The next series of Hints will examine weather and time of day. The 5 time intervals that the writings will be broken down into include morning daylight, afternoon, evening, early night and late night. The series will focus on generalities that can be expected in the weather for each of these time periods. MORNING DAYLIGHT The morning daylight extends from when the sun comes up to local noon time. This range of time is going to depend on season and latitude since sunrise time varies greatly depending on the season and latitude. In general, for many locations this time frame will start from between 5 to 8 am and extend until noon. This 4 or so hour range of time can experience significant changes in the weather. One generality is that the weather tends to be coolest during the early portion of this time frame. Often the overnight low temperature will occur around sunrise. This is because cooling overnight is cumulative. With no solar radiation, the temperature cools during the night and is not able to recover the dropping temperatures until the sun rises and gets high enough in the sky to start warming the surface. Another generality is that the weather tends to be stable during the early portion of this time frame but decreases significantly in stability by noon. Thunderstorms are not as common after sunrise due to it typically being too stable for storms to develop. The colder temperatures at the surface help create an inversion which makes the lower troposphere stable. After sun rise, the increasing sun angle will help warm the surface. By noon, the temperature can be significant warmer than it was at sunrise. Often the greatest temperature jump during the day occurs in the morning hours. The much warmer temperatures at noon mean the troposphere is less stable or may even be unstable. Cumulus clouds will often start developing once the conditions are unstable. A third generality is that the wind tends to be light in the early portion of this time frame but quickly picks up in intensity by late morning. The stable conditions contribute to lighter wind. On many mornings to start, the wind will be calm or nearly calm. Once the sun rises and mixes out the cool and stable air right at the surface, wind speeds can pick up significantly. AFTERNOON Afternoon extends from noon to sunset. Since sunset time varies significantly depending on season and latitude, the sunset time for most locations will be in the range from about 5 pm to 9 pm. Thus, afternoon will typically be short in the winter but can be very long in the summer. This writing will focus on weather generalities that occur in the afternoon hours. This time of day typically has the warmest temperatures. Heating is cumulative throughout the day thus the maximum temperature for the day usually occurs around mid-afternoon. At this point in the day the sun is still high enough in the sky to continue to warm the surface and with the addition of the solar energy that has accumulated up to that point. If the atmosphere becomes unstable, it is the afternoon that the atmosphere is most likely to become unstable and be the most unstable. Thus, thunderstorms and severe thunderstorms are often most frequent this time of day. The atmosphere near ground level tends to be well mixed in the afternoon due to rising thermals from solar heating. There tends to be at least some wind this time of day due to the mixing of the air. Rising thermals will also promote cloud growth this time of day. Many days will start off sunny but be at least partly cloudy by afternoon due to the rising thermals. This is especially true if plenty of moisture is in place. If it is stable with moisture in place, the opposite may occur where the morning starts off cloudy and then it is sunny by afternoon due to solar energy mixing out the saturated air. Since temperatures are typically warmest this time of day, the relative humidity tends to be lowest. The drying power of the air will tend to be highest in the afternoon. Items that need to air dry will tend to dry fastest in the afternoon, especially if exposed to solar energy. Once the sun rises and mixes out the cool and stable air right at the surface, wind speeds can pick up significantly. EVENING Evening has a time frame from sunset to bedtime. Since bedtime is highly variable, a general time frame for the end of evening is sometime between 9 and Midnight. Sunset is highly variable also since it depends on the time of year and latitude. What is considered evening will vary greatly from one person to the next as far as when it begins and when it ends. For this writing, evening will be taken to be from 6 pm to 10 pm. When it comes to weather, this time of day is typically when the temperature cools off, the troposphere starts to stabilize and the character of storms changes. One generality is that temperature gradually cools. Once the sun sets, the ability to warm temperature through solar radiation ends. On a typical day this means the temperature will cool off since the ground cools by emitting longwave radiation. The cooling off tends to not be as abrupt as the warming in the morning hours. Typically the temperature will gradually cool off hour by hour from sunset to sunrise. The low temperature for the day typically does not occur in the evening but occurs around sunrise. Since the temperature cools off, this begins the process of the atmosphere stabilizing. The threat from severe weather tends to be less in the evening as compared to the afternoon. This is a generalization though since strong storms and convective systems do often continue into the evening hours. The cooling of the ground though reduces instability and this contributes to severe storms being less common in the evening as compared to the afternoon. While severe storms are less common in the evening as compared to the afternoon, significant storm events still occur. Storms with heavy rain and lightning can occur from convective systems, lifting mechanisms that move through in the evening and from storms that have held together from the afternoon and into the evening. EARLY NIGHT The early night is the time from about 10 pm to 2 am. Typically, temperatures will continue to cool off during this time frame. This is a time where the atmosphere will often reset from being less stable to much more stable. The wind speeds tend to be lighter due to less mixing of air and stable air. Light wind also helps the temperature at the surface to further cool. When rain and storms occur during this time frame is often occurs from dynamic lifting mechanisms such as fronts. Instability tends to be less due to cooler temperatures, thus thunderstorms are not as common in this time frame. When thunderstorms do occur they will often be from a convective complex that has held together into the early night or from dynamic lifting such as from a cold front. The relative humidity typically increases during this time frame since the air temperature is cooling closer to the dewpoint. Dew or frost can begin to develop in this time frame if there is enough cooling and a high enough dewpoint. If a snow event occurs, this is a good time for the snow to accumulate since there is no solar radiation to melt the snow and the ground can continue to cool from longwave energy emission. LATE NIGHT Late night can be taken to occur from about 2 am to sunrise. This is the time of the day that typically the low temperature will occur. The low temperature often occurs around sunrise. Typically the temperature will cool hour by hour during the night time hours and reach a minimum around dawn. This is the traditional time of the day that the low temperature occurs. If the low does not occur around this time it is known as a “non-traditional” time for a low temperature. This can happen for example when a front comes through and causes the low temperature to occur at a different time of the day. The wind tends to be lightest at this time of night since the air is typically stable. Sunrise is often accompanied with light or calm winds, no precipitation and a high relative humidity. With high humidity, low clouds can be quite common around sunrise. With stable conditions, thunderstorms are much less likely during the late night than in other times of the day. Exceptions can occur such as when a front moves through during the late night hours. This can contribute to stronger wind and storms. The weather tends to not be too threatening during this part of the day and most people tend to be asleep during this time. Thus, it is a time that it is dark and the least amount of people will be observing the weather. It is typically a quiet time of the day weatherwise. The late night is the most common time of the day that dew or frost develop. Dew forms when the temperature drops to the dewpoint and the relative humidity reaches 100%. Continued cooling will cause moisture to condense out of the air. If the dewpoint is below freezing then it is known as a frost point. When the temperature drops to the frost point then frost will develop from moisture going from the vapor state to the frozen state. In some situations the temperature can cool to a dewpoint that is above freezing and then continued overnight cooling causes the temperature to drop below freezing. In this situation dew will become frozen and there can be both frozen dew and frost that is observed. |