Defining spinner in xml with arrow

I am trying to create a Spinner control in my android project.

Here is my xml markup for the spinner

    <Spinner
    android:id="@+id/spinner1"
    android:layout_weight="2"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:background="@color/skyblue"
    android:entries="@array/hour_arrays"
    android:spinnerMode="dropdown"
    android:prompt="@string/hour_select"
    android:gravity="center"/>

This does not have any drop down arrow (which supposedly is there by default?) and as such to the untrained user it does not look like it can be clicked.

What am I missing?