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 2024-12-09.

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% 3 8% 5% 4 4 4 6% 7% 37%	101869
	Night	12% 10% 3 8% 5% 4 4 3 5% 6% 39%	98028
Transparency	Day	46% 3 12% 26% 12% 1	105463
	Night	45% 3 12% 25% 14% 2	102574
Seeing	Day	18% 9% 30% 27% 13% 4	16088
	Night	17% 9% 32% 30% 11% 1	26000

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	8% 16% 9% 19% 6% 5% 5% 3 4 6% 20%	2527
	11th to 20th	Day	7% 12% 5% 14% 8% 7% 6% 7% 9% 4 19%	2534
	21st to end	Day	12% 19% 8% 13% 6% 4 4 3 5% 5% 21%	3135
February	1st to 10th	Day	7% 13% 5% 10% 8% 6% 5% 6% 7% 6% 28%	1710
	11th to 20th	Day	10% 12% 4 10% 5% 5% 5% 4 5% 7% 32%	1664
	21st to end	Day	14% 11% 5% 9% 6% 5% 5% 3 5% 6% 31%	1518
March	1st to 10th	Day	13% 8% 3 8% 4 3 3 3 5% 6% 45%	3302
	11th to 20th	Day	11% 10% 3 9% 4 4 3 4 5% 8% 40%	3480
	21st to end	Day	25% 13% 4 10% 5% 5% 4 3 4 5% 23%	3904
April	1st to 10th	Day	18% 12% 3 9% 4 4 3 3 5% 8% 31%	2280
	11th to 20th	Day	20% 14% 3 8% 4 3 3 3 5% 5% 32%	2254
	21st to end	Day	21% 13% 3 7% 4 4 4 4 5% 5% 30%	2408
May	1st to 10th	Day	18% 7% 2 6% 4 4 5% 4 6% 7% 36%	4291
	11th to 20th	Day	13% 6% 2 7% 4 4 4 4 6% 7% 42%	4399
	21st to end	Day	18% 7% 3 7% 4 4 4 4 5% 6% 38%	4824
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 46%	2660
	11th to 20th	Day	13% 7% 3 6% 5% 4 5% 4 6% 8% 39%	2660
	21st to end	Day	12% 6% 2 7% 4 5% 5% 4 7% 8% 40%	2660
July	1st to 10th	Day	11% 5% 2 6% 5% 6% 6% 5% 8% 9% 37%	4440
	11th to 20th	Day	5% 3 1 4 3 4 5% 3 6% 8% 57%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 8% 44%	4787
August	1st to 10th	Day	11% 6% 2 6% 4 5% 6% 4 6% 8% 42%	4104
	11th to 20th	Day	12% 6% 2 6% 4 3 5% 4 6% 9% 43%	4041
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 8% 46%	4249
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 5% 7% 47%	2280
	11th to 20th	Day	11% 5% 3 5% 5% 4 4 3 6% 7% 48%	2280
	21st to end	Day	13% 7% 3 7% 3 4 4 3 6% 7% 42%	1990
October	1st to 10th	Day	15% 8% 2 5% 4 3 4 3 5% 5% 46%	1900
	11th to 20th	Day	15% 10% 3 6% 4 4 4 4 5% 6% 40%	1900
	21st to end	Day	18% 12% 3 6% 4 4 3 3 4 6% 36%	2046
November	1st to 10th	Day	12% 10% 4 7% 4 3 4 3 5% 7% 40%	1717
	11th to 20th	Day	13% 14% 6% 10% 7% 4 4 3 5% 6% 29%	1600
	21st to end	Day	11% 13% 4 9% 5% 4 3 3 5% 6% 37%	1600
December	1st to 10th	Day	12% 12% 4 11% 8% 6% 6% 4 4 5% 28%	1584
	11th to 20th	Day	10% 12% 7% 9% 6% 6% 4 4 6% 7% 29%	1918
	21st to end	Day	19% 16% 9% 17% 5% 4 3 3 3 4 17%	2783
January	1st to 10th	Night	10% 16% 8% 17% 6% 4 4 3 4 5% 25%	4428
	11th to 20th	Night	8% 13% 5% 13% 7% 7% 7% 6% 9% 5% 20%	4435
	21st to end	Night	14% 18% 6% 11% 5% 4 4 3 5% 5% 24%	4532
February	1st to 10th	Night	6% 15% 5% 10% 6% 6% 4 4 4 6% 34%	2470
	11th to 20th	Night	8% 11% 3 9% 6% 6% 5% 4 4 6% 38%	2403
	21st to end	Night	13% 11% 4 9% 5% 5% 4 4 5% 5% 36%	1935
March	1st to 10th	Night	10% 9% 3 7% 3 3 4 3 5% 6% 46%	3578
	11th to 20th	Night	9% 9% 3 8% 5% 5% 4 4 5% 8% 41%	3480
	21st to end	Night	25% 11% 3 10% 4 3 3 2 4 5% 29%	3727
April	1st to 10th	Night	16% 12% 3 6% 4 4 4 3 5% 6% 37%	1900
	11th to 20th	Night	21% 11% 3 7% 4 3 3 3 5% 6% 36%	1875
	21st to end	Night	19% 10% 3 6% 4 4 4 2 4 6% 38%	1768
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 4 6% 5% 42%	2614
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 5% 7% 50%	2506
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 7% 43%	2743
June	1st to 10th	Night	10% 3 2 5% 5% 4 4 4 6% 9% 49%	1520
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 4 6% 8% 44%	1520
	21st to end	Night	9% 5% 2 4 4 3 4 4 5% 8% 53%	1520
July	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 10% 47%	2530
	11th to 20th	Night	4 3 1 4 3 4 4 4 7% 8% 58%	2530
	21st to end	Night	6% 3 1 5% 4 3 4 4 6% 9% 56%	2772
August	1st to 10th	Night	11% 7% 2 6% 4 4 4 4 4 7% 48%	2836
	11th to 20th	Night	13% 5% 1 5% 3 4 3 3 5% 6% 51%	2798
	21st to end	Night	10% 6% 2 6% 5% 4 4 4 4 7% 49%	3409
September	1st to 10th	Night	9% 4 2 4 3 3 4 3 4 7% 56%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	1900
	21st to end	Night	12% 6% 2 5% 3 3 3 2 5% 6% 53%	2168
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 7% 49%	2280
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	2280
	21st to end	Night	17% 10% 3 7% 4 4 4 4 5% 6% 38%	2551
November	1st to 10th	Night	12% 10% 3 7% 4 3 4 4 5% 6% 43%	2578
	11th to 20th	Night	12% 12% 5% 10% 5% 4 4 3 4 7% 35%	2800
	21st to end	Night	14% 12% 4 9% 5% 4 3 3 4 6% 36%	2800
December	1st to 10th	Night	11% 12% 4 12% 6% 5% 4 4 6% 6% 30%	2779
	11th to 20th	Night	12% 13% 5% 11% 7% 6% 5% 3 5% 6% 28%	3279
	21st to end	Night	21% 15% 5% 13% 6% 4 4 3 4 4 20%	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	67% 2 11% 15% 4	2528
	11th to 20th	Day	62% 4 10% 17% 7%	2534
	21st to end	Day	66% 3 13% 14% 4 1	3135
February	1st to 10th	Day	54% 4 13% 18% 10% 1	1710
	11th to 20th	Day	52% 3 12% 24% 7% 1	1673
	21st to end	Day	55% 2 11% 20% 11% 1	1518
March	1st to 10th	Day	42% 2 13% 28% 14% 1	3302
	11th to 20th	Day	43% 3 10% 32% 11% 1	3480
	21st to end	Day	66% 2 10% 17% 5%	3904
April	1st to 10th	Day	54% 3 11% 21% 10% 1	2280
	11th to 20th	Day	56% 2 10% 20% 11% 1	2254
	21st to end	Day	56% 3 12% 20% 9% 1	2408
May	1st to 10th	Day	46% 3 11% 26% 13% 1	4330
	11th to 20th	Day	40% 4 10% 32% 13% 1	4403
	21st to end	Day	47% 3 10% 26% 13% 1	4837
June	1st to 10th	Day	36% 4 13% 31% 15% 1	2660
	11th to 20th	Day	42% 4 14% 23% 15% 1	2660
	21st to end	Day	41% 4 15% 26% 12% 2	2660
July	1st to 10th	Day	41% 6% 18% 25% 10% 1	4371
	11th to 20th	Day	25% 4 15% 35% 20% 2	4365
	21st to end	Day	35% 6% 14% 28% 16% 2	4636
August	1st to 10th	Day	40% 4 12% 30% 13% 2	4090
	11th to 20th	Day	37% 4 13% 34% 11%	4080
	21st to end	Day	35% 5% 15% 30% 14% 1	4248
September	1st to 10th	Day	37% 4 12% 31% 15% 1	2280
	11th to 20th	Day	35% 4 17% 33% 11% 1	2280
	21st to end	Day	42% 3 11% 33% 12% 1	1990
October	1st to 10th	Day	28% 2 7% 39% 24%	3160
	11th to 20th	Day	39% 2 13% 24% 20% 2	3140
	21st to end	Day	43% 2 12% 32% 10%	3348
November	1st to 10th	Day	44% 3 14% 21% 15% 3	1717
	11th to 20th	Day	57% 3 11% 18% 9% 1	1600
	21st to end	Day	50% 3 18% 20% 8% 1	1600
December	1st to 10th	Day	59% 4 14% 15% 7% 1	1584
	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	64% 2 9% 19% 6%	4428
	11th to 20th	Night	61% 4 11% 17% 7%	4435
	21st to end	Night	64% 2 9% 18% 7% 1	4532
February	1st to 10th	Night	53% 3 12% 22% 9% 2	2470
	11th to 20th	Night	48% 4 12% 25% 10% 1	2404
	21st to end	Night	50% 3 11% 22% 11% 2	1935
March	1st to 10th	Night	40% 2 11% 31% 15% 1	3578
	11th to 20th	Night	41% 4 8% 29% 16% 1	3480
	21st to end	Night	60% 2 8% 21% 8% 2	3727
April	1st to 10th	Night	49% 3 10% 22% 14% 2	1900
	11th to 20th	Night	50% 2 9% 20% 16% 3	1875
	21st to end	Night	49% 2 10% 22% 15% 2	1768
May	1st to 10th	Night	44% 3 10% 20% 22% 1	2640
	11th to 20th	Night	34% 3 10% 29% 19% 5%	2507
	21st to end	Night	42% 3 10% 28% 16% 1	2751
June	1st to 10th	Night	31% 5% 13% 30% 17% 3	1520
	11th to 20th	Night	37% 4 13% 24% 18% 3	1520
	21st to end	Night	29% 4 16% 31% 16% 3	1520
July	1st to 10th	Night	33% 4 22% 28% 11% 2	2490
	11th to 20th	Night	21% 5% 17% 38% 16% 2	2482
	21st to end	Night	25% 5% 18% 30% 20% 2	2684
August	1st to 10th	Night	37% 4 11% 30% 16% 3	2826
	11th to 20th	Night	34% 4 14% 31% 15% 1	2825
	21st to end	Night	35% 4 16% 28% 15% 2	3409
September	1st to 10th	Night	29% 4 11% 36% 20% 1	1900
	11th to 20th	Night	33% 2 15% 33% 16% 1	1900
	21st to end	Night	33% 3 13% 30% 19% 1	2168
October	1st to 10th	Night	26% 2 6% 35% 27% 3	3800
	11th to 20th	Night	31% 3 12% 29% 22% 3	3779
	21st to end	Night	45% 2 10% 27% 15% 1	4199
November	1st to 10th	Night	42% 3 13% 25% 14% 2	2578
	11th to 20th	Night	51% 4 11% 21% 12% 1	2800
	21st to end	Night	51% 3 15% 23% 8% 1	2800
December	1st to 10th	Night	54% 3 15% 20% 7% 1	2779
	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	21% 10% 40% 22% 4 2	270
	21st to end	Day	16% 9% 38% 26% 9% 2	297
February	1st to 10th	Day	16% 14% 44% 19% 5% 1	270
	11th to 20th	Day	17% 20% 36% 21% 5% 1	270
	21st to end	Day	22% 13% 37% 26% 2	225
March	1st to 10th	Day	20% 10% 43% 20% 7% 1	270
	11th to 20th	Day	24% 12% 34% 21% 7% 2	270
	21st to end	Day	28% 13% 27% 21% 6% 4	349
April	1st to 10th	Day	28% 13% 34% 18% 6%	460
	11th to 20th	Day	30% 15% 24% 22% 8% 1	452
	21st to end	Day	29% 11% 26% 24% 7% 2	588
May	1st to 10th	Day	22% 11% 34% 24% 9% 2	653
	11th to 20th	Day	20% 9% 29% 26% 11% 4	675
	21st to end	Day	19% 8% 26% 28% 16% 3	734
June	1st to 10th	Day	11% 8% 28% 31% 16% 6%	680
	11th to 20th	Day	16% 7% 32% 31% 11% 3	680
	21st to end	Day	15% 8% 28% 28% 16% 5%	680
July	1st to 10th	Day	8% 4 22% 32% 23% 10%	671
	11th to 20th	Day	9% 4 28% 31% 20% 8%	668
	21st to end	Day	8% 3 25% 39% 17% 7%	739
August	1st to 10th	Day	10% 3 21% 35% 22% 8%	676
	11th to 20th	Day	15% 3 19% 33% 23% 8%	665
	21st to end	Day	11% 5% 26% 32% 20% 6%	582
September	1st to 10th	Day	17% 5% 26% 30% 16% 7%	490
	11th to 20th	Day	14% 3 28% 31% 20% 4	490
	21st to end	Day	16% 12% 28% 27% 15% 3	323
October	1st to 10th	Day	16% 12% 27% 25% 17% 3	300
	11th to 20th	Day	19% 17% 36% 21% 7%	300
	21st to end	Day	29% 12% 35% 15% 7% 2	330
November	1st to 10th	Day	21% 14% 35% 21% 8% 1	300
	11th to 20th	Day	20% 19% 39% 17% 5%	300
	21st to end	Day	21% 10% 44% 21% 4	300
December	1st to 10th	Day	15% 10% 43% 27% 5% 2	294
	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% 10% 39% 25% 6% 1	950
	21st to end	Night	19% 11% 36% 27% 8% 1	1045
February	1st to 10th	Night	14% 14% 40% 27% 5% 1	950
	11th to 20th	Night	15% 13% 41% 24% 6% 1	929
	21st to end	Night	19% 15% 34% 26% 6%	679
March	1st to 10th	Night	19% 12% 36% 24% 7% 1	670
	11th to 20th	Night	17% 11% 34% 30% 7% 1	570
	21st to end	Night	22% 7% 31% 27% 11% 2	627
April	1st to 10th	Night	22% 12% 34% 26% 5%	570
	11th to 20th	Night	29% 7% 31% 27% 6%	563
	21st to end	Night	27% 6% 32% 25% 8% 1	570
May	1st to 10th	Night	22% 8% 32% 30% 6% 1	570
	11th to 20th	Night	20% 4 30% 34% 11% 1	561
	21st to end	Night	16% 6% 25% 38% 14% 1	617
June	1st to 10th	Night	11% 4 28% 41% 15% 1	570
	11th to 20th	Night	14% 3 33% 38% 12%	570
	21st to end	Night	12% 3 25% 43% 17% 1	570
July	1st to 10th	Night	8% 1 21% 39% 29% 3	555
	11th to 20th	Night	6% 4 23% 41% 23% 2	552
	21st to end	Night	6% 1 17% 52% 20% 4	624
August	1st to 10th	Night	9% 1 16% 47% 24% 2	567
	11th to 20th	Night	13% 3 17% 35% 28% 4	564
	21st to end	Night	10% 2 24% 38% 22% 3	627
September	1st to 10th	Night	12% 2 24% 40% 18% 5%	570
	11th to 20th	Night	14% 2 29% 34% 18% 3	570
	21st to end	Night	13% 9% 30% 32% 15% 1	674
October	1st to 10th	Night	17% 7% 29% 33% 12% 1	760
	11th to 20th	Night	18% 11% 33% 30% 7% 1	760
	21st to end	Night	23% 9% 33% 29% 7%	836
November	1st to 10th	Night	18% 11% 36% 27% 8% 1	822
	11th to 20th	Night	18% 14% 37% 24% 6% 1	1000
	21st to end	Night	19% 14% 36% 22% 8%	1000
December	1st to 10th	Night	17% 10% 39% 26% 7% 1	993
	11th to 20th	Night	21% 9% 38% 24% 7% 1	950
	21st to end	Night	25% 12% 32% 24% 6%	1045

Page generated at 2024-12-06 02:54:47. Version ac07

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