Old Fields Observatory
Clear Sky Chart
History

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

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	100656
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	98342
Transparency	Day	39% 5% 12% 21% 18% 5%	104302
	Night	39% 5% 11% 20% 19% 6%	102835
Seeing	Day	15% 7% 22% 26% 20% 10%	15357
	Night	16% 6% 24% 34% 18% 2	25431

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	17% 5% 1 5% 3 3 3 2 3 5% 52%	2836
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 3 5% 51%	2840
	21st to end	Day	16% 9% 3 6% 4 3 3 3 4 5% 43%	3124
February	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 5% 6% 45%	1710
	11th to 20th	Day	21% 6% 2 5% 3 3 3 3 4 4 47%	1728
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 6% 41%	1652
March	1st to 10th	Day	23% 8% 2 5% 4 3 4 4 5% 6% 36%	3435
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 4 6% 55%	3480
	21st to end	Day	18% 6% 2 5% 4 3 4 4 4 6% 44%	3815
April	1st to 10th	Day	10% 4 2 5% 4 3 3 3 5% 7% 53%	2249
	11th to 20th	Day	10% 5% 2 6% 4 3 4 4 6% 7% 50%	2254
	21st to end	Day	13% 5% 2 6% 4 4 4 4 6% 6% 47%	2280
May	1st to 10th	Day	9% 6% 3 7% 6% 5% 6% 5% 7% 10% 35%	3916
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	4081
	21st to end	Day	7% 4 2 6% 4 4 5% 5% 7% 10% 48%	4476
June	1st to 10th	Day	9% 4 2 6% 6% 6% 6% 5% 8% 9% 39%	2587
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 5% 8% 9% 43%	2660
	21st to end	Day	7% 5% 2 6% 6% 5% 6% 6% 8% 10% 39%	2477
July	1st to 10th	Day	5% 5% 2 6% 5% 6% 7% 6% 9% 13% 36%	4120
	11th to 20th	Day	11% 8% 3 8% 6% 6% 6% 5% 8% 11% 30%	4120
	21st to end	Day	9% 9% 3 8% 6% 6% 6% 5% 8% 10% 29%	4466
August	1st to 10th	Day	19% 10% 3 9% 6% 5% 6% 4 8% 9% 22%	4104
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 5% 8% 10% 34%	3826
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 11% 47%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 4 4 6% 9% 44%	2280
	11th to 20th	Day	15% 6% 2 7% 4 4 5% 3 5% 7% 43%	2229
	21st to end	Day	19% 7% 2 7% 4 5% 4 3 5% 6% 37%	2008
October	1st to 10th	Day	14% 6% 2 5% 3 3 4 3 5% 5% 48%	1900
	11th to 20th	Day	12% 5% 1 5% 4 3 3 3 4 5% 55%	1900
	21st to end	Day	11% 5% 2 6% 4 3 4 2 3 4 56%	2090
November	1st to 10th	Day	15% 5% 1 4 3 3 3 2 3 6% 54%	1908
	11th to 20th	Day	14% 5% 3 5% 4 3 3 3 5% 5% 52%	1858
	21st to end	Day	14% 5% 1 5% 3 3 3 3 5% 6% 52%	1577
December	1st to 10th	Day	16% 8% 2 5% 5% 3 4 3 5% 7% 41%	1520
	11th to 20th	Day	16% 5% 1 5% 3 2 3 2 3 5% 55%	1910
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 7% 42%	3060
January	1st to 10th	Night	14% 7% 2 4 3 3 4 3 5% 6% 47%	4109
	11th to 20th	Night	15% 8% 3 7% 5% 3 3 4 4 5% 43%	4120
	21st to end	Night	17% 7% 2 6% 4 4 3 3 4 6% 44%	4532
February	1st to 10th	Night	17% 7% 3 5% 4 3 3 4 5% 5% 45%	2470
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 5% 5% 42%	2337
	21st to end	Night	16% 7% 2 4 3 3 4 3 6% 6% 47%	1799
March	1st to 10th	Night	25% 8% 3 7% 5% 3 4 3 3 4 34%	3418
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 4 6% 52%	3480
	21st to end	Night	16% 5% 2 5% 3 3 3 3 5% 6% 50%	3817
April	1st to 10th	Night	13% 6% 2 5% 3 4 4 2 6% 8% 48%	1929
	11th to 20th	Night	12% 5% 2 5% 4 3 5% 3 6% 7% 49%	1875
	21st to end	Night	11% 4 2 4 4 4 5% 4 6% 6% 49%	1900
May	1st to 10th	Night	9% 8% 3 9% 5% 4 5% 3 5% 9% 40%	3006
	11th to 20th	Night	18% 8% 2 8% 4 4 5% 4 4 6% 37%	2812
	21st to end	Night	9% 5% 1 5% 5% 4 5% 3 7% 10% 46%	3079
June	1st to 10th	Night	10% 5% 2 6% 5% 4 6% 5% 7% 11% 40%	1582
	11th to 20th	Night	9% 5% 2 7% 5% 5% 6% 5% 7% 9% 40%	1520
	21st to end	Night	7% 6% 2 7% 5% 5% 5% 6% 8% 11% 38%	1690
July	1st to 10th	Night	8% 5% 3 8% 5% 5% 5% 5% 9% 13% 35%	2840
	11th to 20th	Night	13% 8% 3 7% 4 5% 5% 5% 8% 11% 30%	2840
	21st to end	Night	15% 10% 3 10% 5% 5% 6% 4 6% 8% 28%	3087
August	1st to 10th	Night	20% 9% 3 9% 6% 5% 5% 4 6% 9% 24%	2825
	11th to 20th	Night	7% 6% 3 8% 6% 6% 6% 6% 7% 10% 34%	2968
	21st to end	Night	7% 4 2 5% 5% 5% 5% 5% 7% 10% 45%	3476
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 9% 45%	1900
	11th to 20th	Night	14% 6% 3 6% 4 4 4 4 5% 7% 43%	1947
	21st to end	Night	19% 7% 3 7% 5% 5% 4 3 5% 6% 36%	2171
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 7% 48%	2280
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 3 5% 55%	2280
	21st to end	Night	11% 4 1 5% 3 3 3 3 5% 6% 57%	2508
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 5% 4 51%	2292
	11th to 20th	Night	13% 6% 2 4 4 4 3 3 4 5% 51%	2318
	21st to end	Night	13% 5% 2 5% 3 3 4 3 5% 6% 51%	2598
December	1st to 10th	Night	17% 7% 3 7% 4 3 5% 3 4 6% 41%	2660
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 5% 48%	3285
	21st to end	Night	15% 7% 2 6% 3 3 3 3 5% 6% 47%	4592

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	38% 3 5% 24% 25% 5%	2837
	11th to 20th	Day	37% 3 5% 16% 32% 7%	2840
	21st to end	Day	45% 3 6% 18% 20% 8%	3124
February	1st to 10th	Day	41% 3 10% 21% 20% 5%	1710
	11th to 20th	Day	43% 3 6% 19% 23% 6%	1737
	21st to end	Day	45% 4 10% 16% 19% 6%	1652
March	1st to 10th	Day	49% 3 9% 17% 14% 8%	3435
	11th to 20th	Day	31% 3 8% 19% 25% 13%	3480
	21st to end	Day	43% 3 9% 22% 18% 5%	3815
April	1st to 10th	Day	31% 3 9% 25% 24% 7%	2249
	11th to 20th	Day	33% 4 12% 23% 22% 6%	2254
	21st to end	Day	37% 4 13% 23% 18% 4	2280
May	1st to 10th	Day	44% 4 20% 19% 12% 2	3951
	11th to 20th	Day	48% 5% 12% 23% 11% 1	4085
	21st to end	Day	30% 6% 17% 26% 18% 3	4489
June	1st to 10th	Day	39% 6% 16% 22% 15% 1	2587
	11th to 20th	Day	35% 6% 16% 26% 14% 2	2660
	21st to end	Day	36% 8% 18% 23% 14% 2	2477
July	1st to 10th	Day	36% 10% 20% 21% 11% 2	4055
	11th to 20th	Day	46% 8% 24% 15% 5% 1	4051
	21st to end	Day	48% 8% 17% 16% 11%	4324
August	1st to 10th	Day	57% 6% 18% 13% 5%	4091
	11th to 20th	Day	42% 8% 15% 25% 10% 1	3865
	21st to end	Day	31% 5% 16% 31% 15% 2	4180
September	1st to 10th	Day	37% 6% 16% 18% 19% 4	2280
	11th to 20th	Day	41% 5% 10% 20% 20% 3	2229
	21st to end	Day	48% 4 9% 21% 14% 4	2008
October	1st to 10th	Day	33% 4 10% 22% 24% 6%	3160
	11th to 20th	Day	22% 2 5% 20% 27% 24%	3140
	21st to end	Day	26% 3 5% 24% 26% 16%	3428
November	1st to 10th	Day	34% 3 7% 22% 28% 6%	1908
	11th to 20th	Day	35% 3 9% 21% 26% 7%	1858
	21st to end	Day	34% 2 8% 24% 25% 6%	1577
December	1st to 10th	Day	43% 3 11% 21% 17% 4	1520
	11th to 20th	Day	35% 2 6% 27% 22% 9%	1906
	21st to end	Day	42% 4 9% 22% 22% 2	3060
January	1st to 10th	Night	38% 3 7% 21% 27% 5%	4109
	11th to 20th	Night	43% 4 7% 17% 24% 6%	4120
	21st to end	Night	43% 3 6% 19% 23% 7%	4532
February	1st to 10th	Night	42% 2 7% 21% 21% 6%	2470
	11th to 20th	Night	45% 2 8% 16% 22% 7%	2339
	21st to end	Night	38% 4 9% 21% 21% 8%	1799
March	1st to 10th	Night	55% 3 7% 14% 15% 5%	3418
	11th to 20th	Night	34% 4 8% 14% 24% 16%	3480
	21st to end	Night	36% 3 9% 24% 22% 6%	3817
April	1st to 10th	Night	36% 4 9% 17% 25% 8%	1929
	11th to 20th	Night	34% 5% 10% 23% 22% 7%	1875
	21st to end	Night	35% 4 11% 28% 18% 5%	1900
May	1st to 10th	Night	42% 4 16% 23% 12% 2	3036
	11th to 20th	Night	49% 5% 11% 19% 15% 1	2813
	21st to end	Night	32% 7% 18% 24% 18% 2	3087
June	1st to 10th	Night	37% 8% 18% 21% 14% 2	1582
	11th to 20th	Night	39% 7% 16% 20% 17% 2	1520
	21st to end	Night	38% 9% 16% 21% 13% 2	1690
July	1st to 10th	Night	38% 12% 21% 17% 11% 2	2795
	11th to 20th	Night	45% 9% 25% 15% 5% 1	2786
	21st to end	Night	54% 7% 12% 16% 10%	2988
August	1st to 10th	Night	56% 7% 18% 15% 4 1	2814
	11th to 20th	Night	41% 9% 20% 22% 7% 1	2995
	21st to end	Night	32% 6% 14% 30% 16% 2	3476
September	1st to 10th	Night	35% 8% 15% 22% 15% 5%	1900
	11th to 20th	Night	41% 6% 11% 20% 20% 3	1947
	21st to end	Night	49% 4 11% 16% 16% 3	2171
October	1st to 10th	Night	37% 4 8% 21% 26% 5%	3800
	11th to 20th	Night	24% 3 6% 16% 27% 24%	3779
	21st to end	Night	23% 3 7% 28% 23% 17%	4120
November	1st to 10th	Night	36% 3 6% 19% 29% 6%	2292
	11th to 20th	Night	36% 3 9% 16% 25% 11%	2318
	21st to end	Night	35% 3 7% 22% 26% 8%	2598
December	1st to 10th	Night	45% 4 9% 19% 19% 5%	2660
	11th to 20th	Night	39% 2 8% 20% 24% 8%	3288
	21st to end	Night	39% 3 7% 25% 24% 3	4592

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	21% 14% 39% 20% 6%	270
	11th to 20th	Day	20% 21% 33% 19% 7%	270
	21st to end	Day	19% 17% 39% 20% 5% 1	297
February	1st to 10th	Day	18% 11% 43% 21% 6% 1	270
	11th to 20th	Day	31% 11% 37% 18% 2	335
	21st to end	Day	21% 14% 35% 23% 6%	382
March	1st to 10th	Day	19% 12% 33% 26% 8% 1	460
	11th to 20th	Day	17% 15% 37% 21% 8% 1	460
	21st to end	Day	18% 12% 34% 27% 8% 2	506
April	1st to 10th	Day	16% 14% 31% 25% 12% 2	460
	11th to 20th	Day	12% 11% 28% 30% 16% 4	452
	21st to end	Day	16% 6% 25% 30% 20% 3	460
May	1st to 10th	Day	14% 10% 29% 26% 14% 8%	463
	11th to 20th	Day	16% 4 18% 31% 20% 11%	487
	21st to end	Day	17% 4 12% 27% 26% 15%	529
June	1st to 10th	Day	11% 2 12% 32% 28% 15%	490
	11th to 20th	Day	10% 3 14% 27% 27% 19%	490
	21st to end	Day	9% 2 12% 29% 32% 15%	490
July	1st to 10th	Day	9% 1 6% 23% 38% 22%	485
	11th to 20th	Day	8% 1 7% 23% 39% 21%	484
	21st to end	Day	9% 1 9% 30% 35% 16%	532
August	1st to 10th	Day	15% 3 12% 23% 28% 20%	486
	11th to 20th	Day	12% 3 7% 24% 30% 25%	479
	21st to end	Day	10% 2 8% 26% 30% 24%	539
September	1st to 10th	Day	14% 2 13% 20% 29% 22%	490
	11th to 20th	Day	16% 3 11% 26% 29% 16%	490
	21st to end	Day	23% 4 15% 29% 21% 7%	490
October	1st to 10th	Day	17% 4 18% 23% 23% 15%	490
	11th to 20th	Day	13% 6% 23% 30% 22% 6%	490
	21st to end	Day	12% 6% 22% 33% 20% 7%	539
November	1st to 10th	Day	18% 7% 23% 30% 17% 6%	476
	11th to 20th	Day	17% 6% 31% 31% 12% 2	440
	21st to end	Day	14% 12% 23% 29% 19% 3	237
December	1st to 10th	Day	21% 9% 39% 20% 10% 1	180
	11th to 20th	Day	18% 7% 37% 23% 13% 2	180
	21st to end	Day	21% 11% 31% 23% 12% 2	279
January	1st to 10th	Night	20% 10% 37% 27% 6%	760
	11th to 20th	Night	19% 9% 34% 31% 7%	760
	21st to end	Night	19% 8% 40% 30% 3	836
February	1st to 10th	Night	20% 7% 37% 31% 4	760
	11th to 20th	Night	24% 11% 40% 23% 2	743
	21st to end	Night	20% 10% 38% 30% 3	628
March	1st to 10th	Night	16% 11% 34% 33% 6%	760
	11th to 20th	Night	19% 9% 36% 28% 6% 1	760
	21st to end	Night	18% 8% 36% 33% 6%	836
April	1st to 10th	Night	15% 11% 35% 31% 7%	760
	11th to 20th	Night	15% 9% 32% 33% 11% 1	751
	21st to end	Night	14% 5% 31% 40% 11% 1	760
May	1st to 10th	Night	11% 7% 32% 37% 13%	688
	11th to 20th	Night	16% 4 18% 43% 17% 1	561
	21st to end	Night	13% 1 12% 41% 31% 3	617
June	1st to 10th	Night	12% 1 16% 42% 27% 2	442
	11th to 20th	Night	9% 2 15% 42% 28% 3	380
	21st to end	Night	9% 2 12% 43% 31% 2	550
July	1st to 10th	Night	8% 2 7% 38% 41% 4	555
	11th to 20th	Night	7% 2 6% 37% 42% 6%	552
	21st to end	Night	8% 3 10% 40% 36% 4	624
August	1st to 10th	Night	12% 2 7% 39% 36% 4	567
	11th to 20th	Night	11% 2 9% 33% 40% 6%	669
	21st to end	Night	13% 3 6% 31% 40% 7%	836
September	1st to 10th	Night	13% 2 8% 33% 37% 7%	760
	11th to 20th	Night	18% 3 11% 31% 34% 3	760
	21st to end	Night	22% 3 15% 34% 25% 2	760
October	1st to 10th	Night	16% 2 15% 34% 28% 5%	760
	11th to 20th	Night	15% 5% 22% 36% 19% 2	760
	21st to end	Night	12% 5% 24% 35% 22% 2	836
November	1st to 10th	Night	16% 4 24% 37% 16% 3	764
	11th to 20th	Night	17% 7% 27% 34% 14% 1	760
	21st to end	Night	15% 5% 29% 34% 16% 1	760
December	1st to 10th	Night	20% 7% 31% 34% 8%	760
	11th to 20th	Night	19% 7% 29% 36% 8%	760
	21st to end	Night	19% 8% 31% 33% 9%	836

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