Draper Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Draper Park clear sky chart from 2005-11-06 to 2025-08-05.

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% 7% 2 6% 4 4 4 4 6% 7% 39%	125748
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 42%	122331
Transparency	Day	42% 5% 12% 22% 16% 3	129275
	Night	42% 5% 11% 22% 17% 3	126891
Seeing	Day	20% 12% 30% 24% 11% 3	15936
	Night	20% 11% 31% 28% 9% 1	25016

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	13% 6% 2 5% 4 3 4 3 5% 6% 48%	3597
	11th to 20th	Day	14% 9% 3 6% 5% 4 5% 3 4 6% 42%	3570
	21st to end	Day	18% 8% 3 7% 5% 3 4 3 4 5% 39%	4263
February	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 3 6% 6% 38%	1817
	11th to 20th	Day	16% 7% 2 6% 5% 4 4 3 4 5% 44%	1948
	21st to end	Day	19% 8% 2 5% 4 4 4 4 6% 5% 40%	1650
March	1st to 10th	Day	22% 7% 2 5% 3 4 4 3 4 5% 42%	4476
	11th to 20th	Day	12% 7% 2 7% 5% 5% 4 3 6% 7% 43%	4742
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 6% 41%	5958
April	1st to 10th	Day	19% 9% 2 6% 4 3 4 3 5% 6% 39%	2400
	11th to 20th	Day	18% 6% 2 6% 4 4 3 3 5% 8% 40%	2374
	21st to end	Day	19% 6% 2 5% 4 4 4 3 6% 7% 40%	2440
May	1st to 10th	Day	29% 8% 2 5% 3 3 3 4 5% 8% 28%	5886
	11th to 20th	Day	22% 9% 3 7% 4 3 3 3 4 6% 35%	6263
	21st to end	Day	21% 6% 2 5% 4 4 4 4 6% 7% 36%	6949
June	1st to 10th	Day	17% 6% 3 5% 5% 4 4 4 7% 8% 37%	2800
	11th to 20th	Day	16% 7% 2 6% 4 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	12% 7% 2 6% 5% 4 5% 5% 7% 8% 39%	2800
July	1st to 10th	Day	11% 7% 2 7% 5% 5% 6% 5% 7% 10% 35%	6360
	11th to 20th	Day	12% 6% 3 7% 6% 5% 6% 5% 8% 11% 32%	6360
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 5% 6% 8% 37%	6926
August	1st to 10th	Day	20% 7% 2 6% 5% 5% 6% 4 6% 8% 31%	4246
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 7% 8% 41%	3730
	21st to end	Day	9% 5% 2 6% 4 5% 4 3 7% 10% 44%	4180
September	1st to 10th	Day	13% 6% 2 7% 4 4 5% 3 6% 6% 44%	2280
	11th to 20th	Day	14% 4 2 5% 3 3 4 3 5% 7% 49%	2190
	21st to end	Day	17% 6% 2 8% 5% 4 4 4 5% 6% 40%	2090
October	1st to 10th	Day	21% 6% 2 5% 3 3 4 3 5% 6% 42%	1995
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 5% 49%	1900
	21st to end	Day	22% 5% 1 4 3 4 4 4 4 6% 42%	1926
November	1st to 10th	Day	14% 6% 2 5% 3 2 3 4 4 6% 52%	1761
	11th to 20th	Day	19% 6% 2 4 3 2 3 3 5% 6% 48%	1800
	21st to end	Day	16% 6% 2 5% 4 4 3 4 5% 7% 43%	1725
December	1st to 10th	Day	16% 7% 2 5% 4 3 3 3 6% 7% 43%	2600
	11th to 20th	Day	23% 7% 2 4 3 2 3 2 4 5% 45%	2986
	21st to end	Day	19% 8% 3 6% 4 4 4 4 4 6% 39%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	14% 7% 2 6% 3 4 3 3 5% 6% 46%	6315
	21st to end	Night	18% 9% 2 5% 3 4 4 3 4 6% 39%	6692
February	1st to 10th	Night	17% 9% 3 6% 4 3 4 3 4 6% 42%	2581
	11th to 20th	Night	17% 8% 2 6% 4 3 4 4 6% 5% 41%	2338
	21st to end	Night	18% 8% 3 6% 4 4 3 4 5% 5% 41%	1980
March	1st to 10th	Night	21% 8% 2 5% 3 3 3 3 4 6% 42%	5364
	11th to 20th	Night	15% 9% 2 5% 4 3 3 3 5% 6% 44%	5214
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 5% 40%	4973
April	1st to 10th	Night	19% 8% 3 7% 5% 4 4 4 5% 7% 37%	2000
	11th to 20th	Night	20% 7% 2 5% 3 3 4 4 6% 7% 40%	1975
	21st to end	Night	19% 7% 2 6% 3 4 3 3 4 6% 43%	1960
May	1st to 10th	Night	29% 8% 3 7% 4 4 4 3 4 7% 27%	4054
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 4 6% 35%	3672
	21st to end	Night	23% 8% 2 6% 3 4 3 3 5% 7% 36%	3926
June	1st to 10th	Night	16% 6% 2 5% 4 4 5% 4 5% 8% 41%	1600
	11th to 20th	Night	14% 8% 2 6% 3 4 4 3 7% 8% 41%	1600
	21st to end	Night	12% 5% 2 5% 5% 3 5% 5% 7% 9% 43%	1600
July	1st to 10th	Night	11% 7% 3 8% 5% 5% 5% 4 6% 9% 37%	3600
	11th to 20th	Night	16% 8% 3 7% 5% 4 5% 4 7% 8% 33%	3600
	21st to end	Night	14% 6% 3 5% 4 3 4 4 5% 8% 43%	3926
August	1st to 10th	Night	24% 6% 2 7% 5% 4 4 5% 5% 7% 32%	2735
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 4 6% 7% 39%	3097
	21st to end	Night	10% 5% 2 5% 4 3 4 3 5% 8% 52%	3476
September	1st to 10th	Night	15% 5% 2 5% 4 5% 4 3 6% 6% 45%	1900
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 4 6% 53%	1989
	21st to end	Night	18% 6% 2 6% 3 3 3 3 5% 6% 45%	2090
October	1st to 10th	Night	19% 8% 3 7% 4 3 3 2 4 6% 41%	2180
	11th to 20th	Night	16% 5% 2 5% 3 4 3 3 5% 6% 50%	2280
	21st to end	Night	22% 7% 2 5% 2 3 3 3 4 6% 43%	2670
November	1st to 10th	Night	13% 5% 2 4 4 3 3 3 5% 6% 50%	2537
	11th to 20th	Night	20% 7% 2 4 3 2 3 3 4 5% 48%	2600
	21st to end	Night	16% 6% 2 4 4 4 4 4 4 7% 46%	2674
December	1st to 10th	Night	16% 7% 2 4 3 4 3 3 4 5% 48%	4580
	11th to 20th	Night	26% 7% 2 4 3 3 3 2 4 4 42%	5208
	21st to end	Night	20% 8% 3 6% 4 4 3 3 5% 5% 40%	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	35% 3 8% 32% 19% 3	3598
	11th to 20th	Day	43% 4 10% 22% 16% 5%	3600
	21st to end	Day	48% 4 10% 24% 13% 1	4263
February	1st to 10th	Day	45% 5% 13% 22% 13% 2	1817
	11th to 20th	Day	42% 4 10% 25% 16% 3	1957
	21st to end	Day	45% 5% 12% 21% 14% 3	1650
March	1st to 10th	Day	43% 4 12% 19% 14% 8%	4479
	11th to 20th	Day	37% 7% 14% 28% 10% 4	4742
	21st to end	Day	44% 4 9% 13% 24% 6%	5958
April	1st to 10th	Day	45% 5% 11% 19% 15% 5%	2400
	11th to 20th	Day	43% 4 12% 20% 18% 3	2374
	21st to end	Day	42% 6% 10% 21% 17% 4	2440
May	1st to 10th	Day	54% 6% 14% 14% 10% 1	5897
	11th to 20th	Day	49% 6% 10% 20% 14% 1	6267
	21st to end	Day	45% 6% 11% 23% 14% 1	6949
June	1st to 10th	Day	41% 8% 14% 21% 15% 1	2800
	11th to 20th	Day	44% 8% 13% 23% 12% 2	2800
	21st to end	Day	40% 7% 15% 21% 16% 1	2800
July	1st to 10th	Day	44% 7% 12% 21% 15% 1	6290
	11th to 20th	Day	44% 4 19% 22% 10% 1	6285
	21st to end	Day	43% 5% 10% 21% 19% 2	6728
August	1st to 10th	Day	49% 7% 14% 21% 9% 1	4232
	11th to 20th	Day	40% 4 14% 30% 11% 1	3766
	21st to end	Day	36% 5% 10% 28% 20% 2	4180
September	1st to 10th	Day	39% 7% 12% 22% 18% 2	2280
	11th to 20th	Day	33% 6% 11% 24% 24% 2	2190
	21st to end	Day	44% 6% 10% 19% 18% 3	2090
October	1st to 10th	Day	39% 4 9% 19% 23% 6%	3320
	11th to 20th	Day	27% 4 10% 23% 32% 4	3140
	21st to end	Day	29% 4 14% 25% 25% 4	3151
November	1st to 10th	Day	35% 5% 12% 23% 23% 3	1761
	11th to 20th	Day	38% 4 10% 24% 22% 4	1800
	21st to end	Day	39% 5% 14% 28% 12% 2	1725
December	1st to 10th	Day	40% 4 15% 28% 11% 2	2600
	11th to 20th	Day	43% 3 14% 22% 16% 2	2986
	21st to end	Day	45% 6% 8% 23% 16% 3	3960
January	1st to 10th	Night	37% 3 9% 25% 22% 4	6349
	11th to 20th	Night	40% 4 14% 21% 16% 5%	6360
	21st to end	Night	46% 4 9% 20% 16% 5%	6692
February	1st to 10th	Night	45% 5% 12% 24% 12% 2	2581
	11th to 20th	Night	44% 6% 9% 19% 17% 5%	2340
	21st to end	Night	44% 5% 8% 23% 17% 4	1980
March	1st to 10th	Night	43% 5% 10% 20% 13% 8%	5374
	11th to 20th	Night	39% 6% 15% 24% 12% 4	5215
	21st to end	Night	47% 4 6% 14% 24% 5%	4973
April	1st to 10th	Night	47% 5% 10% 19% 15% 4	2000
	11th to 20th	Night	43% 7% 9% 20% 19% 2	1975
	21st to end	Night	42% 6% 8% 21% 19% 4	1960
May	1st to 10th	Night	57% 6% 12% 14% 10% 1	4060
	11th to 20th	Night	51% 6% 13% 14% 15% 2	3673
	21st to end	Night	48% 5% 13% 22% 10% 2	3927
June	1st to 10th	Night	41% 7% 10% 21% 19% 2	1600
	11th to 20th	Night	40% 7% 15% 26% 11% 1	1600
	21st to end	Night	36% 9% 15% 24% 15% 2	1600
July	1st to 10th	Night	45% 7% 13% 20% 14% 2	3560
	11th to 20th	Night	50% 5% 14% 21% 10%	3552
	21st to end	Night	39% 4 13% 23% 19% 1	3809
August	1st to 10th	Night	52% 7% 15% 16% 10% 1	2725
	11th to 20th	Night	43% 6% 12% 28% 10% 1	3127
	21st to end	Night	32% 5% 11% 33% 18% 1	3476
September	1st to 10th	Night	40% 7% 11% 24% 18% 1	1900
	11th to 20th	Night	33% 5% 11% 23% 24% 3	1989
	21st to end	Night	41% 5% 9% 22% 21% 2	2090
October	1st to 10th	Night	40% 4 11% 23% 20% 2	3635
	11th to 20th	Night	25% 4 10% 28% 30% 4	3779
	21st to end	Night	33% 3 12% 29% 22% 2	4395
November	1st to 10th	Night	34% 6% 12% 23% 22% 4	2537
	11th to 20th	Night	39% 4 9% 24% 20% 5%	2600
	21st to end	Night	37% 5% 13% 29% 14% 2	2674
December	1st to 10th	Night	40% 4 12% 30% 12% 2	4580
	11th to 20th	Night	47% 3 10% 21% 16% 3	5208
	21st to end	Night	45% 6% 11% 19% 16% 4	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	22% 28% 31% 16% 4 1	200
	11th to 20th	Day	25% 25% 38% 11% 2 1	200
	21st to end	Day	20% 20% 41% 13% 5% 2	287
February	1st to 10th	Day	27% 16% 39% 16% 2	317
	11th to 20th	Day	25% 20% 36% 17% 2	496
	21st to end	Day	24% 18% 36% 18% 4	414
March	1st to 10th	Day	21% 22% 33% 20% 4	500
	11th to 20th	Day	19% 20% 37% 18% 5% 1	500
	21st to end	Day	26% 20% 31% 17% 5% 1	550
April	1st to 10th	Day	26% 20% 31% 16% 6% 1	500
	11th to 20th	Day	23% 16% 34% 17% 9% 1	492
	21st to end	Day	25% 16% 33% 19% 6% 1	500
May	1st to 10th	Day	32% 13% 29% 18% 7% 2	503
	11th to 20th	Day	22% 12% 32% 22% 10% 2	527
	21st to end	Day	21% 9% 24% 27% 15% 4	573
June	1st to 10th	Day	18% 9% 27% 28% 14% 4	530
	11th to 20th	Day	18% 10% 28% 31% 11% 2	530
	21st to end	Day	15% 8% 29% 27% 17% 3	530
July	1st to 10th	Day	14% 3 24% 35% 18% 6%	525
	11th to 20th	Day	11% 4 23% 35% 23% 5%	524
	21st to end	Day	14% 6% 24% 36% 18% 2	576
August	1st to 10th	Day	13% 4 21% 36% 17% 8%	502
	11th to 20th	Day	15% 4 21% 34% 21% 4	479
	21st to end	Day	12% 5% 25% 32% 17% 8%	539
September	1st to 10th	Day	18% 6% 27% 23% 19% 7%	490
	11th to 20th	Day	15% 5% 27% 26% 16% 11%	490
	21st to end	Day	21% 9% 28% 26% 12% 4	490
October	1st to 10th	Day	26% 8% 24% 21% 16% 4	490
	11th to 20th	Day	18% 13% 29% 24% 13% 2	490
	21st to end	Day	23% 13% 30% 23% 10% 2	452
November	1st to 10th	Day	17% 14% 31% 26% 11% 3	395
	11th to 20th	Day	21% 17% 35% 22% 5% 1	400
	21st to end	Day	19% 20% 32% 24% 5% 1	325
December	1st to 10th	Day	24% 21% 33% 18% 4 2	200
	11th to 20th	Day	30% 22% 34% 12% 3	200
	21st to end	Day	27% 13% 36% 15% 8%	220
January	1st to 10th	Night	20% 15% 42% 19% 3	800
	11th to 20th	Night	23% 16% 34% 23% 4	800
	21st to end	Night	20% 15% 41% 22% 2	880
February	1st to 10th	Night	23% 12% 38% 25% 2	800
	11th to 20th	Night	23% 16% 40% 20% 1	783
	21st to end	Night	24% 14% 36% 21% 4	660
March	1st to 10th	Night	22% 15% 36% 22% 4	800
	11th to 20th	Night	23% 13% 38% 22% 4 1	800
	21st to end	Night	23% 15% 36% 22% 5%	880
April	1st to 10th	Night	22% 15% 35% 22% 5%	800
	11th to 20th	Night	22% 13% 33% 25% 6%	791
	21st to end	Night	21% 12% 34% 27% 6% 1	760
May	1st to 10th	Night	25% 11% 33% 24% 7% 1	600
	11th to 20th	Night	22% 11% 32% 27% 8%	439
	21st to end	Night	21% 5% 18% 40% 14% 1	433
June	1st to 10th	Night	19% 5% 27% 36% 13% 1	400
	11th to 20th	Night	16% 4 26% 38% 16%	400
	21st to end	Night	11% 3 24% 49% 13% 1	400
July	1st to 10th	Night	12% 3 23% 44% 19%	390
	11th to 20th	Night	13% 3 19% 42% 21% 2	388
	21st to end	Night	14% 4 18% 46% 16% 1	438
August	1st to 10th	Night	14% 2 19% 43% 21% 1	492
	11th to 20th	Night	15% 4 17% 36% 25% 2	750
	21st to end	Night	15% 4 17% 37% 24% 2	836
September	1st to 10th	Night	16% 4 24% 34% 19% 2	760
	11th to 20th	Night	17% 4 22% 33% 19% 4	760
	21st to end	Night	21% 5% 23% 33% 16% 2	760
October	1st to 10th	Night	23% 9% 27% 24% 14% 2	760
	11th to 20th	Night	17% 11% 29% 31% 11% 1	760
	21st to end	Night	25% 11% 32% 24% 8%	836
November	1st to 10th	Night	15% 10% 35% 27% 12% 1	780
	11th to 20th	Night	23% 14% 32% 24% 7% 1	800
	21st to end	Night	20% 14% 35% 24% 7% 1	800
December	1st to 10th	Night	24% 14% 35% 21% 6% 1	800
	11th to 20th	Night	24% 14% 35% 24% 4	800
	21st to end	Night	25% 12% 35% 25% 3	880

Page generated at 2025-08-02 01:32:26. Version ac07

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