A Freezing Degree Days (FDD) Calculator is a climate analysis tool that quantifies the severity and duration of cold weather over a specific period. It does not simply count the number of days below freezing; instead, it measures the cumulative “intensity” of the cold. For each day, the calculator takes the average temperature and measures how far below the freezing point (32°F or 0°C) it was. These daily values are then summed over a season. This calculation is critically important in fields like civil engineering for predicting frost depth in the ground, in agriculture for determining winter crop survival, and in hydrology for forecasting the thickness of ice on lakes and rivers.
Formula of Freezing Degree Days Calculator
The calculation of Freezing Degree Days is a cumulative process that begins with finding the value for each individual day.
1. Core Concept: Freezing Degree Day (FDD)
A Freezing Degree Day is a unit representing the difference between a day’s average temperature and the freezing point of water. The calculation is based on a fixed base temperature.
- Base Temperature (Fahrenheit): 32°F
- Base Temperature (Celsius): 0°C
2. Calculating FDD for a Single Day
This is the fundamental calculation you will perform for each day in your chosen period.
2.1. First, Calculate the Average Daily Temperature
You need the high and low temperatures for the day to find the average.
Formula:
Average Daily Temperature = (Daily High Temperature + Daily Low Temperature) / 2
2.2. Then, Calculate the Freezing Degree Days for that Day
Subtract the day’s average temperature from the base temperature. A crucial rule is that if the average temperature is at or above freezing, the FDD value for that day is zero, as no freezing occurred.
Formula (Fahrenheit):
FDD for the Day = 32 – Average Daily Temperature
(If the result is negative or zero, use 0)
Formula (Celsius):
FDD for the Day = 0 – Average Daily Temperature
(If the result is positive or zero, use 0)
3. Calculating Cumulative Freezing Degree Days
To get the total FDD for a specific period (like a week, month, or an entire winter season), you simply sum the FDD values calculated for each individual day within that period.
Formula:
Cumulative FDD = Sum of (FDD for Day 1 + FDD for Day 2 + … + FDD for Day N)
- N: The total number of days in the period.
Sample FDD Calculation for One Week
This table demonstrates how the daily FDD values are calculated and then summed to find the cumulative FDD for a week. The calculations are shown in Fahrenheit.
| Day | High Temp (°F) | Low Temp (°F) | Avg. Temp (°F) | Daily FDD Calculation | Daily FDD Value | Cumulative FDD |
| Monday | 30 | 20 | 25 | 32 – 25 | 7 | 7 |
| Tuesday | 28 | 10 | 19 | 32 – 19 | 13 | 20 |
| Wednesday | 34 | 22 | 28 | 32 – 28 | 4 | 24 |
| Thursday | 38 | 26 | 32 | 32 – 32 | 0 | 24 |
| Friday | 24 | 8 | 16 | 32 – 16 | 16 | 40 |
| Saturday | 20 | 0 | 10 | 32 – 10 | 22 | 62 |
| Sunday | 26 | 12 | 19 | 32 – 19 | 13 | 75 |
Example of Freezing Degree Days Calculator
Let’s calculate the cumulative Freezing Degree Days for a three-day cold snap. We will use Celsius for this example.
The daily temperature data is as follows:
- Day 1: High of -2°C, Low of -8°C
- Day 2: High of -5°C, Low of -15°C
- Day 3: High of 1°C, Low of -7°C
Step 1: Calculate the FDD for Day 1.
- Average Temp = (-2 + -8) / 2 = -5°C
- Daily FDD = 0 – (-5) = 5
Step 2: Calculate the FDD for Day 2.
- Average Temp = (-5 + -15) / 2 = -10°C
- Daily FDD = 0 – (-10) = 10
Step 3: Calculate the FDD for Day 3.
- Average Temp = (1 + -7) / 2 = -3°C
- Daily FDD = 0 – (-3) = 3
Step 4: Calculate the Cumulative FDD.
- Cumulative FDD = FDD Day 1 + FDD Day 2 + FDD Day 3
- Cumulative FDD = 5 + 10 + 3 = 18
Therefore, the total for the three-day period is 18 Freezing Degree Days (Celsius).
Most Common FAQs
They are complementary concepts used to measure different things. A Freezing Degree Day measures the intensity of cold below the freezing point (32°F or 0°C) and is used for applications related to frost and ice. A Heating Degree Day (HDD) measures the intensity of cold below a higher base temperature (typically 65°F or 18°C) and is used to estimate the amount of energy required to heat a building.
In cold climates, civil engineers use cumulative FDD data to predict the maximum depth that the ground will freeze in winter, known as the frost line. This is crucial because building foundations must be placed below this line to prevent them from being lifted or damaged by frozen, expanding soil (a phenomenon called frost heave).
A higher cumulative FDD value indicates a colder winter. The value accumulates faster and reaches a higher total when daily average temperatures are consistently far below the freezing point. A mild winter with few days below freezing would result in a very low cumulative FDD value.