# Expecting fog

When can you expect fog to occur?
A simple rule is: when the difference between the air temperature and the dew point is large and does not decrease, no fog is expected.
When the difference between the air temperature and the dew point is small and decreases, or the difference becomes even zero, you should watch out for fog.
Let’s have a closer look at how you can calculate the dew point.
In nautical publications you can find tables for calculating the dew point.
Here is an example of such a table:
In the left hand column you find the air temperature in Celsius, and at the top of the table you will find the relative humidity as a percentage.
We assume that the temperature is 27 degrees Celsius and the relative humidity is 60 percent. The intersection of the two figures shows that the dew point is reached at a temperature of 18 degrees Celsius or lower.
It is good practice to calculate your dew point at least every hour when you navigate in areas which are known for developing fog.