Thompson Grove Picnic Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thompson Grove Picnic Area clear sky chart from 2005-11-06 to 2025-11-03.

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	5% 4 1 4 3 3 3 3 5% 7% 61%	135097
	Night	5% 4 1 4 3 3 4 3 5% 7% 59%	132344
Transparency	Day	22% 4 9% 22% 28% 15%	130168
	Night	24% 4 9% 21% 26% 15%	128279
Seeing	Day	7% 7% 28% 34% 20% 4	16739
	Night	7% 7% 36% 39% 12% 1	26657

By Month

Percentage Cloud Cover SW (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	5% 8% 3 6% 4 3 3 2 3 5% 58%	3638
	11th to 20th	Day	2 5% 2 5% 2 3 3 3 5% 6% 65%	3714
	21st to end	Day	4 5% 2 5% 3 3 3 3 4 6% 63%	4748
February	1st to 10th	Day	3 7% 2 5% 4 3 3 3 5% 7% 59%	2000
	11th to 20th	Day	3 8% 3 6% 4 3 2 3 4 6% 58%	1949
	21st to end	Day	3 4 1 4 3 3 4 3 4 6% 66%	1650
March	1st to 10th	Day	5% 4 1 4 2 3 3 2 3 5% 66%	4500
	11th to 20th	Day	7% 5% 2 6% 3 4 3 3 5% 6% 58%	4645
	21st to end	Day	3 5% 1 4 3 3 3 3 5% 8% 61%	5961
April	1st to 10th	Day	4 4 1 3 2 2 3 4 4 6% 66%	2400
	11th to 20th	Day	8% 4 1 5% 3 3 3 3 4 6% 60%	2371
	21st to end	Day	7% 4 2 3 3 3 4 3 4 8% 60%	2400
May	1st to 10th	Day	12% 4 1 4 3 3 3 3 5% 6% 56%	5849
	11th to 20th	Day	5% 3 1 3 3 3 3 3 4 6% 66%	5862
	21st to end	Day	7% 4 1 3 3 2 3 3 5% 7% 61%	6446
June	1st to 10th	Day	5% 2 1 3 4 3 4 4 8% 10% 56%	2781
	11th to 20th	Day	4 2 1 3 3 3 4 3 6% 8% 65%	2800
	21st to end	Day	5% 3 1 5% 4 3 5% 4 6% 9% 55%	2800
July	1st to 10th	Day	5% 3 1 4 4 4 5% 4 7% 9% 54%	6339
	11th to 20th	Day	3 3 1 3 3 3 4 3 6% 10% 61%	6340
	21st to end	Day	4 3 1 4 3 4 5% 4 7% 9% 56%	6586
August	1st to 10th	Day	5% 3 2 4 3 4 5% 5% 8% 10% 52%	5348
	11th to 20th	Day	3 2 1 3 3 4 5% 4 7% 10% 58%	5366
	21st to end	Day	5% 4 1 5% 3 4 5% 5% 7% 9% 52%	5984
September	1st to 10th	Day	8% 3 1 3 2 3 4 3 5% 7% 61%	2400
	11th to 20th	Day	7% 4 2 4 2 3 3 3 5% 7% 61%	2249
	21st to end	Day	6% 2 1 3 2 2 3 3 5% 6% 67%	2200
October	1st to 10th	Day	5% 3 1 2 2 2 2 2 3 5% 73%	3590
	11th to 20th	Day	7% 4 2 4 3 3 3 2 4 6% 64%	3331
	21st to end	Day	4 3 1 2 2 1 2 2 2 5% 77%	3304
November	1st to 10th	Day	4 3 1 3 2 2 2 2 4 5% 71%	1781
	11th to 20th	Day	2 4 1 3 2 2 3 3 4 5% 72%	1800
	21st to end	Day	6% 6% 2 3 2 2 2 2 3 5% 66%	1800
December	1st to 10th	Day	1 3 1 3 1 2 2 2 3 5% 76%	2940
	11th to 20th	Day	3 3 1 3 3 3 3 3 4 6% 68%	3221
	21st to end	Day	3 4 2 5% 3 3 3 3 4 5% 65%	4004
January	1st to 10th	Night	3 7% 3 7% 5% 4 4 3 5% 6% 54%	6327
	11th to 20th	Night	2 6% 2 5% 4 3 3 3 4 5% 64%	6296
	21st to end	Night	3 5% 2 5% 4 3 3 3 4 7% 62%	6226
February	1st to 10th	Night	4 5% 2 5% 3 3 3 3 5% 6% 62%	2400
	11th to 20th	Night	5% 7% 2 7% 4 4 3 3 5% 7% 53%	2338
	21st to end	Night	5% 4 1 2 2 2 3 3 7% 6% 64%	1980
March	1st to 10th	Night	5% 4 1 3 2 3 2 2 4 5% 68%	5374
	11th to 20th	Night	5% 6% 2 6% 4 3 3 3 4 5% 57%	5330
	21st to end	Night	4 4 1 4 3 4 3 3 5% 7% 62%	4988
April	1st to 10th	Night	5% 3 1 4 3 3 3 3 5% 7% 65%	2000
	11th to 20th	Night	7% 4 1 4 4 4 4 3 5% 7% 57%	1977
	21st to end	Night	9% 4 2 3 2 4 3 3 5% 7% 59%	2000
May	1st to 10th	Night	12% 5% 1 4 3 3 3 2 4 6% 57%	4131
	11th to 20th	Night	8% 3 1 3 2 3 3 3 5% 6% 64%	4053
	21st to end	Night	10% 5% 2 4 4 4 4 3 4 6% 56%	4451
June	1st to 10th	Night	4 4 1 5% 4 4 4 4 7% 10% 54%	1613
	11th to 20th	Night	3 2 1 3 3 3 4 5% 7% 7% 62%	1600
	21st to end	Night	4 3 1 5% 3 4 6% 5% 7% 10% 51%	1600
July	1st to 10th	Night	7% 4 1 4 3 4 5% 4 8% 11% 50%	3640
	11th to 20th	Night	3 3 2 6% 4 4 5% 5% 8% 14% 48%	3640
	21st to end	Night	5% 4 2 5% 5% 5% 5% 5% 9% 12% 43%	4280
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 5% 9% 10% 46%	4464
	11th to 20th	Night	4 2 1 5% 4 5% 7% 7% 10% 14% 41%	4483
	21st to end	Night	8% 6% 2 7% 5% 5% 6% 5% 7% 8% 40%	4993
September	1st to 10th	Night	6% 3 1 4 3 4 4 4 7% 7% 57%	2000
	11th to 20th	Night	7% 4 1 4 2 3 3 3 6% 7% 59%	2150
	21st to end	Night	6% 3 1 3 3 2 2 3 4 6% 68%	2200
October	1st to 10th	Night	6% 2 1 3 2 2 3 3 4 5% 70%	3590
	11th to 20th	Night	9% 3 1 3 2 3 3 2 5% 6% 62%	3798
	21st to end	Night	5% 3 1 2 2 2 2 2 4 5% 73%	4462
November	1st to 10th	Night	5% 3 1 3 2 2 2 2 3 6% 70%	2562
	11th to 20th	Night	2 3 1 3 2 2 3 2 4 5% 74%	2600
	21st to end	Night	5% 5% 1 3 3 2 2 3 4 6% 65%	2600
December	1st to 10th	Night	2 4 1 3 2 2 3 2 4 5% 72%	4250
	11th to 20th	Night	3 3 1 3 2 2 2 2 4 6% 70%	4974
	21st to end	Night	3 5% 2 5% 3 3 4 4 5% 6% 60%	6974

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	31% 4 5% 24% 29% 8%	3639
	11th to 20th	Day	21% 2 9% 20% 28% 20%	3714
	21st to end	Day	24% 3 6% 20% 37% 11%	4748
February	1st to 10th	Day	26% 3 9% 22% 24% 16%	2000
	11th to 20th	Day	28% 4 7% 23% 24% 14%	1959
	21st to end	Day	20% 4 4 17% 31% 24%	1650
March	1st to 10th	Day	22% 3 3 14% 27% 32%	4500
	11th to 20th	Day	28% 4 5% 15% 30% 18%	4645
	21st to end	Day	21% 3 8% 28% 32% 8%	5961
April	1st to 10th	Day	18% 4 7% 22% 29% 20%	2400
	11th to 20th	Day	25% 4 7% 21% 24% 18%	2374
	21st to end	Day	23% 4 10% 25% 26% 12%	2400
May	1st to 10th	Day	28% 3 12% 21% 19% 16%	5849
	11th to 20th	Day	20% 4 6% 17% 23% 30%	5866
	21st to end	Day	23% 4 12% 21% 19% 21%	6447
June	1st to 10th	Day	21% 7% 17% 25% 19% 11%	2781
	11th to 20th	Day	17% 4 8% 21% 28% 22%	2800
	21st to end	Day	24% 6% 11% 23% 22% 14%	2800
July	1st to 10th	Day	26% 5% 14% 23% 25% 6%	6271
	11th to 20th	Day	19% 4 16% 29% 26% 6%	6265
	21st to end	Day	23% 4 16% 28% 24% 5%	6396
August	1st to 10th	Day	27% 6% 18% 23% 18% 7%	3892
	11th to 20th	Day	20% 4 12% 23% 30% 10%	3882
	21st to end	Day	23% 5% 15% 25% 26% 6%	4311
September	1st to 10th	Day	23% 5% 11% 23% 23% 15%	2400
	11th to 20th	Day	23% 4 9% 20% 27% 17%	2249
	21st to end	Day	18% 4 7% 21% 31% 18%	2200
October	1st to 10th	Day	16% 3 4 16% 38% 22%	3590
	11th to 20th	Day	24% 2 8% 18% 31% 17%	3331
	21st to end	Day	14% 2 3 16% 43% 22%	3304
November	1st to 10th	Day	16% 2 8% 18% 34% 21%	1781
	11th to 20th	Day	15% 3 6% 26% 34% 17%	1800
	21st to end	Day	23% 2 5% 17% 29% 24%	1800
December	1st to 10th	Day	13% 2 7% 28% 38% 13%	2940
	11th to 20th	Day	18% 2 4 19% 43% 13%	3219
	21st to end	Day	22% 4 7% 24% 27% 16%	4004
January	1st to 10th	Night	31% 4 5% 29% 25% 6%	6327
	11th to 20th	Night	23% 3 8% 20% 28% 18%	6296
	21st to end	Night	24% 3 9% 18% 35% 11%	6226
February	1st to 10th	Night	24% 4 9% 21% 29% 14%	2400
	11th to 20th	Night	31% 4 8% 21% 24% 12%	2338
	21st to end	Night	19% 4 4 19% 32% 22%	1980
March	1st to 10th	Night	20% 3 3 15% 26% 33%	5374
	11th to 20th	Night	28% 4 7% 14% 24% 24%	5330
	21st to end	Night	21% 5% 8% 29% 27% 10%	4988
April	1st to 10th	Night	19% 4 8% 23% 27% 19%	2000
	11th to 20th	Night	26% 5% 9% 22% 22% 16%	1977
	21st to end	Night	25% 4 13% 23% 22% 13%	2000
May	1st to 10th	Night	30% 4 11% 20% 17% 18%	4130
	11th to 20th	Night	21% 5% 10% 18% 18% 28%	4054
	21st to end	Night	30% 5% 10% 17% 13% 25%	4452
June	1st to 10th	Night	24% 8% 18% 26% 15% 10%	1613
	11th to 20th	Night	19% 5% 14% 21% 24% 18%	1600
	21st to end	Night	25% 6% 15% 26% 16% 12%	1600
July	1st to 10th	Night	28% 7% 16% 26% 18% 5%	3600
	11th to 20th	Night	26% 5% 20% 28% 18% 3	3592
	21st to end	Night	31% 5% 16% 27% 15% 6%	4157
August	1st to 10th	Night	35% 6% 15% 20% 19% 5%	3244
	11th to 20th	Night	28% 5% 13% 26% 23% 4	3243
	21st to end	Night	33% 5% 14% 23% 20% 4	3597
September	1st to 10th	Night	25% 6% 14% 20% 23% 11%	2000
	11th to 20th	Night	24% 6% 10% 21% 24% 15%	2150
	21st to end	Night	19% 4 9% 21% 32% 15%	2200
October	1st to 10th	Night	17% 3 6% 17% 36% 21%	3590
	11th to 20th	Night	24% 2 7% 21% 27% 19%	3798
	21st to end	Night	16% 2 3 19% 42% 18%	4462
November	1st to 10th	Night	17% 2 7% 19% 32% 23%	2562
	11th to 20th	Night	15% 3 5% 24% 36% 18%	2600
	21st to end	Night	22% 2 6% 20% 28% 23%	2600
December	1st to 10th	Night	17% 2 6% 27% 39% 9%	4250
	11th to 20th	Night	16% 3 4 21% 40% 16%	4975
	21st to end	Night	23% 5% 8% 20% 31% 13%	6974

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	10% 5% 33% 39% 12% 2	300
	11th to 20th	Day	8% 9% 37% 34% 10% 2	300
	21st to end	Day	5% 8% 35% 32% 17% 3	330
February	1st to 10th	Day	5% 11% 45% 29% 9% 1	300
	11th to 20th	Day	7% 10% 48% 26% 7% 3	300
	21st to end	Day	6% 12% 47% 31% 4	249
March	1st to 10th	Day	5% 11% 40% 30% 13% 1	300
	11th to 20th	Day	12% 12% 32% 31% 11% 2	300
	21st to end	Day	11% 17% 38% 24% 9% 1	542
April	1st to 10th	Day	8% 14% 42% 26% 9% 1	500
	11th to 20th	Day	9% 18% 43% 22% 7% 1	492
	21st to end	Day	13% 15% 38% 23% 10% 1	500
May	1st to 10th	Day	8% 10% 35% 32% 12% 2	678
	11th to 20th	Day	10% 8% 29% 34% 17% 3	725
	21st to end	Day	7% 8% 35% 36% 12% 2	789
June	1st to 10th	Day	6% 5% 26% 34% 24% 5%	730
	11th to 20th	Day	5% 4 31% 38% 19% 4	730
	21st to end	Day	5% 4 22% 43% 22% 5%	730
July	1st to 10th	Day	5% 2 17% 38% 31% 7%	721
	11th to 20th	Day	3 1 12% 36% 39% 10%	718
	21st to end	Day	5% 1 7% 38% 39% 9%	780
August	1st to 10th	Day	3 2 14% 35% 36% 10%	527
	11th to 20th	Day	5% 1 13% 40% 32% 9%	517
	21st to end	Day	4 1 12% 45% 28% 9%	583
September	1st to 10th	Day	7% 1 16% 41% 29% 7%	530
	11th to 20th	Day	5% 3 17% 41% 27% 7%	379
	21st to end	Day	9% 3 23% 29% 27% 8%	330
October	1st to 10th	Day	14% 5% 20% 31% 25% 6%	330
	11th to 20th	Day	6% 6% 33% 29% 19% 6%	330
	21st to end	Day	11% 8% 28% 28% 19% 6%	363
November	1st to 10th	Day	10% 8% 29% 31% 20% 2	306
	11th to 20th	Day	5% 9% 32% 35% 15% 5%	300
	21st to end	Day	10% 12% 33% 24% 14% 6%	300
December	1st to 10th	Day	3 6% 36% 36% 16% 3	300
	11th to 20th	Day	8% 8% 36% 37% 12%	300
	21st to end	Day	9% 10% 43% 30% 7% 1	330
January	1st to 10th	Night	6% 8% 37% 39% 9% 1	1000
	11th to 20th	Night	6% 9% 36% 37% 11% 1	982
	21st to end	Night	4 5% 43% 38% 9% 1	880
February	1st to 10th	Night	5% 9% 45% 34% 7% 1	800
	11th to 20th	Night	8% 9% 51% 26% 5%	784
	21st to end	Night	10% 8% 51% 27% 4	660
March	1st to 10th	Night	7% 12% 45% 33% 4	800
	11th to 20th	Night	10% 12% 36% 35% 6% 1	755
	21st to end	Night	6% 15% 50% 25% 4	660
April	1st to 10th	Night	7% 13% 47% 30% 4	600
	11th to 20th	Night	8% 13% 49% 27% 3	593
	21st to end	Night	11% 10% 45% 30% 5%	600
May	1st to 10th	Night	9% 9% 43% 34% 6%	600
	11th to 20th	Night	10% 7% 37% 36% 10% 1	591
	21st to end	Night	8% 7% 45% 33% 7%	650
June	1st to 10th	Night	4 5% 35% 45% 11%	600
	11th to 20th	Night	3 5% 46% 40% 7%	600
	21st to end	Night	5% 2 32% 49% 12%	600
July	1st to 10th	Night	5% 1 20% 53% 20%	585
	11th to 20th	Night	4 2 15% 52% 26% 1	582
	21st to end	Night	7% 1 13% 53% 26%	657
August	1st to 10th	Night	3 1 16% 50% 29% 2	597
	11th to 20th	Night	7% 2 15% 53% 24% 1	594
	21st to end	Night	7% 1 18% 47% 26% 1	660
September	1st to 10th	Night	6% 1 21% 54% 17% 1	600
	11th to 20th	Night	7% 2 32% 45% 13% 1	600
	21st to end	Night	6% 6% 26% 48% 14% 1	600
October	1st to 10th	Night	12% 3 32% 40% 13% 1	600
	11th to 20th	Night	6% 6% 35% 39% 12% 2	739
	21st to end	Night	8% 7% 32% 34% 16% 2	1002
November	1st to 10th	Night	6% 5% 32% 41% 13% 2	986
	11th to 20th	Night	4 8% 38% 37% 12% 1	1000
	21st to end	Night	8% 7% 34% 36% 14% 1	1000
December	1st to 10th	Night	5% 7% 35% 41% 11% 1	1000
	11th to 20th	Night	8% 8% 39% 36% 9% 1	1000
	21st to end	Night	8% 11% 39% 33% 9% 1	1100

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