Medford
Clear Sky Chart
History

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

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	10% 8% 2 6% 4 3 4 3 5% 6% 50%	124184
	Night	12% 10% 3 6% 4 3 4 3 4 6% 45%	123456
Transparency	Day	35% 6% 11% 22% 18% 9%	123936
	Night	40% 7% 11% 20% 15% 7%	123291
Seeing	Day	16% 15% 27% 26% 13% 3	15736
	Night	16% 8% 18% 32% 23% 3	24814

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	19% 16% 5% 11% 5% 5% 5% 4 6% 7% 18%	3639
	11th to 20th	Day	20% 11% 3 6% 3 3 3 2 4 6% 38%	3866
	21st to end	Day	15% 8% 3 6% 3 3 3 3 4 7% 44%	4510
February	1st to 10th	Day	13% 12% 5% 8% 6% 5% 6% 5% 7% 7% 28%	1800
	11th to 20th	Day	13% 13% 3 9% 5% 5% 3 4 5% 7% 33%	1755
	21st to end	Day	13% 17% 4 8% 6% 4 4 3 4 7% 30%	1608
March	1st to 10th	Day	15% 14% 4 9% 5% 5% 5% 3 5% 7% 28%	4825
	11th to 20th	Day	12% 14% 4 8% 5% 4 5% 3 5% 5% 35%	5000
	21st to end	Day	14% 14% 5% 11% 5% 5% 5% 4 6% 8% 24%	5561
April	1st to 10th	Day	13% 14% 4 9% 7% 4 5% 4 6% 7% 27%	2400
	11th to 20th	Day	15% 14% 3 6% 5% 4 4 4 6% 7% 34%	2369
	21st to end	Day	13% 10% 3 7% 4 4 4 4 6% 8% 38%	2509
May	1st to 10th	Day	9% 7% 2 6% 3 3 4 3 5% 6% 53%	5962
	11th to 20th	Day	7% 7% 2 6% 3 4 4 4 5% 6% 50%	6277
	21st to end	Day	8% 5% 1 4 3 2 3 3 5% 7% 59%	6902
June	1st to 10th	Day	10% 8% 2 5% 4 4 4 3 6% 8% 46%	2800
	11th to 20th	Day	8% 5% 2 5% 3 4 4 3 5% 7% 55%	2800
	21st to end	Day	4 3 1 3 2 2 2 3 4 5% 70%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 87%	4491
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 3 80%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 3 86%	4559
August	1st to 10th	Day	2 2 1 3 2 2 2 3 4 4 75%	4091
	11th to 20th	Day	1 2 1 2 1 2 3 2 4 5% 79%	4026
	21st to end	Day	3 2 1 2 2 2 2 2 2 4 78%	4216
September	1st to 10th	Day	2 2 2 2 2 2 2 3 5% 78%	2280
	11th to 20th	Day	4 4 2 4 3 3 4 4 6% 6% 60%	2271
	21st to end	Day	7% 4 1 4 3 2 3 3 3 7% 63%	2003
October	1st to 10th	Day	5% 3 1 3 2 3 3 2 3 5% 70%	3180
	11th to 20th	Day	7% 4 2 6% 4 4 4 3 5% 9% 51%	3161
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 8% 38%	3448
November	1st to 10th	Day	16% 10% 3 7% 5% 4 5% 4 6% 8% 32%	1803
	11th to 20th	Day	18% 10% 3 8% 5% 4 5% 5% 7% 9% 26%	1600
	21st to end	Day	14% 12% 3 8% 5% 5% 5% 3 5% 7% 34%	1600
December	1st to 10th	Day	10% 6% 2 6% 4 4 4 4 6% 10% 45%	2620
	11th to 20th	Day	20% 15% 4 9% 5% 4 5% 4 6% 8% 20%	3028
	21st to end	Day	19% 16% 5% 11% 6% 6% 6% 4 6% 6% 15%	4004
January	1st to 10th	Night	21% 18% 4 9% 5% 4 5% 4 5% 6% 19%	6306
	11th to 20th	Night	17% 13% 3 7% 3 3 3 2 3 7% 37%	6100
	21st to end	Night	18% 12% 3 7% 3 3 3 2 3 5% 40%	6468
February	1st to 10th	Night	14% 13% 4 7% 5% 4 4 3 6% 7% 32%	2600
	11th to 20th	Night	15% 14% 4 8% 5% 5% 4 3 5% 5% 31%	2532
	21st to end	Night	13% 17% 3 8% 5% 4 5% 3 5% 8% 30%	2020
March	1st to 10th	Night	17% 17% 3 10% 5% 4 4 3 5% 7% 26%	5053
	11th to 20th	Night	11% 14% 3 9% 5% 4 4 3 4 5% 37%	5000
	21st to end	Night	14% 16% 5% 11% 7% 5% 5% 4 4 6% 23%	5412
April	1st to 10th	Night	15% 16% 4 7% 5% 4 5% 4 4 7% 29%	2000
	11th to 20th	Night	13% 14% 3 6% 4 4 4 4 5% 6% 37%	1979
	21st to end	Night	18% 10% 2 5% 4 3 3 4 6% 9% 35%	1889
May	1st to 10th	Night	14% 8% 3 5% 3 3 3 3 4 5% 51%	4013
	11th to 20th	Night	13% 10% 3 6% 3 3 4 3 4 4 48%	3616
	21st to end	Night	12% 5% 2 3 3 2 3 2 4 6% 60%	3972
June	1st to 10th	Night	12% 7% 2 5% 4 3 5% 4 7% 6% 47%	1600
	11th to 20th	Night	9% 5% 2 4 4 4 3 3 4 7% 56%	1600
	21st to end	Night	5% 3 1 3 2 2 2 2 4 4 72%	1600
July	1st to 10th	Night	2 1 2 1 2 2 2 2 3 85%	2580
	11th to 20th	Night	2 2 1 1 2 2 1 1 3 4 81%	2540
	21st to end	Night	1 1 1 1 1 1 2 2 3 86%	2998
August	1st to 10th	Night	2 2 2 2 2 2 2 4 4 79%	2848
	11th to 20th	Night	1 1 1 2 2 2 2 3 6% 79%	2812
	21st to end	Night	5% 3 1 3 2 1 2 2 2 3 75%	3460
September	1st to 10th	Night	2 2 1 1 1 1 2 2 3 5% 80%	1900
	11th to 20th	Night	5% 4 1 3 3 2 3 3 5% 8% 64%	1905
	21st to end	Night	8% 4 1 4 3 2 2 2 4 6% 63%	2176
October	1st to 10th	Night	4 3 1 4 3 3 2 2 2 4 71%	3800
	11th to 20th	Night	11% 5% 2 5% 3 2 3 2 5% 6% 55%	3777
	21st to end	Night	10% 8% 3 8% 5% 6% 5% 4 6% 7% 38%	4121
November	1st to 10th	Night	18% 11% 3 7% 5% 4 5% 3 5% 6% 34%	2491
	11th to 20th	Night	19% 9% 3 8% 5% 5% 5% 4 6% 7% 27%	2800
	21st to end	Night	16% 11% 2 6% 4 4 4 3 5% 7% 37%	2800
December	1st to 10th	Night	10% 8% 2 5% 4 3 4 3 5% 9% 48%	4560
	11th to 20th	Night	16% 16% 5% 11% 7% 5% 5% 4 6% 7% 20%	5176
	21st to end	Night	21% 14% 4 8% 5% 5% 4 4 5% 7% 22%	6952

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	62% 8% 9% 14% 6% 1	3639
	11th to 20th	Day	47% 6% 7% 24% 13% 4	3866
	21st to end	Day	39% 7% 10% 13% 19% 12%	4510
February	1st to 10th	Day	52% 9% 15% 13% 10% 2	1800
	11th to 20th	Day	51% 9% 12% 17% 9% 2	1764
	21st to end	Day	54% 7% 11% 15% 10% 2	1608
March	1st to 10th	Day	54% 7% 10% 22% 7%	4825
	11th to 20th	Day	49% 8% 11% 14% 17% 1	5000
	21st to end	Day	55% 11% 13% 17% 3	5561
April	1st to 10th	Day	56% 8% 13% 14% 7% 2	2400
	11th to 20th	Day	50% 8% 14% 14% 10% 4	2372
	21st to end	Day	45% 7% 17% 19% 11% 1	2509
May	1st to 10th	Day	32% 6% 10% 28% 22% 3	5962
	11th to 20th	Day	32% 7% 12% 25% 20% 3	6280
	21st to end	Day	25% 6% 18% 41% 10%	6904
June	1st to 10th	Day	37% 7% 16% 29% 11% 1	2800
	11th to 20th	Day	30% 6% 13% 32% 17% 2	2800
	21st to end	Day	18% 4 12% 31% 27% 8%	2800
July	1st to 10th	Day	7% 2 6% 16% 23% 46%	4422
	11th to 20th	Day	11% 3 4 15% 30% 38%	4345
	21st to end	Day	7% 2 5% 19% 46% 22%	4415
August	1st to 10th	Day	13% 3 6% 18% 40% 20%	4076
	11th to 20th	Day	10% 3 7% 34% 34% 12%	4065
	21st to end	Day	13% 3 7% 31% 33% 13%	4215
September	1st to 10th	Day	11% 3 5% 22% 34% 24%	2280
	11th to 20th	Day	23% 6% 11% 25% 25% 11%	2271
	21st to end	Day	23% 4 11% 25% 23% 14%	2003
October	1st to 10th	Day	20% 4 12% 15% 24% 26%	3180
	11th to 20th	Day	29% 6% 16% 30% 14% 4	3161
	21st to end	Day	41% 8% 11% 19% 15% 5%	3448
November	1st to 10th	Day	48% 8% 13% 16% 12% 2	1803
	11th to 20th	Day	52% 9% 14% 11% 9% 4	1600
	21st to end	Day	48% 7% 13% 14% 13% 6%	1600
December	1st to 10th	Day	34% 6% 9% 24% 20% 6%	2620
	11th to 20th	Day	60% 7% 14% 11% 7%	3028
	21st to end	Day	64% 10% 10% 13% 3 1	4004
January	1st to 10th	Night	63% 8% 7% 13% 6% 1	6306
	11th to 20th	Night	47% 6% 7% 22% 13% 4	6100
	21st to end	Night	47% 6% 6% 10% 16% 15%	6468
February	1st to 10th	Night	52% 7% 14% 14% 11% 2	2600
	11th to 20th	Night	54% 8% 11% 17% 8% 2	2534
	21st to end	Night	51% 10% 13% 15% 8% 2	2020
March	1st to 10th	Night	55% 7% 15% 16% 5% 1	5053
	11th to 20th	Night	47% 8% 11% 19% 14% 2	5000
	21st to end	Night	58% 12% 13% 14% 3	5412
April	1st to 10th	Night	55% 8% 16% 15% 6% 1	2000
	11th to 20th	Night	47% 10% 15% 17% 8% 4	1979
	21st to end	Night	45% 10% 18% 19% 7% 1	1889
May	1st to 10th	Night	36% 7% 12% 27% 18% 1	4010
	11th to 20th	Night	39% 6% 13% 31% 10% 1	3616
	21st to end	Night	27% 6% 26% 36% 4	3972
June	1st to 10th	Night	37% 9% 17% 30% 6%	1600
	11th to 20th	Night	30% 7% 15% 33% 14% 1	1600
	21st to end	Night	19% 3 14% 39% 21% 4	1600
July	1st to 10th	Night	8% 3 6% 18% 21% 43%	2540
	11th to 20th	Night	10% 3 4 17% 36% 30%	2492
	21st to end	Night	6% 2 5% 22% 48% 16%	2904
August	1st to 10th	Night	11% 3 6% 21% 51% 8%	2839
	11th to 20th	Night	8% 3 12% 40% 28% 9%	2839
	21st to end	Night	16% 2 8% 33% 31% 9%	3460
September	1st to 10th	Night	10% 3 6% 26% 36% 19%	1900
	11th to 20th	Night	20% 5% 13% 27% 26% 9%	1905
	21st to end	Night	25% 4 12% 25% 18% 16%	2176
October	1st to 10th	Night	20% 4 8% 19% 26% 23%	3800
	11th to 20th	Night	31% 5% 13% 34% 11% 6%	3777
	21st to end	Night	43% 8% 11% 18% 14% 6%	4121
November	1st to 10th	Night	51% 8% 12% 14% 12% 5%	2491
	11th to 20th	Night	53% 10% 11% 12% 9% 4	2800
	21st to end	Night	47% 6% 12% 12% 15% 7%	2800
December	1st to 10th	Night	34% 7% 8% 22% 25% 5%	4560
	11th to 20th	Night	62% 9% 11% 12% 6% 1	5176
	21st to end	Night	58% 8% 12% 15% 4 1	6952

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	33% 18% 25% 20% 4 2	200
	11th to 20th	Day	27% 11% 29% 19% 10% 4	296
	21st to end	Day	26% 13% 21% 26% 13% 1	440
February	1st to 10th	Day	21% 20% 25% 28% 7%	400
	11th to 20th	Day	27% 22% 23% 21% 7%	395
	21st to end	Day	27% 18% 26% 22% 6% 1	394
March	1st to 10th	Day	27% 20% 28% 18% 7%	500
	11th to 20th	Day	25% 17% 27% 23% 9%	500
	21st to end	Day	25% 24% 27% 18% 6%	550
April	1st to 10th	Day	21% 23% 28% 20% 7% 1	500
	11th to 20th	Day	22% 22% 26% 20% 7% 2	492
	21st to end	Day	22% 19% 26% 22% 9% 2	500
May	1st to 10th	Day	17% 21% 28% 20% 10% 3	503
	11th to 20th	Day	21% 18% 27% 20% 11% 3	528
	21st to end	Day	18% 14% 28% 26% 9% 5%	573
June	1st to 10th	Day	13% 18% 28% 23% 13% 5%	530
	11th to 20th	Day	12% 20% 29% 25% 12% 3	530
	21st to end	Day	4 11% 34% 31% 14% 7%	530
July	1st to 10th	Day	5% 12% 31% 31% 16% 6%	507
	11th to 20th	Day	3 8% 37% 30% 15% 7%	484
	21st to end	Day	1 6% 35% 34% 15% 9%	531
August	1st to 10th	Day	2 7% 33% 33% 16% 9%	487
	11th to 20th	Day	3 11% 25% 35% 18% 9%	477
	21st to end	Day	5% 9% 27% 34% 20% 5%	539
September	1st to 10th	Day	3 9% 28% 36% 20% 4	490
	11th to 20th	Day	8% 11% 28% 33% 18% 2	490
	21st to end	Day	10% 12% 24% 33% 18% 2	490
October	1st to 10th	Day	11% 10% 26% 31% 20% 1	490
	11th to 20th	Day	15% 11% 21% 30% 23% 1	490
	21st to end	Day	18% 15% 26% 24% 16% 2	539
November	1st to 10th	Day	21% 15% 20% 30% 14% 1	341
	11th to 20th	Day	25% 11% 24% 27% 11% 4	200
	21st to end	Day	22% 12% 23% 29% 15% 1	200
December	1st to 10th	Day	22% 12% 23% 25% 14% 5%	200
	11th to 20th	Day	28% 16% 21% 26% 8% 2	200
	21st to end	Day	30% 13% 23% 24% 10%	220
January	1st to 10th	Night	25% 11% 29% 25% 9% 2	800
	11th to 20th	Night	21% 12% 23% 28% 14% 3	800
	21st to end	Night	21% 11% 19% 32% 14% 2	880
February	1st to 10th	Night	22% 10% 21% 36% 10% 1	800
	11th to 20th	Night	23% 12% 21% 30% 13% 2	784
	21st to end	Night	20% 12% 23% 33% 10%	660
March	1st to 10th	Night	26% 12% 22% 28% 10% 2	800
	11th to 20th	Night	20% 11% 22% 29% 16% 2	800
	21st to end	Night	20% 17% 22% 26% 13% 1	880
April	1st to 10th	Night	22% 12% 22% 27% 15% 2	800
	11th to 20th	Night	19% 11% 23% 27% 16% 3	791
	21st to end	Night	22% 8% 17% 30% 20% 3	689
May	1st to 10th	Night	15% 8% 18% 34% 22% 3	565
	11th to 20th	Night	17% 6% 15% 31% 28% 3	394
	21st to end	Night	17% 7% 14% 31% 28% 2	433
June	1st to 10th	Night	15% 6% 10% 34% 33% 2	400
	11th to 20th	Night	11% 6% 16% 40% 25% 2	400
	21st to end	Night	7% 3 9% 39% 40% 2	400
July	1st to 10th	Night	3 2 8% 37% 49% 2	380
	11th to 20th	Night	5% 1 6% 44% 42% 2	368
	21st to end	Night	1 1 4 29% 57% 9%	552
August	1st to 10th	Night	3 1 4 31% 52% 9%	567
	11th to 20th	Night	2 1 6% 31% 52% 8%	564
	21st to end	Night	3 2 15% 34% 41% 5%	811
September	1st to 10th	Night	3 3 13% 36% 40% 6%	760
	11th to 20th	Night	6% 4 13% 32% 37% 7%	760
	21st to end	Night	10% 5% 12% 36% 29% 7%	760
October	1st to 10th	Night	9% 4 15% 38% 31% 4	760
	11th to 20th	Night	14% 5% 14% 36% 27% 5%	760
	21st to end	Night	16% 7% 18% 33% 21% 6%	836
November	1st to 10th	Night	20% 9% 22% 32% 14% 3	780
	11th to 20th	Night	24% 9% 21% 33% 12% 1	800
	21st to end	Night	22% 9% 22% 31% 15% 1	800
December	1st to 10th	Night	20% 11% 20% 33% 14% 2	800
	11th to 20th	Night	24% 14% 24% 24% 12% 2	800
	21st to end	Night	30% 16% 23% 24% 6% 1	880

Page generated at 2025-07-02 22:30:11. Version ac07

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