What Is Millimeter per Hour (mm/h)?
Millimeter per hour is a unit that tells you how many millimeters something changes in one hour. It is a very small speed or rate. People use it for things that change slowly, like rainfall, tiny leaks, or slow moving parts in machines or science experiments.
Definition
One millimeter per hour means that in one hour, the amount or distance has changed by exactly one millimeter.
- Millimeter is a length unit. One millimeter is one thousandth of a meter.
- Hour is a time unit. One hour is 60 minutes or 3 600 seconds.
So, if rain is falling at 5 mm per hour, then in one hour the rain would build up to a depth of 5 millimeters on a flat surface, if nothing runs off or soaks in.
History / Origin
Millimeter per hour comes from two older units.
- The millimeter comes from the metric system, which started in France in the late 1700s. It is based on the meter, the main length unit of the International System of Units, also called SI.
- The hour is an old time unit used for thousands of years in many cultures. One day was divided into 24 hours.
When scientists and engineers began to measure very slow speeds and rates using the metric system, they combined millimeters with hours to form millimeter per hour. This is useful when changes per second are too tiny to talk about easily.
Symbol & Abbreviation
The standard way to write millimeter per hour is:
- mm/h
Key points about the symbol:
- mm means millimeter.
- / means per.
- h means hour.
- Always write the unit in lowercase letters.
- Write it after the number, with a space. For example, 12 mm/h.
Current Use Around the World
Millimeter per hour is used in many fields worldwide, especially where slow changes matter.
- Weather and climate: Meteorologists use mm/h to show how fast rain or snow is falling. Weather radar images often show rainfall rate in mm/h.
- Hydrology and water management: Engineers use mm/h to measure how fast water hits the ground, goes through soil, or falls on roofs and fields.
- Engineering and industry: Used for slow motion of machines, wear on parts, or tiny movements in building structures.
- Medicine and biology: Used for slow liquid flows, like drip rates, or growth rates in some lab tests, when the changes are small.
- Science and research: Used in experiments where very slow speeds or growth must be measured over hours.
Example Conversions
Here are some simple conversions to help you understand millimeter per hour.
Between millimeter per hour and millimeter per second
- 1 hour = 3 600 seconds.
- So, 1 mm/h = 1 mm every 3 600 seconds = 1 ÷ 3 600 mm/s ≈ 0.00028 mm/s.
- 10 mm/h ≈ 10 ÷ 3 600 ≈ 0.0028 mm/s.
Between millimeter per hour and meter per hour
- 1 meter = 1 000 millimeters.
- So, 1 mm/h = 1 ÷ 1 000 m/h = 0.001 m/h.
- 500 mm/h = 500 ÷ 1 000 = 0.5 m/h.
Between millimeter per hour and centimeter per hour
- 1 centimeter = 10 millimeters.
- So, 1 mm/h = 1 ÷ 10 cm/h = 0.1 cm/h.
- 25 mm/h = 25 ÷ 10 = 2.5 cm/h.
Between millimeter per hour and millimeter per day
- 1 day = 24 hours.
- So, 1 mm/h for a full day gives 1 × 24 = 24 mm/day.
- 5 mm/h for a full day gives 5 × 24 = 120 mm/day.
Between millimeter per hour and inch per hour
- 1 inch ≈ 25.4 millimeters.
- So, 1 mm/h = 1 ÷ 25.4 in/h ≈ 0.039 in/h.
- 25.4 mm/h ≈ 1 in/h.
Related Units
Millimeter per hour is part of a family of rate and speed units. Here are some related ones:
- Millimeter per second (mm/s) used for faster motions at small scale in machines or lab tools.
- Centimeter per hour (cm/h) another small scale speed or rate unit, equal to 10 mm/h.
- Meter per hour (m/h) used for very slow speeds over longer lengths, equal to 1 000 mm/h.
- Meter per second (m/s) main SI unit for speed used in physics and engineering.
- Millimeter per day (mm/day) used in climate, soil science, and building movements when changes are very slow.
- Millimeter per year (mm/year) used for tectonic plate motions, sea level change, glacier movement, and land sinking or rising.
FAQs
Q: What does mm/h mean in weather reports?
A: In weather, mm/h shows how quickly rain or snow is falling. For example, 2 mm/h means the rain would make a 2 millimeter deep layer in one hour if it did not drain away.
Q: Is a higher millimeter per hour number always worse for flooding?
A: A higher mm/h rate means heavier rain, which can cause flooding more easily, especially if it lasts a long time or the ground cannot absorb the water.
Q: How do I convert mm/h to mm/day?
A: Multiply by 24. For example, 3 mm/h × 24 = 72 mm/day.
Q: How do I convert mm/h to mm/s?
A: Divide by 3 600, because there are 3 600 seconds in an hour. For example, 18 mm/h ÷ 3 600 ≈ 0.005 mm/s.
Q: Why use millimeter per hour instead of millimeter per second?
A: For very slow changes, the numbers in mm/s would be very tiny and hard to read. Using mm/h gives more comfortable numbers that are easier to understand and compare.
Q: Is millimeter per hour an SI unit?
A: Millimeter and second are SI units. Hour is accepted for use with SI, but it is not the main time unit. So mm/h is an accepted practical unit, but not a base SI unit.