How to sample records

Dear friends, I'm resending my

Dear friends,
owing to a very poor
webmail I'm compelled to use I'm resending my messed-up message of
yesterday.

in postgresql 8.0.7 I have the following table

 \d 
basedati
               Tabella "public.basedati"
 Colonna  
|            Tipo             | Modificatori
----------
+-----------------------------+--------------
 data_ora | timestamp 
without time zone |
 cod_wmo  | character(5)                |
 t_aria   
| numeric(5,1)                |
 ur       | integer                     
|

looking like:

data_ora | cod_wmo | t_aria | ur
---------------------+---------+--------+----
2006-07-05 00:50:00 |
16235 | 22.0 | 74
2006-07-05 02:50:00 | 16235 | 19.0 | 80

2006-07-05 09:50:00 | 16235 | 28.0 | 45
2006-07-05 12:00:00 |
16235 | 31.0 | 41
2006-07-06 00:00:00 | 16235 | 20.1 | 93

2006-07-06 00:50:00 | 16235 | 20.0 | 93
2006-07-06 06:50:00 |
16235 | 25.0 | 65
2006-07-06 11:50:00 | 16235 | 30.0 | 49

2006-07-06 12:00:00 | 16235 | 29.5 | 51
2006-07-05 03:00:00 |
16242 | 19.9 | 64
2006-07-05 03:15:00 | 16242 | 20.0 | 69

2006-07-05 10:15:00 | 16242 | 28.0 | 39
2006-07-05 12:00:00 |
16242 | 28.6 | 39
2006-07-06 00:00:00 | 16242 | 22.2 | 71

2006-07-06 00:15:00 | 16242 | 22.0 | 74
2006-07-06 00:45:00 |
16242 | 23.0 | 64
2006-07-06 01:15:00 | 16242 | 21.0 | 74

2006-07-06 10:15:00 | 16242 | 27.0 | 56
2006-07-06 12:00:00 |
16242 | 25.6 | 72

What I would like to extract (and I'm unable
to!!) is:

for each cod_wmo and each day (that is to_char(data_ora,
'YYYY-MM-
DD')), the complete record in correspondence of the maximum
of t_aria.
e.g.
data_ora | cod_wmo | t_aria | ur
---------------------+---------+--------+----
2006-07-05 12:00:00 |
16235 | 31.0 | 41
2006-07-06 11:50:00 | 16235 | 30.0 | 49

2006-07-05 12:00:00 | 16242 | 28.6 | 39
2006-07-06 10:15:00 |
16242 | 27.0 | 56

Please help this absolute beginner

Ciao
Vittorio