kanji - Partial ひらがな sentences (why ことし, きのう, きょう, あす?)

I was watching a news broadcast on YouTube and when it moved over to the weather report, I noticed that 「今日の天気」was actually written as 「きょうの天気」. Also 「明日の天気」was also written as 「あすの天気」. It wasn't just the weather report but some other parts of the programme such as the main stream headlines, 「きのう」.

Why are these sentences or should I say, just the start of the sentence written in ひらがな and not full 漢字? Does it depend on the news programme? I asked my Japanese friend and he said he didn't know or even realise it.

For reference, I was watching おはよう北海道 (16:15) when I first noticed it.

Thanks

Answer

This is related to the fact that 今日 and 明日 have two readings. According to the July 2008 issue of 放送研究と調査:

放送で，「今年」を「ことし」と表記する理由

「今年」と漢字で書いた場合，「ことし」と読んだらよいのか，「こんねん」と読んだらよいのか，わからなかったり，迷ったりするおそれがある。そのため，放送では「ことし」と，ひらがな書きにすることにしている。同じように「今日（きょう/こんにち）」「明日（あす/みょうにち）」「昨日（きのう/さくじつ）」なども，「きょう」「あす」「きのう」と，ひらがなで表記するようにしている。

EDIT: This is NHK's house rule, so a different media company may have a different rule about this.

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