<s>
A	O
prognostic	B-Application
variable	I-Application
in	O
engineering	O
within	O
the	O
context	O
of	O
prognostics	O
,	O
is	O
a	O
measured	O
or	O
estimated	O
variable	O
that	O
is	O
correlated	O
with	O
the	O
health	O
condition	O
of	O
a	O
system	O
,	O
and	O
may	O
be	O
used	O
to	O
predict	O
its	O
residual	O
useful	O
life	O
.	O
</s>
<s>
An	O
ideal	O
prognostic	B-Application
variable	I-Application
is	O
easily	O
measured	O
or	O
calculated	O
,	O
and	O
provides	O
an	O
exact	O
estimation	O
of	O
how	O
long	O
time	O
the	O
system	O
can	O
continue	O
to	O
operate	O
before	O
maintenance	O
or	O
replacement	O
will	O
be	O
required	O
.	O
</s>
<s>
Real	O
prognostic	B-Application
variables	I-Application
are	O
usually	O
either	O
known	O
with	O
some	O
uncertainty	O
,	O
may	O
be	O
difficult	O
to	O
measure	O
,	O
and	O
their	O
correlation	O
to	O
the	O
state	O
of	O
health	O
of	O
the	O
system	O
may	O
not	O
be	O
exact	O
.	O
</s>
<s>
Examples	O
of	O
prognostic	B-Application
variables	I-Application
are	O
the	O
age	O
of	O
a	O
vehicle	O
and	O
its	O
odometer	O
reading	O
:	O
the	O
older	O
a	O
car	O
is	O
,	O
and	O
the	O
longer	O
it	O
has	O
been	O
driven	O
,	O
the	O
more	O
worn	O
it	O
can	O
be	O
expected	O
to	O
be	O
.	O
</s>
<s>
These	O
prognostic	B-Application
variables	I-Application
are	O
useful	O
,	O
but	O
not	O
ideal	O
,	O
since	O
they	O
do	O
not	O
consider	O
other	O
aspects	O
such	O
as	O
the	O
regularity	O
of	O
the	O
maintenance	O
that	O
was	O
applied	O
on	O
the	O
vehicle	O
,	O
how	O
they	O
were	O
driven	O
,	O
in	O
which	O
weather	O
conditions	O
,	O
etc	O
.	O
</s>
<s>
In	O
climate	O
science	O
,	O
the	O
term	O
indicates	O
a	O
variable	O
that	O
a	O
computer	B-Application
model	I-Application
predicts	O
by	O
integration	O
of	O
a	O
physical	O
equation	O
,	O
typically	O
vorticity	O
,	O
divergence	B-Application
,	O
temperature	O
,	O
surface	O
pressure	O
,	O
and	O
water	O
vapour	O
concentration	O
in	O
atmospheric	O
models	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
there	O
are	O
some	O
other	O
variables	O
that	O
need	O
to	O
be	O
calculated	O
separately	O
as	O
derived	O
variables	O
,	O
such	O
as	O
relative	O
humidity	O
,	O
which	O
may	O
be	O
a	O
diagnostic	O
variable	O
obtained	O
from	O
the	O
model	O
's	O
prognostic	B-Application
variables	I-Application
,	O
temperature	O
and	O
water	O
vapour	O
.	O
</s>
