Sydenham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sydenham clear sky chart from 2005-11-06 to 2025-01-07.

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	17% 10% 3 7% 5% 4 4 3 5% 7% 34%	105199
	Night	18% 10% 3 8% 5% 4 4 3 5% 6% 33%	103839
Transparency	Day	49% 6% 13% 22% 10% 1	108734
	Night	50% 6% 12% 20% 11% 1	108393
Seeing	Day	23% 16% 31% 20% 9% 3	14650
	Night	23% 13% 30% 24% 9% 1	23918

By Month

Percentage Cloud Cover NE (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	16% 17% 4 8% 5% 5% 4 4 4 4 29%	2565
	11th to 20th	Day	14% 16% 7% 13% 6% 5% 5% 3 4 4 21%	2520
	21st to end	Day	18% 20% 8% 12% 6% 4 5% 3 4 4 17%	3028
February	1st to 10th	Day	16% 15% 4 12% 6% 5% 4 3 6% 6% 22%	1710
	11th to 20th	Day	13% 16% 5% 12% 6% 5% 4 4 5% 5% 27%	1663
	21st to end	Day	20% 15% 4 9% 7% 6% 4 3 4 5% 24%	1554
March	1st to 10th	Day	22% 12% 4 8% 6% 4 3 2 4 5% 30%	3207
	11th to 20th	Day	11% 17% 5% 9% 5% 5% 5% 3 4 5% 32%	3480
	21st to end	Day	15% 9% 3 7% 4 4 3 3 4 6% 42%	3966
April	1st to 10th	Day	21% 13% 3 7% 5% 4 3 3 3 5% 33%	2280
	11th to 20th	Day	22% 10% 2 5% 4 4 4 2 4 5% 38%	2254
	21st to end	Day	22% 11% 3 7% 5% 4 4 3 5% 5% 32%	2380
May	1st to 10th	Day	22% 8% 3 7% 5% 4 4 3 5% 6% 32%	4410
	11th to 20th	Day	17% 9% 3 8% 5% 5% 4 4 5% 8% 33%	4398
	21st to end	Day	14% 5% 2 6% 4 4 4 4 6% 8% 42%	4837
June	1st to 10th	Day	20% 7% 3 6% 5% 5% 5% 4 6% 6% 33%	2660
	11th to 20th	Day	15% 5% 2 6% 5% 5% 5% 4 6% 9% 38%	2660
	21st to end	Day	17% 7% 3 6% 5% 5% 4 4 6% 8% 35%	2660
July	1st to 10th	Day	12% 6% 2 6% 5% 4 6% 5% 7% 10% 38%	4440
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 10% 43%	4440
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 8% 9% 41%	4814
August	1st to 10th	Day	17% 7% 2 7% 3 4 4 3 6% 8% 37%	4201
	11th to 20th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 37%	4021
	21st to end	Day	15% 6% 2 6% 4 3 4 4 7% 10% 41%	4180
September	1st to 10th	Day	13% 5% 2 7% 4 4 4 4 6% 8% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 5% 7% 49%	2280
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	2013
October	1st to 10th	Day	21% 6% 3 7% 5% 3 4 3 4 7% 36%	1900
	11th to 20th	Day	22% 7% 3 8% 5% 4 4 3 5% 7% 32%	1900
	21st to end	Day	26% 10% 3 7% 5% 5% 4 4 5% 6% 27%	1995
November	1st to 10th	Day	16% 10% 3 5% 5% 5% 5% 5% 5% 6% 35%	1757
	11th to 20th	Day	16% 12% 4 8% 6% 5% 4 2 5% 7% 31%	1600
	21st to end	Day	21% 14% 4 8% 5% 4 4 4 6% 5% 25%	1600
December	1st to 10th	Day	21% 15% 4 10% 5% 5% 5% 3 5% 6% 21%	2600
	11th to 20th	Day	19% 19% 5% 10% 5% 3 4 3 3 4 24%	2986
	21st to end	Day	23% 16% 5% 10% 5% 4 3 2 3 5% 25%	3960
January	1st to 10th	Night	19% 14% 5% 10% 6% 5% 5% 4 5% 5% 23%	4514
	11th to 20th	Night	15% 14% 6% 14% 7% 6% 5% 3 5% 5% 20%	4440
	21st to end	Night	21% 16% 6% 11% 6% 5% 4 3 4 4 21%	4625
February	1st to 10th	Night	17% 14% 4 10% 7% 5% 5% 4 5% 5% 24%	2470
	11th to 20th	Night	15% 12% 6% 10% 6% 4 4 3 5% 5% 30%	2403
	21st to end	Night	18% 12% 4 8% 6% 5% 3 3 5% 6% 30%	1899
March	1st to 10th	Night	19% 10% 4 7% 4 5% 5% 3 5% 5% 33%	3631
	11th to 20th	Night	20% 15% 4 8% 5% 4 4 3 5% 6% 27%	3479
	21st to end	Night	17% 8% 2 7% 4 3 3 3 4 6% 42%	3665
April	1st to 10th	Night	22% 10% 3 6% 4 4 3 3 5% 5% 34%	1900
	11th to 20th	Night	19% 9% 2 7% 4 4 4 3 5% 5% 38%	1875
	21st to end	Night	21% 8% 3 6% 5% 4 4 3 5% 6% 33%	1815
May	1st to 10th	Night	23% 10% 3 6% 4 4 4 3 6% 6% 31%	2528
	11th to 20th	Night	19% 9% 2 6% 3 3 5% 4 5% 6% 37%	2495
	21st to end	Night	14% 6% 2 5% 4 4 4 4 6% 8% 42%	2738
June	1st to 10th	Night	16% 7% 3 7% 5% 4 6% 4 6% 8% 35%	1520
	11th to 20th	Night	13% 4 2 6% 6% 4 4 4 6% 9% 43%	1520
	21st to end	Night	17% 5% 2 6% 6% 4 5% 4 6% 7% 38%	1520
July	1st to 10th	Night	11% 5% 3 7% 5% 5% 5% 5% 7% 7% 40%	2520
	11th to 20th	Night	15% 6% 2 5% 3 3 3 4 5% 7% 47%	2520
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 6% 8% 47%	2739
August	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 6% 7% 44%	2724
	11th to 20th	Night	14% 6% 3 6% 3 3 3 3 6% 7% 44%	2785
	21st to end	Night	9% 4 2 6% 4 5% 5% 5% 8% 10% 42%	3466
September	1st to 10th	Night	12% 6% 2 6% 3 4 4 3 5% 7% 48%	1900
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 6% 6% 53%	1900
	21st to end	Night	18% 5% 2 7% 4 4 4 3 5% 6% 42%	2165
October	1st to 10th	Night	18% 8% 2 6% 4 5% 4 4 4 6% 38%	2280
	11th to 20th	Night	20% 8% 3 7% 5% 5% 5% 3 5% 6% 34%	2280
	21st to end	Night	22% 9% 3 8% 5% 4 5% 4 5% 5% 30%	2602
November	1st to 10th	Night	19% 9% 3 5% 4 4 4 3 5% 6% 38%	2537
	11th to 20th	Night	18% 11% 3 6% 4 5% 5% 3 6% 6% 34%	2800
	21st to end	Night	21% 12% 3 8% 5% 4 5% 4 5% 5% 27%	2800
December	1st to 10th	Night	20% 16% 4 9% 6% 5% 5% 4 6% 6% 20%	4580
	11th to 20th	Night	23% 15% 4 8% 4 3 3 3 4 5% 27%	5208
	21st to end	Night	27% 15% 4 9% 4 4 3 2 4 4 22%	6996

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	57% 8% 13% 15% 5% 2	2566
	11th to 20th	Day	66% 7% 11% 9% 6%	2520
	21st to end	Day	71% 6% 8% 9% 5% 1	3028
February	1st to 10th	Day	63% 6% 9% 16% 5% 1	1710
	11th to 20th	Day	60% 5% 10% 14% 10% 1	1672
	21st to end	Day	63% 5% 10% 12% 9% 1	1554
March	1st to 10th	Day	58% 4 11% 17% 9% 1	3210
	11th to 20th	Day	55% 5% 16% 12% 10% 2	3480
	21st to end	Day	43% 5% 9% 25% 15% 4	3966
April	1st to 10th	Day	55% 4 8% 17% 13% 3	2280
	11th to 20th	Day	49% 4 8% 21% 15% 2	2254
	21st to end	Day	54% 5% 8% 17% 12% 4	2380
May	1st to 10th	Day	51% 7% 10% 21% 10% 2	4424
	11th to 20th	Day	48% 8% 12% 21% 9% 1	4402
	21st to end	Day	38% 8% 14% 25% 14% 2	4837
June	1st to 10th	Day	48% 8% 12% 18% 12% 1	2660
	11th to 20th	Day	41% 6% 12% 25% 13% 2	2660
	21st to end	Day	45% 8% 14% 21% 11% 1	2660
July	1st to 10th	Day	38% 7% 15% 27% 13% 1	4370
	11th to 20th	Day	35% 6% 17% 32% 9%	4365
	21st to end	Day	38% 7% 15% 29% 10%	4661
August	1st to 10th	Day	45% 6% 15% 24% 10% 1	4187
	11th to 20th	Day	42% 7% 15% 26% 9%	4060
	21st to end	Day	38% 5% 16% 29% 11%	4180
September	1st to 10th	Day	38% 6% 15% 28% 12% 1	2280
	11th to 20th	Day	34% 5% 14% 31% 16% 1	2280
	21st to end	Day	43% 7% 13% 27% 10%	2013
October	1st to 10th	Day	40% 5% 15% 30% 9%	3160
	11th to 20th	Day	45% 5% 15% 27% 7% 1	3140
	21st to end	Day	49% 7% 13% 23% 8%	3272
November	1st to 10th	Day	48% 8% 13% 22% 8% 1	1757
	11th to 20th	Day	53% 6% 10% 18% 11% 1	1600
	21st to end	Day	59% 7% 13% 12% 8%	1600
December	1st to 10th	Day	66% 4 16% 10% 4	2600
	11th to 20th	Day	65% 4 11% 13% 7%	2986
	21st to end	Day	64% 4 10% 16% 7%	3960
January	1st to 10th	Night	61% 8% 14% 12% 5%	4514
	11th to 20th	Night	64% 9% 11% 11% 5% 1	4440
	21st to end	Night	67% 6% 10% 11% 6% 1	4625
February	1st to 10th	Night	62% 6% 10% 16% 6%	2470
	11th to 20th	Night	56% 4 10% 19% 10%	2405
	21st to end	Night	56% 6% 10% 18% 9% 1	1899
March	1st to 10th	Night	53% 5% 9% 22% 10% 1	3641
	11th to 20th	Night	57% 6% 13% 15% 9% 1	3480
	21st to end	Night	43% 3 10% 23% 19% 3	3665
April	1st to 10th	Night	51% 5% 7% 20% 14% 2	1900
	11th to 20th	Night	49% 5% 9% 18% 18% 2	1875
	21st to end	Night	52% 4 9% 15% 16% 3	1815
May	1st to 10th	Night	53% 6% 10% 17% 12% 3	2534
	11th to 20th	Night	46% 9% 11% 22% 9% 2	2496
	21st to end	Night	38% 7% 12% 20% 21% 2	2739
June	1st to 10th	Night	46% 8% 12% 21% 12% 1	1520
	11th to 20th	Night	37% 6% 14% 21% 19% 2	1520
	21st to end	Night	44% 9% 12% 21% 13% 1	1520
July	1st to 10th	Night	41% 6% 12% 22% 18% 1	2480
	11th to 20th	Night	36% 6% 13% 34% 12%	2472
	21st to end	Night	35% 7% 16% 28% 12% 2	2651
August	1st to 10th	Night	39% 7% 13% 27% 13% 1	2714
	11th to 20th	Night	38% 6% 14% 32% 10% 1	2812
	21st to end	Night	35% 8% 16% 29% 11% 1	3466
September	1st to 10th	Night	36% 6% 13% 30% 14% 1	1900
	11th to 20th	Night	31% 5% 15% 33% 14% 1	1900
	21st to end	Night	41% 6% 10% 29% 12% 1	2165
October	1st to 10th	Night	38% 6% 12% 27% 17% 1	3800
	11th to 20th	Night	42% 5% 15% 27% 10%	3779
	21st to end	Night	48% 6% 14% 23% 8%	4275
November	1st to 10th	Night	46% 5% 14% 23% 11% 1	2537
	11th to 20th	Night	49% 7% 13% 19% 11% 2	2800
	21st to end	Night	58% 7% 13% 16% 6%	2800
December	1st to 10th	Night	64% 5% 14% 14% 3	4580
	11th to 20th	Night	60% 6% 10% 14% 9% 1	5208
	21st to end	Night	66% 4 9% 11% 10%	6996

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	27% 21% 33% 14% 5% 1	192
	11th to 20th	Day	33% 18% 32% 14% 2 1	180
	21st to end	Day	28% 19% 31% 18% 4	270
February	1st to 10th	Day	34% 26% 29% 10% 1	270
	11th to 20th	Day	30% 27% 35% 8%	270
	21st to end	Day	34% 20% 31% 12% 2	366
March	1st to 10th	Day	25% 21% 33% 17% 5%	460
	11th to 20th	Day	28% 21% 32% 15% 4	460
	21st to end	Day	32% 21% 28% 12% 6% 1	506
April	1st to 10th	Day	30% 20% 32% 13% 5%	460
	11th to 20th	Day	32% 23% 22% 12% 8% 3	452
	21st to end	Day	28% 24% 29% 12% 6% 2	460
May	1st to 10th	Day	24% 15% 30% 23% 7% 2	463
	11th to 20th	Day	19% 14% 35% 21% 8% 2	487
	21st to end	Day	16% 16% 31% 25% 9% 2	529
June	1st to 10th	Day	23% 12% 34% 21% 7% 2	490
	11th to 20th	Day	16% 15% 33% 22% 11% 2	490
	21st to end	Day	20% 14% 33% 21% 10% 2	490
July	1st to 10th	Day	14% 10% 28% 30% 15% 3	485
	11th to 20th	Day	14% 9% 31% 27% 14% 5%	484
	21st to end	Day	13% 7% 36% 30% 11% 4	532
August	1st to 10th	Day	14% 8% 31% 27% 16% 3	486
	11th to 20th	Day	13% 6% 29% 29% 16% 6%	479
	21st to end	Day	14% 10% 32% 26% 11% 8%	539
September	1st to 10th	Day	14% 10% 31% 26% 12% 7%	490
	11th to 20th	Day	11% 10% 24% 25% 18% 12%	490
	21st to end	Day	17% 13% 28% 25% 14% 3	490
October	1st to 10th	Day	27% 16% 25% 16% 11% 4	490
	11th to 20th	Day	24% 22% 29% 15% 7% 2	490
	21st to end	Day	30% 19% 26% 21% 4	489
November	1st to 10th	Day	27% 19% 29% 18% 8% 1	391
	11th to 20th	Day	26% 17% 38% 11% 7% 2	200
	21st to end	Day	26% 25% 34% 12% 4 1	200
December	1st to 10th	Day	32% 21% 31% 13% 4	200
	11th to 20th	Day	31% 25% 34% 9% 3	200
	21st to end	Day	34% 19% 31% 12% 4	220
January	1st to 10th	Night	30% 17% 31% 18% 4	785
	11th to 20th	Night	29% 20% 28% 18% 5%	760
	21st to end	Night	22% 16% 35% 22% 5%	836
February	1st to 10th	Night	26% 18% 33% 20% 2	760
	11th to 20th	Night	22% 18% 39% 17% 3	743
	21st to end	Night	26% 20% 32% 18% 3	628
March	1st to 10th	Night	23% 18% 35% 21% 3	760
	11th to 20th	Night	28% 17% 30% 21% 4 1	760
	21st to end	Night	24% 14% 36% 19% 7%	836
April	1st to 10th	Night	25% 14% 36% 21% 3	760
	11th to 20th	Night	23% 18% 34% 17% 8% 1	751
	21st to end	Night	26% 12% 30% 23% 9%	675
May	1st to 10th	Night	26% 8% 28% 25% 12% 1	394
	11th to 20th	Night	15% 10% 35% 32% 8%	374
	21st to end	Night	12% 8% 36% 30% 12% 1	411
June	1st to 10th	Night	16% 8% 33% 32% 11% 1	380
	11th to 20th	Night	14% 8% 31% 33% 13% 1	380
	21st to end	Night	18% 4 26% 39% 13%	380
July	1st to 10th	Night	10% 5% 27% 38% 19%	370
	11th to 20th	Night	13% 4 28% 37% 17% 1	368
	21st to end	Night	13% 3 27% 41% 15% 1	416
August	1st to 10th	Night	13% 3 25% 35% 23% 1	505
	11th to 20th	Night	13% 4 23% 33% 23% 3	564
	21st to end	Night	10% 6% 30% 35% 17% 2	826
September	1st to 10th	Night	15% 6% 28% 33% 15% 3	760
	11th to 20th	Night	14% 7% 21% 27% 25% 6%	760
	21st to end	Night	19% 9% 25% 27% 18% 3	760
October	1st to 10th	Night	21% 13% 28% 20% 17% 2	760
	11th to 20th	Night	25% 15% 28% 23% 9% 1	760
	21st to end	Night	26% 16% 28% 24% 6%	836
November	1st to 10th	Night	26% 14% 30% 22% 8%	780
	11th to 20th	Night	25% 17% 32% 18% 8% 1	800
	21st to end	Night	30% 17% 30% 16% 7%	800
December	1st to 10th	Night	31% 18% 29% 19% 4	800
	11th to 20th	Night	28% 20% 33% 16% 3 1	800
	21st to end	Night	34% 15% 30% 17% 3	880

Page generated at 2025-01-04 03:24:01. Version ac07

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