 6% 49%	1980
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 6% 6% 49%	1980
	11th to 20th	Night	17% 9% 3 6% 3 4 4 3 4 6% 42%	2157
	21st to end	Night	21% 9% 3 6% 5% 4 3 3 5% 6% 36%	2574
November	1st to 10th	Night	13% 9% 2 7% 4 4 3 3 6% 6% 43%	2406
	11th to 20th	Night	14% 13% 4 8% 4 4 4 4 5% 8% 33%	2470
	21st to end	Night	18% 14% 4 8% 5% 4 4 3 5% 6% 29%	2470
December	1st to 10th	Night	17% 13% 5% 8% 6% 5% 5% 3 5% 7% 26%	2531
	11th to 20th	Night	16% 16% 7% 11% 5% 5% 3 3 5% 4 23%	3279
	21st to end	Night	26% 19% 7% 10% 5% 4 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	64% 7% 12% 12% 5%	2528
	11th to 20th	Day	69% 8% 9% 9% 5%	2530
	21st to end	Day	76% 4 7% 10% 3	2810
February	1st to 10th	Day	63% 5% 10% 13% 7% 1	1851
	11th to 20th	Day	58% 7% 11% 17% 7%	1858
	21st to end	Day	58% 6% 12% 16% 6% 1	1570
March	1st to 10th	Day	60% 6% 8% 18% 7%	3120
	11th to 20th	Day	45% 7% 13% 27% 8% 1	3367
	21st to end	Day	58% 9% 14% 16% 3	3991
April	1st to 10th	Day	53% 6% 12% 19% 11%	2280
	11th to 20th	Day	48% 6% 11% 21% 11% 3	2258
	21st to end	Day	48% 8% 13% 18% 11% 3	2470
May	1st to 10th	Day	47% 10% 13% 22% 8%	4120
	11th to 20th	Day	42% 9% 15% 25% 9% 1	4085
	21st to end	Day	33% 9% 13% 23% 20% 2	4691
June	1st to 10th	Day	37% 8% 12% 24% 16% 2	2549
	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	63% 8% 11% 13% 5%	4428
	11th to 20th	Night	73% 3 9% 11% 4	4440
	21st to end	Night	71% 4 8% 12% 4	4853
February	1st to 10th	Night	59% 6% 12% 14% 8% 2	2327
	11th to 20th	Night	53% 6% 11% 20% 10% 1	2219
	21st to end	Night	56% 5% 11% 15% 10% 2	1884
March	1st to 10th	Night	49% 7% 12% 23% 8% 1	3733
	11th to 20th	Night	36% 8% 12% 26% 14% 2	3591
	21st to end	Night	50% 8% 14% 21% 6% 1	3668
April	1st to 10th	Night	44% 8% 13% 20% 14% 1	1900
	11th to 20th	Night	45% 7% 11% 19% 17% 2	1870
	21st to end	Night	46% 6% 12% 20% 13% 3	1710
May	1st to 10th	Night	47% 10% 10% 21% 11% 1	2840
	11th to 20th	Night	42% 9% 13% 26% 10% 1	2813
	21st to end	Night	35% 9% 12% 20% 20% 3	2844
June	1st to 10th	Night	32% 6% 13% 27% 19% 3	1461
	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	23% 15% 42% 16% 3 1	270
	21st to end	Day	31% 14% 31% 19% 4	297
February	1st to 10th	Day	25% 17% 36% 17% 4 1	270
	11th to 20th	Day	26% 17% 36% 17% 4	270
	21st to end	Day	25% 20% 41% 11% 4	225
March	1st to 10th	Day	23% 20% 37% 17% 3	270
	11th to 20th	Day	24% 15% 32% 22% 7% 1	386
	21st to end	Day	31% 16% 27% 18% 6% 2	506
April	1st to 10th	Day	21% 15% 34% 21% 8% 1	460
	11th to 20th	Day	19% 19% 31% 19% 9% 3	452
	21st to end	Day	26% 13% 31% 19% 8% 2	460
May	1st to 10th	Day	24% 13% 31% 20% 10% 3	463
	11th to 20th	Day	21% 8% 27% 23% 17% 4	487
	21st to end	Day	12% 6% 29% 26% 18% 9%	665
June	1st to 10th	Day	12% 8% 26% 29% 19% 6%	641
	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	24% 14% 38% 20% 4 1	950
	21st to end	Night	22% 14% 36% 24% 4	1027
February	1st to 10th	Night	27% 11% 35% 24% 4	760
	11th to 20th	Night	21% 13% 39% 24% 3	743
	21st to end	Night	27% 15% 37% 18% 3	628
March	1st to 10th	Night	20% 12% 36% 26% 5% 1	760
	11th to 20th	Night	18% 13% 37% 26% 6%	760
	21st to end	Night	26% 11% 31% 27% 5%	751
April	1st to 10th	Night	18% 13% 37% 26% 6% 1	570
	11th to 20th	Night	21% 12% 35% 25% 7% 1	563
	21st to end	Night	21% 9% 35% 25% 9% 1	570
May	1st to 10th	Night	21% 8% 32% 30% 8%	570
	11th to 20th	Night	20% 5% 28% 31% 15% 1	561
	21st to end	Night	12% 4 25% 39% 18% 1	617
June	1st to 10th	Night	11% 3 25% 43% 17%	548
	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

Page generated at 2024-05-31 06:01:40. Version ac07

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