\begin{table}%t1
\caption{The effect of different environmental parameters on the differential
refraction. The effects of changes in the atmospheric parameters
on the differential refraction are shown using our observations of the
Lockman hole with Keck. As a reference point, 2.5~$^\circ$C
temperature, 61.5 kPa ambient pressure, 40\% relative humidity
and 450~ppm CO$_2$ concentration is used. The effect of changing \emph{one} of
these four parameters (while keeping the other three at the nominal
value) is shown below. Only the differential refraction for 3500~{\AA}
is shown, $\Delta R(3500~\hbox{\AA})$ in arc seconds. The hour angle is
18~h.}
\label{modeltable}      % is used to refer this table in the text
%\centering
                          % used for centering table
\begin{tabular}{l c}        % centered columns (4 columns)
\hline\hline    
\noalign{\smallskip}             % inserts double horizontal lines
Parameter change & $\Delta R(3500~\hbox{\AA})$ \\    % table heading 
\hline                        % inserts single horizontal line
nominal & 4.53\\
$t=-10$ $^\circ$C & 4.75 \\
$t=20$ $^\circ$C & 4.27 \\
$p=30$~kPa & 2.21 \\
$p=100$ kPa & 7.37 \\
${\rm RH}=0$\% & 4.53 \\
${\rm RH}=90$\% & 4.54 \\
$x_{\rm c}=300$ ppm & 4.53 \\
$x_{\rm c}=500$ ppm & 4.53 \\
\hline                                   
\end{tabular}
\end{table}