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 2024-03-18.

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	12% 8% 3 5% 5% 3 4 4 5% 6% 46%	78716
	Night	14% 9% 3 6% 5% 3 4 4 5% 6% 42%	81098
Transparency	Day	39% 6% 12% 20% 17% 7%	78427
	Night	43% 7% 12% 19% 14% 5%	80908
Seeing	Day	16% 15% 27% 26% 13% 3	14015
	Night	17% 8% 18% 31% 23% 3	23243

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	18% 16% 5% 10% 7% 3 5% 5% 5% 7% 19%	2539
	11th to 20th	Day	29% 14% 3 6% 5% 3 4 3 4 5% 23%	2696
	21st to end	Day	20% 11% 4 7% 6% 3 5% 5% 5% 7% 28%	3146
February	1st to 10th	Day	12% 11% 5% 7% 6% 4 5% 6% 7% 7% 29%	1710
	11th to 20th	Day	13% 11% 3 9% 6% 4 4 5% 6% 7% 34%	1665
	21st to end	Day	14% 16% 6% 8% 6% 3 4 4 4 7% 28%	1530
March	1st to 10th	Day	17% 13% 4 9% 6% 4 5% 5% 5% 7% 26%	2033
	11th to 20th	Day	14% 12% 5% 7% 6% 3 6% 4 5% 6% 32%	2059
	21st to end	Day	16% 12% 5% 9% 6% 3 6% 5% 6% 8% 24%	2202
April	1st to 10th	Day	14% 14% 5% 8% 7% 3 5% 5% 6% 7% 26%	2160
	11th to 20th	Day	16% 13% 4 6% 5% 3 4 4 6% 7% 31%	2129
	21st to end	Day	13% 10% 3 5% 5% 3 4 4 6% 8% 39%	2257
May	1st to 10th	Day	11% 8% 2 6% 4 3 3 4 4 6% 46%	2370
	11th to 20th	Day	12% 8% 3 6% 5% 4 4 5% 5% 7% 42%	2477
	21st to end	Day	17% 8% 3 5% 5% 2 4 4 5% 8% 38%	2723
June	1st to 10th	Day	12% 7% 3 5% 5% 3 4 3 6% 8% 44%	2520
	11th to 20th	Day	9% 5% 2 4 4 3 4 4 4 7% 54%	2520
	21st to end	Day	5% 3 1 2 3 2 2 3 3 5% 70%	2520
July	1st to 10th	Day	2 2 1 2 2 1 2 3 3 4 78%	2520
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 3 79%	2520
	21st to end	Day	2 1 2 2 1 2 2 3 3 81%	2611
August	1st to 10th	Day	2 1 1 2 2 1 3 3 4 5% 75%	2328
	11th to 20th	Day	1 2 1 2 2 1 2 3 4 5% 79%	2298
	21st to end	Day	3 2 1 2 2 1 2 3 2 5% 79%	2399
September	1st to 10th	Day	3 2 2 2 2 2 3 3 5% 78%	2160
	11th to 20th	Day	4 3 2 3 3 2 3 5% 6% 6% 61%	2152
	21st to end	Day	7% 4 1 4 4 2 2 4 4 7% 61%	1898
October	1st to 10th	Day	9% 4 2 4 4 3 4 3 4 6% 56%	1800
	11th to 20th	Day	10% 5% 2 4 4 3 5% 4 5% 7% 51%	1800
	21st to end	Day	12% 8% 3 5% 5% 3 4 3 4 6% 46%	1980
November	1st to 10th	Day	17% 10% 3 6% 5% 3 4 5% 6% 8% 32%	1712
	11th to 20th	Day	18% 8% 2 6% 6% 3 5% 6% 7% 9% 28%	1520
	21st to end	Day	15% 10% 4 7% 6% 3 5% 5% 4 7% 34%	1520
December	1st to 10th	Day	15% 9% 3 6% 5% 3 4 4 5% 8% 38%	1520
	11th to 20th	Day	18% 13% 4 6% 7% 3 4 5% 6% 9% 24%	1928
	21st to end	Day	18% 14% 4 8% 7% 4 6% 5% 6% 8% 21%	2794
January	1st to 10th	Night	19% 17% 5% 8% 6% 4 6% 5% 5% 6% 19%	4406
	11th to 20th	Night	25% 15% 4 6% 5% 3 4 3 4 6% 25%	4265
	21st to end	Night	25% 15% 4 8% 5% 3 3 3 4 5% 24%	4521
February	1st to 10th	Night	13% 13% 4 6% 6% 3 4 5% 6% 7% 33%	2470
	11th to 20th	Night	15% 14% 4 7% 5% 3 4 4 5% 6% 32%	2402
	21st to end	Night	13% 15% 4 7% 5% 4 5% 4 5% 8% 30%	1922
March	1st to 10th	Night	18% 14% 3 8% 5% 3 5% 5% 5% 7% 27%	2142
	11th to 20th	Night	13% 13% 3 8% 6% 3 6% 4 5% 6% 33%	2067
	21st to end	Night	16% 13% 6% 8% 7% 4 5% 5% 5% 7% 25%	2152
April	1st to 10th	Night	16% 14% 5% 6% 6% 3 5% 6% 4 7% 29%	1800
	11th to 20th	Night	15% 13% 4 5% 5% 3 5% 5% 5% 6% 35%	1779
	21st to end	Night	18% 9% 3 5% 4 3 4 5% 5% 9% 36%	1701
May	1st to 10th	Night	12% 8% 3 5% 4 4 4 3 5% 6% 46%	1589
	11th to 20th	Night	14% 5% 3 5% 5% 3 6% 6% 5% 7% 40%	1416
	21st to end	Night	15% 8% 4 5% 5% 3 4 4 5% 8% 39%	1552
June	1st to 10th	Night	13% 7% 2 5% 4 2 4 5% 6% 6% 44%	1440
	11th to 20th	Night	10% 5% 2 4 4 3 3 3 4 7% 54%	1440
	21st to end	Night	5% 3 1 3 2 2 2 2 4 4 71%	1440
July	1st to 10th	Night	3 2 1 2 2 2 3 4 4 4 74%	1440
	11th to 20th	Night	3 2 1 2 3 1 2 2 2 4 79%	1440
	21st to end	Night	1 1 2 1 1 2 2 2 3 85%	1704
August	1st to 10th	Night	2 2 1 2 1 2 2 4 5% 77%	1611
	11th to 20th	Night	1 1 1 2 1 3 2 2 5% 80%	1602
	21st to end	Night	2 1 1 1 2 1 2 3 3 5% 80%	1956
September	1st to 10th	Night	3 2 1 1 1 1 1 2 3 5% 80%	1800
	11th to 20th	Night	5% 3 1 3 3 1 3 3 5% 8% 65%	1804
	21st to end	Night	9% 4 1 4 3 2 2 3 4 6% 62%	2061
October	1st to 10th	Night	7% 5% 2 4 4 2 3 4 4 6% 59%	2160
	11th to 20th	Night	11% 5% 2 4 5% 2 3 3 5% 6% 54%	2160
	21st to end	Night	13% 6% 2 5% 4 3 4 4 5% 6% 46%	2376
November	1st to 10th	Night	18% 9% 3 6% 5% 3 5% 5% 5% 6% 34%	2362
	11th to 20th	Night	20% 8% 3 6% 6% 3 5% 5% 6% 8% 29%	2660
	21st to end	Night	17% 10% 3 6% 4 3 5% 4 5% 7% 37%	2660
December	1st to 10th	Night	16% 9% 3 5% 6% 3 4 5% 5% 7% 38%	2660
	11th to 20th	Night	16% 12% 4 9% 8% 4 5% 5% 6% 7% 25%	3276
	21st to end	Night	18% 14% 4 8% 6% 4 5% 5% 5% 6% 25%	4862

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	63% 9% 10% 13% 4 1	2539
	11th to 20th	Day	63% 7% 7% 11% 7% 5%	2696
	21st to end	Day	53% 9% 13% 15% 7% 3	3146
February	1st to 10th	Day	51% 9% 16% 13% 10% 2	1710
	11th to 20th	Day	50% 9% 12% 18% 9% 2	1674
	21st to end	Day	55% 7% 11% 15% 9% 2	1530
March	1st to 10th	Day	60% 7% 13% 14% 7%	2033
	11th to 20th	Day	53% 7% 12% 16% 10% 1	2059
	21st to end	Day	59% 9% 13% 13% 5%	2202
April	1st to 10th	Day	58% 8% 12% 13% 7% 2	2160
	11th to 20th	Day	53% 8% 14% 13% 8% 4	2132
	21st to end	Day	45% 7% 17% 19% 11% 1	2257
May	1st to 10th	Day	39% 6% 15% 26% 12% 1	2370
	11th to 20th	Day	42% 8% 13% 25% 10% 2	2480
	21st to end	Day	46% 7% 18% 21% 8%	2724
June	1st to 10th	Day	40% 7% 16% 28% 9%	2520
	11th to 20th	Day	31% 7% 13% 33% 15% 2	2520
	21st to end	Day	19% 4 11% 32% 26% 8%	2520
July	1st to 10th	Day	12% 3 10% 28% 30% 17%	2451
	11th to 20th	Day	13% 3 6% 21% 35% 22%	2445
	21st to end	Day	10% 2 6% 18% 44% 20%	2466
August	1st to 10th	Day	12% 4 10% 29% 31% 14%	2312
	11th to 20th	Day	11% 2 8% 23% 42% 14%	2298
	21st to end	Day	12% 2 7% 26% 37% 15%	2399
September	1st to 10th	Day	11% 3 6% 23% 34% 24%	2160
	11th to 20th	Day	22% 6% 11% 24% 26% 12%	2152
	21st to end	Day	24% 5% 11% 24% 22% 14%	1898
October	1st to 10th	Day	30% 5% 13% 18% 20% 14%	1800
	11th to 20th	Day	33% 6% 12% 21% 20% 7%	1800
	21st to end	Day	40% 6% 12% 17% 16% 9%	1980
November	1st to 10th	Day	48% 8% 13% 16% 13% 2	1712
	11th to 20th	Day	50% 9% 14% 12% 10% 4	1520
	21st to end	Day	48% 7% 13% 13% 12% 6%	1520
December	1st to 10th	Day	44% 8% 12% 19% 14% 3	1520
	11th to 20th	Day	55% 8% 16% 12% 8% 1	1928
	21st to end	Day	57% 10% 11% 16% 4 1	2794
January	1st to 10th	Night	63% 10% 10% 12% 4 2	4406
	11th to 20th	Night	61% 7% 8% 12% 8% 5%	4265
	21st to end	Night	62% 7% 7% 12% 9% 3	4521
February	1st to 10th	Night	50% 8% 15% 15% 11% 2	2470
	11th to 20th	Night	54% 8% 10% 18% 8% 2	2404
	21st to end	Night	52% 11% 13% 15% 7% 2	1922
March	1st to 10th	Night	56% 8% 13% 15% 7% 1	2142
	11th to 20th	Night	53% 7% 11% 16% 11% 1	2067
	21st to end	Night	60% 9% 14% 12% 4	2152
April	1st to 10th	Night	55% 9% 14% 15% 6% 1	1800
	11th to 20th	Night	50% 10% 15% 15% 7% 4	1779
	21st to end	Night	45% 10% 18% 19% 6% 1	1701
May	1st to 10th	Night	41% 8% 16% 24% 9% 2	1589
	11th to 20th	Night	44% 8% 16% 26% 5% 2	1416
	21st to end	Night	45% 9% 21% 21% 4	1552
June	1st to 10th	Night	40% 9% 17% 29% 4	1440
	11th to 20th	Night	31% 8% 16% 32% 12% 1	1440
	21st to end	Night	19% 3 14% 39% 21% 4	1440
July	1st to 10th	Night	14% 5% 11% 32% 24% 15%	1400
	11th to 20th	Night	12% 3 6% 26% 38% 15%	1392
	21st to end	Night	8% 2 5% 24% 49% 12%	1609
August	1st to 10th	Night	11% 4 10% 34% 33% 8%	1602
	11th to 20th	Night	10% 3 7% 32% 38% 10%	1602
	21st to end	Night	10% 3 10% 30% 35% 12%	1956
September	1st to 10th	Night	10% 3 6% 26% 35% 20%	1800
	11th to 20th	Night	19% 5% 13% 26% 27% 9%	1804
	21st to end	Night	26% 4 12% 24% 18% 16%	2061
October	1st to 10th	Night	27% 7% 12% 19% 24% 11%	2160
	11th to 20th	Night	32% 5% 13% 21% 19% 10%	2160
	21st to end	Night	37% 9% 13% 15% 15% 11%	2376
November	1st to 10th	Night	51% 8% 11% 14% 12% 5%	2362
	11th to 20th	Night	52% 10% 12% 12% 10% 5%	2660
	21st to end	Night	48% 6% 12% 12% 15% 8%	2660
December	1st to 10th	Night	44% 9% 10% 19% 16% 3	2660
	11th to 20th	Night	57% 10% 10% 13% 10% 1	3276
	21st to end	Night	57% 8% 12% 16% 6% 2	4862

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% 24% 19% 4 2	180
	11th to 20th	Day	30% 12% 28% 17% 9% 4	271
	21st to end	Day	28% 14% 21% 25% 12% 1	407
February	1st to 10th	Day	18% 21% 26% 28% 7%	370
	11th to 20th	Day	26% 21% 22% 22% 7%	365
	21st to end	Day	28% 18% 27% 21% 5% 1	364
March	1st to 10th	Day	28% 20% 28% 17% 7%	460
	11th to 20th	Day	25% 16% 26% 24% 8%	449
	21st to end	Day	26% 21% 29% 18% 6%	462
April	1st to 10th	Day	22% 23% 29% 19% 7% 1	420
	11th to 20th	Day	25% 23% 25% 18% 6% 2	412
	21st to end	Day	22% 21% 25% 20% 9% 2	420
May	1st to 10th	Day	17% 21% 29% 20% 10% 3	423
	11th to 20th	Day	23% 19% 25% 19% 11% 3	448
	21st to end	Day	20% 14% 26% 25% 10% 5%	485
June	1st to 10th	Day	14% 20% 27% 21% 14% 4	450
	11th to 20th	Day	13% 20% 28% 25% 11% 3	450
	21st to end	Day	3 12% 35% 30% 14% 6%	450
July	1st to 10th	Day	5% 12% 34% 30% 14% 5%	446
	11th to 20th	Day	3 8% 38% 29% 14% 8%	444
	21st to end	Day	1 6% 35% 33% 15% 9%	487
August	1st to 10th	Day	1 7% 32% 34% 17% 9%	447
	11th to 20th	Day	3 9% 25% 36% 18% 9%	437
	21st to end	Day	4 9% 28% 35% 19% 5%	495
September	1st to 10th	Day	4 10% 28% 36% 19% 4	450
	11th to 20th	Day	8% 10% 27% 34% 18% 2	450
	21st to end	Day	10% 13% 25% 32% 18% 2	450
October	1st to 10th	Day	12% 11% 26% 30% 19% 1	450
	11th to 20th	Day	16% 10% 20% 29% 23% 1	450
	21st to end	Day	17% 15% 26% 24% 16% 2	495
November	1st to 10th	Day	20% 15% 20% 29% 15% 1	310
	11th to 20th	Day	21% 9% 26% 28% 12% 4	180
	21st to end	Day	22% 12% 23% 30% 13% 1	180
December	1st to 10th	Day	23% 13% 23% 24% 12% 4	180
	11th to 20th	Day	28% 15% 21% 26% 8% 2	180
	21st to end	Day	30% 12% 22% 25% 11% 1	198
January	1st to 10th	Night	25% 10% 28% 25% 9% 3	760
	11th to 20th	Night	22% 12% 22% 28% 13% 3	760
	21st to end	Night	22% 11% 19% 32% 14% 3	836
February	1st to 10th	Night	21% 10% 20% 37% 11% 1	760
	11th to 20th	Night	23% 11% 20% 31% 14% 2	744
	21st to end	Night	20% 12% 24% 33% 10%	628
March	1st to 10th	Night	25% 12% 22% 28% 10% 3	760
	11th to 20th	Night	20% 11% 21% 29% 17% 2	752
	21st to end	Night	21% 15% 22% 27% 13% 1	792
April	1st to 10th	Night	23% 13% 21% 26% 16% 2	720
	11th to 20th	Night	20% 12% 22% 26% 17% 3	711
	21st to end	Night	23% 6% 17% 30% 21% 3	621
May	1st to 10th	Night	15% 8% 17% 33% 24% 3	509
	11th to 20th	Night	16% 6% 15% 30% 30% 3	354
	21st to end	Night	18% 7% 14% 31% 27% 3	389
June	1st to 10th	Night	16% 7% 10% 33% 33% 2	360
	11th to 20th	Night	11% 6% 17% 37% 27% 2	360
	21st to end	Night	6% 3 9% 38% 42% 2	360
July	1st to 10th	Night	3 1 8% 34% 52% 2	350
	11th to 20th	Night	5% 1 7% 42% 44% 2	348
	21st to end	Night	1 1 4 27% 58% 9%	522
August	1st to 10th	Night	3 1 4 29% 54% 9%	537
	11th to 20th	Night	2 1 5% 29% 54% 9%	534
	21st to end	Night	3 2 15% 34% 41% 5%	768
September	1st to 10th	Night	3 3 13% 34% 40% 6%	720
	11th to 20th	Night	5% 4 13% 32% 39% 8%	720
	21st to end	Night	11% 5% 12% 35% 29% 8%	720
October	1st to 10th	Night	9% 4 15% 37% 30% 4	720
	11th to 20th	Night	13% 5% 14% 36% 27% 5%	720
	21st to end	Night	17% 6% 16% 33% 22% 6%	792
November	1st to 10th	Night	20% 9% 22% 31% 14% 4	740
	11th to 20th	Night	23% 8% 20% 35% 12% 1	760
	21st to end	Night	22% 8% 23% 31% 15% 1	760
December	1st to 10th	Night	21% 11% 19% 32% 14% 2	760
	11th to 20th	Night	25% 13% 24% 24% 13% 2	760
	21st to end	Night	29% 16% 23% 25% 7% 1	836

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