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 2024-11-12.

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	14% 9% 4 8% 5% 4 4 4 6% 7% 35%	101211
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	98017
Transparency	Day	45% 7% 12% 21% 13% 1	104824
	Night	46% 6% 11% 21% 14% 2	102545
Seeing	Day	17% 10% 28% 24% 15% 6%	15722
	Night	18% 9% 29% 29% 12% 1	25551

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%	4716
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 5% 8% 43%	2660
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 4 7% 8% 44%	2660
	21st to end	Day	11% 6% 2 6% 6% 5% 5% 4 6% 9% 40%	2660
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 7% 11% 46%	4440
	21st to end	Day	7% 7% 2 7% 6% 6% 6% 4 6% 8% 41%	4768
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 6% 9% 46%	4104
	11th to 20th	Day	9% 4 2 6% 4 5% 5% 4 7% 8% 46%	3955
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 10% 48%	4180
September	1st to 10th	Day	10% 6% 2 6% 4 4 5% 3 5% 8% 46%	2256
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 8% 47%	2090
	21st to end	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 42%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2090
	11th to 20th	Day	16% 7% 3 7% 4 4 4 5% 6% 6% 38%	1898
	21st to end	Day	20% 10% 2 9% 6% 4 4 4 5% 7% 29%	1881
November	1st to 10th	Day	11% 8% 2 7% 4 4 4 3 4 7% 45%	1762
	11th to 20th	Day	16% 14% 4 8% 5% 4 4 3 4 7% 31%	1719
	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%	2844
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 9% 49%	1520
	11th to 20th	Night	9% 6% 2 7% 4 4 4 4 5% 9% 46%	1520
	21st to end	Night	10% 5% 2 6% 5% 4 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	9% 5% 2 6% 5% 5% 4 4 6% 9% 45%	2530
	11th to 20th	Night	7% 4 2 4 4 4 4 3 6% 9% 53%	2530
	21st to end	Night	7% 6% 2 6% 5% 5% 6% 4 8% 9% 43%	2780
August	1st to 10th	Night	11% 5% 3 5% 4 5% 4 5% 6% 9% 44%	2825
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 49%	2871
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 51%	3476
September	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 4 7% 52%	1905
	11th to 20th	Night	9% 4 2 4 3 3 3 3 4 9% 56%	2090
	21st to end	Night	13% 7% 2 5% 4 3 4 3 5% 6% 47%	2090
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 2 6% 6% 50%	2090
	11th to 20th	Night	16% 9% 3 7% 3 4 4 3 4 6% 43%	2278
	21st to end	Night	20% 8% 3 6% 5% 3 4 3 5% 7% 37%	2717
November	1st to 10th	Night	13% 9% 3 7% 4 4 3 3 6% 6% 43%	2536
	11th to 20th	Night	14% 13% 4 8% 4 4 4 4 5% 8% 33%	2484
	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	4729
June	1st to 10th	Day	37% 8% 12% 24% 16% 2	2660
	11th to 20th	Day	35% 7% 12% 24% 19% 3	2660
	21st to end	Day	39% 9% 16% 18% 15% 3	2660
July	1st to 10th	Day	42% 6% 13% 21% 16% 3	4371
	11th to 20th	Day	29% 7% 16% 29% 18% 1	4365
	21st to end	Day	39% 7% 15% 17% 19% 3	4618
August	1st to 10th	Day	34% 7% 16% 27% 15% 1	4091
	11th to 20th	Day	33% 7% 14% 28% 17% 1	3994
	21st to end	Day	30% 7% 14% 26% 20% 3	4179
September	1st to 10th	Day	36% 7% 15% 22% 17% 3	2256
	11th to 20th	Day	32% 7% 12% 26% 21% 2	2090
	21st to end	Day	39% 6% 11% 23% 19% 2	2090
October	1st to 10th	Day	28% 4 11% 32% 24% 1	3480
	11th to 20th	Day	37% 6% 10% 21% 26%	3127
	21st to end	Day	45% 4 10% 27% 12% 2	3081
November	1st to 10th	Day	39% 6% 13% 24% 16% 1	1762
	11th to 20th	Day	54% 4 11% 19% 10% 1	1719
	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	2852
June	1st to 10th	Night	32% 6% 13% 28% 18% 3	1520
	11th to 20th	Night	34% 7% 12% 25% 19% 3	1520
	21st to end	Night	35% 9% 16% 21% 15% 3	1520
July	1st to 10th	Night	37% 5% 13% 25% 14% 6%	2490
	11th to 20th	Night	27% 5% 15% 26% 24% 2	2482
	21st to end	Night	35% 6% 16% 16% 22% 4	2691
August	1st to 10th	Night	34% 8% 15% 29% 14% 1	2814
	11th to 20th	Night	32% 7% 12% 30% 18% 1	2898
	21st to end	Night	31% 7% 11% 27% 21% 3	3476
September	1st to 10th	Night	31% 7% 13% 24% 19% 6%	1905
	11th to 20th	Night	25% 7% 9% 31% 26% 3	2090
	21st to end	Night	37% 7% 11% 23% 20% 3	2090
October	1st to 10th	Night	23% 4 8% 33% 30% 1	3480
	11th to 20th	Night	36% 3 8% 23% 29% 1	3788
	21st to end	Night	37% 4 10% 28% 20% 1	4467
November	1st to 10th	Night	40% 5% 12% 24% 18% 2	2536
	11th to 20th	Night	49% 5% 12% 20% 12% 1	2484
	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% 19% 9%	665
June	1st to 10th	Day	12% 9% 27% 28% 19% 5%	680
	11th to 20th	Day	10% 6% 29% 28% 20% 7%	680
	21st to end	Day	14% 6% 27% 31% 16% 6%	680
July	1st to 10th	Day	11% 3 19% 28% 23% 16%	671
	11th to 20th	Day	8% 3 25% 30% 23% 12%	668
	21st to end	Day	8% 4 20% 32% 26% 10%	718
August	1st to 10th	Day	8% 2 15% 28% 31% 16%	486
	11th to 20th	Day	11% 3 19% 30% 25% 12%	479
	21st to end	Day	9% 2 15% 30% 31% 13%	539
September	1st to 10th	Day	13% 6% 19% 26% 26% 11%	490
	11th to 20th	Day	8% 5% 14% 26% 30% 17%	490
	21st to end	Day	16% 10% 21% 25% 21% 7%	490
October	1st to 10th	Day	13% 5% 28% 29% 17% 8%	490
	11th to 20th	Day	17% 19% 28% 25% 9% 2	400
	21st to end	Day	28% 14% 33% 21% 3 1	330
November	1st to 10th	Day	12% 14% 35% 26% 10% 3	300
	11th to 20th	Day	23% 18% 31% 19% 8%	273
	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% 40% 18% 1	617
June	1st to 10th	Night	11% 4 26% 42% 17% 1	570
	11th to 20th	Night	10% 4 26% 42% 19% 1	570
	21st to end	Night	12% 3 24% 37% 23% 1	570
July	1st to 10th	Night	8% 2 16% 42% 28% 4	555
	11th to 20th	Night	10% 3 16% 39% 29% 3	552
	21st to end	Night	6% 2 15% 43% 32% 2	624
August	1st to 10th	Night	8% 1 11% 44% 31% 5%	567
	11th to 20th	Night	9% 3 14% 41% 29% 4	564
	21st to end	Night	9% 2 15% 35% 35% 4	627
September	1st to 10th	Night	11% 4 16% 38% 28% 3	575
	11th to 20th	Night	10% 4 15% 36% 31% 4	760
	21st to end	Night	16% 6% 24% 32% 20% 3	760
October	1st to 10th	Night	13% 6% 25% 36% 18% 3	760
	11th to 20th	Night	20% 13% 30% 28% 10%	760
	21st to end	Night	24% 13% 31% 26% 6%	836
November	1st to 10th	Night	17% 12% 34% 26% 10% 1	780
	11th to 20th	Night	21% 16% 32% 24% 6% 1	765
	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 2024-11-08 22:45:37. Version ac07

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