Why Does the Low Temperature Often Occur Just After Sunrise?
by Steve Horstmeyer, Meteorologist, Cincinnati, OH
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A Few Important Points
- LOW TEMPERATURES often occur shortly after sunrise, that is shortly after the sun has begun to heat earth.
- THIS IS TRUE mainly for clear and calm nights.
- ON CLOUDY NIGHTS clouds reflect heat re-radiated by the ground and clouds themselves radiate heat downward, this tends to even the temperature fall so the post-sunrise downward spike on the graph above may not be present.
- ON WINDY NIGHTS the air is constantly stirred, that is mixed so the temperature tends to fall more evenly, and not get as cool.
The Sun Does NOT Heat the Air (Directly, That Is)
The radiant energy from the sun (meteorologists call it insolation) does not heat the air directly, it heats the ground which then through conduction heats the lowest layers of the air. As the day progresses the air is heated from below and the air warms up from the ground. On windy days there is considerable mixing and the pattern is obscured, but on days that are calm, or nearly so, the upward warming can be easily predicted.
This is NOT Instantaneous it takes some time for this to happen and for the heat to reach thermometer level (about 5' above the ground.)
This is called a TEMPERATURE TIME LAG and the amount of time lag there is between heating the air and when the sun rises is least over warm, dry soil and greatest over snow covered ground.
Because some heat goes to EVAPORATE SOIL MOISTURE the lag time is longer when the soil is wet.
Because FRESH SNOW COVER reflects almost all insolation the ground may never begin to heat the air over fresh snow.
This explains why THE HIGH TEMPERATURE OCCURS LATER THAN the time of maximum solar heating, (at local noon - In Cincinnati this is about 12:37PM EST, and hour later EDT and is determined by longitude east or west of the meridian that determines your time zone - the 75th in the case of the U.S. Eastern Time Zone).
AT NIGHT THE PROCESS IS REVERSED the ground cools by re-radiation, and heat is conducted FROM the air TO the ground.
So the coolest temperatures are found near the ground. As the sun rises it takes a few minutes for the ground to heat the air, and the heat to work its way upward to thermomenter level.
This may not be the case on windy and/or cloudy nights. On windy nights turbulence mixes the air, so the air cooled by the ground is mixed with air that is warmer a bit higher, this keeps the temperature warmer near the ground and a bit cooler higher. Cool dense air cannot pool as easily near the ground so mixing actually keeps the air within a few feet of the ground warmer.
On cloudy nights clouds maintain a higher temperature two ways:
- Clouds reflect heat radiated by the ground
- Clouds radiate heat back towards the ground
In this way clouds act as a blanket ot keep the air and ground below them warmer at night. Thicker and lower clouds retain the heat Earth gained during the day more effectively than higher, thinner clouds.
© 2008 Steven L. Horstmeyer, all rights reserved
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