Birdland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birdland Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 37%	74756
	Night	13% 9% 4 7% 6% 3 4 4 5% 7% 39%	75077
Transparency	Day	47% 4 13% 24% 11% 1	74457
	Night	46% 3 12% 24% 13% 2	74896
Seeing	Day	18% 9% 30% 27% 13% 4	14806
	Night	17% 9% 32% 30% 11% 1	24823

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	9% 10% 8% 9% 8% 5% 7% 5% 5% 8% 25%	1517
	11th to 20th	Day	10% 14% 7% 13% 8% 4 6% 4 6% 5% 22%	1484
	21st to end	Day	11% 13% 5% 13% 9% 5% 4 5% 6% 7% 20%	1782
February	1st to 10th	Day	8% 11% 6% 9% 8% 5% 6% 7% 6% 6% 27%	1620
	11th to 20th	Day	10% 12% 5% 9% 6% 4 5% 5% 5% 7% 31%	1574
	21st to end	Day	16% 11% 5% 7% 7% 4 6% 4 5% 6% 30%	1431
March	1st to 10th	Day	13% 11% 5% 10% 7% 3 4 3 5% 5% 33%	1898
	11th to 20th	Day	15% 11% 4 7% 5% 3 5% 4 4 7% 34%	1980
	21st to end	Day	20% 11% 4 6% 5% 4 5% 4 6% 6% 30%	2227
April	1st to 10th	Day	18% 11% 4 8% 5% 3 4 4 5% 8% 31%	2160
	11th to 20th	Day	20% 13% 3 7% 5% 3 3 4 5% 5% 32%	2134
	21st to end	Day	22% 12% 4 6% 5% 3 4 4 6% 5% 30%	2281
May	1st to 10th	Day	18% 8% 3 7% 6% 4 5% 5% 6% 8% 30%	2452
	11th to 20th	Day	17% 7% 3 6% 6% 3 5% 4 5% 7% 36%	2479
	21st to end	Day	16% 5% 3 6% 5% 3 5% 5% 6% 6% 39%	2725
June	1st to 10th	Day	12% 5% 3 5% 4 3 4 5% 6% 8% 46%	2520
	11th to 20th	Day	13% 6% 3 5% 6% 3 5% 5% 5% 8% 40%	2520
	21st to end	Day	11% 6% 3 6% 5% 3 5% 5% 6% 8% 40%	2520
July	1st to 10th	Day	7% 3 2 5% 5% 3 5% 5% 6% 9% 48%	2520
	11th to 20th	Day	8% 4 2 4 5% 3 5% 4 6% 10% 50%	2520
	21st to end	Day	7% 5% 2 4 5% 3 5% 6% 7% 10% 47%	2730
August	1st to 10th	Day	8% 5% 2 5% 5% 4 6% 6% 7% 10% 42%	2328
	11th to 20th	Day	13% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	2301
	21st to end	Day	9% 5% 2 4 5% 3 5% 4 5% 8% 49%	2417
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 5% 5% 7% 45%	2160
	11th to 20th	Day	11% 5% 3 4 5% 3 4 4 6% 6% 48%	2160
	21st to end	Day	14% 7% 3 6% 5% 3 4 4 6% 7% 41%	1886
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 5% 5% 44%	1800
	11th to 20th	Day	15% 10% 3 5% 5% 3 4 5% 5% 7% 39%	1800
	21st to end	Day	19% 12% 4 5% 4 3 3 4 4 5% 36%	1939
November	1st to 10th	Day	11% 10% 4 6% 5% 2 4 4 5% 7% 41%	1630
	11th to 20th	Day	13% 12% 8% 9% 8% 4 4 4 6% 6% 28%	1520
	21st to end	Day	12% 11% 5% 8% 5% 3 4 4 4 6% 38%	1520
December	1st to 10th	Day	13% 11% 5% 10% 9% 4 6% 5% 4 5% 28%	1520
	11th to 20th	Day	10% 11% 8% 8% 6% 4 5% 5% 6% 7% 29%	1918
	21st to end	Day	19% 16% 9% 16% 7% 3 3 4 3 4 18%	2783
January	1st to 10th	Night	9% 10% 6% 10% 8% 4 6% 5% 5% 6% 31%	2648
	11th to 20th	Night	12% 13% 6% 10% 8% 5% 5% 5% 6% 8% 24%	2598
	21st to end	Night	12% 14% 6% 11% 8% 4 5% 5% 6% 5% 24%	2574
February	1st to 10th	Night	6% 14% 5% 9% 7% 4 5% 5% 4 6% 34%	2340
	11th to 20th	Night	8% 10% 4 8% 7% 4 6% 4 4 6% 38%	2273
	21st to end	Night	14% 10% 5% 8% 7% 3 5% 4 4 5% 35%	1824
March	1st to 10th	Night	13% 11% 4 6% 5% 3 4 5% 5% 6% 37%	2075
	11th to 20th	Night	12% 11% 4 7% 6% 3 4 4 4 7% 37%	1980
	21st to end	Night	19% 9% 3 7% 6% 2 5% 4 6% 7% 33%	2129
April	1st to 10th	Night	16% 10% 4 6% 4 3 4 4 5% 6% 37%	1800
	11th to 20th	Night	21% 10% 3 6% 5% 2 3 3 4 6% 35%	1775
	21st to end	Night	20% 10% 3 5% 4 3 4 3 4 6% 37%	1675
May	1st to 10th	Night	19% 7% 3 6% 4 3 5% 4 6% 6% 39%	1508
	11th to 20th	Night	16% 5% 2 4 5% 3 4 5% 6% 8% 42%	1416
	21st to end	Night	14% 6% 2 6% 5% 3 5% 4 5% 9% 41%	1551
June	1st to 10th	Night	9% 3 2 5% 5% 3 4 4 6% 9% 49%	1440
	11th to 20th	Night	11% 5% 2 5% 6% 3 5% 4 6% 9% 45%	1440
	21st to end	Night	8% 4 2 3 5% 1 4 5% 5% 9% 54%	1440
July	1st to 10th	Night	5% 4 1 3 3 2 4 4 6% 10% 57%	1440
	11th to 20th	Night	5% 3 1 4 4 3 4 4 7% 8% 57%	1440
	21st to end	Night	6% 2 1 4 3 2 3 5% 5% 10% 59%	1584
August	1st to 10th	Night	6% 5% 2 5% 4 3 5% 6% 5% 9% 51%	1611
	11th to 20th	Night	11% 5% 2 4 5% 2 4 4 5% 6% 53%	1600
	21st to end	Night	8% 4 2 4 5% 3 4 4 4 7% 55%	1939
September	1st to 10th	Night	10% 5% 2 4 3 3 4 4 5% 7% 55%	1800
	11th to 20th	Night	12% 4 2 5% 4 2 3 4 5% 7% 52%	1800
	21st to end	Night	13% 6% 2 4 4 2 3 3 5% 7% 51%	2053
October	1st to 10th	Night	15% 6% 2 5% 4 2 3 4 4 7% 48%	2160
	11th to 20th	Night	15% 7% 3 6% 4 3 5% 4 5% 8% 41%	2160
	21st to end	Night	17% 10% 3 6% 5% 3 4 4 5% 6% 38%	2416
November	1st to 10th	Night	12% 9% 3 6% 4 2 4 4 4 7% 44%	2445
	11th to 20th	Night	13% 11% 5% 9% 6% 3 4 4 4 7% 34%	2660
	21st to end	Night	14% 10% 4 7% 6% 3 4 4 4 6% 38%	2660
December	1st to 10th	Night	11% 12% 5% 10% 8% 4 4 5% 5% 6% 30%	2660
	11th to 20th	Night	12% 12% 6% 9% 8% 4 6% 4 5% 6% 28%	3279
	21st to end	Night	22% 14% 6% 12% 7% 3 4 4 4 4 21%	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	58% 3 12% 18% 7%	1518
	11th to 20th	Day	64% 3 10% 13% 8%	1484
	21st to end	Day	62% 3 13% 13% 8% 1	1782
February	1st to 10th	Day	54% 4 14% 18% 9% 1	1620
	11th to 20th	Day	53% 4 12% 23% 8% 1	1583
	21st to end	Day	57% 3 10% 19% 11% 1	1431
March	1st to 10th	Day	53% 3 14% 21% 8% 1	1898
	11th to 20th	Day	51% 3 14% 22% 10%	1980
	21st to end	Day	56% 3 11% 23% 7% 1	2227
April	1st to 10th	Day	54% 3 11% 21% 10% 1	2160
	11th to 20th	Day	55% 3 10% 20% 11% 2	2134
	21st to end	Day	56% 3 12% 19% 8% 1	2281
May	1st to 10th	Day	52% 4 14% 21% 8% 1	2452
	11th to 20th	Day	47% 4 13% 24% 11% 1	2483
	21st to end	Day	46% 3 12% 23% 14% 2	2725
June	1st to 10th	Day	36% 4 13% 31% 14% 1	2520
	11th to 20th	Day	41% 5% 14% 23% 15% 1	2520
	21st to end	Day	41% 4 15% 27% 12% 2	2520
July	1st to 10th	Day	33% 5% 16% 31% 14% 1	2451
	11th to 20th	Day	31% 5% 14% 29% 18% 3	2445
	21st to end	Day	31% 5% 16% 31% 14% 3	2578
August	1st to 10th	Day	36% 5% 15% 33% 10% 2	2314
	11th to 20th	Day	39% 4 15% 29% 12% 1	2301
	21st to end	Day	33% 4 12% 31% 18% 2	2417
September	1st to 10th	Day	39% 4 12% 30% 15% 1	2160
	11th to 20th	Day	35% 4 16% 33% 11% 1	2160
	21st to end	Day	43% 3 11% 33% 10% 1	1886
October	1st to 10th	Day	42% 3 10% 30% 14% 1	1800
	11th to 20th	Day	45% 3 14% 23% 14% 1	1800
	21st to end	Day	51% 3 11% 25% 10%	1939
November	1st to 10th	Day	44% 3 14% 22% 15% 3	1630
	11th to 20th	Day	57% 3 12% 17% 10% 1	1520
	21st to end	Day	49% 4 18% 21% 8% 1	1520
December	1st to 10th	Day	60% 4 14% 15% 8% 1	1520
	11th to 20th	Day	54% 4 19% 17% 7%	1915
	21st to end	Day	74% 3 7% 13% 4	2783
January	1st to 10th	Night	54% 4 10% 21% 10% 1	2648
	11th to 20th	Night	59% 4 13% 16% 8% 1	2598
	21st to end	Night	61% 3 11% 16% 8% 1	2574
February	1st to 10th	Night	53% 2 12% 22% 9% 2	2340
	11th to 20th	Night	48% 4 12% 24% 10% 1	2274
	21st to end	Night	52% 3 10% 21% 12% 2	1824
March	1st to 10th	Night	48% 4 15% 23% 9% 1	2075
	11th to 20th	Night	49% 2 12% 23% 12% 1	1980
	21st to end	Night	51% 3 12% 23% 9% 2	2129
April	1st to 10th	Night	49% 3 10% 22% 14% 2	1800
	11th to 20th	Night	52% 2 9% 19% 15% 3	1775
	21st to end	Night	49% 2 10% 22% 14% 2	1675
May	1st to 10th	Night	46% 4 12% 23% 14% 1	1508
	11th to 20th	Night	40% 4 15% 24% 14% 2	1417
	21st to end	Night	42% 4 13% 24% 16% 1	1552
June	1st to 10th	Night	31% 6% 14% 30% 17% 2	1440
	11th to 20th	Night	37% 4 13% 25% 18% 3	1440
	21st to end	Night	28% 5% 17% 32% 16% 3	1440
July	1st to 10th	Night	24% 3 15% 38% 17% 3	1400
	11th to 20th	Night	24% 5% 15% 34% 18% 3	1392
	21st to end	Night	21% 3 18% 35% 18% 4	1496
August	1st to 10th	Night	30% 4 14% 32% 16% 3	1601
	11th to 20th	Night	33% 4 15% 30% 17% 2	1600
	21st to end	Night	30% 4 13% 30% 19% 4	1939
September	1st to 10th	Night	30% 4 10% 35% 20% 1	1800
	11th to 20th	Night	33% 2 14% 33% 17% 1	1800
	21st to end	Night	35% 3 14% 30% 18% 1	2053
October	1st to 10th	Night	38% 3 9% 32% 16% 2	2160
	11th to 20th	Night	42% 3 12% 26% 16% 1	2160
	21st to end	Night	48% 3 11% 24% 14% 1	2416
November	1st to 10th	Night	42% 3 13% 25% 15% 3	2445
	11th to 20th	Night	52% 4 11% 20% 12% 1	2660
	21st to end	Night	49% 3 16% 23% 8% 1	2660
December	1st to 10th	Night	55% 3 15% 20% 7% 1	2660
	11th to 20th	Night	57% 4 13% 18% 7% 1	3281
	21st to end	Night	69% 2 9% 15% 5%	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	15% 12% 38% 23% 10% 3	270
	11th to 20th	Day	22% 9% 39% 22% 5% 2	255
	21st to end	Day	17% 9% 41% 23% 9% 1	264
February	1st to 10th	Day	17% 14% 45% 19% 4	240
	11th to 20th	Day	18% 20% 37% 20% 5% 1	240
	21st to end	Day	24% 14% 34% 26% 2	198
March	1st to 10th	Day	20% 11% 42% 20% 7% 1	240
	11th to 20th	Day	25% 11% 35% 20% 8% 2	240
	21st to end	Day	27% 15% 27% 20% 7% 4	313
April	1st to 10th	Day	27% 13% 34% 19% 7%	420
	11th to 20th	Day	29% 14% 24% 23% 9% 1	412
	21st to end	Day	30% 10% 26% 24% 8% 2	541
May	1st to 10th	Day	22% 10% 34% 24% 8% 2	603
	11th to 20th	Day	21% 10% 28% 25% 11% 5%	625
	21st to end	Day	19% 7% 26% 28% 16% 3	679
June	1st to 10th	Day	10% 7% 27% 32% 17% 6%	630
	11th to 20th	Day	15% 7% 31% 32% 12% 3	630
	21st to end	Day	14% 8% 28% 28% 16% 6%	630
July	1st to 10th	Day	7% 4 21% 33% 24% 10%	621
	11th to 20th	Day	10% 4 28% 32% 19% 7%	618
	21st to end	Day	8% 4 26% 39% 17% 6%	684
August	1st to 10th	Day	10% 2 21% 36% 23% 8%	626
	11th to 20th	Day	15% 2 18% 34% 23% 8%	615
	21st to end	Day	12% 4 24% 33% 20% 7%	536
September	1st to 10th	Day	18% 5% 25% 31% 16% 6%	450
	11th to 20th	Day	14% 3 28% 31% 19% 4	450
	21st to end	Day	17% 12% 29% 27% 12% 2	292
October	1st to 10th	Day	17% 11% 26% 26% 17% 4	270
	11th to 20th	Day	19% 17% 35% 21% 7%	270
	21st to end	Day	30% 13% 33% 15% 7% 2	297
November	1st to 10th	Day	20% 14% 35% 21% 9% 1	270
	11th to 20th	Day	19% 20% 38% 19% 5%	270
	21st to end	Day	21% 10% 43% 21% 5%	270
December	1st to 10th	Day	16% 10% 41% 27% 5% 2	270
	11th to 20th	Day	20% 13% 39% 23% 5%	270
	21st to end	Day	29% 12% 38% 19% 3	297
January	1st to 10th	Night	14% 13% 39% 26% 7% 1	950
	11th to 20th	Night	19% 11% 37% 26% 6% 1	930
	21st to end	Night	19% 11% 36% 26% 7% 1	990
February	1st to 10th	Night	14% 14% 40% 26% 4 1	900
	11th to 20th	Night	15% 13% 42% 22% 6% 1	879
	21st to end	Night	20% 15% 34% 25% 7%	640
March	1st to 10th	Night	20% 12% 34% 25% 8% 1	635
	11th to 20th	Night	18% 11% 34% 30% 7% 1	540
	21st to end	Night	22% 8% 30% 28% 11% 2	594
April	1st to 10th	Night	21% 12% 35% 26% 5%	540
	11th to 20th	Night	30% 7% 30% 27% 7%	533
	21st to end	Night	27% 6% 32% 26% 9% 1	540
May	1st to 10th	Night	22% 8% 32% 31% 6% 1	540
	11th to 20th	Night	20% 5% 31% 33% 11% 1	531
	21st to end	Night	16% 6% 24% 38% 14% 1	584
June	1st to 10th	Night	11% 4 27% 41% 16% 1	540
	11th to 20th	Night	13% 3 34% 38% 12%	540
	21st to end	Night	11% 3 24% 44% 17% 1	540
July	1st to 10th	Night	7% 1 21% 38% 29% 3	525
	11th to 20th	Night	7% 4 23% 42% 22% 2	522
	21st to end	Night	6% 1 18% 52% 19% 3	591
August	1st to 10th	Night	8% 1 15% 47% 26% 2	537
	11th to 20th	Night	12% 3 18% 34% 29% 4	534
	21st to end	Night	10% 2 25% 37% 23% 3	594
September	1st to 10th	Night	13% 2 23% 40% 17% 5%	540
	11th to 20th	Night	14% 2 29% 34% 17% 3	540
	21st to end	Night	14% 9% 31% 31% 14% 1	638
October	1st to 10th	Night	18% 7% 29% 33% 13% 1	720
	11th to 20th	Night	19% 12% 32% 29% 7% 1	720
	21st to end	Night	23% 9% 31% 29% 7%	792
November	1st to 10th	Night	17% 11% 36% 26% 9% 1	779
	11th to 20th	Night	18% 14% 37% 24% 6% 1	950
	21st to end	Night	19% 14% 37% 22% 8%	950
December	1st to 10th	Night	17% 11% 39% 25% 7% 1	950
	11th to 20th	Night	21% 9% 38% 24% 7% 1	950
	21st to end	Night	25% 12% 32% 24% 6%	1045

Page generated at 2024-01-13 08:37:54. Version ac07

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